# "Normal" Lens

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

1. ### Gary MorrisonGuest

Really bsaic question here: Is there any reasonably-precise mathematical
formulation of what focal length constitutes a "normal" lens (i.e., the
breakpoint between wide-angle and telephoto), or are numbers for that
fundamentally approximate and conventional, and derived basically
intuitively or inductively? I'd imagine that that could get interesting
when you bring wide-screen media into the question, for example.

Gary Morrison, Aug 14, 2003

2. ### photo35744Guest

For 35 mm a normal lens is 50 mm for medium format it is 80 mm.

photo35744, Aug 14, 2003

pentax reckons that based on the human AOV it's about 43mm

4. ### Gary MorrisonGuest

For 35 mm a normal lens is 50 mm for medium format it is 80 mm.

Sure, and around 30 for my new D100.

My question though is how those numbers are calculated, or whether they really
even can be calculated.

Gary Morrison, Aug 14, 2003
5. ### BrianGuest

Sorry bud you lost me right after "For 35mm, yes", LOL
Ok lets try this then,
Ansel Adams defines a "Normal" lens as "one whose focal lenght is about
equal to the diagonal of the film format. Such a lens will have an angle of
view of about 50 degrees to 55 degrees, comparable to what we consider
normal human vision. A 50mm (2-inch) lens is considered normal for 35mm
cameras (although the actual film diagonal is about 42mm); 80mm is normal
for 2 1/4 x2 1/14 format; and 150mm to 165mm is normal for 4x5."

From his book The Camera.

Brian.................

Brian, Aug 14, 2003
6. ### John GarrisonGuest

I did a dirty check on what was normal for my field of view, by taking a
35mm camera with the lens on, and composing with one eye and looking out
with the other. When what I saw in the viewfinder was what I saw with the
open eye, I found the lens was at about 50mm!

John Garrison, Aug 14, 2003
7. ### FrederickGuest

I don't know any formula used to calculate it, but on a 35mm camera
the standard "normal" lens is 50mm. Less would be Wide angle, and more
would be telephoto or Zoom.

Frederick, Aug 14, 2003
8. ### John O.Guest

I think you should try a different forum for this one. A medical, math
or science group would probably be better.

John O., Aug 14, 2003
9. ### JeremyGuest

x-no-archive: yes
The word "normal" refers to the lens' focal length--the distance between a
point in the lens and the film when the lens is focused on infinity.

A "normal" focal length lens on any camera is considered to be a lens whose
focal length closely approximates the diagonal of the picture area produced
on the film. With 35mm cameras this actually works out to be about 43mm,
generally considered to be a little too short to produce the best angle of
coverage and the most pleasing perspective. Makers of 35mm cameras have
generally standardized on 50mm or 55mm as normal lenses. Pentax even made
some normal lenses in 58mm.

Pentax currently does have a 43mm normal lens in their "Limited" series, and
it has gotten excellent reviews.

Jeremy, Aug 14, 2003
10. ### Sanda & Georg PolenikGuest

Hallo Gary,
the definition of "normal lens" depends on the format of used picture
format in camera and corresponds approximately to the diagonale of the slide
/ negative. This has been defined in the early days of photography and it
was assumed, that this will simulate a natural view of the taken pictures,
i.e. the perspective corresponds to the natural view angel of our eyes in
real world.
So, if you look at the most common slide/negative format of 36mm x 24 mm,
the diagonale is calculated (due to the formula from Pythagoras): sqare root
of (24*24 + 36*36) which gives something about 43. Actually the value of
50mm is a rounded up one, I don´t know the reason for that now.
Considering large format negatives / slides you should be aware of various
available formats (6x4 cm, 6x6 cm, 6x9cm , 9x13 cm etc). So the individual
"normal lens focal length" for large format cannot be given with one
value...
greetings Georg

Sanda & Georg Polenik, Aug 14, 2003

Standard NOW...but in times past some camera makers have sold 45mm or
55mm lenses as "standard" with their 35mm bodies. My understanding as
someone else mentioned earlier in this thread is that approximately
43mm would be considered technically "standard" since that most
closely approximates the field of view of a single human eye.

12. ### Gary MorrisonGuest

Matching magnification doesn't seem to work for me on my D100 at least. When I
matched the magnification of what I saw through my open eye with what I saw
through the lens, it turned out to be 62mm, clearly too large when a normal
lens is accepted to be around 30mm or thereabouts.

Matching the height or width of the image appears to be the right approach,
except, as in 70mm or IMAX where the image is specifically designed to involve
a lot of peripheral-vision area.

Gary Morrison, Aug 15, 2003
13. ### Gary MorrisonGuest

I don't think that'll work for IMAX.

Gary Morrison, Aug 15, 2003
14. ### Gary MorrisonGuest

(The diagonal approach I mean.)

