Fuji APS-C vs FF for portrait DOF

Started 6 months ago | Discussions thread
OP razorfish Contributing Member • Posts: 675

...to everyone who replied to this thread. I've read through every post, and hopefully I am a bit wiser than I was. What I gather from this discussion, as well as reading reviews and viewing images on Flickr, is that two thirds of a stop (as is the difference between Sony's and Fuji's best options today) does not make a very big difference to the amount of background blur you can achieve, and that photographer skill and location is much, much more important. At the same time, depending on conditions, there is an observable difference. And, if two thirds of a stop is negligible, why not just change to micro four thirds, where the 45mm f1.2 has just about the same gap up to Fuji's 56mm? To me, the portability gains of micro four thirds over Fuji are actually greater than what Fuji has over Sony FF (because Fuji still neglects to design the ideal telephotos for the format).

Thanks to ProstheticEmpathy in particular for the link to the HowMuchBlur graphical interface. Even though it's purely theoretical, it was very interesting to input various values and observe the differences. A couple of interesting observations:

  • As I tried to explain in my opening post, it is vastly more difficult to obtain a nice background blur for full-body portraits than it is for head-and-shoulders only. Just increasing the captured width to 2 meters (which is almost too little for a full body) will diminish background blur (e.g. at 10 meters behind the subject) with the same amount as changing the aperture from f1.2 to f3.5 would. It is very interesting to observe on paper what I always felt was the case.
  • The Fuji 56mm (with a captured width of 2 meters) has as much background blur 10 meters behind the subject, as Sony's 85mm f1.4 has 5 meters behind. A micro four thirds f1.2 85mm equiv. will not reach the same amount of blur until 40 meters behind the subject. Again, it's very theoretical, but to me illustrates what I have observed in some Flickr examples, that Sony's 85mm has the best ability to obliterate the background immediately behind the subject.

My contribution to the DOF discussion is this: It is pretty obvious that DOF is a function of only two parameters: Distance to focused subject, and physical aperture opening. As physical aperture opening is again a function of focal length divided by f-stop, if you increase the focal length from e.g. 50mm f2 to 100mm f4, the physical opening remains the same (25mm) and so does DOF. In practical photography you of course will need to keep your main (focused) subject the same size in the frame, and if you increase the focal length, the only way to accomplish this is to also increase the distance to subject. So with the focused subject appearing the same size, you actually will need a 100mm f2 in order to obtain the same DOF as a 50mm f2, leaving f-stop the only real parameter available for changing DOF. Perspective will of course also change by moving back and increasing focal length, and with less background in the photo (or what many call more compression) the 100mm f2 background (at a greater distance to the focused subject) will still appear more blurred than the 50mm f2 background (but DOF is actually the same).

In practice, in order to obtain the most background blur, it is therefore a good idea to use the longest lens that can capture your entire subject in the frame, with as low f-stop as possible. 85mm equiv. is pretty ideal for full-body portraits (even 50mm equiv. could be needed sometimes) while for head-and-shoulders you can often go longer (but for mirrorless, 85mm equiv. is the longest native lens you can get without losing f-stop).

The existence of various sensor sizes further complicates things. With DOF being a function of the physical aperture opening, which again is focal length divided by f-stop, a sensor with crop factor 1.5 will require a 1.5 times faster lens (f-stop divided by 1.5) in order to keep DOF the same at the same FOV (e.g. the Fuji 56mm would need to be approximately f0.95 in order to match the shallow DOF of the Sony 85mm f1.4). This is actually just keeping the physical aperture opening the same. I feel Fuji could do this with a select few primes in order to accommodate the most demanding photographers. At longer focal lengths (200mm+ equiv.) where DOF is always sufficient anyway, Fuji's X-system has a definite size advantage over FF, so matching FF performance for those uses where it's currently at a slight disadvantage (ultra shallow DOF portraiture) would leave no real incentives for choosing FF over Fuji.

Lastly, I am aware of the importance of actual bokeh quality, but this discussion was intended for purely DOF (and the amount of background blur you can obtain). But rest assured I would never buy a portrait prime with bad bokeh. And in order to see the beautiful bokeh of e.g. the 56mm APD, you still need to first blur the background sufficiently.

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