Does the 85mm f/1.8mm Produce that much shallower DOF than the 70-200 f/4 ?

thaBuzzed wrote:

I already own a 70-200 f/4 so I'm wondering if the 85mm prime would help me make significantly different images from a DOF perspective? I must say I'm not exactly blown away by the DOF control on the 70-200 in the 70-140mm range.

Great as this lens is, it's still only f/4 - no faster than the $199 EF-S 55-250 at the short end of the range. The 85/1.8 is more than two stops faster so the visible difference is huge.

But there are two aspects to this. When people talk about "depth of field control", what they are often looking for is subject isolation from the background. This doesn't necessarily require less depth of field, it requires more background blur which is not quite the same thing. You can achieve this two ways - by using a larger aperture at the same focal length (as with the 85/1.8) or the same aperture at a longer focal length, while shooting from further away to maintain subject framing (as with the 70-200/4 at 200 mm).

The difference between the two solutions is important to understand, because if you use the longer focal length you get the extra background blur without losing depth of field. Often, this means a sharper subject and therefore better subject separation. On the other hand you also get a flatter perspective and this is not always desirable.

Using the 85/1.8 wide open for portraits gives you lots of background blur but it also gives you blurry hair and, not infrequently, one sharp eye and one blurry eye. What you'll probably find yourself doing is stopping down to f/2.5 or f/2.8 which I find to be a sweet spot for conventional portraits, and of course that loses half the background blur advantage.

I'm not saying the 85/1.8 wouldn't be a great lens to have - just that you need to understand the pros and cons.