Depth of Field Due to Sensor Size

[Zenmervolt]

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

Same focusing distance? If you're shooting a subject at 5 meters with the 50mm f/1 and a subject at 50 meters with the 200mm f/4, you will have a larger DOF for the photo taken with the 200mm f/4 because of differing focusing distances.

ZV

Yup, same focusing distance. 5m for the 50mm and 5m for the 200mm, or Xm for the 50mm and Xm for the 200mm. It involves cropping the 50mm image to match the FOV of the 200mm image (a pretty substantial crop, and you may run into resolution limitations?), which is making it hard for me to visualize concretely which would have the greater DOF. A test would be great, but from general experience, a large part of me just feels that the 200mm f/4 will have greater DOF.

Wasn't thinking about cropping, and really have no way to test that with the sort of files I'll get from Walgreens (my pro lab will be closed by the time I can get this test done). I do think that croping changes the image though because it effectively reduces sensor size, which would mean I was testing two variables (sensor area and focal length/F-number) rather than just focal length/F-number.

ZV

 

[Zenmervolt]

Originally posted by: ElFenix

Originally posted by: fuzzybabybunny
Yup, same focusing distance. 5m for the 50mm and 5m for the 200mm, or Xm for the 50mm and Xm for the 200mm. It involves cropping the 50mm image to match the FOV of the 200mm image (a pretty substantial crop, and you may run into resolution limitations?), which is making it hard for me to visualize concretely which would have the greater DOF. A test would be great, but from general experience, a large part of me just feels that the 200mm f/4 will have greater DOF.

i think that after the crops, the picture taken with the 50mm lens will look exactly the same as long as the absolute apertures are the same, the focusing distances are the same, and overall exposure is the same.

Actually, I think that after the crop, the 50mm will have a larger DOF because it is effectively using a much smaller sensor area than the full picture from the 200mm lens. But I'm not sure anymore, all mental play at this point.

ZV

 

[fuzzybabybunny]

Originally posted by: ElFenix

Originally posted by: fuzzybabybunny
Yup, same focusing distance. 5m for the 50mm and 5m for the 200mm, or Xm for the 50mm and Xm for the 200mm. It involves cropping the 50mm image to match the FOV of the 200mm image (a pretty substantial crop, and you may run into resolution limitations?), which is making it hard for me to visualize concretely which would have the greater DOF. A test would be great, but from general experience, a large part of me just feels that the 200mm f/4 will have greater DOF.

i think that after the crops, the picture taken with the 50mm lens will look exactly the same as long as the absolute apertures are the same, the focusing distances are the same, and overall exposure is the same.

If this were indeed true, it'd be kinda cool. But like I mentioned above, testing this may be difficult because of the extreme 4x crop and the tester may run into too low of a resolution on the cropped photo to even determine if something's in sharp focus or not.

Or, the DOFs are different, but are close to being the same, in which case I still ask myself does this DOF debate even matter practically? I mean, the original thread is about DOF and sensor size... and a teacher making vast generalizations...

 

[ElFenix]

whatever it is, it's an arcane subject.

optical engineers are frickin' geniuses, i know that

 

[Madwand1]

Originally posted by: fuzzybabybunny
does this DOF debate even matter practically? I mean, the original thread is about DOF and sensor size... and a teacher making vast generalizations...

Before we got into the nonsense of focal length not mattering, focal length was clearly given as the reason for DoF being greater with a smaller sensor, other factors being held constant. Of course aperture makes a difference as well. The nonsense part is not in claiming that aperture matters, but in claiming that focal length doesn't matter.

The proof that this is nonsense is as follows: There are no DoF calculation formulas that are independent of focal length. Zenmervolt's claims about 50/2 and 100/4 giving the same DoF are simply wrong, as the existing DoF formulas easily show.

Originally posted by: Madwand1
Essentially, subject distance, focal length, aperture, and circle of confusion all matter, and make the topic difficult to grasp. When we factor out aperture and subject distance to compare different format sizes, the smaller circle of confusion penalizes smaller format size, but the smaller focal length for the same field of view turns it around, giving smaller formats greater depth of field.

Moral? Take it easy on the teacher, it's not as if this subject is trivial and commonly well-understood. Considering the teacher's recent remark, perhaps the teacher's gotten it right after all, and the wrong part is in the interpretation and context.

 

[ElFenix]

i'll see your circle of confusion, and raise you a cone of silence

 

[Zenmervolt]

Forgot that the camera is loaded with Velvia. Will have to postpone the test until after this weekend, I'm not wasting chromes on a test.

ZV