Depth of Field (Crop vs FF)

[yjt]

http://www.dofmaster.com/dofjs.html

by using this calculator it shows that crop (7D) has a shallower depth of field than (5D II)
Anyone care to explain?

I thought full frame has a shallower depth of field.

[Wilt]

I bet you made the mistake of taking a single lens (like 50mm) and doing calculations with the same FL on both 7D and 5DII...this is in error because the AOV is not the same on the two formats. You should have used 31mm on the APS-C body and 50mm on the FF body for the calculations of DOF, so that you have the same AOV on both frames.

[Calicajun]

Makes no sense to me, maybe bad math.

[yjt]

well... Isn't Focal Length is same no matter which camera it's on?

I'm not getting the "AOV", assuming it's angle of view, can you explain more on this?

[krb]

Yes, AOV is angle of view. APS-C is referred to as a "crop" sensor because it is smaller than a 35mm-sized "full frame" sensor so the image is effectively being cropped when the two are compared.

I'm pretty sure that if you click on some of the "related threads" at the bottom of this page, or do a search on the phrase depth of field, then you will find plenty of existed threads where the differnces in DOF, AOV, perspective, etc. between these two sensor formats have been beaten into the ground. Complete with illustrations, graphs and charts.

[yjt]

well on the website it states that

"Use the actual focal length of the lens for depth of field calculations. The calculator will automatically adjust for any "focal length multiplier" or "field of view crop" for the selected camera."

[krb]

Yes, use the actual focal length of the lens while calculating DOF. But in order to have the same framing/composition between the two formats you must either use a different focal length or you must change the camera position which would change the perspective. Like I said, click on some of the "related threads." There is -always- a thread about some variation of this subject that is less than a week old.

[krb]

****, just read the thread you started on this same subject back in December.
http://photography-on-the.net/forum/...d.php?t=973999

[xarqi]

Four factors affect DoF: FL, f ratio, subject distance, and format size (or CoC).

If you vary only the format size, yes, the smaller format has the shallower DoF. The resaon is that the image must be magnified more to reach the correct size for DoF evaluation. Of course, if you do this, the two images will not be the same since the smaller sensor will have captured less of the image circle.

If you vary both the sensor size, and the focal length so as to make the images essentially identical compositionally, then the one taken with the longer focal length will have the shallower DoF. That will be the one with the larger sensor, but it is the change in focal length that is responsible as the change in sensor size alone would have increased the DoF, as described above.

[yjt]

Ok, after reading this I get it now hopefully this can help other people.

source:http://www.bobatkins.com/photography...igitaldof.html


the dof calculator is correct, it's just that the FF picture would have a different FOV than the Crop. the crop has a shallow DOF if they are taken at the same distance from the subject.

[Wilt]

Originally Posted by yjt

well... Isn't Focal Length is same no matter which camera it's on?

I'm not getting the "AOV", assuming it's angle of view, can you explain more on this?

The first principle that must be understood by everyone is that the size of the subject (where you focus) affects the perception of 'out of focus' vs. 'in focus'. If something is a greater percentage of the frame size, the out-of-focus disks (which are blurry versions of the original sharp point) are similarly larger...this is why telephoto shot has less DOF than a wide angle shot of the same object shot from the same location. This is also why DOF for prints other than 8x10 are different from the DOF of the 8x10 print, and calculators like DOFmaster are useless when you intend on making a print size other than 8x10.

If you use FF to shoot a 10' tall object from 21' away, that 10' tall object is the full height of the short dimension of the frame. Then when you enlarge that image 8.5x to make an 8x10" print (which is the standard size which is assumed by most DOF calculators), the DOF calculator says things from 18.6'-24.0' are sufficiently sharp (shooting at f/2) to fool your brain into calling them 'in focus'
But if you use APS-C to shoot the 10' tall object from 21' away, that 10' object must be shot with a 30mm lens in order for the full height to fill the shorter short dimension of the APS-C frame. Then when you enlarge that image 13.5x to make an 8x10" print (which is the standard size which is assumed by most DOF calculators), the DOF calculator says things from 17.1'-26.9' are sufficiently sharp (shooting at f/2) to fool your brain into calling them 'in focus'.

To compare DOF of any two formats, you must do the comparison which assume comparable FL to achieve the same Angle of View from the same camera position... in this case, 30mm with 7D vs. 50mm with 5DII.