Digital 17mm Is Not Equivelent 27mm on 35mm Film

"David J. Littleboy" wrote in message
...
SNIP
Thinking of it as a "crop factor" is what caused both the original
poster's error and the common error in which people think the
DOF stays the same for the same lens. That's because it focuses
people's minds on a given lens being used on both cameras and
not realizing that the same lens is functions differently on the
different cameras.

True, but that's only because DOF is a function of output
size/magnification as well, which has little to do with perspective
(depending on how the other variables are chosen).

The best term would be "format conversion factor", since that's
what it is. That would make people realize that the 1.6x cameras
are a different format from 35mm and allow them to think about the
different photographic functions of a given lens on the different
formats.

I agree, but it would probably result in even longer threads... ;-(

Bart

"Mark=B2" mjmorgan(lowest even number wrote:

"A" wrote in message

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor),=
you
still get distorted pics just like 17mm lenses on 35mm film cameras.

The "equivalents" really only apply to *field of view,* since you're re=
ally
just cropping out the middle portion of the normal 35mm film camera's i=
mage.

Is it also true that if you take a portrait of a person with a big nose
using a 50mm lens on your DSLR instead of an 85mm(*) lens on your SLR,
the nose will look bigger in the DSLR picture?

(*) The two aren't quite equivalent, 80mm would be.

"Mark=B2" mjmorgan(lowest even number wrote:

"A" wrote in message

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor),=
you
still get distorted pics just like 17mm lenses on 35mm film cameras.

The "equivalents" really only apply to *field of view,* since you're re=
ally
just cropping out the middle portion of the normal 35mm film camera's i=
mage.

Is it also true that if you take a portrait of a person with a big nose
using a 50mm lens on your DSLR instead of an 85mm(*) lens on your SLR,
the nose will look bigger in the DSLR picture?

(*) The two aren't quite equivalent, 80mm would be.

bob wrote:

QUOTE:
If you shoot tall buildings at 17mm on digital (with 1.6 crop
factor), you still get distorted pics just like 17mm lenses
on 35mm film cameras.

Sounds like a complaint about barrel distortion to me.

Every digicam and DSLR test I have seen so far
shows some kind of barrel distortion, usually much worse than moderate,
often objectionable, at wide angle.

This is even true of overpriced lenses like the Nikon 17-55/2.8 DX.

The "big nose" effect is due to camera position. Therefore given the same
cropping (image size) of the person and 50mm lens on a 1.5 factor digital
camera would give the perspective of a 75mm lens on a 35mm camera. (same
distance from subject for same image size as the 75 lens on 35mm camera) A
digital with a 1.6 factor would yield an 80mm perspective. This is all based
on camera distance to subject. Longer lenses flatten perspective thus
allowing nose and ears to appear more correct in their relationship to each
other. (Hope I didn't jumble that too badly)

Hope this helps,
Dave

The "big nose" effect is due to camera position. Therefore given the same
cropping (image size) of the person and 50mm lens on a 1.5 factor digital
camera would give the perspective of a 75mm lens on a 35mm camera. (same
distance from subject for same image size as the 75 lens on 35mm camera) A
digital with a 1.6 factor would yield an 80mm perspective. This is all based
on camera distance to subject. Longer lenses flatten perspective thus
allowing nose and ears to appear more correct in their relationship to each
other. (Hope I didn't jumble that too badly)

Hope this helps,
Dave

"Bill Tuthill" wrote in message ...
"Mark²" mjmorgan(lowest even number wrote:

"A" wrote in message

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor),
you
still get distorted pics just like 17mm lenses on 35mm film cameras.

The "equivalents" really only apply to *field of view,* since you're
really
just cropping out the middle portion of the normal 35mm film camera's
image.

Is it also true that if you take a portrait of a person with a big nose
using a 50mm lens on your DSLR instead of an 85mm(*) lens on your SLR,
the nose will look bigger in the DSLR picture?

(*) The two aren't quite equivalent, 80mm would be.

It would give you the same perspective because you would likely back away to
the same distance you would have if you used the 80mm on a film camera.
Remember, the nose:face size ratio (big or smaller nose) is NOT determined
by the focal length, but instead, the relative difference in distance to the
nose tip compared with the distance to the rest of the face.

This is easier to conceptualize if you imagine an EXTREME close-up.
Imagine yourself taking a picture of someone's face only one inch away from
a person's nose. If you did this, the nose would be one inch away, and the
rest of their face would be perhaps two inches and further away...TWICE the
distance away that their nose was from you.

This will greatly exaggerate the size appearance of the nose because there
is a nose:face distance ratio of 1:2. This difference will remain true
regardless of what focal length you have mounted.

Now imagine backing away from the nose/face. As you back away to a full one
foot (12 inches) from the nose, the ratio of distance from nose:face has now
changed to a far more similar 12:13 ratio (12 inches from the nose, and 13
inches from the face). As you move back to 8 feet away (for example), the
ratio of distance from nose vs. distance from face will become tiny (96:97).
You can see from this extreme example how the farther you back up, and less
out of whack the ratio of distance between (distance to) nose and (distance
to) face will become.

