A Simple DOF Question

If I had a FF camera with the 135L and a 1.6 crop camera with my Sigma 85mm in the same position, both at F/2, and aimed at the same subject, would the DOF in both pictures be the "exact" same?

Is this no different than, let's say... having a 70-200mm on a FF camera and shooting two pictures: one at 85mm and one at 135mm (same aperture & location). Then, crop the 85mm picture to have the same frame as the one shot at 135mm.

I'm debating on whether I should just stick my Sigma 85mm on my 40D, or actually get a 135L.

I read before that some people on this forum said that the Sigma 85mm at F/1.4 has roughly the same DOF as the 135L at f/2. Why is that?

Thanks!

There would be a difference between the two images.

Try entering your specifications into a depth of field calculator such as the DOFMaster on-line DOF calculator.

There are two distinct reasons for the difference. One is the focal length difference. The other, which many people don't understand, is the circle of confusion. The circle of confusion factor involves how much the in-camera image has to be enlarged to fill the standard sized print used as the reference (typically 8x10 inches or so).

I believe the 85mm @ f/1.4 on the 40D will have an equivalent DOF to the 135mm @ f/2.2ish on the 5D, when enlarging to the same size. The 40D and 5D2 have about the same pixel density, so I'm thinking that you can just use it on the 5D and crop if needed.

For some reason, I thought that DOF and background blur was dependent on the camera-to subject and subject-to-background differences. Correct me if I'm wrong, but wouldn't the different focal lengths not make a difference on the DOF if the camera-to-subject distance was the same on both cameras?

XxDJCyberLoverxX wrote in post #14964199
For some reason, I thought that DOF and background blur was dependent on the camera-to subject and subject-to-background differences. Correct me if I'm wrong, but wouldn't the different focal lengths not make a difference on the DOF if the camera-to-subject distance was the same on both cameras?

..... Background blur is enhanced my increasing the subject to background distance. Let's just finish off your first question and avoid some of the complexity for now.

If subject to camera is 10ft and assuming both cameras are same spot and at f2

1.6 crop + 85mm = Dof 0.31ft, near limit 9.85, far limit 10.2
35mm sensor + 135mm = Dof 0.19, Near limit 9.91, far limit 10.1

A six pack and a google search on circle of confusion, plus differences in effective focal length, etc.....

You would have the same framing and perspective because of equivalent focal length and subject distance.

However. On the FF camera you have a 135mm f2 lens. 135mm/2 = 67.5mm aperture. On the Crop camera, you have a 85mm f2 aperture. 85mm/2 = 42.5mm aperture. That means approximately 60% more subject isolation on the 135mm f2 which is exactly what you would expect from the crop factor of 1.6.

However, your question was about whether to just use your Sigma 85mm f1.4 on crop instead of a 135L on FF. If you used 85mm f1.4 on the 40D you would end up with a 60mm aperture - so very close to 135mm f2 on FF. You would actually need 85mm f1.2 to get the same exact subject isolation as 135mm f2 on FF, but 85mm f1.4 is close enough if you ask me. Now the 135mm would probably be sharper at f2 than the 85 at 1.4, but from a DOF/subject isolation point of view they are about the same.

There is some enlargement factor (CoC) at play here, but aperture size is the biggest contributor.

^
DOF and amount of blur in background are two very different concepts.

• DOF zone at subject distance of 10', with APS-C 85mm f/1.4 is about 63mm while DOF zone for FF 135mm f/2 is about 57mm.
  
• However far field blur is much greater with the FF combination, with infinity blur circle of 3.2mm vs. only 1.8mm with APS-C.
  

Wilt, trust me, I understand DOF and BG Blur are not the same thing.

How are you calculating "infinity blur". Everything I have seen makes far field blur a basic function of aperture size. I would not expect to see 3x the infinity blur where only a 10% aperture size difference exists.

tkbslc wrote in post #14965757
Wilt, trust me, I understand DOF and BG Blur are not the same thing.

