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by Wildflower-nut on Mon May 13, 2019 8:24 pm
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I've been struggling with this and thought I would see what input you might have. 

Lets say we have a full frame camera 5D and a aspc camera 7d.  We photograph a flower from 2ft.  The full frame 5D has a 100mm lens and the 7d camera a 62.5mm lens (the perspective should be the same).  We take a picture at f8.  The digital files are both enlarged to an 8x10 print and viewed from the same distance.  Which will have the greater DOF or will they be the same.
 

by DChan on Mon May 13, 2019 8:36 pm
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Some related materials:


Depth of Field Calculator


“Equivalence” in a Nutshell
 

by Scott Fairbairn on Tue May 14, 2019 10:49 am
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No expert, but f8 on a 62.5mm lens will give greater DOF than a 100mm at f8 at the same distance.
 

by Anthony Medici on Tue May 14, 2019 1:09 pm
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To make the DOF the same with the two cameras you'd need to use different apertures on those two different lenses.

For macro work, for which you usually want more DOF, the camera with the smaller sensor has the advantage. And when you want less DOF, the camera with the bigger sensor has the advantage.
Tony
 

by Wildflower-nut on Tue May 14, 2019 3:16 pm
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The shorter lens gives you more DOF (less magnification implies greater DOF).  However when you print an 8x10 from the smaller sensor it requires more enlargement (larger circle of confusion can be tolerated on larger negative since enlargement of the negative is less to make same size print).  I've printed out a few DOF charts on one of the websites DChan suggested and have come to believe it is a wash.  It seems that for a given perspective and f stop, if the magnification of the subject in the final print remains the same, the DOF is constant and not dependent on sensor size.   Have not had the time to do the math.
 

by Scott Fairbairn on Tue May 14, 2019 8:38 pm
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Wildflower-nut wrote:
The shorter lens gives you more DOF (less magnification implies greater DOF).  However when you print an 8x10 from the smaller sensor it requires more enlargement (larger circle of confusion can be tolerated on larger negative since enlargement of the negative is less to make same size print).  I've printed out a few DOF charts on one of the websites DChan suggested and have come to believe it is a wash.  It seems that for a given perspective and f stop, if the magnification of the subject in the final print remains the same, the DOF is constant and not dependent on sensor size.   Have not had the time to do the math.




Not sure I understand your reasoning. To make things simpler, if you have a 24-megapixel full frame and a 24-megapixel crop sensor, printing will be identical will it not? Even if it wasn't, it won't change the DOF by how you print it. DOF is a function of the lens and f-stop. The crop sensor camera with the same FOV as the FF will give greater a DOF whether you use a shorter lens or move further away.
 

by E.J. Peiker on Tue May 14, 2019 8:58 pm
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DOF is only dependent on aperture and subject magnification. It is not directly dependent on focal length. In other words if you adjust your distance to compensate for a different focal length and shoot both at the same aperture, on the same camera, the DOF will be the same (background coverage will be different but DOF on the subject is the same). The only reason that a print with the same subject size on the print but taken with a crop camera vs a FF camera is different is because the subject magnification on the full frame sensor has to be bigger to have the same frame coverage. This is achieved either with a longer focal length lens or by moving closer - in either case, subject magnification is greater for the size in the frame of the subject if on a full frame sensor over a cropped sensor.
 

by Wildflower-nut on Wed May 15, 2019 10:35 am
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EJ, I follow what you say about the taking of the picture. DOF is only a function of magnification and f stop. Greater magnification results in less depth of field.  In the taking, the image on the small sensor should be sharper because less magnification is involved for a given composition or perspective.  But when you print it, the magnification required of the cropped camera image is greater so its dof (sharpness) will be reduced in the printing more than a FF camera's image.  The cropped sensor has a smaller circle of confusion in the taking but then that smaller circle gets enlarged more in the printing.  Thank you very much with your input.  I'll ponder what you said some more.
 

by E.J. Peiker on Thu May 16, 2019 5:35 am
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Hmm, now you are talking about sharpness which is different than DOF - assuming a totally non-moving subject and a totally stable camera, sharpness can only be measured at the exact plane of focus and is dependent primarily on the optical qualities of lens and secondarily on the strength of any AA filter on the sensor.
 

by DChan on Thu May 16, 2019 10:43 am
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I think it's perceived sharpness that Wildflower-nut is talking about here?

Print size does not change the depth of field.



.
 

by Wildflower-nut on Thu May 16, 2019 1:48 pm
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Depth of field to me is the limits of apparent sharpness (closest point and farthest point that appear to be in focus). This depends on visual acuity. What your eyes are able to resolve will depend on the size of the print and the distance it is viewed at. This is reduced for practical purposes to a circle of confusion size that can be tolerated which will produce a fixed size print which when viewed at a fixed distance will appear sharp. The size of the circle of confusion of course becomes a function of the size of the sensor, the print size and the viewing distance. For a full frame camera, .03mm is suggested. For aspc .025mm is suggested when determining depth of field. The point I was trying to make was that less magnification is required to produce the negative but then more magnification is then required to produce the print from that image (hence the smaller circle of confusion requirement). More magnification implies less DOF. I guess my question is as long as the size of the subject in both prints is the same (ie net magnification of the subject is equal) is the DOF observed in the prints the same. I guess I need to study the math more deeply.
 

by E.J. Peiker on Thu May 16, 2019 3:57 pm
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If you are using those CoC numbers you are basically accepting an 8 megpixel level of resolution. The 40+ megapixel sensors, if you want to get the level of resolution that those sensors are capable of you need to use 0.013.
 

by Wildflower-nut on Thu May 16, 2019 7:40 pm
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Thanks everyone.  I found this a good read.  Really does not answer my question of whether dof is influenced by sensor size when producing the same size print.  it does effect the circle of confusion size the mfg uses in calculating the DOF tables.


https://cpn.canon-europe.com/content/education/infobank/depth_of_field/depth_of_field.do
 

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