# How to Calculate Zoom

Discussion in 'Digital SLR' started by Dave, Jun 13, 2006.

1. ### DaveGuest

Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
with a 4X or ~7X zoom equil. How do you calculate the ?X zoom
factor for a zoom lense?

thanks

Dave, Jun 13, 2006

2. ### Jeremy NixonGuest

Dave <> wrote:

> Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> with a 4X or ~7X zoom equil. How do you calculate the ?X zoom
> factor for a zoom lense?

Maximum focal length divided by minimum focal length.

Most SLR lenses aren't rated that way, though. They usually just give the
actual focal length numbers, which are a lot more meaningful.

--
Jeremy |

Jeremy Nixon, Jun 13, 2006

3. ### Marc SabatellaGuest

> Looking a purchasing a new zoom lense for a rebel xt. I see some
> lenses with a 4X or ~7X zoom equil.

??? This is not how lenses for SLR's are usually labelled. They are
labelled with the actual focal length range: 18-55, 70-300, 28-80, etc.
A lens with a range of 18-55 means its focal length is 18mm when zoomed
"in" and 55mm when zoomed "out". If you wanted to calculate a single
number to say how much more magnification you can get when zoomed "out"
versus "in", simply divide. The 18-55 lens that usually comes with the
camera is basically a 3X zoom. But then, so is a 100-300 zoom - just
when in which even zoomed "in" provides more magnification than the
former lens is zoomed "out". A point & shoot camera with a 3x zoom is
likely to be similar to the 18-55 in terms of what focal lengths it is
providing.

---------------
Marc Sabatella

Music, art, & educational materials
Featuring "A Jazz Improvisation Primer"
http://www.outsideshore.com/

Marc Sabatella, Jun 13, 2006
4. ### All Things MoparGuest

Today, with great enthusiasm and quite emphatically, Dave laid
this on an unsuspecting readership ...

> Looking a purchasing a new zoom lense for a rebel xt. I see
> some lenses
> with a 4X or ~7X zoom equil. How do you calculate the ?X
> zoom
> factor for a zoom lense?
>

divide the longest focal length in mm by the shortest - voila! zoom
ratio! The "equivalent" refers to the smaller frame size for most
digital cameras requiring you to convert the focal lenghts you want
for your non-full frame digital to their true visual effect when
used on a full-frame digital or a 35mm film SLR

--
ATM, aka JerryR

"Those who can, do. Those who can't, teach. Those who can do
neither, administrate and become 'experts'" - National Education
Association

To reply by E-mail, replace "SPAM" with "com"

All Things Mopar, Jun 13, 2006
5. ### Gerrit 't HartGuest

"Marc Sabatella" <> wrote in message
news:...
> > Looking a purchasing a new zoom lense for a rebel xt. I see some
> > lenses with a 4X or ~7X zoom equil.

>
> ??? This is not how lenses for SLR's are usually labelled. They are
> labelled with the actual focal length range: 18-55, 70-300, 28-80, etc.
> A lens with a range of 18-55 means its focal length is 18mm when zoomed
> "in" and 55mm when zoomed "out". If you wanted to calculate a single
> number to say how much more magnification you can get when zoomed "out"
> versus "in", simply divide. The 18-55 lens that usually comes with the
> camera is basically a 3X zoom. But then, so is a 100-300 zoom - just
> when in which even zoomed "in" provides more magnification than the
> former lens is zoomed "out". A point & shoot camera with a 3x zoom is
> likely to be similar to the 18-55 in terms of what focal lengths it is
> providing.
>

Have I had it wrong all these years?

I have always believed that "zooming in" means that you magnify the picture.
To do that you use the larger focal length number end of the zoom lens. So
on a 28 - 200 lens zoomed "in" would be at the 200mm end of the zoom.

Gerrit

Gerrit 't Hart, Jun 13, 2006
6. ### Guest

In message <mmnjg.900\$>,
Dave <> wrote:

>Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> with a 4X or ~7X zoom equil. How do you calculate the ?X zoom
>factor for a zoom lense?

You divide the longest focal length by the shortest.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

, Jun 13, 2006
7. ### Marc SabatellaGuest

> Have I had it wrong all these years?
>
> I have always believed that "zooming in" means that you magnify the
> picture.

Actually, in that context, that's how I would use the term too. But
here I was thinking more of the physical action of the lens - one "zooms
in" on a scene in the sense you mean by twisting the zoom ring to make
the lens extend "out" from the camera. In any case, the term isn't
really a technical one, nor is the idea of computing a "zoom ratio
factor" one that ordinarily concerns SLR photographers - we're generally
more concerned with the specific focal lengths than what the ratio of
biggest to smallest is. But in a point & shoot world, where it's assume
that all cameras will zoom from some sort of wide angle to some sort of
telephoto, and in which differing sensor sizes make specific focal
length numbers meaningless for comparison purposes (unless converted to
35mm equivalents), it is of course a very common marketing metric.

