50 1.2L vs 85 1.2L - effective dof/ bokeh- same?

Started Jul 18, 2011 | Discussions thread
Great Bustard
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DOF vs background blur vs bokeh
In reply to MX_shooter, Jul 18, 2011

MX_shooter wrote:

question for people who have or have used both (on a 5d2)

If you are using a 50L at 1.2 and you step forward to have the same kind of framing as an 85L , will you end up having the same bokeh amount / thin DOF (the at 1.2) ?

The distance of subject from camera would be shorter , so i am thinking that this give you mroe blur.

(lets not consider the compression advantage of using an 85L instead of using 50L, ive got a 70-200 if i want compression)

For a given sensor size, the same framing and f-ratio will result in the same DOF, but not the same background blur. The lens with the larger aperture diameter will have the larger background blur for portions of the scene "close enough" to infinity.

For example, the 50 / 1.2L at 10 ft f/1.2 will have the same framing and DOF as the 85 / 1.2L at 17 ft f/1.2. The 70-200 / 2.8L (IS) at 40 ft, 200mm, f/2.8 will have the same framing as the 50 / 1.2L at 10 ft and 85 / 1.2L at 17 ft, but a deeper DOF.

The aperture diameter of the 50 / 1.2L wide open is 50mm / 1.2 = 42mm. Likewise, the aperture diameters for the 85 / 1.2L and 70-200 / 2.8L (IS) at 200mm wide open are 85mm / 1.2 = 71mm and 200mm / 2.8 = 71mm.

So, the background blur (for the portions of the scene "close enough" to infinity) will be the least with the 50 / 1.2L, whereas the 85 / 1.2L and 70-200 / 2.8L at 200mm will have the same background blur.

Thus, it's important to distinguish between DOF (Depth of Field -- the depth from the focal plane where the scene is considered to be "in focus") and background blur (the diameter the disk from a point light source infinitely far from the focal plane).

Lastly, bokeh is the quality of the background blur, as opposed to the quantity of the background blur. In other words, one lens may have more blur than another, but the lens with the lesser amount of blur may render that blur in a more pleasing fashion. See this excellent article for more on this:

http://www.rickdenney.com/bokeh_test.htm

Thus, the 85 / 1.2L is clear winner when it comes to DOF / background blur, and, in the opinion of many, also the clear winner when it comes to bokeh.

That said, I use, and love, the 50 / 1.2L, and don't own the 85 / 1.2L, nor do I have any plans to get it (I do, however, have some interest in the Sigma 85 / 1.4).

The simple question would be, "Why the 50 / 1.2L over the 85 / 1.2L?" The simple answer is, "Because I have a 'need' for 50mm more than I have a 'need' for 85mm."

I am on the 24-50-100 plan, whereas many prefer the 35-85-135 route. While the 35-85-135 route has the "better" lenses, myself, I find that the 24-50-100 route to be "satisfactory":

http://forums.dpreview.com/forums/read.asp?forum=1032&message=38275990

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