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<blockquote data-quote="WayneF" data-source="post: 207256" data-attributes="member: 12496">As you pointed out,&nbsp; I don't think there is any one technically &quot;correct&quot; answer. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile&nbsp; &nbsp; :)" loading="lazy" data-shortname=":)" />The concept of a normal lens is not related to frame size, except to the extent it tries to match the same frame size viewing width that the human eye perceives to see.&nbsp; &nbsp; But human eyes can only focus sharply in a very tiny center spot (that spot we actually look at).&nbsp; We do have peripheral vision that can detect light and movement at wider angles, but what that dimension is - is pretty vague.&nbsp; &nbsp; &nbsp;&nbsp; If you &quot;look at&quot; the edge of your monitor screen, you cannot read text in the center of the screen - but you are sure that you can see it.But someone claimed if the focal length were about same as the diagonal, then this match to the eye is reasonably close - meaning the camera picture matches what we thought we saw when standing there.&nbsp; &nbsp; &nbsp; But 50mm was already the assumed &quot;kit lens&quot; for 35mm film much earlier, and it stuck.The horizontal view of a 50mm lens on FX is 39.6 degrees&nbsp; (which is not far from 43mm diagonal).&nbsp; But I think most of us would imagine we see wider, maybe 90 degrees, even if we really cannot &quot;see&quot; stuff clearly at half of that 40 degree width.&nbsp; My own guess and perception is that 40 degrees is not much stretch to &quot;see&quot; if we move our eyes, but not our heads. But, we are trying to precisely dimension something in mm, which is in fact quite vague and undefined.&nbsp; And also of course, we might choose for our picture to show a scene much wider (wide angle) or more narrow (telephoto), than what our eyes normally saw there.&nbsp; Nevertheless, a &quot;normal&quot; lens does turn out to be quite handy for many pictures.Here is a chart of FX viewing angle for common focal lengths<a href="http://en.wikipedia.org/wiki/Angle_of_view#Common_lens_angles_of_view" target="_blank">http://en.wikipedia.org/wiki/Angle_of_view#Common_lens_angles_of_view</a></blockquote>

[QUOTE="WayneF, post: 207256, member: 12496"] As you pointed out, I don't think there is any one technically "correct" answer. :) The concept of a normal lens is not related to frame size, except to the extent it tries to match the same frame size viewing width that the human eye perceives to see. But human eyes can only focus sharply in a very tiny center spot (that spot we actually look at). We do have peripheral vision that can detect light and movement at wider angles, but what that dimension is - is pretty vague. If you "look at" the edge of your monitor screen, you cannot read text in the center of the screen - but you are sure that you can see it. But someone claimed if the focal length were about same as the diagonal, then this match to the eye is reasonably close - meaning the camera picture matches what we thought we saw when standing there. But 50mm was already the assumed "kit lens" for 35mm film much earlier, and it stuck. The horizontal view of a 50mm lens on FX is 39.6 degrees (which is not far from 43mm diagonal). But I think most of us would imagine we see wider, maybe 90 degrees, even if we really cannot "see" stuff clearly at half of that 40 degree width. My own guess and perception is that 40 degrees is not much stretch to "see" if we move our eyes, but not our heads. But, we are trying to precisely dimension something in mm, which is in fact quite vague and undefined. And also of course, we might choose for our picture to show a scene much wider (wide angle) or more narrow (telephoto), than what our eyes normally saw there. Nevertheless, a "normal" lens does turn out to be quite handy for many pictures. Here is a chart of FX viewing angle for common focal lengths [URL]http://en.wikipedia.org/wiki/Angle_of_view#Common_lens_angles_of_view[/URL] [/QUOTE]

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