---------------
Marc Sabatella

Music, art, & educational materials
Featuring "A Jazz Improvisation Primer"
http://www.outsideshore.com/

Marc Sabatella, Jun 13, 2006
8. ### Jeff RifeGuest

Marc Sabatella () wrote in rec.photo.digital.slr-systems:
> But
> here I was thinking more of the physical action of the lens - one "zooms
> in" on a scene in the sense you mean by twisting the zoom ring to make
> the lens extend "out" from the camera.

Although this is the way it works for probably 99% of zoom lenses, I have
a Sigma 24-70/f2.8 that is shortest at 60mm. It lengthens very slightly
(less than 5mm) as you move to 70mm, and lengthens a *lot* (about 40mm)
when you move to 24mm.

Yeah, it's weird, but it's a good lens, and in the Minolta AF world,
there's really nothing else in that range that comes close in quality.

--
Jeff Rife |

Jeff Rife, Jun 13, 2006
9. ### My Names NobodyGuest

"Dave" <> wrote in message news:mmnjg.900\$...
> Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor for
> a zoom lense?
>
> thanks

I think that the answers being given aren't answering the question being

OK lets say some simple point and shoot camera (or any camera/lens that is
not magnifying/zooming) does not magnify the object you are photographing.
We will call this Zero Magnification (What we see with the naked eye?).

The question I want an answer to, is not what the zoom ratio of a lens
compared to itself, but rather the number of times that "Zero Magnification"
photograph would be "zoomed" multiplied if that photo were taken with a lets
say 400mm lens (lets forget the digital crop factor for now, that is an
easy equation)?

We all know that a Canon EOS 30D with a 400mm lens zooms in a lot closer
(magnifies the object many more times) than say a 12x lens on a Kodak P850.

These point and shoot cameras and big lenses for SLR cameras magnification
certainly can be compared, as one would compare small and large binoculars
magnification.

Does anyone have any way to calculate a base line "Zoom" factor in this
manor rather than comparing the range of a specific lens?

Thanks

My Names Nobody, Jun 13, 2006
10. ### Guest

My Names Nobody wrote:
> "Dave" <> wrote in message news:mmnjg.900\$...
> > Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor for
> > a zoom lense?
> >
> > thanks

>
>
> I think that the answers being given aren't answering the question being

(...snip...)

> Does anyone have any way to calculate a base line "Zoom" factor in this
> manor rather than comparing the range of a specific lens?

Sure - but you need to pick a baseline for comparison.

What you're really looking at here is the angle of view - how much of
the world in front of you is ending up in the final picture. A very
long lens / a very powerful zoom will give you a very small angle of
view - a very small amount of the world in fornt of you ends up on the
photo, getting "enlarged" in the process.

The two things that affect angle of view are the lens's focal length
(measured in mm) and the size of the image sensor or film frame. To
make things easier to compare, people often talk about "35mm
equivalent" focal length - comparing the focal length to the focal
length required to get the same image on a standard 35mm film camera.
For instance, a 50mm lens on a 35mm camera is roughly equivalent to a
31mm lens on some DSLR cameras, or an 8.3mm lens on a Canon S3-IS. A
site like dpreview.com (and some camera spec sheets) will give you the
35mm equivalent focal lengths for compact cameras: DPReview describes
the Canon S3 IS as having a "36-432mm equiv." lens (the actual lens is
6-72mm). The Digital Rebel's 18-55mm kit lens is a 29-88mm equiv. when
mounted on a Digital Rebel.

So...once you have 35mm equivalents for all the cameras/lenses you want
to compare, you need a baseline to work from (assuming you want numbers
of the type "3X"). A reasonable starting place is 50mm - this is
generally considered to be a "normal" lens on a 35mm camera.

Working from that: the Canon S3 IS is 432/60 = 8.6X when zoomed in all
the way, and 36/50 = 0.7X when zoomed out all the way. The Digital
Rebel kit lens, mounted on a Digital Rebel, is 1.8X when zoomed in, and
29/50 = 0.6X when zoomed out.

But really, once you have 35mm equivalents, there's not much benefit to
calculating them as numeric "X" factors, especially since they need to
be calculated from an arbitrary base point. The equivalent focal
lengths alone are all the information you need, and this is why you'll
often see them quoted.

