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Archive 2012 · Question about the crop factor

rprouty
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p.1 #1 · p.1 #1 · Question about the crop factor


Say these three images were taken ju st like they are but each camera was a, say, 20MP and you made 24x30 inch prints and you wanted pretty much a close up of the cats face would one produce better prints than the other two.
When I took these a couple of years ago the cat cooperated and stayed in the same place and all I did was change camera bodies and not move the lens or tripod.

I'm trying to understand what some folks call the crop factor.

Please only serious replys.

Thanks

Rod

http://www.pbase.com/rprouty/image/117468685/original.jpg

http://www.pbase.com/rprouty/image/117468686/original.jpg

http://www.pbase.com/rprouty/image/117468688/original.jpg




Nov 03, 2012 at 08:59 AM
Monito
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p.1 #2 · p.1 #2 · Question about the crop factor


The crop factor is a linear factor. Your shots illustrate it well.

Almost always the reference is with respect to the "full-frame" 36 x 24 mm film ("35mm" film) or sensor.

So the first one, the 5DII, has a 36 x 24 mm sensor and a crop factor of 1.0. The second one is has a crop factor of 1.3 and the third of 1.6. That means that 36 and 24 divided by 1.6 yields 22.5 x 15 mm approximately.

You have to multiply the crop factor by itself to get the effect on MegaPixels, because they cover an area. So if you have a 20 MPx crop factor camera, you'd have to have 1.6 x 1.6 x 20 MPx in full-frame to crop out the same number of MPx. That would be 2.56 x 20 = 51 MPx.

You can also compute the other way: if you crop a 20 MPx full-frame sensor to match the 1.6x crop, you have to divide by 1.6 squared = 2.56. So 20 / 2.56 = 7.8 MPx or not much resolution.

To approximate the crop, you can multiply or divide the focal length by the crop factor. So to equal the composition of the 500mm on the crop factor, you have to have a 500 x 1.6 = 800mm lens. Or to get the same angle of view on the crop factor as the 500mm on the full-frame, you have to mount a 500 / 1.6 = 312 or 300mm lens onto the crop factor.

You can't shift your position to compensate for the crop factor because that would change the perspective (making a different composition and therefore different photograph) or it would scare the cat.



Nov 03, 2012 at 09:07 AM
jcolwell
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p.1 #3 · p.1 #3 · Question about the crop factor


Hi Rod,

The crop factor is all about magnification. I've taken the liberty of hacking up your cat (henceforth known as Rod's Cat). Seeing as you're taking the three images with the same lens from the same distance, the size of the image is the same on all three camera sensors. I've added rectangles to show the relative sensor size of the three cameras. So far, so good - no "crop factor effect".

The crop factor effect becomes apparent when you show the images from the three different CF cameras at the same output size - just as you have done with your three Rod's Cat images. The FF image has the least magnification and the CF = 1.6 image has the greatest magnification. In cases where the CF = 1.6 camera has higher pixel density on the sensor, then you can get "more reach", because the higher pixel density lets you magnify the image more than a camera with lesser pixel density (assuming that the more dense pixels have the same "per pixel" IQ as the less dense pixels - often not the case).

Hope this helps.

Cheers, Jim







Nov 03, 2012 at 09:24 AM
mitesh
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p.1 #4 · p.1 #4 · Question about the crop factor


Great examples and great explanation! Thanks to both of you, Rod and Monito.


Nov 03, 2012 at 09:24 AM
Ian.Dobinson
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p.1 #5 · p.1 #5 · Question about the crop factor


While the examples above show things pretty well I will muddy the waters a little bit.

If you are just talking about the whole image as above then the explanation above is as far as you need to go .

BUT
If you want to talk about reach (or 'pixels per duck' as its. Been called ) then things blurr slightly . If you were to include an old 8mp APS-c body in the mix the that has NO extra reach over the 5D2 even though you will view an image like the 50D's one above. This is because you could crop the 5D2 image down to APS-c and have the same number of pixels left.
So Infact in your 3 images above the 10mp 1D3 has the least 'reach' of the 3



Nov 03, 2012 at 09:34 AM
BrianO
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p.1 #6 · p.1 #6 · Question about the crop factor


rprouty wrote:
...each camera was a, say, 20MP and you made 24x30 inch prints and you wanted pretty much a close up of the cats face...


If each camera was 20MP (or any other equal number), the one with the largest cat's face would make the better print -- all else being equal -- because you'd be using more of those 20MP than if you took one with a smaller face and cropped & enlarged just that portion of the image.

The problem is that all else is seldom equal. For example, to get the same number of pixels on a smaller sensor, each sensel has to be smaller and as a result is less light-sensitive. That can result in noisier images.



Nov 03, 2012 at 10:25 AM
Eyeball
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p.1 #7 · p.1 #7 · Question about the crop factor


Here are my quick calculations, which show the 50D to be the winner when only pixels per inch in the final print is the consideration.

Someone let me know if I have made any major errors. There are some minor errors due to rounding and the way Canon produces effective pixels from the sensor area. I used 24" as the pertinent dimension for the print with the assumption that the sides would be cropped.

http://dl.dropbox.com/u/12145446/CropComp.gif





Nov 03, 2012 at 10:32 AM
Imagemaster
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p.1 #8 · p.1 #8 · Question about the crop factor


A Google search of "crop factor" brings up 956,000 results.


Nov 03, 2012 at 10:46 AM
jcolwell
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p.1 #9 · p.1 #9 · Question about the crop factor


Imagemaster wrote:
A Google search of "crop factor" brings up 956,000 results.


76 of which are about farming.



Nov 03, 2012 at 10:49 AM
bbasiaga
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p.1 #10 · p.1 #10 · Question about the crop factor


You can also think about it from a print perspective. The number of pixels per inch (think dpi) to make a high quality photo is arguably around 300. The pixels are physically smaller on a 1.6 crop camera, but still take up just one dot on the print. Therefore there will be more pixels on your subject and it will appear larger (or more zoomed in) in a display or print. Of course, as atated aqbove, that assumes the crop body does have a higher pixel density than a FF, which is true if we are within a generation or two of technology.

My 40d, for instance, has pixels barely smaller than my 5DIII, so the effect on final output size is much less.

-Brian



Nov 03, 2012 at 11:24 AM
BrianO
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p.1 #11 · p.1 #11 · Question about the crop factor


Eyeball wrote:
...Someone let me know if I have made any major errors.


Only that the OP specified different sensor sizes but equal pixel counts. Your examples have different pixel counts.



Nov 03, 2012 at 12:00 PM
Eyeball
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p.1 #12 · p.1 #12 · Question about the crop factor


BrianO wrote:
Only that the OP specified different sensor sizes but equal pixel counts. Your examples have different pixel counts.


You're right. I don't know how but I missed that assumption. The smallest sensor would come out on top in that case.

Here is a revised table:

http://dl.dropbox.com/u/12145446/CropComp2.gif

How dramatic the pixel per inch differences will be seen will depend on the amount of fine detail in the image and the viewing distance.

"what camera produces the best 24x30 print" can also be influenced by other factors related to the sensor such as noise and dynamic range.





Nov 03, 2012 at 12:45 PM
BrianO
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p.1 #13 · p.1 #13 · Question about the crop factor


Eyeball wrote:
..."what camera produces the best 24x30 print" can also be influenced by other factors related to the sensor such as noise and dynamic range.


^ This. ^

It's something that is often overlooked when someone is trying to pick a camera, and is swayed by advertising hype about megapixels, zoom range, etc.



Nov 03, 2012 at 01:01 PM





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