alexol

This has probably been asked somewhere else but I wasn't able to find an answer - apologies for boring everyone if this is a repeat.

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? I'm disregarding vibration/shake etc - I know that higher powered binoculars and scopes etc require tripods or some form of stabilization.

Thanks in advance...

JimH123 Loc: Morgan Hill, CA

A quick rule of thumb is that a 50mm lens on a full frame camera approximates what the human eye can see pertaining to angle of view and size of objects being viewed. Thus a 500mm lens on a full frame camera would require about 10x magnification. A problems with this method is that two binoculars may have the same power which is calculated from objective focal length divided by eyepiece focal length. But the Field of View can be wider on one compared to the other which makes this method somewhat subjective.

Another way I like to consider it is to compare the Field of View of the binoculars with the Field of View of the camera plus lens. When they are about equal, what you looking through the binoculars or telescope is about what the image in the camera will also show. Binoculars, since they are using two eyes with some overlap and some non-overlapping to both sides, actually produces a wider FOV. The camera image may be set to be as wide, but the height of the image may be greater.

Of course, some individuals may interpret this different. For instance, someone may look though binoculars or a telescope at some object and to then look through the camera's viewfinder and expect to see the object look the same size.

So the answer is that there is no one answer that everyone agrees upon. My personal favorite is the 2nd option I gave about using the FOV. You can usually find the FOV for a given lens on the internet. For binoculars, some give the FOV (I have some 8 x 42mm binoculars that say that the FOV is 8.1 degrees), others give the width of the scene seen in feet at 1000 yards (such as 900 feet at 1000 yards), and a little math can produce the actual FOV. For telescopes, the actual FOV can also be difficult to pin down. Some eyepiece may have a small apparent FOV such as 45 degrees. And another eyepiece may have a much wider apparent FOV, for instance, 68 degrees or even wider. These two eyepieces can have the same focal length, but the one with the wider apparent FOV will actually produce a wider actual FOV.

Ourspolair

This has to do with what is considered a "normal" lens on a 35mm diagonal film or image sensor and the "normal" angle of view of the human eye. From memory, a 28mm lens gives about the same angle of view as the human eye. 1:1 = 28mm 1:8 =224 mm 1:40 =1120 mm So it looks as though you would be looking for an optic which had 18x magnification. I did not do the geometry to confirm this, I am sure there are some Hoggers out there who have a more precise answer, but this should put you in the ballpark.
I based this on an fov of around 60 degrees for the human eye.
https://www.nikonians.org/reviews/fov-tables

SuperflyTNT Loc: Manassas VA

Actually the width of the scene at a particular distance is the FOV. The AOV, or Angle of View is what’s given in degrees.

JimH123 Loc: Morgan Hill, CA

Yes, I should have said AOV, not FOV. Thanks for the correction.

dpullum Loc: Tampa Florida

Alexol said in part: I know that higher-powered binoculars and scopes etc require tripods orsome form of stabilization.

Increased ISO and Faster Shutter freeze the photo eliminating much of the blur with long lenses. So that i may brag I bought a tripod and mono-pod... never used them.... too much trouble to lug around when my ISO and shutter speed do the job. It would be better to have one of those folding 3 leg stools and sit.

John N Loc: HP14 3QF Stokenchurch, UK

Looking forward to the answers.

starlifter Loc: Towson, MD

I have found about 21 or22mm = 1x. So an 8x set of binoculars would be about 163mm or so.

Ourspolair

Binocular vision, which is the basis for stereopsis and is important for depth perception, covers 114 degrees (horizontally) of the visual field in humans, according to Wikipedia. FOV = 2xarctan (sensor size/2f) where f is the focal length. For a normal human eye, the central vision is usually taken to be 60 degrees in the horizontal plane. If you trust Wikipedia. IMy calculation was based on "eyballing" 67 degrees. Stay well everyone.

John N Loc: HP14 3QF Stokenchurch, UK

I just found this which may help.

http://petapixel.com/2012/11/09/digiscoping-a-look-at-using-a-spotting-scope-as-a-telephoto-lens/

alexol

I did find this which is detailed & very informative but I think left me more confused than before!

https://photo.stackexchange.com/questions/87917/can-i-convert-binocular-zoom-to-equivalent-lens-focal-length

2Wheeler

Funny but true!

BebuLamar

The focal length on a camera only dictates the angle of view and not how large you would see the subject. That depends on the viewfinder magnification too.
The magnification number on a binocular dictates how large you see the subject but not the angle of view and in fact the larger angle of view is the better.

MrPhotog

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.

alexol

Thanks to everyone for all the informative replies!

Much appreciated.