how do lens zoom #s compare to bin zoom #s?

[BirderSean]

I'm used to the zoom factors on scopes and bins e.g. 8x, 10x etc.
On camera lenses it's a mm measurement, correct? Someone recommend I get at least a 300 mm zoom lens for photographing birds. Just curious how the two measurements compare, i.e. does a 300mm lens equal a 20x zoom on a scope?

enlighten me please.

[njlarsen]

If you were using an old film SLR or possibly a full frame dSLR, 300 mm would correspond to about 6x, because the length given would have to be divided by about 50mm. With most other dSLRs your would take that number and multiply by the crop-factor, which is about 2 for olympus 4:3rds cameras, and about 1.6 on several other consumer dSLRs. (on an olympus 4:3rds camera, 300mm corresponds to 12x). If you are prepared to use only the central part of an image file, then you can "zoom in" (really crop) using the computer and thereby possibly be happy with less magnification than on your scope.

Niels

[LHB1]

Sean,
I think you are confusing terms.
a. The 8x or 10x on binoculars is the magnification factor. An 8x bino magnifies the image 8 times larger than seen with the naked eye. A 20x eyepiece on spotting scope typically yields a magnification of 20x unless used with a scope body having different focal length than that for which the lens was designed/calculated.

b. For 35mm cameras the magnification factor is typically the focal length, i.e. 300 mm, divided by the accepted normal lens length of 50 mm. Thus a 300 mm lens on 35mm camera is rated as 6x magnification. Note that the std length lens changes for different film formats (35mm, 2 1/4 x 2 1/4, etc.) and the magnification factor also changes with smaller size digital light sensors.

c. Zoom lenses/eyepieces are those which can change magnifications, i.e. 20x-50x (spotting scopes), 3x-12x (rifle scope sights), or 75mm-200 mm (camera lens, equal to 1.5x-4x magnification).

LB

[BirderSean]

Sean,
I think you are confusing terms.

LB


I was confusing terms, because I AM confused! I don't understand what size lens on a camera would produce the equivalent magnification on a bin that I am used to. Your response was helpful, thank you.

[hollis_f]

I was confusing terms, because I AM confused! I don't understand what size lens on a camera would produce the equivalent magnification on a bin that I am used to. Your response was helpful, thank you.

Well, I would hazard a guess that a 400mm lens on my 50D produces an image in the viewfinder roughly the same size as my 8x bins. But the viewfinder image isn't the final image from a photo - because you add magnification after the shot, either by printing or viewing on the screen. So, with that same 400mm lens I can see detail that just wouldn't show up in the bins.

So your viewfinder/bins image may look like the smaller attached image. But when you prepare it to view onscreen, or print, it looks like the larger one.

So what's the magnification of the 400mm lens? In the viewfinder, about 8x. But in the final image around 100x.

[Crodol]

Regarding the magnification in the viewfinder: this depends not only on your lens but also to a certain degree on your camera. There is a good link below for Canon:

www.the-digital-picture.com/Canon-Lenses/Canon-Lens-Magnification-Value.aspx