Lens perspective distortion and DSLR crop factor

Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

Regards,
Paul

wrote in message
oups.com...
Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

You're correct - wide angle distortion is a function of distance to subject
only, and the zoom settings you describe should perfectly cancel out the 1.6
crop factor, modulo lens distortion and shifting of the nodal point.

This is a very common misconception, similar to the belief that DOF is less
for a telephoto than a wide angle.
--
Mike Russell
www.curvemeister.com/forum/

wrote:
: Hi all,

: I attended the Rocky Mountain School of Photography last week and
: their equipment guru made a claim that is inconsistent with my earlier
: understanding of an issue related to Crop Sensor DSLRs. Maybe someone
: out there could clarify for me.

: I have always been under the understanding that if I took a Canon
: 24-70mm f/2.8 zoom and took photos with the following two bodies, the
: resulting images would be exactly the same if we were to ignore pixel
: density and quality differences of the zoom at different focal
: lengths:

: Canon 30D @ 31.25mm focal length, 5 feet from subject
: Canon 5D @ 50mm focal length, 5 feet from subject

: According to the individual at Rocky Mountain, the lens (perspective)
: distortion normally associated with wide-angle shots (traditional 35mm
: film) would be evident on the 30D example above because this
: perspective distortion is inherent to the wide angle lens. My
: understanding was that this was false and that the distortion is a
: result of distance to subject. Could somebody please clarify this
: issue for me?

: Regards,
: Paul

The answer is kinda maybe.

Yes the field of view will be the same for both and at these values the
spherical distortion (most noticable at extra wide angle or fisheye)
differences will be minimal. So the two images will be close enough to
identical as to be not noticed. But as you get further toward any extreme,
lens based distortions can become more problematic. For example if it was
only field of view, a fisheye photo of a closeup human face that shows the
dramatic exagerating of physical features that is then cropped to an image
with the same field of view as a longer lens image would be identical. But
the simple cropping will not correct the exageration of features. So at
extremes there will be a difference. But when the lens lengths are fairly
close, the crop of one to match the FOV of another will be very close to
eachother.

So yes it is probable that the two images you propose will be visually
similar enough that you wouldn't likely notice the difference. Of course
each combination of camera and specific lens will have its own set of
plusses and minuses and so even using the same body, several different
lenses at the same focal length could have some slight variation in the
image distortion from trying to represent a spherical view on a flat
medium.

JMHO

Randy

==========
Randy Berbaum
Champaign, IL

On May 13, 7:08 pm, wrote:
Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?
You are right, he is wrong.

There might be some small differences in the amount of pincushion
distortion, but this is not the
same as perspective.

Scott



Scott

On May 14, 12:08 am, wrote:
Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

Regards,
Paul

Ah, the problem may be that there are TWO definitions of distortion.
To an optical engineer and lens designer, distortion means that the
field angle to any point in the image is not exactly the same as the
field angle of the input point. This kind of distortion IS reduced by
reducing the format used by a lens, and hence by the cropping done by
digicams. That is, the distortion increases with distance from the
optical axis, which intersects the image plane at the center of the
image for most cameras. Thus cropping does not use the wide field
angles and does reduce this form of distortion.

Perspective distortion is another issue. This is the APPARENT
distortion of perspective caused by viewing an image from a different
effective vantage point than what it was taken at. This is especially
due to the fact that wide angle prints would require viewing from a
distance closer to the print than our eyes can focus at. This type of
distortion is ONLY a function of the taking and viewing distances, and
has no dependence on focal length of lens.

Don Stauffer in Minnesota writes:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

Ah, the problem may be that there are TWO definitions of distortion.
To an optical engineer and lens designer, distortion means that the
field angle to any point in the image is not exactly the same as the
field angle of the input point. This kind of distortion IS reduced by
reducing the format used by a lens, and hence by the cropping done by
digicams. That is, the distortion increases with distance from the
optical axis, which intersects the image plane at the center of the
image for most cameras. Thus cropping does not use the wide field
angles and does reduce this form of distortion.

That would be pretty much always true if both cameras were using the
same lens, but the 30D was using less of the image circle than the 5D.

However, that's not the case here. The two cameras are using different
focal length lenses in order to match the field of view, and so the field
angles at the corner of the image are exactly the same for both of the
proposed cases. So either camera could see slightly more distortion
than the other, depending on the distortion performance of the two lens
designs.

Dave

writes:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

You're right, he's wrong. The wide-angle perspective distortion is only
caused by a difference in field angle between taking and viewing the
image. In this case, you are carefully matching the field angles
between the two cameras, so you will get exactly the same perspective in
images from both. If you make prints of the same size and view them
from the same distance, you will get the same amount of perspective
distortion from both.

On the other hand, the image shot with the 30D will have slightly
greater depth of field if shot at the same aperture as the 5D one.

Dave

wrote:
Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

Regards,
Paul

If both - or any - cameras are at the same distance from the subject,
the perspective drawing is the same for all, regardless of focal length,
wide angle or not, and regardless of image or film/sensor size. Any
part of the scene common to both images will have the same drawing,
which is determined solely and only by the camera position, and nothing
else.

It is commonly stated that certain lenses produce optimum perspective
for portrait shots, typically 85mm on a 35mm camera. This is fallacial;
the reality is the desired perspective is obtained from the shooting
distance, and that distance needs an 85mm lens to fill the frame. If
you shot with a shorter lens from the same viewpoint, the perspective
will be identical - but the image will be smaller. If you go closer to
fill the frame, then the perspective will change, and this gives rise to
the commonly but wrongly held belief that the lens controls perspective.
It does not; distance from subject controls perspective.

Minor distortions with particular lenses are not relevant to your question.

Colin D.

--
Posted via a free Usenet account from http://www.teranews.com

On 13 May 2007 22:08:43 -0700, wrote:
................................................. .............................. the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

Paul

Actually the distortion you see in a wide angle lens image does not
exist. It just looks like there is a distorted wide angle look and if
you know how to look it wouldn't look like there is wide angle look
to look at.

Ben

wrote in message
oups.com...

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

Yes, with two qualifications:

1) The lens might have different rectilinear distortion characteristics
at different zoom settings. So, for example, it might have some barrel
distortion at 31.25mm that it does not have at 50mm.

2) To get the same depth of field, you would have to adjust the aperture
to compensate for the difference in focal length. Since subject-camera
distance is the same, you would need the absolute size of the aperture to be
the same. So, for example, if the 31.25mm setting was at f/4, then the
aperture would be 7.8125mm in diameter. To obtain the same diameter at a
50mm setting, you would need f/6.4.

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens.

Well, now you know one instructor not to believe.