Photography Forums > How to Calculate Zoom

# How to Calculate Zoom

Dave
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Posts: n/a

 06-13-2006, 12:11 AM
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

Jeremy Nixon
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 06-13-2006, 12:17 AM
Dave <(E-Mail Removed)> 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 | (E-Mail Removed)

Marc Sabatella
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 06-13-2006, 12:25 AM
> 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
(E-Mail Removed)

Music, art, & educational materials
Featuring "A Jazz Improvisation Primer"
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All Things Mopar
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 06-13-2006, 12:59 AM
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"

Gerrit 't Hart
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Posts: n/a

 06-13-2006, 01:32 AM

"Marc Sabatella" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed). ..
> > 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

JPS@no.komm
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 06-13-2006, 03:55 AM
In message <mmnjg.900\$(E-Mail Removed)>,
Dave <(E-Mail Removed)> 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 <(E-Mail Removed)>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

Marc Sabatella
Guest
Posts: n/a

 06-13-2006, 06:04 PM
> 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
(E-Mail Removed)

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

Jeff Rife
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 06-13-2006, 07:20 PM
Marc Sabatella ((E-Mail Removed)) 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 |

My Names Nobody
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Posts: n/a

 06-13-2006, 07:41 PM

"Dave" <(E-Mail Removed)> wrote in message news:mmnjg.900\$(E-Mail Removed)...
> 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

Guest
Posts: n/a

 06-13-2006, 08:11 PM

My Names Nobody wrote:
> "Dave" <(E-Mail Removed)> wrote in message news:mmnjg.900\$(E-Mail Removed)...
> > 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

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