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Author Topic: APS-C vs megapixels for zoom  (Read 1578 times)

corey.kaye

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APS-C vs megapixels for zoom
« on: September 28, 2012, 06:03:09 AM »
Hi !

I've got a 40D APS-C with a 1.6x crop factor.  That means my 70-200 is actually x1.6 or 112-320mm. 

Given that i'm shooting at 10Mp, if i upgrade to FF a 5Dmk3 given the grater number of pixels how would I calculate the zoom of a 10Mp image cropped out of the center?  I'm thinking it's greater than 1.6 but not sure how the math works out.


dtaylor

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Re: APS-C vs megapixels for zoom
« Reply #1 on: September 28, 2012, 06:18:25 AM »
It works out to about 8 MP. The math? Compute the pixels-per-mm of the 40D, then multiply by the 5D3's sensor dimensions.

bainsybike

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Re: APS-C vs megapixels for zoom
« Reply #2 on: September 28, 2012, 07:01:03 AM »
Divide the square root of 22 by the square root of 10. It works out at about 1.48. Note that a 10mp area cropped from the centre of a 5DIII sensor will be larger than APS-C, because the pixels are larger than those of a 40D

viggen61

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Re: APS-C vs megapixels for zoom
« Reply #3 on: September 28, 2012, 09:24:51 AM »
Hi !

I've got a 40D APS-C with a 1.6x crop factor.  That means my 70-200 is actually x1.6 or 112-320mm. 

No, the 70-200 is still actually a 70-200. Mounting it on an APS-C sensored body gives it an effective focal length of 112-320mm.

Quote
Given that i'm shooting at 10Mp, if i upgrade to FF a 5Dmk3 given the grater number of pixels how would I calculate the zoom of a 10Mp image cropped out of the center?  I'm thinking it's greater than 1.6 but not sure how the math works out.

You need to know, or figure out, the size of the pixels. The 40D has a sensor of 10MP, measuring 22.2x14.8mm, or in pixels 3888x2592. So, to get the size of the pixels, you divide 22.2 by 3888, giving you 0.00571 mm, or 5.71 microns (or micrometers). You can do the same for the vertical measurement, but since Canon uses square pixels, it's really not necessary.

The 5DIII has a sensor of 22.3 "effective" Megapixels, on a sensor 36mm x 24mm. Largest image size is 5760x3840. We can do the math, but Canon very nicely publishes the pixel dimensions on the 5DIII spec page as 6.25 microns. So, the 5DIII pixels are larger.

So you want a 10MP, 3888x2592 image out of that? 3888x6.25 microns is 24.3mm, and 2592x6.25 is 16.2mm, about a 10% larger dimension for the 10MP "crop" out of a 5DIII. On the other hand, a 22.2x14.8mm crop gives you 3552x2368 pixels, or about 8 MP total.

If you are looking to upgrade, what is it that you shoot? The 5DIII is a great camera, well suited to many things, but if the effective focal length matters that much to you, maybe you would be better off upgrading to a 7D, which would give you nearly double the number of pixels, and keep the crop multiplication factor.
Canon 7D

corey.kaye

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Re: APS-C vs megapixels for zoom
« Reply #4 on: September 30, 2012, 02:57:20 AM »
Thanks for all the replies.  I shoot mostly landscape/travel and have done several weddings.  Considered going full time but stayed with my "day job".

I'm really looking forwarded to getting my 16-24 back.  I rented a 5D a month back and was amazed how wide it was!  Given that I was wondering how "short" my 70-200 would appear.

Sounds like it'll be pretty close to the same, 1.4 vs 1.6.  I know it's not an exact comparison but close enough.

Bob Howland

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Re: APS-C vs megapixels for zoom
« Reply #5 on: October 01, 2012, 10:35:39 AM »
I thought about using a 5D3 that way. As it turned out at ISOs of 800 and below, my old 40D looked better than the 5D3 image cropped to APS-C size. The same is true comparing cropped 5D3 to 7D, although the 7D improvement over the 40D has been something of a disappointment.