ksbal wrote in post #18349940
Reasons to buy full frame:
1. shallower dof
2. better high iso performance
Why are those separate?
Larger sensors only collect more light through shallower DOF, for the same FOV and shutter speed!
I don't know why people are so resistant to adopt the fact that normalized for DOF, FOV, diffraction spot size (relative to the full image), and with the same shutter speed, sensor size is totally irrelevant to total light collection.
From any given subject, with the same illumination, the number of photons you collect per millisecond from the subject depends on just two things: the area of your aperture (adjusted for transmission factor and vignetting, of course, if applicable), and your distance from the subject. Nothing about the size of your sensor, or even the size of your pixels, affects how many photons you get from the subject, or any given part of the scene. Sensor size only affects how wide your recording canvas is, in that paradigm.
Any person who thinks that they are going to get more light from a subject or composition just by having a larger sensor is sadly mistaken (unless they are shooting in Av mode at ISO 100, and doesn't care about exposure length). Nevertheless, many people fall prey to the illusion that the subject gets more light in the higher ISO ranges, because larger sensors almost always have larger pixels, and larger pixels means that the default limit to the magnification, 100%, presents sensor surface area less magnified, yielding a sharper and less noisy view "at the same ISO". The fact is, unless you are shooting that Av mode at ISO 100, "the same ISO" does not do "the same photographic thing".
If shooting above 6400 iso, and doing the 'in thing' with shallow dof portraits is your primary concern, then absolutely a FF is a serious consideration.
Yes. Shallow DOF is where FF shines best. Some reasons are that shallow DOF is often more obtainable with realistic lens choices for FF. You won't find an f/0.75 lens to do the same APS-C1.6 DOF job as an f/1.2 lens of 1.6x the focal length with the same aperture. On top of that, current microlens designs for DSLRs tend to lose light progressively at f-numbers below 2.8 for larger pixels and 2.0 for smaller pixels, so at f/1.2, a 20MP FF is losing about 1/2 stop less light than a 20MP APS-C. That lost light, I believe, is mostly light that makes the DOF shallower, so I think that along with this light loss, there is loss of blur outside the plane of focus, as well, but I have not verified this. It would be very interesting to compare the DOF with the same f/0.9 lens on a current 12MP FF and a 50MP FF.