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What is the Focal Length of…


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20 replies to this topic

#1
Nikon Shooter

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… your eye?

And do you know how to measure it?



#2
krag96

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Great question!  I've pondered it many times and don't have a clue as to how to measure it other than look at an object about 20 feet away and compare it by looking through a zoom lens to get the same composition size.  I know my eyes see a wider view, but as for size of en object, I would say my eyes are about equal to a 100mm focal length FX lens. 



#3
Nikon Shooter

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I would say my eyes are about equal to a 100mm focal length FX lens. 

Cool, you got the method right, krag!
…but not so your conclusion.  :o



#4
krag96

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I know what they say it should be, 50mm, but to me 50mm is still in the wide angle realm compared to size of an object at 20ft. as seen through the human eye.  Things appear smaller than they actually are from, say 20ft. yet the angle of vision is narrower, (different than focal length).   Our eyes are rather panoramic and marvelous creations for sure! 

 

I'm guessing about a 15mm FX lens would see close to the same angle as our eye, but not the same objective size.  Create a lens that does both and you may win the, ''I wish they could make'' lens prize!

 

Anyhow, looking again I see a very close object size at 20ft. with my 24-120 f4 lens between 85 and 100mm.  Am I missing something here? 

 

(don't give the answer until we've had some more input, I'm interested to read what others see)



#5
Nikon Shooter

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Am I missing something here? 

 

(don't give the answer until we've had some more input, I'm interested to read what others see)


I won't… I promise!

Btw, 50 mm was an arbitrary consensus!



#6
krag96

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Btw, 50 mm was an arbitrary consensus!

 

I agree, it never made sense to me.  



#7
Merco_61

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When you look through a viewfinder, you also have the magnification to think about. The D700 has .72x magnification to achieve the 18 mm eyepoint it has. The same goes when comparing using prints.



#8
Merco_61

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krag96, you need two objects of similar size at different distances. When the size and the relationship between the objects look the same with the naked eye as captured through a lens and printed at a reasonable size for the viewing distance, you have found the equivalent focal length in that system. There are far too many variables to make general observations. 

 

The reason it is so difficult to compare is that the eye uses a spherical projection and the camera a flat one.



#9
krag96

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The reason it is so difficult to compare is that the eye uses a spherical projection and the camera a flat one.

 

That I had not considered. 



#10
Nikon Shooter

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Nevertheless krag, your conclusion is waaaaay too high!



#11
Jerry_

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So we should look into the wide angles :), in that range I have a couple of lenses with spherical front elements ...

#12
krag96

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Apparently I'm confused between an object size at a given distance and over-all angle of view, or field of view/vision.  If angle of view I would choose a lens of around 16mm. 



#13
Nikon Shooter

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AoV is of no help in this and the object distance neither. :P
I think you're trying too hard.



#14
krag96

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This is an intriguing conversation.  Eyes-eyes...we have two of them, complete with lenses and pupils which open and close like a camera lens, (f stops in the human eye) throw in the brain as a processor, mix well and come up with a number.  I'm going to take two focal lengths here since neither seem to make sense to me by themselves, round the numbers up or down, find the middle and call it good, (this probably won't be precise)!  For angle of view I'm going with 20mm and for size perspective, 80mm. Right in the middle we have 40mm, a compromise at best since no lens made can do both and see what we see, the way we see it.  So, I say 40mm, (roughly) as a compromise as our eyes and brain can see and process much more than we can expect from a single camera lens.



#15
Nikon Shooter

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… I say 40mm, (roughly) …

 

That sounds more reasonable, Sir!

 

… our eyes and brain can see and process much more than we can expect from a single camera lens.

Neuro- or organic, chemically based transmission is waaaay slower than
electronic transmission. The processing power of our brain doesn't have
the improvisation limits of a program but the camera is much faster and
complete in its restricted operation in terms of whole image recordings.

Our brain, for the sake of speed, will work a lot on assumptions — even
filling the gaps with details from the memory —  as the camera won't.

Since both have different reasons, missions, and motivations, the brain will
"only process" what is different between two stimuli… it is a matter of sur-
vival. 

False positive conclusions have lesser dramatic consequences on one's life.



#16
Merco_61

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From a perspective standpoint, my perception is that a lens whose focal length is equal to the diagonal of a square inside the negative format feels the most like the naked eye. For FX, that would be a 34 mm focal length (the diagonal of a 24 mm square). For DX, we land at 22.5 mm as that sensor is 16 mm on the short edge.

 

Another interesting side note in the same vein as the 50 mm "normal" lens is the reason why the standard aspect ratio on 35 mm film became 2:3. Clue: there are two reasons, one for marketing and one for engineering reasons in the 20-s



#17
krag96

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Are we comparing a camera single lens to two eye vision, or single lens vision?  Thinking about it, it makes quite a difference.



#18
Nikon Shooter

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Are we comparing a camera single lens to two eye vision, or single lens vision?  Thinking about it, it makes quite a difference.


No matter mono or stereoscopic vision krag, each of our
eyes have similar focal length.

Not really, the same difference than spyglass vs binocular.



#19
Merco_61

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Are we comparing a camera single lens to two eye vision, or single lens vision?  Thinking about it, it makes quite a difference.


I think you are back to thinking of the fov as a factor.

#20
krag96

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I think you are back to thinking of the fov as a factor.

Yes, I am in fact.  I consider fov an interesting factor in that it seems impossible to have both fov and a focal length that both match the human eye, or eyes. It seems we can achieve one or the other, but not both.