- Darryl

, Jun 13, 2006
11. ### DoN. NicholsGuest

According to My Names Nobody <>:
>
> "Dave" <> wrote in message news:mmnjg.900\$...
> > Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor for
> > a zoom lense?
> >
> > thanks

>
>
> I think that the answers being given aren't answering the question being
>
> OK lets say some simple point and shoot camera (or any camera/lens that is
> not magnifying/zooming) does not magnify the object you are photographing.
> We will call this Zero Magnification (What we see with the naked eye?).
>
> The question I want an answer to, is not what the zoom ratio of a lens
> compared to itself, but rather the number of times that "Zero Magnification"
> photograph would be "zoomed" multiplied if that photo were taken with a lets
> say 400mm lens (lets forget the digital crop factor for now, that is an
> easy equation)?
>
> We all know that a Canon EOS 30D with a 400mm lens zooms in a lot closer
> (magnifies the object many more times) than say a 12x lens on a Kodak P850.
>
> These point and shoot cameras and big lenses for SLR cameras magnification
> certainly can be compared, as one would compare small and large binoculars
> magnification.
>
> Does anyone have any way to calculate a base line "Zoom" factor in this
> manor rather than comparing the range of a specific lens?

Well ... a so-called "normal" lens on full-frame 35mm is a 50mm
(or close to that), so you can calculate the number you want by dividing
50 into 400, giving a "magnification factor" of 8.

However, for a Nikon digital SLR, with a crop factor of 1.5, a
"normal" lens would be about 33.3mm, so that, divided into 400 would
give a magnification factor of 12, on *that* camera.

Since the Cannon seems to more often have a crop factor of 1.6,
on that camera, a "normal" lens would be closer to 31.25mm, and that,
dividied into 400mm would give a magnification factor of 12.8

Of course, this makes a 50mm lens on a Nikon DSLR a mild
telephoto (giving you the 1.5 crop factor as the magnification factor.)

So -- you have to re-calculate it for each sensor size.

But -- you have the tools above to allow you to do so for each
camer and lens combination you come across.

I hope that this helps,
DoN.
--
Email: <> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

DoN. Nichols, Jun 13, 2006
12. ### My Names NobodyGuest

<> wrote in message
news:...
>
> My Names Nobody wrote:
>> "Dave" <> wrote in message
>> news:mmnjg.900\$...
>> > Looking a purchasing a new zoom lense for a rebel xt. I see some
>> > lenses
>> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor
>> > for
>> > a zoom lense?
>> >
>> > thanks

>>
>>
>> I think that the answers being given aren't answering the question being

>
> (...snip...)
>
>> Does anyone have any way to calculate a base line "Zoom" factor in this
>> manor rather than comparing the range of a specific lens?

>
> Sure - but you need to pick a baseline for comparison.
>

(...snip...)
> - Darryl
>

The baseline for comparison is equipment non specific, ZERO magnification,
what the naked eye sees.

Why is it that binocular, rifle scope, spotting scope and video camera
makers all seem to easily come up with a baseline for comparison, and yet
that seems to be near impossible for SLR camera people?

Can't the baseline for comparison be equipment non specific?
Say a 1 inch bumble bee (or white square) at 30 feet is 1/32 of an inch at
100% with no magnification (what the naked eye sees). With X lens/camera
that same 1 inch bumble bee at 30 feet is 1/2 of an inch at 100% 15X zoom?
Or with X lens/camera that same 1 inch bumble bee at 30 feet is 1 foot at
100% 384X zoom?

Then you could rather easily chart that say a 400mm lens is the equivalent
to 260X (or whatever it is) zoom on a 35 millimeter camera body (isn't that
the de facto camera body baseline anyway?)
IE: A 35 millimeter camera body with a 400mm lens is the equivalent to 260X
(or whatever it is) zoom.

Seems someone should have already done this?

Does anyone have any way to calculate a equipment non specific, "Zoom"
factor in this manor rather than comparing equipment specifics?

My Names Nobody, Jun 13, 2006
13. ### Dr. BoggisGuest

In article <4yEjg.37573\$%m5.15520@trnddc04>,
"My Names Nobody" <> wrote:

> "Dave" <> wrote in message news:mmnjg.900\$...
> > Looking a purchasing a new zoom lense for a rebel xt. I see some lenses
> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor for
> > a zoom lense?
> >
> > thanks

>
>
> I think that the answers being given aren't answering the question being
>
> OK lets say some simple point and shoot camera (or any camera/lens that is
> not magnifying/zooming) does not magnify the object you are photographing.
> We will call this Zero Magnification (What we see with the naked eye?).

