Background blur: more important than DOF???

In the discussion following the definitive photo.net article he wrote

on digital DOF (www.photo.net/learn/optics/dofdigital/), Bob Atkins

wrote:

 

"Distant background blur is a function of the physical size of the

aperture, which is usually pretty much the same as the actual

physical diameter of the front element for prime lenses used wide

open. The larger the diameter, the more background blur you get.

 

Background blur should be greater for DLSRs with small sensors

because the image has to be enlarged more (and thus the blur is

enlarged more) to get to a given print size compared to full frame

sensors or 35mm film."

 

I think the conclusion here is wrong. To take the same shot at the

same f stop, you use a shorter lens on the digital camera. But this

means that the physical size of the aperture is reduced

proportionally to the format. Of course, the extra magnification

required gets the background blur back.

 

So the correct answer seems to be: background blur is roughly

proportional to aperture used, regardless of format.

 

If this is correct, it is _amazingly_ good news for consumer digital

photography, because it means that you get the DOF where you need it

(the subject, where DOF, especially with MF, is often inadequate) but

still get the background blur you want.

 

Hmm. This sounds much too good to be true. (Part of that may be the

definition of "distant", and that the background in most shots isn't

far enough away to meet the criteria for being distant.)

 

What's the right answer here???

I think my comment was intended to answer a question about using the same lens on a full frame SLR and a small sensor DSLR.

 

In that case, since you have the same lens on each camera, and the physical size of the blur circle for distant objects doesn't care about the sensor size, enlarging the image to the same final print size results in more background blur for the small sensor DSLR. Of coure the "effective focal length" is longer too - so the comparison has to be made with that in mind.

 

If you use a shorter focal length lens to compensate for the "cropping factor", if it has the same sized physical aperture it will be faster. If it has the same speed (f-stop) the physical aperture will be smaller.

 

Off the top of my head, if you use a 50/1.8 on a 10D and an 80/1.8 on a 1Ds you get the same field of view and the same speed lens. The physical aperture on the 50/1.8 is 27.78mm, on the 80/1.8 it's 44.44mm, so the 80/1.8 will give more 1.6x more background blur. However to get to the same print size the 10D image has to be enlarged 1.6x more, so the two effects cancel and you get exactly the same amount of distant background blur in the print in each case.

 

"Distant" can be defined as a distance much greater than the rear limit of the depth of field. I'm not sure exactly what range "much" has. It may well depend on the focal length, I haven't tried to do the analysis.

I think you'll find that you really do get more DOF & less background blur with the smaller format. Consider this: you would get the same degree of background blur if you a) halved the sensor size, b) halved the focal length, c) halved the physical aperture size (i.e. used the same f-number), d) halved the size of the subject, and e) halved the subject's distance from the camera. Since we're talking about photographing a subject that's the same size, we get a change in DOF and background blur depending on the format. Larger formats give shallower DOF (and more background blur), smaller formats give deeper DOF (and less background blur) at the same f-number.