Question on "normal" focal length vs. sensor size

CamaroX84

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I have a Nikon D3100 with the 18-55mm DX lens. From what I can tell from reading this forum and other places, a normal (not wide angle or telephoto) focal length is equal to the diagonal dimension of the sensor. For a DX sensor, this should be about 30 mm.

When I look through the view finder with the lens set at 30 mm, it appears to be more wide angle (or zoomed out). If I set it at around 50 mm, it seems like I am just looking through the view finder without any "zoom" at all. In other words, if I keep both eyes open, I don't see any zoom effect through the lens when set at 50 mm, but there is an effect at 30 mm, which should be the normal focal length.

Does a normal focal length correlate with how it looks through the view finder like I am describing? Or am I misunderstanding the concept?

The reason I am inquiring about this is because the photos I take need to be as "normal" as possible. I am an accident investigator and the photos should show distances between objects with as little distortion as possible.

Thanks in advance!
 

Derrel

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What the view through an SLR camera "looks like" with both eyes open is determined both by lens focal length AND by the viewfinder's magnification. The D3100 has a magnification of 0.8x, as measured with a 50mm lens focused at Infinity.Nikon D3100 Review Full Review - Viewfinder

To answer your question, NO, the normal lens for a particular camera has absolutely no fixed or specific relationship to what that lens "looks like through the finder". Some cameras have lower viewfinder magnification than other cameras have. Nikon has slowly been inching the viewfinder magnification levels upward in its 3xxx and 5xxx series bodies. According to Nikon USA, the D3300 has about a 0.85x magnification with the 50mm lens at Infinity.

At one time, years and years ago, Nikon SLR cameras fitted with, I want to say a 58mm lens, offered "equal-eye" magnification when looking through the original Nikon F camera; the 50mm lens was not quite long enough to give that perfect "equal-eye" magnification level. This was in the early to mid-1960's as I recall, before my time as a photographer.

The "normal lens length" for a particular camera format is more or less arrived at by measuring the diagonal of the film or sensor, and using a lens of, approximately, the same length; this is not set in stone however, and there is some variation. With larger format cameras, the variation has sometimes been a bit more so than with the smaller, miniaturized format cameras.
 

480sparky

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........The "normal lens length" for a particular camera format is more or less arrived at by measuring the diagonal of the film or sensor, and using a lens of, approximately, the same length........

That's my understanding as well.
 

dennybeall

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The early to mid 60's were well in my time as a photographer and in those by-gone days, when we took pictures of our horses and buggies, a 50mm was the "normal" lens and the 85mm was the portrait lens on the Nikon "F". Still have all those.......
 
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CamaroX84

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Thank you for the very detailed response, Darrel. That is very informative. I had no idea the viewfinder had magnification to it, so it makes perfect sense now.
 

TCampbell

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As Derrell points out, the finder has it's own magnification. You're fitting an area of view that the eye can comfortably view (without looking around) all crammed into a tiny viewfinder and that gives you the impression that the viewfinder has an angle of view which is wider than the human eye.

Human vision is a bit tricky because our eyes only show strong detail for subjects located directly in front of our eyes (near the center axis of vision). We see good contrast, but not good detail a little farther out, and out at the periphery we are particularly good at detecting motion, but not good at detail or even contrast. Our brains "assemble" what we see from multiple images created by our eyes. The "framing" that our eyes see in no way resembles the nice rectangle that the camera presents.

BUT... this "normal" angle of view is an approximation of what an eye can comfortably take and, more importantly, without stretching or compressing our sense of depth.
 

table1349

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If you like math or physics this is a good read on your question as well as other information on lenses.

- Lenses -

If you don't like math or physics, skip the link.
 

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