Defraction Question 20D vs 5D

[Slough]

Agree totally and that is the complete correct answer -- but keep in mind that subject distance, lens focal and sensor size all factor into image magnification.
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As stated in the rest of my post (apart from focal length).

[Ray]

1) Smaller sensors (including film) will always diffraction limit sooner than larger ones due to the greater enlargement factor required to get to the same final image size. 

2) Smaller pixels will always diffraction limit sooner than larger ones if you use pixel diameter (or a linear function of it) instead of a fixed numerical constant for CoC. 

Combine the two and yes, the 20D will always be handicapped compared to the 5D.
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To get back on track, not always, Jack. The 20D will not always be handicapped compared to the 5D. The circumstances where it won't be handicapped are as described in previous posts, namely when the telephoto reach is not long enough with the 5D resulting in cropping of the image in post processing to the same size (or smaller) than the 20D image. In such circumstances, the 5D will never have an advantage at any aperture, but the 20D might. My tests have shown that even at a very diffraction limited stop of f22, the 20D can be sharper than the cropped 5D image (but not by much, of course. For normal size prints, both cameras would be equal, in my tests).

[Jack Flesher]

To get back on track, not always, Jack. [a href=\"index.php?act=findpost&pid=71514\"][{POST_SNAPBACK}][/a]

Sorry Ray.  Taken exactly as I wrote my two statements above, they will *ALWAYS* be true.

If you add caveats about when and if you have to crop or increase magnifications, then you might have arguable points, but those extraneous conditions were not part of the statements I made.

Cheers,

[bjanes]

1) Smaller sensors (including film) will always diffraction limit sooner than larger ones due to the greater enlargement factor required to get to the same final image size. 

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Enlargement is not the proper term for a digital image, which has pixels but no physical dimension. That is why digital camera resolution is rated in pixels/picture height. For example, the Canon 5D has 2912 pixels/picture height and the Nikon D2x has 2848 pixels/picture height (the picture heights are 23.9 and 15.7 mm respectively). Once the images are brought into Photoshop, the sizes of the sensors are not relevant and both cameras can produce equally large pictures at a resolution of 300 pixels/inch on the picture. For a larger picture, you would have to upres.

The Nikon lens would have to have better resolution in terms of lp/mm, but it would have to cover a smaller sensor, simplifying optical design in the case of a DX lens.

[Jack Flesher]

Once the images are brought into Photoshop, the sizes of the sensors are not relevant [a href=\"index.php?act=findpost&pid=71517\"][{POST_SNAPBACK}][/a]

Okay Bill.  I should have added to #1 "assuming pixel pitch remains smaller than CoC"  

Then sensor sizes are absolutely relevant if you made the same composition with both and then intend to print out the same size image from both.  That simple process circumvents the entire pixel-per-image-height discussion.  

Cheers,

[dwdallam]

And 5D vs 20D DoF is very interesting to debunk, also for picture quality when you must crop and upres from teh 5D when printing larger images.

But my question is--CAN YOU SAY HIJACK?

LOL

[Ray]

If you add caveats about when and if you have to crop or increase magnifications, then you might have arguable points, but those extraneous conditions were not part of the statements I made.

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Jack,
And that's why it is not correct to write always. Cropping is a very common activity in image processing. We crop for all sorts of reasons. If you want to make a definitive statement free of caveats, conditions and exceptions, I suggest something along the lines of, "The sensor with the smaller pixels, whatever the size of the sensor, will be affected by diffraction first, when stopping down.

Now dwdallam is right that we are hijacking his thread. His original question was,

can someone shed light on the aperture setting where both cameras become defracted?

The answer is, the aperture at which the 5D becomes diffracted is also the aperture setting at which both camera's resolution is limited by lens diffraction.

As to what that aperture is precisely in terms of f stop# is difficult to say because of lens quality variation and the gradual change, over a number of stops, from mostly aberration limited to mostly diffraction limited. Even at f22 a lens can have a resolution of 72 lp/mm at 10% MTF.

[Jack Flesher]

CAN YOU SAY HIJACK?

[Jack Flesher]

Ray:

Speaking of hijacking, I'm not sure why you insist on adding your personal what if scenario on cropping and uprezzing to my comments.  Within the parameters I very clearly stated, what I said will *ALWAYS* be true. Period.  

As for you and Bill and your insistence on including cropping... I think what you guys are essentially saying is "if you compare crops at actual pixel view, the sensor with the smaller pixels will always -- err make that ususally -- win."   And I am sure this will be a critical revelation to a few readers, so thanks for sharing.

,

[dwdallam]

All of this is pertinent, but the question was based on my using the calculator I linked to in the opening post. It seems that the 5D has a much greater latitude of sharp F stops compared to the 20Ds F8-F11. When I used the calculator and input the CoC of the 5D and other specific information, it looked like the 5D would perform as well as the 20D at apertures from as small as 3.5 to F18 (or something incredible like that). Is this what you all get using that calculator?

Hijacking my own post, I think pixel density and resolution are even more important for people to understand. Most people don't realize that doubling pixel density does not double resolution, nor does doubling pixel density double print quality. All things being equal, its all about resolution. That's what I just could not communicate to a friend who has a degree in computer science, and is a photographer. I kept saying that even though you get a roughly 30% increase in pixel density from the 20D to the 5D you don't get a 30% jump in resolution, nor a 30% increase in printed picture quality.