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Old Jul 15, 2005, 6:36 PM   #1
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This is sort of a follow up to a couple of previous posts I've made. I'm about to buy my first DSLR, the Canon 20D, and am stuck on choosingwhich telephoto lens to buy at the same time.

As I've asked for your advice I've had several people mention to me that using a 300mm lens, for example, will result in something like a 480mm lens when used on a DSLR due to a digital camera's inherent magnification or crop factor, or something like that. I don't understand what that means.

This is pretty important because I will be using my telephoto to shoot surfing from the beach and am trying to figure out what size will be big enough while still being affordable. If a 400mm will be enough reach, that would help alot as those are more affordable than a 600mm, for example.

Please provide me with as in-depth an explanation as you can. Thanks.

Rob

p.s. and I'm still taking recommendations for a good telephoto (prime or zoom) for surf photography, so don't be afraid to giveme your favorites!
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Old Jul 15, 2005, 10:27 PM   #2
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Hi Rob.

The 'crop factor' or 'focal length multiplier' depends on the size of the sensor in a DSLR as compared to 35mm film (36mm x 24mm)and can vary with different cameras.

The Canon 20D's sensor is an APS-C, measuring 22.5mm x 15mm, which gives itafocal length multiplierof 1.6

As the sensor is smaller than 35mm film, it has a smallerangle of view. Consequently, lenses will behave as if they were a longer focal length lens.

This is all good, until you want wide angle. As the focal length multiplier affects these in the same way. You can pay good money to get a really wide angle.

Sorry, no recommendations for your needs.

Hope this helps.

Stevekin.
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Old Jul 16, 2005, 2:46 AM   #3
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Not to hijack this thread, but further to what Stevekin said, Tamron has just released a11-18 wide-angle which sells for under $600 US$. At f/4.5-5.6, it is slower than most in the field, but according to reports, it works very well. Just to keep this in line with the rest of the thread, at a 1.6 focal multiplier, this lens behaves as 17.6-28.8. Hence the frustration of photographers who want a really wide lens.

Cheers,

Tom, on Point Pelee, Canada
http://tomoverton.myphotoalbum.com
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Old Jul 20, 2005, 12:46 PM   #4
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I've yet to hear anyone say that this magnification factor is a bad thing (other than the wide angle issue). I mean, it sorta sounds too good to be true, that is if you're into telephoto shooting like me.......1.6x more zoom for free??

I mean, it sounds so similar to digital zoom where you lose quality hand over fist- but clearly it's not like that, I realize that. But I'm still wondering if there is no quality loss during this crop "process" vis a vis an APS sensor. In other words, would sharpness/image quality be better on a camera w/o the digital crop factor but then using a larger focal length lens to balance equate to what a dslr would have done?

Rob

I guess in a way it's moot since you get it with a camera like the 20D whether you want it or not.
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Old Jul 20, 2005, 1:10 PM   #5
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Rob, my explanation was a somewhat simplified one to try to explain the basics.

Have a look here, it may help you to understand a little more.

http://www.dpreview.com/learn/?/Glos...tiplier_01.htm

Stevekin.
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Old Jul 20, 2005, 1:19 PM   #6
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From the studies I've seen posters do, there is virtually no difference between an image taken on a DSLR with a given lens and that same photo taken with that same lens on a film / full size sensor and 'cropped' in processing. In other words, you don't really get extra zoom - it's a field of view difference. Or, another way:

If you had two lenses of exactly equal quality - one a 200mm lens and one a 320mm lens that were the same quality at their maximum focal range. Now assume you have two cameras - one with a full size sensor and one with APS-C (with 1.6 factor) - The full sized sensor camera with the 200mm lens should take sharper images than the APS-C sensor with the 200mm - both situations have an 'effective' focal length of 320. Now, if you took that 200mm lens and took the same shot on both bodies and cropped the photo from the full sized body the quality should be the same.

I'm confident that if I misstated here that someone more experienced will jump in and correct me

As for a good lens for surf photos - I would suggest taking a look at DP Review - they seem to have a lot more sports shooters than here (I can't say I recall too many here that are into surfing)and I seem to remember several threads discussing surfing in either their 300D or 20D or Lens forums. Good Luck!
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