Gary Morrison, Aug 15, 2003
15. ### Gary MorrisonGuest

IMAX is clearly intended to be viewed much closer than a typical 35mm
print. Its purpose is to entirely fill peripheral vision and to allow you
to turn your head and see new stuff. So the diagonal doesn't quite work in
that case, as far as I can tell, nor does the angle approach as such.

Gary Morrison, Aug 15, 2003
16. ### Sanda & Georg PolenikGuest

Hallo Gary,

the focal length of a "normal lens" is defined as the diagonale of the
slide / negative.
This has been set up in the early days of photography and it was assumed,
that this will
simulate a natural view of the taken pictures, i.e. the perspective will
correspond to the
natural view angel of our eyes inreal world.

So, if you look at the most common slide/negative format of 36mm x 24 mm,
the diagonale is calculated (due to the formula of Pythagoras): sqare root
of (24*24 + 36*36) which gives something about 43 (someone else has told us
of such a lens
from Pentax in this thread). Actually the value of 50mm is a rounded up
one, I don´t know the
reason for that now.
Concerning large format negatives / slides you should consider the various
available formats (6x4 cm, 6x6 cm, 6x9cm , 9x13 cm etc). So an individual
"normal lens focal length" for large format is always associated with given
conditions.
greetings Georg

Sanda & Georg Polenik, Aug 15, 2003
17. ### JeremyGuest

x-no-archive: yes
You asked a question in a photographic newsgroup--we gave you the correct
answer. If you are concerned with IMAX lens properties, you are in the
wrong place.

Most of us have never even seen an IMAX film, let alone know anything about
the technical end.

Normal is defined as a lens whose focal length is approximately equal to the
diameter of the film. That has been THE textbook answer for many decades.

Jeremy, Aug 15, 2003
18. ### Gary MorrisonGuest

No, not the height or width of an object, but the height and width of the field
of vision.

The going theory is appears to be that human non-peripheral vision has a
right-to-left (probably at least, rather than top-to-bottom) of around 50
degrees. A hypothetical experiment for determining this would be:

1. Tell somebody to stare straight ahead,
2. Place one object to the right of that line of vision and one to the left,
3. Gradually move those objects to the right and left,
4. Stop when they say that the objects are clearly only in their peripheral
vision field. That is, you stop when they report that it's iffy as to
whether they even see it.
5. A "normal" lens with the focal point right where your eyes are will put
those objects right on edge of the frame.

The angle at their eye between those two objects, allegedly at least will be
between 48 and 55 degrees.
I think the answer would be yes, but I certainly don't know that.
That seems reasonable, except it certainly has to be measured in proportion to
the size of the retina relative to the size of the film.

Still, there has to be a factor related to how you're viewing the presentation
of the picture as well. My illustration earlier was that, if you look at an
IMAX frame printed on an 8x10 sheet of paper, it'll look very-wide-angle, but
when viewed in the way it's intended to be viewed, muuuuch closer up, it'll
look normal.

Gary Morrison, Aug 15, 2003
19. ### Gary MorrisonGuest

A hypothetical experiment for determining this would be...

Or at least what I described here is what I interpret the concept to be.
More likely it would be at a point where the object becomes hard to recognize,
since we can see in our peripheral vision objects that we can't clearly recognize.

Gary Morrison, Aug 15, 2003
20. ### J CGuest

Let's put an end to this thread....

I'd disagree with that statement. What's considered normal for any
specific camera format does not equal what the human eye sees.
Example, a 50 mm lens on a 35 mm does not capture the entire field of
view of the human eye (i.e., from left to right periphery). In truth a
"normal" lens is one that does not distort the image. For example,
straight horizontal and vertical lines are captured on film as
straight lines! The field of view is irrelevant.

AND There is a formula for calculating a normal lens and that involves
the ratio of the lens diameter to the focal length. Put another way,
the normal lens is one wherein the focal length equals the diagonal of
the image placed on the film.

So here's the calculation using a 2.25 inch film format and the
equation for a calculating the hypotenuse of a triangle:

2.25 inches (squared) + 2.25 (squared) = the diagonal (squared)

5.0625 + 5.0625 = diagonal (squared)

The Square root of 10.125 = 3.1819

Since this is in inches, convert to cm (using 1 inch = 2.54 cm) and

3.1819 inches = 8.082 cm OR 80.82 mm

So a normal lens for the 2.25 inch format is an 80 mm lens.

However, note that there is actually a range (about +/- 5%) that would
be considered normal because a lens will actually shine a circle of
light and the only reason the camera takes square or rectangular
pictures is because the camera's shutter is shaped that way. In some
constructions the square or rectangle of the image will not form cords
across this circle and therefore the diagnal of the image area on the
film will be less than the diameter of the circle.

-- JC

J C, Aug 18, 2003