Crop vs FF question

OK, first of all let me apologize for what some may think is a stupid question, but this is something I just can't wrap my head around and am therefore hoping someone can clarify it for me. I currently have a 50D, which of course has Canon's APS-C crop sensor. I get how the crop works with respect to lenses due to the physical size of the sensor compared to the full frame CMOS sensor. What I don't understand is why, say for example, a 20M APS-C sensor does not have better resolution than a 20M CMOS sensor. Logic seems to suggest putting the same number of pixels in a smaller area would give you better resolution.

Thanks for helping me to sleep better at night!

jrk said:

jrk wrote:

OK, first of all let me apologize for what some may think is a stupid question, but this is something I just can't wrap my head around and am therefore hoping someone can clarify it for me. I currently have a 50D, which of course has Canon's APS-C crop sensor. I get how the crop works with respect to lenses due to the physical size of the sensor compared to the full frame CMOS sensor. What I don't understand is why, say for example, a 20M APS-C sensor does not have better resolution than a 20M CMOS sensor. Logic seems to suggest putting the same number of pixels in a smaller area would give you better resolution.

Thanks for helping me to sleep better at night!

Click to expand...

I would tend to disagree. If you are cramming the same number of pixels into a smaller area, something has to change in order to fit all those pixels into a smaller area. In the case of sensors, you can only shrink the pixels so much, or in the case of a smaller area, cram the same pixels closer together. The problem that image sensors have when shrinking the space between 'cells' is ambient light will fall onto the adjacent sensor as a lens cannot focus light directly onto each pixel, just onto a sensor. Sony came out with Super HADs, which basically mounted a small, curved lens over each pixel to focus the light it was interested in, while ignoring the ambient light from an adjacent sensor. This is one reason why more pixels in the same sensor size is not always a good thing.

jrk said:

jrk wrote:

OK, first of all let me apologize for what some may think is a stupid question, but this is something I just can't wrap my head around and am therefore hoping someone can clarify it for me. I currently have a 50D, which of course has Canon's APS-C crop sensor. I get how the crop works with respect to lenses due to the physical size of the sensor compared to the full frame CMOS sensor. What I don't understand is why, say for example, a 20M APS-C sensor does not have better resolution than a 20M CMOS sensor. Logic seems to suggest putting the same number of pixels in a smaller area would give you better resolution.

Click to expand...

First, I assume when you say "CMOS" you really mean full-frame, as most (all?) DSLR sensors these days are CMOS, whether they're APS-C or full-frame.

Now, as for the resolution -- a higher pixel pitch sensor can, in fact, have higher resolution even if it is smaller, subject to the constraints of diffraction and lens resolution.

You just have to make sure you're making the right comparison. There are two scenarios to consider:

Scenario one -- same lens, same aperture setting, same camera-to-subject distance, different sensor sizes. In this case, the shot from the APS-C camera is cropped tighter, and you have more pixels covering a certain area of the subject. In this case, you could very well have higher resolution with the crop sensor camera, assuming that you are shooting at an aperture that doesn't cause undue diffraction effects, and that the lens itself is not limiting the resolution. In this case, using the same lens and same aperture, the APS-C sensor resolution can be no worse than that of the FF sensor, and even then that is only in the case where diffraction and lens aberrations limit the resolution.

Scenario two: same camera to subject distance, different lens, same framing. In this case, you have the same number of pixels on a given area of the subject in both the APS-C and FF cases, so the resolution of the APS-C system can be no better than that of the FF system (assuming the resolution of the FF system is not optics-limited). In this situation, the effects of diffraction and lens aberration can potentially have a larger effect on the APS-C system as the tighter physical pixel pitch demands better lens resolution in order for the entire system to not be limited by the optics. However, in this scenario, like-to-like comparisons between the APS-C and FF cases are not so straightforward as the lenses must necessarily be different. You could have an APS-C system with an excellent lens and a FF system with a crap lens coated in vaseline and the APS-C system would probably win in terms of resolution.

I think it is a fair statement that the larger the sensor, the worse the lenses can be without being the limiting factor in the total system resolution (for a given total sensor pixel count).

jrk said:

jrk wrote:

OK, first of all let me apologize for what some may think is a stupid question, but this is something I just can't wrap my head around and am therefore hoping someone can clarify it for me. I currently have a 50D, which of course has Canon's APS-C crop sensor. I get how the crop works with respect to lenses due to the physical size of the sensor compared to the full frame CMOS sensor. What I don't understand is why, say for example, a 20M APS-C sensor does not have better resolution than a 20M CMOS sensor. Logic seems to suggest putting the same number of pixels in a smaller area would give you better resolution.

