Cropping vs. lens selection

Group,
Here is a really dumb question from a newbie. I can't really afford a 500 or 600mm Canon lens now. I'm currently shooting a Canon 40D at 10meg with a 70-200 2.8 and 2Xtc for an effective 400mm. How many more megapixils would I need to be able to crop enough to equal stepping up to a 500mm lens? A 600mm lens? For example, would I benefit greatly by getting a 50D or a 7D? It appears to me that it would be much cheaper to add megapixils with a new body and crop more than go for a very expensive lens. There must be some simple mathematical equation to show a comparison. I'm assuming that this approach won't really work since it is so affordable! Thanks!
Marv

Marv,

First, crop factor does not affect telephoto reach. The keys for pixels on subject are:
1) distance to subject, 2) focal length, and 3) pixel pitch.

So moving to a camera with a smaller pixel pitch will help. You might check the recent thread on a birding setup:
http://www.birdphotographers.net/for...ad.php?t=50170

This is from that thread (with some editing for clarity):
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A couple of years ago a cutting edge combination was a 500 mm on a 1D Mark II with 8 frames per second. The key in this is the 8.2 micron pixels. The means that one pixel sees 206265*.0082/500 = 3.4 arc seconds.
The rig weighs about 11 pounds. (smaller arc-seconds means more pixels on subject)

Then came the 1D Mark III (7.2 micron pixels) + 500 mm which = 2.97 arc-seconds and weighs about the same as the combination above. (1DIII = 2.54 lbs + 500mm f/4 = 8.53 lbs = 11.07 lbs)

Now look a 7D (4.3 micron pixels) with a 300 mm f/4 L IS lens: 1 pixel is 2.95 arc-seconds.
7D = 1.8 pounds + 300 mm f/4= 2.6 pounds = 4.4 pounds.

The 300 f/4 +1.4x TC = 420 mm f/5.6 and autofocuses on the 7D and the 300+1.4 is very sharp.
And you have 8 frames per second. less than $3,000 for camera + lens + 1.4x TC.

Much lower cost, much lower weight. What do you lose over the 500 mm + 1D Mark III? Weight and a little bit of high ISO performance, and 2 extra frames per second; but you save thousands of $ and you get more pixels on the subject than either of the above 1D + 500 mm combinations.
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Note, a 50D also has small pixels and will save some money.

Back to crop factor. This web page explains the effects and gives the equations for the number of pixels on the subject:
http://www.clarkvision.com/photoinfo/cropfactor

Roger

Hey Marv -- I agree with Roger that if you are on a budget you should consider the 300 F4 L IS along with the 1.4 TC. I have used this combo for years with my 40D and you will get great shots and w/the 1.6 crop factor on the camera you will have excellent reach. Although I love my 500 F4, I still use my 300 a lot, especially in the spring because you can use it as a close-up lens as well. I also own the 70 - 200 F2.8 but rarely try to reach out for birds with it and in the long run you'll find that cropping is never the answer for obtaining reach. Good luck.