Canon says: more EF-S lenses

Started Oct 6, 2004 | Discussions thread
Doug Kerr
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Perspective issues
In reply to ChuckH, Oct 8, 2004

Hi, Chuck,

ChuckH wrote:

Hi, Doug,
I composed that post in a bit of a rush and didn't choose my words
as carefully as I should have.

It's tough to be precise without being very "wordy"! (As you know, I just don't worry abouyt that!)

To answer your question, I was thinking of 2 different types of
distortion. The geometric (i.e. barrel) distortion that you
referred to as well as the tendency of WA lenses to distort certain
features of the subject.
The first, geometric distortion, as well as vignetting, is somewhat
alleviated by the fact that the sensor captures a smaller portion
of the image circle. However, I still think it will always be more
pronounced in the case of a "normal-equivalent" 28mm lens (whether
EF or EF-S) on a 1.6X crop-factor body than in the case of a
"normal" 50mm lens on a FF body.

That will be the tendency, but of course it varies so much with teh design of the individual lens.

The second type of distortion is even more of a problem, if I am
not mistaken, since it would not be reduced by use of a smaller
sensor. I don't know a great deal about optics or lens design, so I
could be entirely wrong in my assessment. It wouldn't be the first
time, I assure you (grin). But, it seems to me that the tendency of
a wide-angle lens to make closer objects appear much larger than
distant objects and thus result in long noses, large heads, small
torsos and the like would be just as pronounced when used with a
1.6X body as with a FF body.

My point in that regard is that the "big head, long nose" appearance does not result per se from a short focal length, but only from the camera position beiong close to the subject.

Now of course in reasonable practice, if we shoot from close to the subject, we will probably want to/have to use a short focal length lens, else we will not get into the picture as much of the subject as we wish. That is why we associate this particular appaearance with wide angle lenses.

But, to illustrate my point, imagine that take a picture of a face from, say, a distance of 20 feet with a 20 mm lens, and then crop the image (severealy) to get a typical "portrait" composition. The appearance of the face (in the way we are discussing) is the same as we would have it we had taken the shot from the same distance with an 85 mm lens and cropped the print (less severely) to the same composition. (The appaerance wouild have been the same even without the cropping, but of course might be harder to visualize.)

This is a result of the basic principles of the geometry of vision.

My understanding is that a 28mm lens
would, therefore, give roughly equivalent perspective to a 50mm
lens on a FF body, but would distort the perspective in the way we
normally associate with WA images. Am I incorrect in that
assumption? I would be interested in hearing your opinion on the
matter.

It's important to note that the "big head, long nose" appearance isn't the result of any "distortion of perspective". Its just the result of perspective itself, when the "point of perspective" (camera's vantage point) is relatively close to the subject. Of course we think of it as a distortion of appearance, compared to the appearance we see when viewing someone's face from a more customary distance.

Now back to your comparison between the 28 mm lens on a "factor 1.6" body and a 50 mm lens on a FF35 body.(Let's make it a 45 mm lens, so the field of view of the two are essentially identical.) Suppose we in each case position ourselves so that the subject's head occupies the same portion of the height of the frame. We would then find that we took both shots from the same subject distance.

Having done that, we would find that the two images were (from the standpoint we are discussing) identical. The faces would be the same size (assuming prints of equal size, of course) and the visual effect of "bigness-of-head-and-longness-of-nose would be identical.

(This all assumes no gigantic geometric distortion on the part of either lens, of course.)

To go back to geometric distortion for a moment, it just seems to
me that they will never be able to produce WA lenses that, when
used with 1.6X crop-factor bodies, will equal the performance of
normal lenses on FF bodies, much less super-WA lenses to substitute
for their WA counterparts.

That may be the practical reality, especially if you mean with comparable zoom ratio and any reasonable comparison of price. Still, for example, my Sgma 12-24 at 12 mm has pretty low geometric distortion.

From all that I have read and from the numerical ratings given the
various lenses on the market, it seems that normal to moderate
telephoto lenses virtually always rank much higher optically than
their WA counterparts, which in turn rank much higher than their
super-WA counterparts. I realize that those rankings are generally
based upon FF film camera use and that the WA and super-WA ratings
would be somewhat higher if restricted to 1.6X digital use, but the
discrepancy between the various classes (i.e. moderate tele/normal
--> WA --> super-WA) is so great that I don't think the higher
ratings would totally eliminate the performance gap.

So, in summary, I find that we have to pay substantially more and
still settle for lesser performance at the WA to normal end of the
spectrum. This is balanced somewhat, of course, by the ability to
save some money at the extreme telephoto end where the crop factor
wins us some free added reach. That is useful for sports and nature
photographers, no doubt, and shouldn't be discounted. But, I would
wager that the vast majority of users take far more photos at the
WA to normal side of the equation.

I certainly agree with your practical assessment of our situation. I just wanted to be certain that inappropriate technical concepts didn't get would up in the matter.

Thanks for your inputs.

Best regards,

Doug

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