Thanks for helping me to sleep better at night!

Click to expand...

Canon's APS-C crop sensors are also CMOS, not just the full frame. When used with the same lens focal length, a 20Mp APS-C will have better resolution than a 20Mp full frame. When used with different focal lengths to give the same field of view, they will have near the same resolution. Variations in the latter case can be seen in DPR comparisons.

I think you need to add a third scenario:

Same lens on both cameras and the subject doesn't fill the frame in either case. The captured images from both would be cropped so that the subject would fill the frame. The image from the full frame would come from a smaller area of the sensor and would need heavier cropping, so the final pixel dimensions for the subject (at the same viewing size) would be lower for the full frame than the crop body, so the crop body would have more resolution (all other things being equal). This is exactly why many bird shooters prefer crop bodies, as their subjects rarely fill the frame, even with the longest lens they can afford/manage.

Jeff Peterman said:

Jeff Peterman wrote:

I think you need to add a third scenario:

Same lens on both cameras and the subject doesn't fill the frame in either case. The captured images from both would be cropped so that the subject would fill the frame. The image from the full frame would come from a smaller area of the sensor and would need heavier cropping, so the final pixel dimensions for the subject (at the same viewing size) would be lower for the full frame than the crop body, so the crop body would have more resolution (all other things being equal). This is exactly why many bird shooters prefer crop bodies, as their subjects rarely fill the frame, even with the longest lens they can afford/manage.

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I agree, but the APS-C already has more resolution with same lens before or after you crop image.

Jeff Peterman said:

Jeff Peterman wrote:

I think you need to add a third scenario:

Same lens on both cameras and the subject doesn't fill the frame in either case. The captured images from both would be cropped so that the subject would fill the frame. The image from the full frame would come from a smaller area of the sensor and would need heavier cropping, so the final pixel dimensions for the subject (at the same viewing size) would be lower for the full frame than the crop body, so the crop body would have more resolution (all other things being equal). This is exactly why many bird shooters prefer crop bodies, as their subjects rarely fill the frame, even with the longest lens they can afford/manage.

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I'm pretty sure that's more or less equivalent to my scenario 1 (and in fact I was thinking exactly of that scenario of having more effective "reach" with an APS-C camera!)

altair8800 said:

altair8800 wrote:

Canon's APS-C crop sensors are also CMOS, not just the full frame. When used with the same lens focal length, a 20Mp APS-C will have better resolution than a 20Mp full frame.

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This is not correct. Each 20 MP camera will resolve 20 MP one does not have "better resolution" than the other. All you are getting is different FOV if you slap a 50mm lens on aps-c and 35mm dSLRs.

altair8800 said:

When used with different focal lengths to give the same field of view, they will have near the same resolution. Variations in the latter case can be seen in DPR comparisons.

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That's why the subject/final image is important. If you use the same lens on both bodies, the subject will fill more of the frame with the crop body (assuming distance is fixed), so the image from the crop body will have more pixels, and when both images are cropped/displayed so that the subject is the same size, the one from the crop body has the potential for more resolution.

The most obvious example is for a birder: if he shoots a bird in the distance using a 500 mm lens using a 20 MP full frame or a 20 MP crop body, and then produced a final image of where the bird is shown the same size, he's more likely to see fine detail from the crop body than from the full frame - which is why so many bird shooters prefer crop bodies.

RedFox88 said:

RedFox88 wrote:

RedFox88 said:

altair8800 wrote:

Canon's APS-C crop sensors are also CMOS, not just the full frame. When used with the same lens focal length, a 20Mp APS-C will have better resolution than a 20Mp full frame.

Click to expand...

This is not correct. Each 20 MP camera will resolve 20 MP one does not have "better resolution" than the other. All you are getting is different FOV if you slap a 50mm lens on aps-c and 35mm dSLRs.

Click to expand...

If you use the same lens and photograph a resolution chart, you will see better resolution with APS-C. You would need to crop the FF if you want same FOV and you would have only 8 Mp. The 70D has 243 pixels per milimeter and the 5DIII has 160 pixels per milimeter. The only way 5DIII wins is put a longer lens on it, but that was not the question.

RedFox88 said:

RedFox88 said:

When used with different focal lengths to give the same field of view, they will have near the same resolution. Variations in the latter case can be seen in DPR comparisons.

Click to expand...

Click to expand...