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-   -   What is the "focal length" of our eye? (

BenjaminXYZ Oct 23, 2006 9:02 AM

I am planning to buy a 35 mm lens for my next 1.5 crop factor dSLR camera; that would equate to 52.5 mm in 35 mm equivalent terms. Am I right to assume that I would be achieving the human's eye field of vision (focal length) by doing so?

eric s Oct 23, 2006 9:59 AM

Yes, the human is is roughly 50mm (I think your number is even more precise than my "loose" number. But 50mm is close enough.)

It's actually fairly simple math, once you know what to do. A camera with a sensor crop of 1.5 you want to solve the equation
Z x1.5 = 52.5mm

where z is the focal length of the lens you want. 52.5 / 1.5 = z, therefor z must be 35.
So on a camera with a 1.5 crop factor you want a 35mm lens to match the human eye.


hgernhardtjr Oct 23, 2006 10:13 AM

The human eye is 50mm? Not quite ... as an amateur astronomer, this is what my various recent (and old) factbooks list and are considered to be the accepted focal length(s):

Object focal length of the eye = 17 mm
Image focal length of the eye = 22 mm
Dark adapted eye is about an ISO of 800
MP equivalent estimate is just under 600.

Here's just one of many places to check:

On edit, here's an astronomy reference, also:

BenjaminXYZ Oct 23, 2006 10:26 AM

Eh, without getting too complex with the astronomy stuffs etc...:lol:I am just curious to know what is the focal length of our eyes in terms of mm. (Just as a dSLR lens have 20, 30, 35, or 50 mm etc...) :) So our eyes is like a lens with __ mm? :-)

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hgernhardtjr Oct 23, 2006 10:42 AM

22mm is the most accepted scientific value, but various researchers list it as being between and 22-24mm (the image focal length referred to above).

kenbalbari Oct 23, 2006 11:00 AM

A normal focal length is generally considered to be one at which the focal length is equal to the diagnol of the film or sensor. So for 35mm film this is technically about 43mm. Conventionally, it is 50mm.

The diagnol is used because this produces an angle of view of about 45 degrees, which is presumably similar to that of the eye. I'm not sure the actual focal length of the eye matters here (but if you wanted to figure out how the eyes optics compare, you would probably have to take into account the size of the retina, as well as the focal length).

Nagasaki Oct 23, 2006 11:05 AM

You're confusing focal length with field of view.

A 50mm lens on a 35mm camera gives roughly the same field of view as the human eye. A 35mm lens on a 1.5 crop sensor will give approximately the same field of view.

The focal length of the human eye is about 22mm but a 22mm lens is wide angle on either 35mm or DSLR because on those formats it gives a wider field of view than the human eye. I guess you could consider the human eye as having a 2.5 crop when compared to a 35mm camera.

BenjaminXYZ Oct 23, 2006 11:14 AM


A 50mm lens on a 35mm camera gives roughly the same field of view as the human eye. A 35mm lens on a 1.5 crop sensor will give approximately the same field of view.

Actually that may be what I am interested when we look in front of us right now, our perspective of vision is at 50 mm?

BenjaminXYZ Oct 23, 2006 11:44 AM

Sorry guys, I know my question at the beginning sounds confusing.

I actually don't know how many mm(s) we are actually seeing. :roll:So I was asking whether the 52.5 mmwas = to "our human eye" or do you call that perspective orfield of view?

Sorry for the confusion.


OK, never mind already; I took the effort to find it all out myself.

I shall past it here for anyone who wants to know too>>>


(From Pentax lens literature) The Pentax SMC P-FA 50mm f/1.4 lens provides an angle of view and perspective similar to the human eye. Its natural perspective of subjects makes it ideally suited for everyday use including indoor photography, traveling, and hiking. Like every Pentax lens, the SMC P-FA 50mm f/1.4 lens features our acclaimed SMC multi-layer coating to lower surface reflection, reduce ultraviolet rays, and deliver clear, high-contrast images.


{From Nikon} Normal lenses are so called because they provide a 46 [suP]°[/suP] picture angle, for an angle of view that approximates that of the human eye. They are useful for many applications, from landscapes to candid shots. Other advantages include wide maximum apertures.

Alright, I got it!

blindsight Oct 23, 2006 3:02 PM

Different opinion about "50 mm" and "22-24 mm":

Is it possible that both are correct, but

50 mm is the focal length of ONE eye (we only use one eye to look through the camera viewfinder)

22-24mm is the "focal length" (note the quote) of TWO eyes, representing the field of natural human vision


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