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I grew accustomed to the notion that what one sees through a normal lens equates (or is close to) what can be seen with the naked eye (although that is not the "pure" definition of a normal lens, which is when the focal length and the sensor's diagonal are the same or close enough).

However, while playing with a zoom lens (on Canon APS-C, 1.6 crop) and keeping both eyes open, both views perfectly overlapped (and "merged") at 50mm (you get interesting effects when defocusing the lens at that stage, although you can't capture what you see).

That's a long stretch from what's considered normal on APS-C formats (between 25 and 35mm), so how could this be? Do full-frame DSLRs experience the same effect somewhere around 80mm?

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What you are seeing is the effect of viewfinder magnification. For whatever reason (probably simply to make the numbers sound better), this spec is usually given for a 50mm lens, even on APS-C. The Canon 60D, for example, has a 0.95x magnification with a 50mm lens focused at infinity. And that's why around 50mm gives you the magic double-vision effect. There's more on this in Stan's helpful answer to What does "viewfinder magnification" mean?.

On full frame, the numbers are also given with a 50mm lens, so assuming a high decent magnification, you'll get the effect right around the normal length.

This is different from the idea that a normal lens produces output with a normal perspective, which should still hold true for around 30mm on an APS-C camera, assuming a typical viewing distance for the size of your prints. (Approximately arms' length for an 8x10, for example.)

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  • \$\begingroup\$ And another reason to go full frame.. Thanks for your answer! \$\endgroup\$
    – Berzemus
    Aug 16, 2012 at 13:24
  • \$\begingroup\$ @Berzemus — indeed. In fact, since APS-C sensors are getting so technically amazing, I think that except for corner cases and extreme use, a bigger, better viewfinder is the primary advantage of full-frame. \$\endgroup\$
    – mattdm
    Aug 16, 2012 at 14:44
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    \$\begingroup\$ Theoretically speaking there's always a sharpness advantage to larger formats, as there's only so many linepairs/mm you can resolve on the sensor plane, a bigger sensor means more lines per picture height. This has been confirmed in every test I've seen, hence you don't have to put your FF to any extreme use or be a DOF junkie for there to be an advantage. Plus provided you have the pixel density you can crop your images and retain almost all the APS-C advantages. Personally I find the bigger viewfinder a disadvantage! \$\endgroup\$
    – Matt Grum
    Aug 16, 2012 at 15:30
  • \$\begingroup\$ @Matt Didn't mean to get into a full-frame argument — clearly having more surface area is an inherent advantage; my point is that technology is at the point where APS-C (and for that matter Four Thirds) is amazingly good and can provide technical image quality beyond what's required for almost all mainstream use (including enthusiast, professional, and artistic mainstream). I'll happily concede that I'm engaging in some degree of hyperbole in calling the viewfinder the primary advantage, though. :) \$\endgroup\$
    – mattdm
    Aug 16, 2012 at 16:25
  • \$\begingroup\$ I don't buy the "aps-c getting amazing vs FF idea " :) \$\endgroup\$
    – Michael Nielsen
    Feb 13, 2013 at 21:11
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I know this is an old post, but the comments are so off the mark I thought I would chime in. Everyone seems stuck on this idea that a "normal" lens equates to the human eye in someway, a normal lens is a lens with the focal length the same as the diagonal measurement of your film plane. It allows us to compare across mediums, and usually has listed distortion.

In relation to your eye, your eye gives you around the same magnification as an 80mm lens (on a full frame) - hold one up to your eye and look around, everything will be about the same size through both eyes, but an angle of view like a 17mm lens (on a full frame).

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    \$\begingroup\$ Since there's only one other answer — mine — I assume you're suggesting that what I'm saying is "off the mark". But, I don't make the claim that a normal lens equates to a human eye at all, so I'm puzzled. Do you mean "comments on the internet in general" (in which case I agree with you), or is there something in specific? \$\endgroup\$
    – mattdm
    Oct 17, 2015 at 16:50
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    \$\begingroup\$ I also don't think your 80mm lens example is helpful. Looking directly through a lens is very different from the way a lens is used when taking a photograph, and I don't think it relates in any meaningful way. (Holding a flat glass to one eye gives the same "magnification" as holding no glass to the other...) \$\endgroup\$
    – mattdm
    Oct 17, 2015 at 16:53

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