No magnification would be magnification x1, surely? If it's zero times
the amount you see with the naked eye, you'd see nothing
--
My random photostream: http://www.flickr.com/photos/boggissimo/

Dr. Boggis, Jun 13, 2006
14. ### DoN. NicholsGuest

According to My Names Nobody <>:
>
> <> wrote in message
> news:...
> >
> > My Names Nobody wrote:
> >> "Dave" <> wrote in message
> >> news:mmnjg.900\$...
> >> > Looking a purchasing a new zoom lense for a rebel xt. I see some
> >> > lenses
> >> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor
> >> > for
> >> > a zoom lense?
> >> >
> >> > thanks
> >>
> >>
> >> I think that the answers being given aren't answering the question being

> >
> > (...snip...)
> >
> >> Does anyone have any way to calculate a base line "Zoom" factor in this
> >> manor rather than comparing the range of a specific lens?

> >
> > Sure - but you need to pick a baseline for comparison.
> >

> (...snip...)
> > - Darryl
> >

>
> The baseline for comparison is equipment non specific, ZERO magnification,
> what the naked eye sees.

*Whose* naked eye? The actual size of the image on the retina
is a function of the size of the individual eye. You are not seeing a
*size* at a distance, but an *angle*.

> Why is it that binocular, rifle scope, spotting scope and video camera
> makers all seem to easily come up with a baseline for comparison, and yet
> that seems to be near impossible for SLR camera people?

Quite simple -- the instruments which you make (other than the
video camera) are designed to go in front of human optical equipment
(eyes), and they *increase* the size of the image by a specific factor
on *anybody's* retina.

And the video cameras are not giving you zoom related to a
"normal" view, but rather a ratio between the widest and the narrowest
angles of view.

35mm camera lenses, are designed to produce an image on a
sensor. Originally film, and those lenses are now being used to form
images on the digital sensors of cameras made to accept the same lens
mount.

Note that you can't simply hold a telephoto lens in front of
your eye and see a larger image. It has to be formed on something, even
a virtual image (in air) with a secondary lens to reformat it for
feeding into the individual's eye. And if you use the secondary lens,
the magnification which you see looking though the combination of the
secondary lens and your camera's lens is going to be a function of both
the focal length of the camera's lens and the magnification of the
eyepiece. (This is equivalent to changing the size of the sensor in the
camera.

And photographers who have come to this from the 35mm world have
a natural feel for what a given focal length lens will give them, and
only have to adjust their expectations for the size of the crop factor
for their particular camera.

Now -- there are optical devices for 35mm and DSLR cameras which
*are* rated with a magnification factor. Those are telextenders, which
go between the body and the lens which you normally use, and they
increase the size of the image from a given lens by such a factor. (I
think that the most common factors are 1.5X and 2.0X.) In the process,
they also reduce the effective maximum aperture of the lens -- you get
less light in exchange for greater image size.

> Can't the baseline for comparison be equipment non specific?
> Say a 1 inch bumble bee (or white square) at 30 feet is 1/32 of an inch at
> 100% with no magnification (what the naked eye sees).

1/32 of an inch on *what*? As I mentioned above, the size of
the image on the retina of the eye depends on the physical dimensions of
the individual's eye. The one inch bumble bee at 30 feet is not a
*size*, it is an angle. In particular, nine minutes, 32 seconds, or
0.1592 degrees in decimal. The size of the image produced from that
angle is a function of the focal length of the lens, but "no
magnification" is meaningless here. In my previous followup elsewhere
in this thread, I explained how you need to pick a standard 50mm for a
35mm film camera) which is close to producing the same angle of view (on
a print of a given size, viewed at a specific distance). It is this
which defines a "normal" lens for a given sensor. And everything else
needs to be compared to that.

> With X lens/camera
> that same 1 inch bumble bee at 30 feet is 1/2 of an inch at 100% 15X zoom?
> Or with X lens/camera that same 1 inch bumble bee at 30 feet is 1 foot at
> 100% 384X zoom?

Again -- a function of what you call a "normal" for that
camera/sensor.

> Then you could rather easily chart that say a 400mm lens is the equivalent
> to 260X (or whatever it is) zoom on a 35 millimeter camera body (isn't that
> the de facto camera body baseline anyway?)

A 50mm focal length lens is considered by most to be "normal" on
a 35mm film camera body. There are others who prefer something
somewhere in the range between 42mm and 50mm.

A normal lens on other cameras is a function of the size of the
film. IIRC, a "normal" lens on a 2-1/4" square camera (TLR or something
like a Hasselblad) is 80mm. The lens which came with my 4x5" Crown
Graphic is 135mm -- certainly within the "telephoto" range for 35mm
film.

