depth of field 50d vs rebel xt

Yup, this viewing at 100% on screen comparisons of images from different cameras has made some folks lose sight of the geometric enlargement issue and how it's dependent on sensor pixel density, for a particular monitor. They're all at 100% so directly comparable!? Errr, not-so-much

stsva wrote in post #8423656
I feel like I've been circled by confusion!

Heh! These discussions can get like that!

I believe the OPs question was likely answered by suggesting that the two images were being compared at a 100% view, meaning that the 15MP image was being viewed at a much larger size than the XT image (6 megapixels?). Apparent DOF will be different, but if you view the two images at the same size, it will be the same.

In practice, I take a rule-of-thumb approach to DOF, because I don't shoot things that require scientific calculations but I know by experience what approach to take for, say, a macro or a flower blossom or a landscape/scenic photo. Plus, where there is a possibility that things might not turn out exactly the way I wish (a flower is a good example) I'll frequently "bracket" both by using different apertures and often by shifting the plane of focus a bit. I don't carry a ruler and a calculator for working out depth of field.

AJSJones wrote in post #8422694
...... Did I pass?? (I know I prolly used more than the allotted time )

Let's see, ... paper turned in late ... we may have to deduct a few points.

BTW, concerning the discussion of dot sizes in the neighbor hood of .1 mm, we have gone beyond the resolution of the human eyeball, especially mine.

In the most sensitive area of our eyes, the resolution is approximately one milliradian in a person with perfect vision. I have far from perfect vision, but at a typical reading distance of sixteen inches (that is my typical reading distance, but not everyone reads at the same distance), that translates to approximately 0.4 millimeters or 1/64 inch. That's why for really small things we get our eyeballs really close or whip out the trusty magnifying glass. The significance of this is that size no longer matters when at 0.4 millimeters and below because everything will look the same size if looking at a single point in isolation or in the case of an image composed of dots we will not be able to see the individual dots without resorting to the looking glass.

However, this whole discussion is mainly an academic discourse into DOF as neither I, nor anyone else I know, resorts to obsessing over DOF except in a few specialized situations such as macro photography and sometimes bird photography with a super telephoto lens at very close range. For essentially all other situations, I have an intuitive sense of what lens along with FL and aperture to use for a given situation. A basic example is that I frequently use a very wide angle lenses (i.e., short FL) for landscape images where I am able to get sharp focus from just a few feet to infinity at wide aperture. Also, I will sometimes stop down to improve sharpness and increase DOF when that is important, but sometimes I don't want maximum DOF.

Bill Boehme wrote in post #8424327
Let's see, ... paper turned in late ... we may have to deduct a few points.

BTW, concerning the discussion of dot sizes in the neighbor hood of .1 mm, we have gone beyond the resolution of the human eyeball, especially mine.

In the most sensitive area of our eyes, the resolution is approximately one milliradian in a person with perfect vision. I have far from perfect vision, but at a typical reading distance of sixteen inches (that is my typical reading distance, but not everyone reads at the same distance), that translates to approximately 0.4 millimeters or 1/64 inch. That's why for really small things we get our eyeballs really close or whip out the trusty magnifying glass. The significance of this is that size no longer matters when at 0.4 millimeters and below because everything will look the same size if looking at a single point in isolation or in the case of an image composed of dots we will not be able to see the individual dots without resorting to the looking glass.

However, this whole discussion is mainly an academic discourse into DOF as neither I, nor anyone else I know, resorts to obsessing over DOF except in a few specialized situations such as macro photography and sometimes bird photography with a super telephoto lens at very close range. For essentially all other situations, I have an intuitive sense of what lens along with FL and aperture to use for a given situation. A basic example is that I frequently use a very wide angle lenses (i.e., short FL) for landscape images where I am able to get sharp focus from just a few feet to infinity at wide aperture. Also, I will sometimes stop down to improve sharpness and increase DOF when that is important, but sometimes I don't want maximum DOF.

I picked the 254 dpi (i.e. 0.1mm per pixel) mainly because it's simple from inches to mm and is close to the acuity of 300 dpi that I've always seen thrown around, but clearly the absolute numbers are irrelevant to the discussion: DoF is determined by enlargement from sensor size to viewing size. If I multiplied all the numbers by 4 or 8, it would not affect the logic!

But that's what digital images are and it's the size of the dots that determines whether the eye can see a pattern or just a blur. If the blurry-edged line being photographed is narrower than the threshold of detail perception, it will , in your words "all look the same size" - the threshold, i.e. appear sharp in prints UNTIL it is ENLARGED enough to be perceived as blurry.

Because the OP was concerned about the softness in the comparison mentioned, it was important that there was a proper understanding of what 100% on screen view means and what its effect is on how sharp details appear. In other words, the blurry line (e,g, bird feather detail) might appear sharp in one image but become soft when enlarged more, as it crosses that threshold.

True only until it is enlarged to the point where its true size can be perceived and then it very much matters - it's perceived as out of focus - or "blurry". If the OP compared images at the same size, the difference in sharpness will decrease some. Until we see some good comparison shots we won't know if it all will go away. Alternatively, the OP will not have to stop down more to increase DoF to get more things to look sharp.

Now you come to mention it, DoF is pretty important even in birds that are not very close, especially given how detailed the feathers can be and how crummy they look when not in focus - that's often at the threshold too.