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Let's use a specific example 7D with 400f5.6 and 5D3 with 400f5.6 and 1.4 extender. If on the 7D the bird is 1000 pixels tall how tall will it be on the 5D3.
First take the 5D3 vertical Pixels and divide by 7d vertical pixels = 3,840/3,456 = 1.11% pixel on the 5D3 in one direction compared to 7D.
Now multiply 1.11 by 1.4 (Extender added to 5D3) = 1.56
Now compare to crop factor = 1.56 / 1.6 = 97.5%
Therefore, your bird on a 5D3 in this example would be 975 pixels tall compared to the 1,000 on the 7D.
On the 5D3...Show more →
(note that, after posting, I realized that my explanation in the first part of this post is not correct, and I have corrected my error in a subsequent post.)
No, your math is wrong again. The 1.4x is not increasing the field of view by 1.4x in both the horizontal and vertical directions. It is increasing the total focal length by 1.4x. The increase in terms of increased horizontal size or increased vertical size is the square root of 1.4x, which is 1.18. If you do your calculations and replace your 1.4 with 1.18, the 5D Mark III will have about .8 times the number of horizontal pixels as the 7D, and it will have about.8 times the number of vertical pixels. When you multiply .8 times .8, the total number of pixels on the subject with the 5D Mark III and a 1.4x will be about 64% of what you would have with the 7D and the bare lens, which is consistent with what I said earlier when I said that the 5D Mark III with a 1.4x is still only going to put about 2/3 the number of pixels on the subject as the 7D will with the bare lens.
Further, you have the option of using the 7D with a 1.4x, too, and if you use both bodies with the same lens and a 1.4x, you will only be putting roughly 1/2 as many pixels on the subject with the 5D Mark III combination as you will be putting on the subject with the 7D combination.
I will avoid the temptation of posting bird-in-flight shots taken with the 7D and a bare 400 mm lens because they would take the thread further off topic and would prove nothing. Only comparisons of the same subject taken from the same distance would be meaningful, and the web is full of such comparisons that show the advantages of the 7D in focal length-limited situations. The point of my post is not to contend that you can't take good bird-in-flight pictures with a 5D Mark III. Nor is it to claim that a 5D Mark III can't do better at a many types of photography than a 7D. Instead, it is, as I said in my previous post, to point out that you cannot deny, either with correct math or with actual photo comparisons of the same subject taken from the same distance, that the 7D gives advantages in focal length-limited situations. Further, I fail to understand why some cannot accept the possibility that, while an excellent camera, such as the 5D Mark III, is capable of doing better than another camera in many situations, it may not be able to do better in all situations. If you wish to use faulty math to convince yourself of something that isn't true, be my guest. I don't expect to convince you that you don't have the perfect camera for absolutely every type of photography. Others, however, might be interested in a more balanced assessment and one that is not based on faulty math.
Edited on Mar 25, 2013 at 11:46 PM · View previous versions