# "Normal" Lens

Discussion in 'Photography' started by Gary Morrison, Aug 14, 2003.

1. ### john grimanisGuest

It is my understanding that the term "NORMAL" lens refers to the lens that
approximates on film the same image as viewed by the unaided eye. Any other
lens enhances the viewed image. The terminology is somewhat vague and
subjective.

While your replies note that the medium format lens is 80mm, that really
depends on the format.
A 6x4.5 normal might be a 75mm, a 6x6 is sold with an 80mm as the "normal",
however the 6x7 that I shoot for wedding work comes standard with a 90mm,
while larger formats move up.

As to formulas, if you take the 50mm as a standard for the 35mm camera which
has a negative approximately 25% of the size of a 6x6 negative, and then
extrapolate the ratio of the focal length of the lens to the size of the
film plane, you may be able to intuit the formula.

I had been taught that the relationship was somewhat more subjective, since
the correlation of an 80mm or 90mm lens on the medium format camera brings
the same result as the 50mm lens on the 35mm camera *which was sold in the
1940's with in some cases a 45mm lens as "normal". That result, of course,
is an image of "normal" viewing size, IE: what you see through the lens is
the SAME as what you see when you look at your subject from the camera
position NOT looking through the lens.
JGG

john grimanis, Aug 19, 2003

2. ### J CGuest

No. The terminology is neither vague nor subjective. There's a formula
for calculating a the minimum size of a lens that will project a
normal image. See my above posts.

That is an unnecessary workaround. In addition I'd point out that its
much less precise as that calculation would need to assume as a given
(taken on faith) what is stated as a "normal" lens for the 6x6 format.
There is actually a way to calculate normal for any given format.
Again, see above posts.

-- JC

J C, Aug 19, 2003

3. ### John GarrisonGuest

All this makes my head hurt a bit, especially when we detoured into square
"routes" I guess I will have trust the all the camera models will be
available in at least one kit with a "normal" lens, and take it on faith
that what they say is normal indeed is.

Nevermind at all I thought I knew what was normal for a given format, I was
right, but Oh......as usual, more deep a subject than I thought.

John Garrison, Aug 19, 2003
4. ### Jan PhilipsGuest

I once read that it would be so that the focal length of the lens
would be the same as the diagonal measure of the film frame. For
example, a 35mm film frame is approximately 24mm x 36mm. The diagonal
is approximately 43.3mm. The image on the film plane is a circle, and
the thing I read said that when the diameter of that circle was the
same as the diagonal size of the film, it would be "normal". (I don't
know why that is true.) So about 43-44mm would be "normal" for 35mm,
but 50mm and 55mm are close enough to be considered "normal".

Jan Philips, Aug 19, 2003
5. ### Jan PhilipsGuest

....

One thing I used was the assumption that the lens yields a circle at
the film plane whose diameter is equal to the focal length of the
lens. Is that correct?

Jan Philips, Aug 19, 2003
6. ### J CGuest

Assuming the size circle's diameter as the first step in determining
what a "normal" lens might be is not the correct way to proceed. If
you used your assumption and started with a 35 mm lens on a 35 mm
camera, you'd not get the answer you need.

What you actually need to do is calculate (not assume) the circle's
diameter based on what you know about the largest square that the
lens could project. (the diagonals of that square then are equal to
the diameter of the smalles possible circle). When you do the
calculation that way, the diameter of the circle projected by a 50mm
lens comes out to 50.91. This calculation assumes that a max square
image of 36mm x 36mm is possible when in fact, the len might project a
larger circle and therefore a slightly larger image might be possible.

Someone that has a professional understanding of lens optics would be
better at letting us in on the mysteries of lens design (for example
how much "rounding up" is done when you slap the designation "50 mm"
on the lens barrel).

Also, related to that larger image circle: As I've been given to
understand the reason it might be larger is that there is (with some
lenses, particularly cheap, poorly manufactured lenses) some fall off
in the light intensity toward the edges of the optical elements. So
the actual circle of light projected by the lens might be larger to
compensate -- so that this area where the light falls off does not
appear on the film. If it did fall on the film then the four corners
of the square inscribed in the circle would be 1. less exposed on the
film and 2. dimmer on the prints. But then, perhaps this isn't much of
a problem because a lot of pro art photogs recommend burning in the
four corners, and sometimes the edges of a print slightly (the idea
being to keep the viewer's eye on the main portion of the image).

-- JC

J C, Aug 19, 2003
7. ### Jan PhilipsGuest

I found this

http://www.wikipedia.org/wiki/Normal_lens

"In still photography, a normal lens is a lens whose focal length is
roughly equivalent to the diagonal of the image projected within the
camera. This roughly approximates the perceived field of view of the
human eye."

Jan Philips, Aug 19, 2003
8. ### J CGuest

Well, to be truthful, I only retained the stuff I use frequently. All
those proofs bored the crap out of me.

-- JC

J C, Aug 19, 2003
9. ### John GarrisonGuest

Me too, about 3000 theorems and at least as many postulates, and our our
esteemed teacher wanted us commiting each and every one to memory. Ouch. I
got maybe ten of them.

John Garrison, Aug 20, 2003
10. ### Jan PhilipsGuest

I'm getting off-topic, but there really is no point in having students
memorize the proofs. The point is to be able to discover proofs.
Back in my day, we didn't memorize proofs, and geometry was the first
step towards real math.

Jan Philips, Aug 20, 2003
11. ### DTJGuest

Not any more.

I recently heard of a high school in Illinois that has lower
requirements than when I graduated in the 80's. Geometry and Algebra?
No way should a kid need to take those classes, that is only for
scientists...

DTJ, Aug 30, 2003
12. ### Omer KGuest

Actually if it's circular it would be 36mm. since the diagnal in a
circle is just the radius itself. If you count the diameter as being
normal then of course it's 72mm.

I don't understand your calculation. Maybe I'm missing something.

Omer

Omer K, Sep 9, 2003
13. ### J CGuest

Say what? I'm not certain what your post means as I can't follow your
logic for how you come up with 72 as the answer.

I think you are missing something. Draw a square with sides of 36 mm.
Now draw a circle touching each of the four corners. The square is
inscribed in the circle and the sides of the square are cords of the
circle NOT the diameter.

Now if you calculate the square's diagonal, well... THAT IS the
diameter of the circle.

And to find the square's diagonal, AKA the circle's diameter, use the
Pythagorean theorem.

-- JC

J C, Sep 9, 2003