> IE: A 35 millimeter camera body with a 400mm lens is the equivalent to 260X
> (or whatever it is) zoom.

Actually -- it is closer to an 8X magnification -- when compared
to a 50mm lens.

> Seems someone should have already done this?

It seems to me that the normal users of SLRs have found that the
existing information -- the actual focal length range of the zoom -- is
much more usable information.

And remember that the built-in zoom lenses in digital cameras
are rated not in magnification relative to "normal", but rather
magnification change from widest to narrowest field of view.

> Does anyone have any way to calculate a equipment non specific, "Zoom"
> factor in this manor rather than comparing equipment specifics?

You *must* include equipment specifics to get the kind of number
which you are looking for. Anything else is meaningless.

Enjoy,
DoN.
--
Email: <> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

DoN. Nichols, Jun 13, 2006
15. ### Steve LeoGuest

Wouldn't you divide the "normal" lens for your camera (in the case of Full
Frame Camera -- 50MM into the longest zoom say 300mm & get 6X zoom? Or in
the case of a 1.6 sensor which has a "normal" lens of about 30mm divided
into 300mm would give you a 10X zoom. I believe that is what the OP wanted
to know. Of course I could be wrong on this.

"All Things Mopar" <> wrote in message
> Today, with great enthusiasm and quite emphatically, Dave laid
> this on an unsuspecting readership ...
>
>> Looking a purchasing a new zoom lense for a rebel xt. I see
>> some lenses
>> with a 4X or ~7X zoom equil. How do you calculate the ?X
>> zoom
>> factor for a zoom lense?
>>

> divide the longest focal length in mm by the shortest - voila! zoom
> ratio! The "equivalent" refers to the smaller frame size for most
> digital cameras requiring you to convert the focal lenghts you want
> for your non-full frame digital to their true visual effect when
> used on a full-frame digital or a 35mm film SLR
>
> --
> ATM, aka JerryR
>
> "Those who can, do. Those who can't, teach. Those who can do
> neither, administrate and become 'experts'" - National Education
> Association
>
> To reply by E-mail, replace "SPAM" with "com"

Steve Leo, Jun 14, 2006
16. ### Jeremy NixonGuest

My Names Nobody <> wrote:

> Why is it that binocular, rifle scope, spotting scope and video camera
> makers all seem to easily come up with a baseline for comparison, and yet
> that seems to be near impossible for SLR camera people?

It's not impossible, it's just not a useful metric, so no one does it.

> Can't the baseline for comparison be equipment non specific?

Yes, it can, but for YOU to figure out the optical magnification given
a lens and camera spec, you need to define 1x magnification in terms of
something you know. So you start with the "normal" lens focal length
for the format, which is 1x magnification, and then you can figure out
the magnification factor provided by a given focal length.

> Then you could rather easily chart that say a 400mm lens is the equivalent
> to 260X (or whatever it is) zoom on a 35 millimeter camera body (isn't that
> the de facto camera body baseline anyway?)
> IE: A 35 millimeter camera body with a 400mm lens is the equivalent to 260X
> (or whatever it is) zoom.

Camera lenses are *never* specified with that kind of magnification factor,
so no, you can't do that kind of comparison. When a P&S camera is said to
have a "10x zoom", that does *not* mean 10x magnification; it means that
the longest focal length is 10x the shortest focal length.

If you do the above procedure to figure out the optical magnification, the
resulting number is not useful for comparison with any camera's zoom specs.

--
Jeremy |

Jeremy Nixon, Jun 14, 2006
17. ### My Names NobodyGuest

"Dr. Boggis" <> wrote in message
news:...
> In article <4yEjg.37573\$%m5.15520@trnddc04>,
> "My Names Nobody" <> wrote:
>
>> "Dave" <> wrote in message
>> news:mmnjg.900\$...
>> > Looking a purchasing a new zoom lense for a rebel xt. I see some
>> > lenses
>> > with a 4X or ~7X zoom equil. How do you calculate the ?X zoom factor
>> > for
>> > a zoom lense?
>> >
>> > thanks

>>
>>
>> I think that the answers being given aren't answering the question being
>>
>> OK lets say some simple point and shoot camera (or any camera/lens that
>> is
>> not magnifying/zooming) does not magnify the object you are
>> photographing.
>> We will call this Zero Magnification (What we see with the naked eye?).

