Lens Focal Length vs. Binocular Magnification

Here's a question for you optics experts. Comparing the

magnification power of binoculars vs. a camera lens, what is the

relative power of a 300mm or 400mm lens compared to typical 8x30

binoculars? Just trying to figure out equipment for a safari!

the 8 in the 8x30 is the magnification. If you assume normal vision is about 50mm, then 8x50=400mm.

Mike,

 

Divide the lens focal length by 50. A 300mm lens would be 6X. A 400mm lens would be 8X. That's how I do it.

 

Roger

Although I am definitely not an optics expert, the 50mm figure in the above answers assumes you are comparing magnification relative to a normal lens on a 35mm full frame camera.

 

You need to take the camera film or sensor size into account to make a proper calculation. For instance, a 300mm lens on a 1.5 FOV factor digital camera would be roughly equivalent to a 9x binocular. And a 300mm lens on a 4x5 would be roughly equivalent to a 2x binocular.

With a 50mm lens on your camera; the rig afocally looks like a 400mm lens with an 8x binos. Here the camera looks into the one of the binos eyepieces. This is how some folks made a poor mans telephoto long ago. It acts like a 400mm, no matter if the camera is a 8mm, 16mm 1/2 frame, full frame, cropped dslr.

The 8x binos just multiply the angle by 8x. the moon is about 1/2 across to the unaided eye, with the 8x binos the moon appears to be 4 degrees across.

I don't know if I'm adding any wisdom to this discussion but I have the impression that the image size in my 8X binoculars is larger than the

image size with a 300mm + 1.4 TC.

 

I've checked this several times as I almost always have the binoculars with me, camera or no camera and the image looks larger than the 420mm the above lens + TC affords me.

 

Whether this is my imagination, or optics or the fact that both sides

of the brain are engaged for binocular vision, I don't know.

 

All I can tell you is that it is always a pleasure to look through

the glasses at whatever I'm looking at and it makes me want to

renew my acquaintance with a MF/WLF camera.

Most binoculars, even "wide field" binoculars, have a smaller field of view than a camera lens with similar magnification, which obscures the comparison. The magnification of a 35mm camera lens is generally taken as the focal length (in mm) divided by 50 (the "normal" focal length). The magnification of binoculars is the objective diameter divided by the exit pupil diameter.

I don't know where you're buying your binoculars, Edward, but my Nikon Superior E's have a much wider view than any camera viewfinder.

The "magnification" of the camera lens depends on the final print size, and viewing distance. However if you wish to compare the observed magnification in the camera's viewfinder, you need to know the focal length of the eyepiece. For example, the focal lengths eyepieces of my F4 and F5 are both 70mm. If I mount my 500mm on the camera the magnification is 500mm/70mm, or about 7X. As I usually have a 1.4X TC on the 500, this gives 700/70=10X. I will sometimes lay my 10X40 binos on top of the camera so I can watch the subject more easily at the same magnification, especially if the mirror is locked up.

Ocean,

 

What's your point? Do you want to know where I shop?

 

My Nikon wide-field 7x35 "E-type" binoculars have a 7.3 degree FOV, which is comparable to the diagonal FOV of a 350mm lens on a 35mm camera. The 8x35 "Superior E" have a FOV of 7.5 degrees. This hardly qualifies as "much bigger" than a camera's view. Read further for the justification (sorry, no pictures).

 

Mike and others,

 

If one accepts that a 50mm lens is considered "normal" (a tradition based on the diagonal dimension of the film frame), then a 400mm lens would be the equivalent of 8x. By this criteria, a "normal" lens for a Nikon DSLR would be about 35mm, and an 300mm (266mm) lens would be the equivalent of 8x.

 

I think it is confusing to compare the appearance through the viewfinder with the same view through binoculars of "comparable" power. Most viewfinders have a net magnification factor less than one with a "normal" lens, and different cameras can vary significantly. This, of course, makes no difference whatsoever in the final image. Cameras don't make very good telescopes, but that's not what cameras are for.

 

It doesn't matter what the print size is, or how far the viewing distance, or the net magnification through the viewfinder. All else being equal, the subject will be 8x as large (linear dimensions) with a 400mm lens as with a 50mm lens.