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

thaBuzzed wrote:

I have a friend's wedding coming up and I'm toying with the idea of an 85mm f/1.8 (I'd love a f/1.2 but I simply don't have the budget right now due to a major gear acquisition!).

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. Do any of you own both of these lenses have any comparison images to share?

Uh yeah the difference is going to be huge,  they're not even on the same universe let alone planet.

Personally though I would get the Sigma 85mm 1.4, I sold my 85mm 1.2 for it due to image quality and purple fringing. It's cheaper and at f/1.4 you start to see a really huge difference in DOF. Compared to 1.8 (1.2 gives you little benefit). I've owned both the 1.8 1.2 and Sigma 1.4, and the sigma is the best by far.

Hope that helps.

Another thing you can look at is an online DOF calculator, e.g.

http://www.dofmaster.com/dofjs.html

Plug in (say) full frame, 2 meter subject distance:
DOF @f4 =   29cm
DOF @1.8 = 13cm

DOF is very sensitive to subject distance. The sample pictures above may help more than the calculator (although the image at 1.8 looks to be front-focused to me though...)

-V

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.

Thank you for all your inputs -very insightful. I've a lot to think about, including working on technique with the lenses I do own.

Steve Balcombe wrote:

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.

dofmaster.com