« on: August 11, 2013, 10:31:15 AM »
Note: I signed up to this forum just so that I could reply to this thread, because I need to clarify the technical issues being discussed.
The formula referred to applies only when the background object is located at infinity, and shows that it is proportional to the entrance pupil diameter. For a given fixed magnification, then, the compiled list is merely a list of lenses sorted by decreasing entrance pupil size. Unfortunately, this information frequently fails to capture the most interesting behavior of the background blur of a lens, which is its diameter as a function of the distance away from the subject in focus.
For instance, it is possible to have two lenses, say Lens A and Lens B, such that for a given subject magnification the blur circle for objects "close behind" the subject is larger for Lens A than for Lens B, but the reverse is true for objects at infinity. This occurs because (informally speaking) there are competing factors that contribute to the size of the blur disk. To complicate matters further, the background distance at which this "switch" occurs is itself a function of the subject magnification.
One such example of this phenomenon is an 85/1.2 versus a 300/2.8 lens. When both are shot around 1:10 magnification (which is near MFD for both real-world implementations), the former is predicted to have about 2x the blur circle diameter up to about 1 foot behind the subject, decreasing until the two have equal blur at about 11-12 feet behind the subject, after which the 300/2.8 will dominate. What is happening is that a faster f-number will increase the blur at distances close to the subject, but a longer focal length will increase the blur of very distant objects because of perspective.
To further illustrate, suppose we compare a 50/1.0 against a 200/4 lens. Both lenses have the same entrance pupil diameter at infinity focus (P = 50mm), so at the same subject magnification, a very distant background should have approximately the same amount of blur. But which lens should blur objects closer to the subject more? The answer to this question is one of the reasons why the (out of production) EF 50/1.0L is especially coveted for the way it images--it's not merely for the light-gathering ability of f/1.0. The combination of a relatively short focal length and a very fast aperture can result in images with a distinctive look, because it simultaneously delivers background blur while showing more of the background scene (owing to perspective), compared to a telephoto lens. By no means is this everyone's cup of tea, but there is a technical explanation for this behavior.
Of course, the entire complexity of the lens design itself must be taken into account for a more real-world understanding of its blur characteristics. Aberrations such as Petzval curvature, astigmatism, and spherical aberration, can significantly affect the way the blur looks off-axis. But for most well-corrected designs, the above holds true, especially for paraxial rays.
This is a very clear and necessary addition to the conversation.