alexol wrote:
. . . My question: If a telescope has a magnification of 8 (in the case of 8x5 binoculars) or 40x in the case of a say a spotting scope, how would I compare this with a 35mm equivalent camera lens?
Reversing this, if I like the view through say a 500mm lens, what power binoculars would i be looking for?
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A *precise* answer to your first question is really difficult, but you can come close. The problem is that what you see through one model of 35 mm camera will vary, depending on the magnification of the viewfinder. This happens between manufacturers, and also among several models made by one manufacturer. In fact, with old Nikons that had interchangeable viewfinders you could see 3 magnification with the unmagnified waistlevel finder, the metering pentaprism finder, and the large eye relief sportsfinder. Same lens, same camera, 3 sizes. ( 4 if you added the pop up magnifier in the waistlevel finder).
Despite that, most manufacturers figured that the cameras would be sold with (nominal) 50mm lenses, and created the viewfinder magnification to come close to appearing ‘lifesize’.
Take your camera and check this yourself. Hold the camera and look through it at some scene. Then close that eye and look at the sane scene using your other eye. Then repeat the process. After a few blinks you should be able to detect that the image in your viewfinder is a wee bit different (probably smaller) than what you see with the unaided eye.
If you are using a zoom lens, set it near 50 mm to start and tweek it if want to see how this changes.
I’ll assume it is close. If so, consider that to be a magnification factor of 1x. After that, others have pointed out the simple math: twice the focal length is 2x, 3 times is 4x, and so on. It works the other way too. If your experiment gave you 1x view with a 50 mm lens, then a 25 mm lens would be 1/2x.
For telescopes and binoculars, just turn the equation around to get an approximate idea.
The problem with comparing a telescope or binoculars with a camera lens is that the camera uses only a small portion of the image that the optics produce.
For example, a 135 mm lens would give 2.7x magnification on a 35mm frame, but it produces a cone of light large enough to cover a 4x5” piece of film—wider than three frames of 35 mm film. how much of that cone of light can be captured and magnified by the eyepiece will affect the magnification and the field seen through the binoculars.