LeonardLeo Loc: Warren Michigan

50mm lens works for a 35mm film camera, but most digital cameras need a bit longer lens. Maybe about 75mm. You can judge for yourself if you look at a scene with a nearby object and also something distant. Draw a little sketch with both objects relative to their apparent size. Then look through your camera lens and see if they appear to be relatively the same size as in your sketch. You may use something like an 8 foot step ladder at 20' and a 3 foot stool at 10' .
Perspective makes a difference. I used a 35 to 85mm Zoom on my film camera for most shots. But I also liked a 75 to 300mm. Zoom. I have an 18mm f1.2 and rarely use it. I think my camera came with a 55mm or 58mm 1.4 lens. With digital I don't use many lenses. And those expensive film camera lenses aren't any use on my digital camera.
Len

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

The 50 mm thing is about angle of view. But no lens can "see" the eyes can. For any number of reasons. Binocular vision of the eyes, constant focus changes. Plus "optical illusion" plays a role. The closer the moon is to the horizon the larger it looks to the eye. The eyes want to see in 3 dimensions and a lens doesn't have those problems. Technology attempts to duplicate physiology. But, so far at least; it cant

kenArchi Loc: Seal Beach, CA

My client wanted the moon included in the picture.
Of course the near subject was correct size as you see it but the moon was a dot.
Zoomed in on the moon and ps it in.

LeonardLeo Loc: Warren Michigan

I also had an old camera that took 3D pictures. It had two lenses a bit apart. And yes the eye is 'auto focus' 'auto Iris' and can't be duplicated.
Even when the optometrist says you need glasses to see 20 20 the brain compensates and 'sees' fine without them. I remember as a kid, seeing the smoke stacks of Cement City 20 miles away.
The lens for the Hubble telescope was not ground correctly, but the software was adjusted so the computer could correct for it.

LeonardLeo Loc: Warren Michigan

I also had an old camera that took 3D pictures. It had two lenses a bit apart. And yes the eye is 'auto focus' 'auto Iris' and can't be duplicated.
Even when the optometrist says you need glasses to see 20 20 the brain compensates and 'sees' fine without them. I remember as a kid, seeing the smoke stacks of Cement City 20 miles away.
The lens for the Hubble telescope was not ground correctly, but the software was adjusted so the computer could correct for it.

David in Dallas Loc: Dallas, Texas, USA

Actually, the Hubble was given some "glasses" to correct its vision. I think analogous to the "corrector plate" found on Schmidt-type telescopes to fix the use of spherical mirrors.

Longshadow Loc: Audubon, PA, United States

Yea but...

At some point in time one's mind will not be able to "auto correct" and glasses will be required.
Maybe I can get a software update and I won't need glasses.

kymarto Loc: Portland OR and Milan Italy

It's not an easy question because the eye has only a small area of maximum resolution, the fovea, with increasingly lesser resolution out to the cutoff point of peripheral vision. We generally look by moving the eyes so that what we want to see is in the area of the fovea. With normal vision, the edge of peripheral vision is around around 120 degrees for each eye, with overlap in the center, so that the field of view is 170-180 degrees horizontal and 120 vertical, but there is steadily decreasing resolution starting just a few degrees off center.

Longshadow Loc: Audubon, PA, United States

Interesting.

Such a simple question,
such complicated answers.

k2edm Loc: FN32AD

watching a football game where a long lens is used, object farther away seem larger. this distortion is least(35mm full frame) when a 50mm lens is used, the main reason why it is so popular. for portrats, (for whatever reason) most use 100mm lens..(something to do with making the nose look smaller).

burkphoto Loc: High Point, NC

It's called foreshortening. The 2X focal length forces you to back up, so the relative distance between the tip of the nose and the eyes is reduced, "flattening" the face. It's really the shift in your position that causes the perspective change. The 100mm lens is just a 2X crop of the 50mm view. If you DON'T back up, the perspective does not change, but you see only 1/4 of the subject area covered by the 50mm lens from that same distance.

Pablo8 Loc: Nottingham UK.

I, like you , always referred to the diaganol of the 'Negative , would determine what the 'normal' focal length should be. But , because I have Macular degeneration in my left eye, I only view everything with with a Single eye. I am learning to cope with this. Those with good vision in two eyes, just count your blessings, and stop quibbling over small degrees of difference in vision versus lens focal length.

stan0301 Loc: Colorado

The Sigma 8-16 does this very well - and you have the ability to adjust the angle to suit your taste - I tend to use the 8mm end mostly - enlarges to 20x30 with no problem

Longshadow Loc: Audubon, PA, United States

Depends on full frame; crop sensor; size of sensor. You can't say 75mm off the bat. I definitely would NOT say most would be about 75mm! (Might be close for a 2-1/4 camera though.)
With my crop sensor camera, to make an object 20 fee away look the same distance away in the camera is about 32-34mm, give or take.
One doesn't have to draw a sketch. look at the subject, then look through the viewfinder. Using a zoom is simpler than changing lenses. Keep alternating viewfinder/eye and adjusting the zoom until the subject looks the same distance away. Very simple. Remember, comparing what one sees with the eye vs. how it looks in the camera, closer or farther. It's real easy to see - look at an object, then look at that object with the camera. Does that object appear closer or farther away in the camera?

Perspective of a smaller object that is closer looking as large as a larger object that is farther away is a separate subject unrelated to comparing the distance an object is in the camera to an unaided eye.

Also, binocular vision simply provides depth perception, again, unrelated to comparing the distance an object is in the camera to an unaided eye. Binocular vision does not make things look closer or farther away. It just makes it easier to differentiate two different distances in one's mind. Things do not look closer or farther away with binocular vision, one's mind allows the interpretation of the information in the view as to the distance separation of different objects.

rmalarz Loc: Tempe, Arizona

The human eye has a nominal focal length of approximately 17mm.
--Bob