Distorted images

Hi,
My wife and I are fairly experienced 35mm photographers and for many
years we used a Canon EOS 100 with some excellent results. Recently we
decided to 'upgrade' to a digital camera (I know this is a 35mm forum
but please stick with me) buying a Canon EOS 350D but we noticed that
many of our new pictures have significant distortion of the horizontal
and vertical lines where they are near the edge of the frame.
The effect is much more noticeable (to the point where it is a problem,
ruining many shots) than it used to be with our EOS 100 despite the
fact the we are still using the same lens (a 28 to 80 zoom).
I think the problem is that we have to zoom out more on the 350D than
we do on the 100 to take the same shot meaning that more of our
pictures are now being taken at the wide angle end of our zoom lens
than the quivalent shot on the 100.
Can anyone explain what is going on here, why does a lens at 50mm show
less of the image on one camera than it does on another. Would buying a
better quality lens help with this?
Thanks in advance.
Si.

Simon_Keep@hotmail.com

Re: Distorted images

Thanks, Chris.
This is a perfect example of exactly the sort of good faith, unbiased reply
to a posters legitimate question - digital or film - that I was referring to
in that other thread. Your preference is for film, but you don't let that
get in the way of being objectively helpful. And being helpful instead of
adversarial is what will keep new people coming to this group in the future
for assistance.
And I especially like the fact that you didn't feel the need to start
calling the OP derogatory names because of his personal choice of imaging
technology. Nice...
Ken


Ken Nadvornick

Re: Distorted images

Thanks everyone, excellent answers I guess I need to start saving!


Simon_Keep@hotmail.com

Re: Distorted images

What you are seeing is called the "effective crop." The lens projects
the same size image onto the film plane regardless of whether the body
is digital or film. However, the size of the digital sensor is smaller
than the size of 35mm film. Thus the sensor captures only the central
portion of the image projected by the lens leading to a crop. Some
people call this "focal length multiplying" because the effect of the
crop makes it look like you have the taken the picture with a lens of
longer focal length. In an EOS-350D the focal length multiplier is 1.6.
Thus a lens with a 28mm focal length on 35mm film has an effective
focal length (or crop) of 1.6 * 28 or 44.8 mm.
This effective crop is present in all DSLRs where the sensor is smaller
than the area of a 35mm film. The EOS 350D has what they call an APS-C
sized sensor. Canon also sells the Canon EOS 5D where the sensor is as
large as 35mm film. Such DSLRs like the 5D are called Full Frame DSLRs.
Hope this helps.


Anthony

Re: Distorted images

Because the sensor is smaller than film. This difference means that a
lens' apparent focal length is multiplied by about 1.5 (depending on the
model).
Your zoom used at 28mm on a digital sensor is giving you the same
coverage that it did at 42mm with film.
One of the reasons I'm sticking with film...


Chris Loffredo

Re: Distorted images

Thanks for that, so why does my new camera body effectively force me to
take the same photo at a wider angle than my old camera body? I guess
this is just down to design but it is quite annoying.


Simon_Keep@hotmail.com

Re: Distorted images

It sounds like the reason is what you just guessed: Using it at a
shorter focal length than usual. Zooms typically have more distortion at
the ends than in the middle.
Solution: Zooms aren't known for their low distortion. Some expensive
professional zooms may do better, but little or no distortion at 28mm
really is prime lens country - and if you're really picky about
distortion, you'll need one of the best 28mm primes (talking Zeiss,
Leitz and a few chosen others here).
If you really want to stick with zooms and 28mm is your favorite FL, you
might do better with a zoom where it lies somewhere in the middle of the
range (for example 20-35mm). Again, lower distortion at mid-range is
typical for zooms, but is not always the case. You need inform yourself
on a case by case basis.


Chris Loffredo

Re: Distorted images

You spent the money for a new camera body, now you may need to spend a
bit more for a lens that is 'newer' in technology and that may work
better with a digital camera.
The sensor (electronic film) is smaller and the lens has to be
'crisper' in the central portion of focus. Your old lens may/may not
work really well on the new digital Canon.
= = =


Jerry L

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