>
> No magnification would be magnification x1, surely? If it's zero times
> the amount you see with the naked eye, you'd see nothing
> --
> -Take out Ron to reply-
> My random photostream: http://www.flickr.com/photos/boggissimo/

Zero times? I wrote zero magnification, not zero times.
As defined by the dictionary, zero magnification is correct, surely?

magnification
noun
1 The act of magnifying or the state of being magnified.
2 The process of enlarging the size of something, as an optical image.
3 Something that has been magnified; an enlarged representation, image, or
model.
4 The ratio of the size of an image to the size of an object.
5 A photographic print that has been enlarged

zero
noun
1 A cardinal number indicating the absence of any or all units under
consideration
2 An ordinal number indicating an initial point or origin
Zero magnification means just what is says, What we see with the naked eye
is not magnified, not enlarged.

Can you add any input as to a way to calculate a universal, equipment non
specific, "Zoom" factor?

My Names Nobody, Jun 14, 2006
18. ### Guest

My Names Nobody wrote:
> <> wrote in message
> news:...
> >
> > My Names Nobody wrote:
> >> Does anyone have any way to calculate a base line "Zoom" factor in this
> >> manor rather than comparing the range of a specific lens?

> >
> > Sure - but you need to pick a baseline for comparison.
> >
> > - Darryl
> >

>
> The baseline for comparison is equipment non specific, ZERO magnification,
> what the naked eye sees.
>
> Why is it that binocular, rifle scope, spotting scope and video camera
> makers all seem to easily come up with a baseline for comparison, and yet
> that seems to be near impossible for SLR camera people?

Because you look through a telescope, but you print out a photo. When I
look through a pair of binoculars, I am enlarging the image on my
retina, to be (say) 10x the size of the original. "Huh," I say, "these
binoculars have 10x magnification."

But when I take a picture, I can print it at wallet size, or 4x6, or
8x10, or 20x30. I can look at it from six inches away or from the other
side of the room. Every time I do that, the image changes size on my
retina, occupying more (or less) of my field of view.

Now, this doesn't mean you can't define a 1x magnification. I could
(watch me!) arbitrarily say that "A 1x lens will allow me to take a
photo of a tree, print it out at 8"x10", hold it in landscape
orientation at arm's length, and have the resulting image look exactly
the same size as it did when I was standing there."

No problem; we can do that. Assuming your arm is one metre long, that
10" wide photo is going to take up about 10 degrees of your field of
view ( atan(17.5 cm / 100 cm) = 9.93 degrees ). So, if the camera lens
had a 10 degree field of view, I should be able to print out the
picture, hold it in front of me, and have it blend seamlessly into the
surroundings. Tada, it's 1x magnification! As it turns out, a 200mm
lens on a 35mm camera has about a 10-degree field of view, so without

A 200mm lens on a 35mm camera is a 1x lens
A 50mm lens on a 35mm camera is a 0.25x lens
A 200mm lens on a 35mm camera is a 1.5x lens
A 18mm lens on a APS-C camera is a 0.14x lens
A 6mm lens on a Canon S3 IS is a 0.18x lens

As it turns out, these are pretty silly numbers; more less than 1x than
greater. So apparantly I picked bad assumptions; maybe assuming an
11x14 print at half a meter would have given me numbers more in line
with what we're looking for.

But that's the problem right there. The size of the print, and the
distance at which the print is viewed, changes the apparant
magnification of the image. A telescope doesn't have this problem: You
look through it, and your eye is always in the same place with respect
to the lens. Nothing changes size, nothing changes distance.

That doesn't mean it's not possible; I've just proven that it is. But
it's arbitrary. Change the assumptions you make about print size and
viewing distance, and you change the magnification numbers you get. And
when you examine the process, you realize that all those assumptions
did is tell me that a 200mm lens on a 35mm camera is 1x, and all the
other calculations were made off of that. This is exactly what I
originally suggested: You need to pick a baseline for comparison. My
original choice of a 50mm lens on a 35mm camera was based on a common
photographic tradition; my new choice is based on some calculations

So instead, we measure lens magnification based on the lens's focal
length. This is an inherent property of the lens. A 35mm photographer
will know that a 100mm lens makes things big and a 28mm lens makes
things small. As odd-sized digital sensors got common, the idea of a
"35mm equivalent" focal length gained popularity, exactly so that
people had a good non-ambiguous way of understanding the optical
effects of their lenses when combined with their cameras.

> Can't the baseline for comparison be equipment non specific?

Not really; if nothing else, the camera is important. I have a 50mm
lens that I use on my Digital Rebel. If I took a picture with it, and
then gave the lens to my sister, she could take a picture with it with
her 35mm film camera from the same location, and when we printed the
pictures, hers would look more "zoomed out" than mine. Canon can't
stamp a big "1x" on the side of the lens, because it's going to behave
differently depending on the camera you put behind it. But they can
engrave "50mm" on the lens; that never changes, and it's up to the
photographer to understand what that means when it's combined with his
or her camera.

