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

"Chris Brown" wrote in message
...
SNIP
Well, the depth of field might change, that depends on the apertures
used in each case.

Obviously, since the aperture number is the physical size related to
the focal length, the larger focal length also has a larger physical
aperture size than the shorter focal length lens has at the same
aperture value.

There is a myth that the choice of focal length has some sort of
magic effect on the perspective of a scene, and I think that's where
a lot of this squabbling over terms comes from.

IMO it is all due to some people changing *two* parameters at the same
time, both focal length *and* shooting position to achieve the same
field of view in the cropped image. Changing the shooting position
changes the magnification factor between near and far objects, and
thus perspective. Cropping is just that, cropping.

SNIP
Once you understand this, you'll see why it doesn't make a blind
bit of difference whether you think about this phenomenon as a "crop
factor", a "focal length magnifier", or anything else. They're all
equivalent.

But calling it a "focal length magnifier" is what started the whole
confusion, so I'd avoid it. "Crop factor" (compensation) is
technically correct and it describes what happens. To compensate for
the crop factor, without changing perspective/shooting position, a
lens with a wider FOV needs to be used.

Bart

"Chris Brown" wrote in message
...
SNIP
Well, the depth of field might change, that depends on the apertures
used in each case.

Obviously, since the aperture number is the physical size related to
the focal length, the larger focal length also has a larger physical
aperture size than the shorter focal length lens has at the same
aperture value.

There is a myth that the choice of focal length has some sort of
magic effect on the perspective of a scene, and I think that's where
a lot of this squabbling over terms comes from.

IMO it is all due to some people changing *two* parameters at the same
time, both focal length *and* shooting position to achieve the same
field of view in the cropped image. Changing the shooting position
changes the magnification factor between near and far objects, and
thus perspective. Cropping is just that, cropping.

SNIP
Once you understand this, you'll see why it doesn't make a blind
bit of difference whether you think about this phenomenon as a "crop
factor", a "focal length magnifier", or anything else. They're all
equivalent.

But calling it a "focal length magnifier" is what started the whole
confusion, so I'd avoid it. "Crop factor" (compensation) is
technically correct and it describes what happens. To compensate for
the crop factor, without changing perspective/shooting position, a
lens with a wider FOV needs to be used.

Bart

"Bart van der Wolf" wrote:

SNIP
Once you understand this, you'll see why it doesn't make a blind
bit of difference whether you think about this phenomenon as a "crop
factor", a "focal length magnifier", or anything else. They're all
equivalent.

But calling it a "focal length magnifier" is what started the whole
confusion, so I'd avoid it.

No! This whole confusion is because the original poster didn't realize that
lenses function differently on different formats. His claim was that the
"focal length multiplier" didn't apply!

Since "focal length multiplier" tells you (or should tell youg) that the
lens _functions_ differently, it's the better term.

"Crop factor" (compensation) is
technically correct and it describes what happens. To compensate for
the crop factor, without changing perspective/shooting position, a
lens with a wider FOV needs to be used.

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.

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.

David J. Littleboy
Tokyo, Japan

"Chris Brown" wrote in message
...
In article et,
Robert Scott wrote:

It's the biggest misconception in digital photography.

Indeed it is, and if you, like the original poster, think that you'll get
the same area distortions from rectilinear projection with a given lens,
then I'm afraid you're suffering from said misconception yourself. ;-)


Your math certainly seems sound and your explanation is compelling enough to
convince me I had the wrong idea about what was happening.

But I just need to do a simple test to prove it to myself. I'm reserving
further comment on this thread until I take a pair of identical images at
18mm with the D70 and 28mm with the F4S.

:-)

Thanks for taking the time to lay it out the way you did. That was very
enlightening.

Happy Holidays everyone!

Bob
the stubborn cynic

In article et,
Robert Scott wrote:

"Chris Brown" wrote in message
...
In article et,
Robert Scott wrote:

It's the biggest misconception in digital photography.

Indeed it is, and if you, like the original poster, think that you'll get
the same area distortions from rectilinear projection with a given lens,
then I'm afraid you're suffering from said misconception yourself. ;-)


Your math certainly seems sound and your explanation is compelling enough to
convince me I had the wrong idea about what was happening.

But I just need to do a simple test to prove it to myself. I'm reserving
further comment on this thread until I take a pair of identical images at
18mm with the D70 and 28mm with the F4S.

Go for it, and do keep a link to the images. This does, as you pointed out,
cause a lot of confusion, and seeing is believing. :-)

Thanks for taking the time to lay it out the way you did. That was very
enlightening.

Glad you found it useful.

Happy Holidays everyone!

Likewise

In article et,
Robert Scott wrote:

"Chris Brown" wrote in message
...
In article et,
Robert Scott wrote:

It's the biggest misconception in digital photography.

Indeed it is, and if you, like the original poster, think that you'll get
the same area distortions from rectilinear projection with a given lens,
then I'm afraid you're suffering from said misconception yourself. ;-)


Your math certainly seems sound and your explanation is compelling enough to
convince me I had the wrong idea about what was happening.

But I just need to do a simple test to prove it to myself. I'm reserving
further comment on this thread until I take a pair of identical images at
18mm with the D70 and 28mm with the F4S.

Go for it, and do keep a link to the images. This does, as you pointed out,
cause a lot of confusion, and seeing is believing. :-)

Thanks for taking the time to lay it out the way you did. That was very
enlightening.

Glad you found it useful.

Happy Holidays everyone!

Likewise

"David J. Littleboy" wrote in
:

No! This whole confusion is because the original poster didn't realize
that lenses function differently on different formats. His claim was
that the "focal length multiplier" didn't apply!


Actually, the OP only said that his 17mm lens has distortion.

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.

Bob

Finally, a rational post of how the same focal length behaves with different
film/digital formats.

Having shot from 8x10 down to Nikon Digital (1.5 factor, don't remember
actual sensor size right now). I have seen this for myself and have found a
great deal of misunderstanding in this group. Particularly the silly theory
that focal length in itself affects perspective.

Good on you David

Best,
Dave

Good on you too Chris.

(See my post below)

Best,
Dave

PS - I think that the 35mm equivalent thing is just about how a lot of us
think in terms of 35 regarding which lens for which shot. 75-135 for
portraits (waist up to tightly cropped head shot) while maintaining the
correct PERSPECTIVE between the nose and ears. Try changing that
relationship by changing focal length but maintaining the same camera
distance.

"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