**This ratio will remain consistent regardless of what lens you use. What
WILL change is how much of your subject is visible within the frame.
So...when you've got your 50mm lens attached, and you add the 1.6 crop
factor, you will have to back up to a similar distance than you would with
your ~80mm lens.
--This will give you the same perspective.

It's all about relative distance to the subject's closest attribute (in this
case, the nose) compared with the distance to the rest of the subject.

You've probably seen those funny snap-shots taken of cows, or dogs, where it
seems like their nose stretches right out to you? -Those look that way
simply because they were taken at such close range that the distance ratio
was extreme...and that they used such a wide-angle lens, that you could
still see the entire face/head to appreciate the distorted difference.
-Mark

"Mark²" mjmorgan(lowest even number wrote in message
news:Gp_xd.60904$ka2.26406@fed1read04...

"Bill Tuthill" wrote in message
...
"Mark²" mjmorgan(lowest even number wrote:

"A" wrote in message

If you shoot tall buildings at 17mm on digital (with 1.6 crop factor),
you
still get distorted pics just like 17mm lenses on 35mm film cameras.

The "equivalents" really only apply to *field of view,* since you're
really
just cropping out the middle portion of the normal 35mm film camera's
image.

Is it also true that if you take a portrait of a person with a big nose
using a 50mm lens on your DSLR instead of an 85mm(*) lens on your SLR,
the nose will look bigger in the DSLR picture?

(*) The two aren't quite equivalent, 80mm would be.

-----------
My above post began he
(for some reason, Bill's post didn't have "" preceding it in my other
response to him)

It would give you the same perspective because you would likely back away
to
the same distance you would have if you used the 80mm on a film camera.
Remember, the nose:face size ratio (big or smaller nose) is NOT determined
by the focal length, but instead, the relative difference in distance to
the
nose tip compared with the distance to the rest of the face.

This is easier to conceptualize if you imagine an EXTREME close-up.
Imagine yourself taking a picture of someone's face only one inch away
from
a person's nose. If you did this, the nose would be one inch away, and
the
rest of their face would be perhaps two inches and further away...TWICE
the
distance away that their nose was from you.

This will greatly exaggerate the size appearance of the nose because there
is a nose:face distance ratio of 1:2. This difference will remain true
regardless of what focal length you have mounted.

Now imagine backing away from the nose/face. As you back away to a full
one
foot (12 inches) from the nose, the ratio of distance from nose:face has
now
changed to a far more similar 12:13 ratio (12 inches from the nose, and 13
inches from the face). As you move back to 8 feet away (for example), the
ratio of distance from nose vs. distance from face will become tiny
(96:97).
You can see from this extreme example how the farther you back up, and
less
out of whack the ratio of distance between (distance to) nose and
(distance
to) face will become.

**This ratio will remain consistent regardless of what lens you use. What
WILL change is how much of your subject is visible within the frame.
So...when you've got your 50mm lens attached, and you add the 1.6 crop
factor, you will have to back up to a similar distance than you would with
your ~80mm lens.
--This will give you the same perspective.

It's all about relative distance to the subject's closest attribute (in
this
case, the nose) compared with the distance to the rest of the subject.

You've probably seen those funny snap-shots taken of cows, or dogs, where
it
seems like their nose stretches right out to you? -Those look that way
simply because they were taken at such close range that the distance ratio
was extreme...and that they used such a wide-angle lens, that you could
still see the entire face/head to appreciate the distorted difference.
-Mark

"DSphotog" wrote:

The "big nose" effect is due to camera position.

Exactly!

Therefore given the same
cropping (image size) of the person and 50mm lens on a 1.5 factor digital
camera would give the perspective of a 75mm lens on a 35mm camera. (same
distance from subject for same image size as the 75 lens on 35mm camera) A
digital with a 1.6 factor would yield an 80mm perspective. This is all
based
on camera distance to subject.

Still spot on, but:

Longer lenses flatten perspective thus
allowing nose and ears to appear more correct in their relationship to
each
other. (Hope I didn't jumble that too badly)

And I hope you don't mind the following quibbleg.

The longer lens does not "flatten perspective": it provides magnification
(relative to a shorter lens) so that you can achieve the framing you want at
a (longer) distance such that the perspective is "flatter".

David J. Littleboy
Tokyo, Japan

"DSphotog" wrote:

The "big nose" effect is due to camera position.

Exactly!

Therefore given the same
cropping (image size) of the person and 50mm lens on a 1.5 factor digital
camera would give the perspective of a 75mm lens on a 35mm camera. (same
distance from subject for same image size as the 75 lens on 35mm camera) A
digital with a 1.6 factor would yield an 80mm perspective. This is all
based
on camera distance to subject.

Still spot on, but:

Longer lenses flatten perspective thus
allowing nose and ears to appear more correct in their relationship to
each
other. (Hope I didn't jumble that too badly)

And I hope you don't mind the following quibbleg.

The longer lens does not "flatten perspective": it provides magnification
(relative to a shorter lens) so that you can achieve the framing you want at
a (longer) distance such that the perspective is "flatter".

David J. Littleboy
Tokyo, Japan