Looking at it another way: 35mm equivalent focal lengths -are-
equipment non-specific. Every lens/body combination has a 35mm
equivalent focal length, which describes exactly how much "zoom" you
will get. Why 35mm? Why not? Most photographers are familiar with it,
and those that aren't can learn it or convert it to some system they
know better (like APS-C equivalent, or
angle-of-view-when-printed-at-8-x-10-and-held-a-meter-away equivalent).

> Say a 1 inch bumble bee (or white square) at 30 feet is 1/32 of an inch at
> 100% with no magnification (what the naked eye sees). With X lens/camera
> that same 1 inch bumble bee at 30 feet is 1/2 of an inch at 100% 15X zoom?

This goes back to the bulk of my response, up above: 1/2 an inch on
what size print? On the film negative (or digital sensor) itself? The
frame is rather small, and is almost always enlarged to at least some
degree before you actually look at it.

> Then you could rather easily chart that say a 400mm lens is the equivalent
> to 260X (or whatever it is) zoom on a 35 millimeter camera body (isn't that
> the de facto camera body baseline anyway?)

It's only the common baseline among people that use 35mm cameras,
compact digital cameras, and DSLRs. Talk to a medium- or large-format
photographer (I am neither) and I'd be very surprised if they use 35mm
equivalents when discussing their lenses.

> Seems someone should have already done this?

I just did, results are above, free for the world to use I think
they suck, though. Really, 200mm as a 1x lens? That's just silly.
Improvement on this is left as an exercise for the reader...

> Does anyone have any way to calculate a equipment non specific, "Zoom"
> factor in this manor rather than comparing equipment specifics?

I've only seen this in one photographic context: Macro lenses. They're
commonly quoted as being 0.5x (low-end ones), 1x (most decent ones),
and I've seen as high as 5x. This is comparing the size of the object
in real life to the size of the image on the film/sensor. This means,
though, that when a print is made from a picture taken with a 1x macro
lens, the image on the print will be larger than the object actually is
in reality. The 1x is very counterintuitive here. They're also
described as ratios (e.g. 1:2, 1:1, 5:1), possibly to try to avoid
exactly this confusion.

- Darryl

, Jun 14, 2006
19. ### Guest

My Names Nobody wrote:
> "Dr. Boggis" <> wrote in message
> news:...
> >
> > No magnification would be magnification x1, surely? If it's zero times
> > the amount you see with the naked eye, you'd see nothing

>
> Zero times? I wrote zero magnification, not zero times.
> As defined by the dictionary, zero magnification is correct, surely?
>
> Zero magnification means just what is says, What we see with the naked eye
> is not magnified, not enlarged.
>

Well, to have any type of coherent discussion, it's pretty important to
agree on the basic terms.

Magnification is a ratio. 1:1 magnification is no magnification; the
final image is the same size as the starting image. 2:1 magnification
makes the final image twice as big as the initial image, etc. Those
ratios are equivalent to magnification factors of 1.0 and 2.0; a
magnification of 1.0, or 1:1, or 1x, or however you write it, is indeed
"no magnification".

The problem with writing "zero magnification" is that, while it's
perfectly valid on its own (i.e. it's synonymous with "no
magnification"), the usage of a number opens the door to potential
confusion. The phrase "0.5 magnification" is commonly understood to
mean "half the size"...this raises the question of what "0
magnification" should be.

The phrase "zero magnification" has a perfectly valid meaning according
to the strict dictionary definition, but in this context something like
"1x magnification" or "no magnification" is less likely to cause
confusion.

- Darryl

, Jun 14, 2006
20. ### Marc SabatellaGuest

> The baseline for comparison is equipment non specific, ZERO
> magnification, what the naked eye sees.

That might be the baseline *you'd* like to use for comparison, but it is
*not* the baseline actually used when you see cameras with zoom lenses
quote magnification ratios. A point & shoot camera that advertises a
"7X" zoom is not saying that it magifies things seven times more than
"what the naked eye sees", but rather, that its maximum magnification is
seven times its minimum magnification. Most likely, the minimum
magnification is a little more than half "what the naked eye sees" (to
the extent it makes sense to define this at all - see below), and the
maximum magnification three to four times "what the naked eye sees".
Whereas another zoom lens might be only "4X", but that might mean
minimum is twice "what the naked eye sees" and the maximum is eight
times "what the naked eye sees". Meaning the 4X zoom would actually
magnify more than the 7X. That's why you don't see SLR lenses using
ratios like these. You see real, hard numbers to give focal length.
The 7X zoom I mentioned would actually be sold as 28-200, the 4X as
100-400. Then we could compare the focal length ranges directly to see
that the former did a lot more at the wide angle end of the range but
the latter did a lot more at the telephoto end. This is all relevant
information, and that's why camera lenses are labelled with all of it.