How are you calculating "infinity blur". Everything I have seen makes far field blur a basic function of aperture size. I would not expect to see 3x the infinity blur where only a 10% aperture size difference exists.

My point was not to 'correct' anything in your post (I used the up caret to express agreement!), but to amplify the fact that DOF zone size and the degree of background blur were two distinct concepts.

Your point about aperture diameter comparison applies in the 'near background'. In the 'far background' only FL is the determinant. In this article by Zeiss on page 29 of the PDF, you can clearly see the degree of background blur difference visible when shooting the same shot with different formats (on the vertical scale, a higher value is less blurred)
http://www.zeiss.com …/$File/CLN35_Bo​keh_en.pdf

The background blur calculator which I used is found here... http://toothwalker.org​/optics/vwdof.html

Wilt wrote in post #14965796
Your point about aperture diameter comparison applies in the 'near background'. In the 'far background' only FL is the determinant.

That is not true. We all know that a 300mm at f22 will not have larger blur disc at infinity than a 50mm f1.4. FL is the determinant only in that it increases the aperture size at a given f-ratio.

My math is correct when talking about a single sensor size. Blur is just (magnification x focal length) / f-stop. You can get the same magnification by changing distance. Focal length/f-stop is just the aperture size. So preserving the same magnification and using the same aperture size, the blur will be identical. For example, 50mm f2 at 2m subject distance has the same blur as 100mm f4 at 4m distance because both have the same subject magnification and aperture size.

However, I had not realized how large a factor CoC as a result of enlargement was until reading through those equations. If you kept the enlargement factor the same, then it is only a function of aperture. SO that would mean printing the APS-C shot at about 5x7 while the FF shot gets printed 8.5x11.

However, when preserving output size, your numbers are correct. When moving between sensor sizes, the difference in blur is nearly double on FF.

One thing to consider is that for near background the DOF is more important than the "infinity blur".

tkbslc wrote in post #14965905
That is not true. We all know that a 300mm at f22 will not have larger blur disc at infinity than a 50mm f1.4. FL is the determinant only in that it increases the aperture size at a given f-ratio.

My math is correct when talking about a single sensor size. Blur is just (magnification x focal length) / f-stop. You can get the same magnification by changing distance. Focal length/f-stop is just the aperture size. So preserving the same magnification and using the same aperture size, the blur will be identical. For example, 50mm f2 at 2m subject distance has the same blur as 100mm f4 at 4m distance because both have the same subject magnification and aperture size.

However, I had not realized how large a factor CoC as a result of enlargement was until reading through those equations. If you kept the enlargement factor the same, then it is only a function of aperture. SO that would mean printing the APS-C shot at about 5x7 while the FF shot gets printed 8.5x11.

However, when preserving output size, your numbers are correct. When moving between sensor sizes, the difference in blur is nearly double on FF.

One thing to consider is that for near background the DOF is more important than the "infinity blur".

Maybe I should have stated it differently...far field blur is directly related to FL, when the f/stop is the same on both lenses.

There was a discussion some months back, and tonylong correctly stated,
"Note the equation in Post #1:

D=((f2-f1)*d*F1*F2)/(i*f1*F2-i*f2*F1)

Note that in the Numerator part of the equation, you have the two apertures at the different focal lengths subtracting, which when they are both the same, you get zero, then when you multiply the rest of the variables you still end up with 0.

So, if the equation is accurate, whenever you are consider matching apertures with two focal lengths, the longer lens always wins. 'Course we understand that as far as DOF, but for this discussion I'd say it is of interest..."

Thanks for the clarification everyone. It makes a lot of sense now. I think I was definitely getting DOF mixed up with frame coverage and framing/perspective.

So in a nutshell: basically, the larger the aperture diameter (affected by Focal Length and F-stop), the more "blur?"

SkipD wrote in post #14963929
The other, which many people don't understand, is the circle of confusion. The circle of confusion factor involves how much the in-camera image has to be enlarged to fill the standard sized print used as the reference (typically 8x10 inches or so).

I'm "Tempted"......ok...mu​ch like the "Ball of Confusion".... that's what the world is today