Why does 50mm look life-size through my APS-C DSLR viewfinder?
Asked 8/16/2012
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On a Canon APS-C DSLR, I noticed that if I set a zoom lens to about 50mm and keep both eyes open, the view through the optical viewfinder seems to overlap closely with what my other eye sees. But I thought a "normal" lens on APS-C was more like 25–35mm, based on sensor size and angle of view. Why does 50mm appear life-size in the viewfinder, and would the same effect happen at a different focal length on full-frame cameras?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
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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.)
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
14y ago
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What you’re noticing is mainly a viewfinder magnification effect, not proof that 50mm is the “normal” focal length on APS-C.
On many DSLRs, viewfinder magnification is specified using a 50mm lens focused at infinity. Because of that, a lens around 50mm can make the finder image appear close to life-size when compared with your other eye, creating that overlap effect.
That is separate from the photographic meaning of a “normal lens,” which is based on format size and angle of view. On APS-C, a normal focal length is still roughly around 30mm (give or take), while on full frame it’s around 50mm.
So:
- APS-C: 50mm may look life-size in the finder because of the camera’s viewfinder optics/specification.
- Full frame: the same overlap effect is more likely to happen closer to the actual normal focal length, around 50mm, depending on the finder magnification.
Also, human vision doesn’t match any single lens exactly: our central vision feels more “telephoto” in scale, while our total field of view is much wider.
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