Telephoto Lens Power Compared to Binoculars

How do I figure out how powerful a telephoto lens for a digital SRL is?
How do the millimeters given related to powers given for telescopes
and binoculars?

What telephoto lens provides 5X, 7X, 10X, 15X, 25X, 50X, 150X, and even
250X? What is the largest power of telephoto lens commercially
avialable? What is the most popular size? What is the most popular
size for birding?


John Freck

 

For full frame 35mm images you can take the focal length of the lens and
divide it by about 47.5 to give you a number that is somewhat comparable.
but they are really not the same thing and you can't really do a comparison.

The most powerful telephoto lens commonly available is going to be
something around 1,200 mm to maybe 5,000 and will cost more than a $1.00 per
mm, often much more.

Considering that Hubble is a commercially made camera lens as are many
observatories you can get some very large ones.

Bird photography would normally start in the 1,200mm range, or you work
from a blind where you may do well with say a 200 mm.

 

Yep, for a *full-frame* 35mm SLR you divide the focal length of the
lens by 50 to give a rough approximation of the equivalent power of
binoculars/scopes.

So, to get the equivalent magnification of 7x binocs on a full-frame
SLR, you would need about a 350mm telephoto (7x50).

But then you have to take into account the crop factor on most DSLRs,
so in fact, if you had say a 1.5 crop, then you would only need about
230mm.

(And even if I got all that right, it is further complicated by the
differeing field-of-views that you get, depending on the
binoc/telescope design..)

So it's a bit tricky to generalise..

 

I feel I should add a little to my comments.

The human eye and brain are marvelous tools. When you look though a
pair of binoculars you often select just a small part of the total image and
you see it in far more detail than you might imagine. You can even see the
effect using the old paper tube trick like you did as a kid. Things really
did look larger and more detailed using one. That is because your brain
only focused on the subject and was not distracted. The camera can not do
the same thing. So you will generally need a more powerful telephoto lens
to record the same detail you see with a less powerful pair of binoculars.

 

A "1X" (for 1.5X cropped sensor) would be roughly 35mm.

A 9X would be a 300mm lens.

The above refers to field of view rather than magnification, as the
prints you make at various enlargements are another stage of magnification.

Hope that helps. Go to a birding website for advice on binoculars and
cameras lenses.

Cheers,
Alan

 

Well, now I'm confused (but what's new). Are you saying that the 1.5X
factor for digital cameras does not give the same magnification at the
equivalent lens length for film cameras. For instance a 300mm lens
magnification on a 35mm film camera would not be basically the same as a
200mm on a digital. Are you saying that this equivalency only applies to
FOV? I've never really understood the true comparison of lens on both types
of cameras.

 

Nope, backwards....

http://www.lookbefore.wading-in.net/lookbefore/Digital_Crop.htm

Should help clarify it for you.

Jim Kramer

 

So as I understand the example you posted is that the only thing that
changes is the FOV, ie the image is cropped but the actual magnification of
the image remains the same.

 

Only the FOV changes, "magnification" is subjective unless you are
specifically talking about the actual image that is projected by the lens on
to the film/sensor and then yes, the magnification remains the same.

I'm not trying to be obtuse or overly wordy but many of people seem to have
problems with this so I'd like to be clear.

Jim

 

Yes.

Put it this way: a given lens on 35mm film camera produces an image
circle that is a bit over 43mm in dia. just fitting the diagonal of the
35mm fram (36x24). So a moose some distance away makes (eg) a 5mm tall
moose on the film.

Same lens on a cropped sensor: the image is still a bit over 43mm in
diameter. But the sensor is now smaller (say 28m dia for a 1.5x crop).
So only that smaller bit gets recorded.

The moose over yonder still makes a 5mm tall moose on the sensor.

So, for that the magnification is the same.

But, if you print both the film version and the digital image of the
full frame to, say an 8x10, then you will end up with a bigger moose on
the digital version. as the enlargement factor would 1.5X greater.

Is that clearer or muddier?

Cheers,
Alan.

 

Is that clearer or muddier?

Depends how much Moose Head he had...

 

I didn't know you were that kinky...

 

Kinky? What's kinky about beer?

 

"Moose head"

 

LOL. You used to be so reserved, too!