ELI5: Why is the 50mm camera lens considered closest to human vision when the average human eye image focal length is actually much smaller at 22mm?

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Because when you look at a photo you don’t hold it directly up to your eye, and it doesn’t fill your field of view. Think of a long lens image as just being a wide angle image cropped in and blown up. So a 50mm image (on a full size sensor) looks good when it fills the same amount of your vision as the same part of a wide angle image that would fill your vision.


Its only estimated that a 50mm is close to a human eye on a 35mm sensor. If using a smaller sensor, say a APS-C sized one, you’d need about a 35mm lens to get the same focal length. If you use a large format camera, you’d need something like an 80mm lens for the same results. Its not about the specifics of the lens in comparison to the eye. Its about what the internal optics of the lens, and the end results once the light hits the sensor. All of the optics on a particular focal length gives some sort of compression to whatever its seeing. Telephoto lenses, like anything over 100mm will compress things so objects in the distance will appear closer to you, and flattened out, conversely a wider angle lens like a 25mm will make things appear further away and you also start to get distortion due to the angle of the lens. A 50mm equivalent lens is closer to a human eye because thats about what kind of compression our eyes see, give or take. You don’t get the flattening out or distortion of other focal lengths.


It’s about the optics proportions. Its the ratio between lens distance and sensor width. A 50mm on a full frame sensor/film A 35mm for a Aps-c size sensor A 25mm for a half frame (Olympus uses them a lot) And the human eye. They all have the same distance/size ratio. What it gives: If you move the lens more distant from sensor, you zoom in more giving the picture a “flat-ish” feeling Shorter lens distances gives the picture a fish eye / curved feeling to the picture. The 50mm on a full frame gives the exact same curvature our eyes do, so that feels natural and perfect. Edited for clarity.


Because the standard film diagonal size is 50 mm. The retina is much smaller than that so a smaller focal length gives a similar field of view. Actually the eye has a much wider field of view than a standard lens but much of that is peripheral vision. To some extent the standard lens length is an arbitrary choice that has a simple formula (equal to film diagonal) and looks to us to be in between what we see as wide angle and telephoto lenses.


So if you look through a camrea (as mentioned in other comments this is a full frame 35mm camrea) eyepiece with a 50mm lens and open your other eye the image through the camera is the same magnification as your eye. Yes the field of view is much smaller in the camera than your eye. It’s the magnification that counts.