Common photography myths

Common photography myths (4)

Myth #3: A 50 mm lens on 35 mm systems is called "normal" because it delivers about the same view as the human eye

September 5th, 2009 - 08:32:23 PM:

You have already guessed it, this isn't really the case. Human vision is much different from that of a camera. The horizontal angle of view is about 120 to 140 degrees, which translates to 13 mm to 9 mm focal length with a 35 mm camera. However, we don't see very sharp over this entire field of view but only at the center. This part of our retina has an angle of view of about 60 degrees, which translates to 35 mm focal length. The area with the sharpest vision, the one that you use right now to read these lines, is even smaller. Its angle of view is only 2 degrees, equivalent to 1100 mm focal length.

The lateral field of vision is mainly used to detect motion while the central part is used for precise visual inspection. So you see, human vision is quite different from the view through a 50 mm lens.

Some other theory says it's the standard focal length because it's the diagonal size of the 35 mm frame [and even that isn't precisely true; the diagonal size is about 43.27 mm]. Well, that's how we calculate what the standard focal length for a given format is, but it's not the reason why it was actually selected as the standard.

The real reason is much more mundane. When SLRs (or rather: system cameras) became widely availably to a large number of amateur photographers, the makers of these systems had to select one lens that they could sell as a standard set together with the camera. Following the logic of economics, it had to be one that could be made well for little money. 50 mm lenses fit these criteria perfectly. They typically are focused by extension (simple mechanism), don't require aspheric elements (simple lens element shapes), they don't require elements made of glass with anomalous dispersion (simple materials), and they're not zooms (fewer elements and simpler mechanism). It's easy to make a really good and fast 50 mm lens for little money. Also for this reason, the 50 mm standard lenses are often among the best lenses of one maker's lens lineup. That's why most 35 mm cameras came with a 50 mm lens until recently. The makers could have also selected 40 mm or 55 mm as the standard, but 50 mm probably looked more “even”.

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Readers' comments

#1: Comment posted by Jay on May 25th, 2013 - 05:16:57 AM:

As for myth #3 (50mm lens on 35mm camera), indeed the angle of view is not the reason why it is considered a normal lens, and the angle of view or FOV is not what people mean when they say "it delivers about the same view as the human eye".

It is about 3D perspective, about the relation of distances. When you shoot objects with a wide angle lens, the perspective is stretched away from you. Objects that are far away appear even further away. Likewise, when you shoot with a telephoto lens, distance appears to be compressed, tightened. Think of formula 1 cars on TV, shot with telephoto, driving towards you. They're going faster than 300km/h, yet they seem to hardly move.

(by the way, this effect is being used in a cinematographic technique called 'dolly zoom'. You zoom in while backing off, or zoom out while getting closer, so your subject stays in the frame as it is but the whole background is getting stretched or compressed)

So, both wide angle and telephoto lenses distort the perception of distance against what we humans are used to see. A 50mm lens on 35mm cameras is as close as you can get to reproduce the normal perception of distance on photographs (it's slightly above 48mm, so 50mm comes very close).

Michael Hohner answers:
I think you should go right to myth #8 and read that. The reason why the F1 car seems to hardly move is not that it was shot with a telephoto lens, but that it is far away. If you looked at that scene with your bare eyes and concentrated only on the car, it would look the same. Perspective is not influenced by the lens.

#2: Comment posted by Jay on May 25th, 2013 - 04:32:32 PM:

@Michael Hohner:
Yes, that is true of course. As such, it is no "special thing" about the 50mm lens. But still, when you are using a telephoto lens, you are bringing the subject closer.

Let me explain it like this.
When you shoot very wide angle (and let's just assume it is free of lens distortion) and want the picture to look "natural" to our eyes, to get that effect of "natural view", you could just crop the picture in post. So yes, technically that has nothing to do with the lens used. But when you're shooting telephoto, for the same thing, you would have to extend the picture, which you cannot do, unless you shoot a panorama with a certain number of rows and columns so you get the same field of view as you would with a 50mm. So, again, yes, it's not something special about the lens.
But the point is, the human vision or the human perception thereof is not used to seeing just a crop of its normal vision, but that's what you do with a telephoto lens.
Our two-eye stereo view allows us to have "real" 3D vision only within a certain range from where we stand, because the distance between our eyes is kind of small. Objects further away appear flat to us. We don't actually notice because our brain knows they're 3D too. But seeing only flattened objects in the frame is not what our normal vision is used to see, especially in relation to the distance. Because as soon as the objects get closer to us (and that is, by size, what a telephoto lens does), they "un-flatten" and start to look threedimensional. They don't do that when you use telephoto.
So, to be absolutely precise, I will say: The *field of view* you get when you use a 50mm lens on a 35mm camera is what best reproduces the human perception of distance. You could as well crop a wider shot to the same FOV or stitch a panorama of telephoto shots to the same FOV, but when you don't want to do that and just ask for a lens that does this without any post production, it's the 50mm.

The best proof that it is not about the lens: To get the same FOV on a crop camera like entry level DSLRs with a 1.5x or 1.6x crop factor, you'd have to use a 30mm lens. You could as well use a 30mm on a full frame camera and then crop the picture to compensate for the larger sensor. It is all the very same. So, once again, yes, it's not the lens but the FOV, but you only get that FOV with a 50mm/30mm lens unless you put more work in it than just taking the picture.
Can we agree on this?