|GG| Why does a normal lens' focal length equal to the sensor'sdiagonal size?

Anoop Saxena wrote:
Hi,
Perhaps a photographer need not know it but I am very curious,
why is a lens with a focal length that matches the diagonal length of
a sensor or film considered normal? I understand that it roughly
matches the normal field of view of the Human eye. But I believe that
is the outcome of focal length matching the sensor diagonal size and
not the other way round.

Thanks in advance,
Anoop.

Try this test, put your hands up in front of you and box in what you
think is a 'normal' field of view, like a movie director frames a
scene... most people guess larger than normal... photographers at least
have a chance of guessing something about right. If you take a photo
with a normal lens, print it out to 8x10 and hold it at a comfortable
viewing distance, it probably matches up with the real scene pretty
well, and that's a rather narrow angle compared to all you can see with
two eyes!

Now shoot something super wide, print it out & the perspective looks
wrong. Now, move your eyes right up a few inches away from the print &
the perspective looks alright.

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

Normal is what most folks see ... about 50mm on a full-frame dslr.

Angle of view is what is important.

Use a normal lens and you will capture the same field (angle) of view of
what you see.

Charles wrote:

Normal is what most folks see ... about 50mm on a full-frame dslr.


I thought that normal on the standard 35mm frame was 43mm (the diagonal).
However, in practice 50mm is considered normal. I suppose with a 1.5 crop
for APS-C size sensors that translates to 29mm and 33mm respectively.

--
http://www.petezilla.co.uk

Paul Furman wrote:
Anoop Saxena wrote:
Hi,
Perhaps a photographer need not know it but I am very curious,
why is a lens with a focal length that matches the diagonal length of
a sensor or film considered normal? I understand that it roughly
matches the normal field of view of the Human eye. But I believe that
is the outcome of focal length matching the sensor diagonal size and
not the other way round.

Thanks in advance,
Anoop.

Try this test, put your hands up in front of you and box in what you
think is a 'normal' field of view, like a movie director frames a
scene... most people guess larger than normal... photographers at least
have a chance of guessing something about right. If you take a photo
with a normal lens, print it out to 8x10 and hold it at a comfortable
viewing distance, it probably matches up with the real scene pretty
well, and that's a rather narrow angle compared to all you can see with
two eyes!

Now shoot something super wide, print it out & the perspective looks
wrong. Now, move your eyes right up a few inches away from the print &
the perspective looks alright.

The theoretically correct viewing distance for an image is just the
focal length of the taking lens times the magnification of the image you
are viewing. A 50mm lens on 35mm film enlarged to 8x12 - about an 8x
enlargement - is 50 x 8 mm, or 400 mm, about 16 inches viewing distance.
A 'standard' lens is one where the natural viewing distance more or
less coincides with the calculated distance as above.

For a wide-angle lens the calculated distance will probably be less then
the natural viewing distance, which causes perspective distortion in the
eye of the viewer - and the same for telephoto shots.

--

Colin D.