> Why is it that binocular, rifle scope, spotting scope and video camera
> makers all seem to easily come up with a baseline for comparison, and
> yet that seems to be near impossible for SLR camera people?

Video camera zoom ratios are exactly like point & shoot zoom ratios.
That is, they aren't telling you how much a camera magifies comared to
what the naked eye sees, but simply telling how its maximum
magnification compares to its minimum magnification. As for binoculars
and scopes, it is possible to come up with a definitive number for these
because the images are being projected on the same screen: your retina.
Now, if you want to take the lens off your camera and hold it up to your
eye like a telescope, you could similar come up with an absolute number
to measure the magnification. But that number would only be meaningful
when projecting the image onto your retina. It would cease to have
meaning when projecting the image onto a piece of film or a camera
sensor. Because in order for that image to eventually get on to your
retina, it's going to have to be displayed or printed somehow, and you
have no way of knowing how much it is going to be magnified in the
process of displaying or printing it.

> Can't the baseline for comparison be equipment non specific?

Not really, at least, not in the way you want. See below. The best
convention we have is that of a "normal" lens, which as has been
mentioned here, is considered around 50mm on a 35mm film camera, or
around 30-35mm on most digital SLR's. What is relevant in making this
"normal" isn't so much magnification, since any picture can be blown up
bigger, but rather, angle of view - how much you can actually take in at
once without moving your eyes. A normal lens is one that approximates
the angle of view that we typically see. Of course, that differs
between people, and since our vision isn't constant across our field of
view but rather drops off toward the edges (and at different rates for
things close up than things far away, most likely), there is some leeway
in defining even for a given person what his angle of view is. But
there is general agreement that somewhere around 50mm is a "normal"
focal length for most people when projecting an image onto a piece of
35mm film. And since most digital sensors are smaller than that, a
correspondingly smaller focal length is "normal". Of course, it order
for that image to actually *look* normal, we'd have to project them onto
a wall that just barely fills out field of vision. When looking at the
image printed 4x6, we sort of have an inutive sense of what looks
"normal" and what looks like it was taken with a longer or shorter focal
length length, but this can be deceiving. It can be hard to tell the
difference between an object taken with a 50mm from 10 feet away versus
the same object taken with a 200mm from 40 feet away (actually, I'm not
sure I have the math right there - it might be 20 feet, or some other
number involving the square root of 2, to get the object to appear the
same size on a same sized print, but whatever). Still, the notion of a
"normal" lens is the closest you're going to get to "what the naked eye
sees".

So if you want to compute a "magification" factor in this way, divide
the focal length by, say, 33 for most digital SLR's or 50 for a 35mm
film SLR or "full frame" digital. Thus, the aforementioned 28-200 zoom
would not be called a "7X" zoom, but rather, a ".6X - 4X" zoom on a 35mm
camera, or a ".8X - 6X" on most digital SLR's. That is, you'd list the
magnifications at both ends of the zoom range, not just the ratio
between those magnifications. And indeed, one *could* do this. But I'm
not sure I have any intuitve sense of what ".6X" or "4X" looks like.
The only way to know would be get a feel for it through experience. And
one can develop a feel for what "200mm" looks like just as surely as one
can what "4X" looks like. So I don't see what is gined by this
approach.

> Say a 1 inch bumble bee (or white square) at 30 feet is 1/32 of an
> inch at 100% with no magnification (what the naked eye sees).

1/32 of an inch where? On your retina? If you hold out a ruler at
arm's length? This is problem #1 with computing what sort of
magnification we are talking about in absolute terms.

> With X lens/camera that same 1 inch bumble bee at 30 feet is 1/2 of
> an inch at 100% 15X zoom?

1/2 an inch where? On the camera sensor? When you display it on your
comuter screen? When you make a 4x6 print? When you project it on a
wall? That's problem #2 with computing what sort of magification we are

> Does anyone have any way to calculate a equipment non specific, "Zoom"
> factor in this manor rather than comparing equipment specifics?

No, because such a thing is quite impossible to do for the reasons
described. Howeve,r it is trivially simple to divde focal length by 50
or 33 to get something resembling what you are talking about if calling
a 200mm lens a "4X" zoom has more meaning to you than simply calling it
a 200mm lens.

---------------
Marc Sabatella

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Featuring "A Jazz Improvisation Primer"
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Marc Sabatella, Jun 14, 2006