why not f/1.2 by Sony?

Started May 14, 2013 | Discussions thread
ilza Regular Member • Posts: 107
Re: why not f/1.2 by Sony?

tomtom50 wrote:

The Sony 35mm f1.4 lens works pretty well on the a900 (T1.6) and not so well on the a380 (T2.2).


As for DOF, the same problem applies. The more angled rays that are lost come from the outside of the lens, so not only do you get less light you get deeper DOF. Oh well.


Sony seems perfectly happy to sell 35mm f1.4 lenses to a380n owners without warning them.

I especially enjoy this one!

Focus reducers are also concentrating light onto a smaller area, which should (theoretically at least) result in faster shutter speeds than would be achieved on FF.

Well, this one at least looks like an honest misunderstanding, not a blatant misunderstanding…

I’m afraid you need to get your knowledge of basic physics of photography up a little bit.  What I would agree with is that the whole subject of comparing different form-factors it too obfuscated; too many people talk about things they do not have any clue about.

It helps if one thinks in correct terms:

  • What part of the subject plane is captured;
  • number of captured photons per an image of some area on the subject;
  • Diffraction circle mapped back to the subject plane;
  • Circle of confusion mapped back to the subject plane(s);
  • (Averaged) readout noise of photocells per an image of some area on the subject, in electrons [*];
  • Full well capacity of photocells per an image of some area on the subject, in electrons;
  • QE: fraction of photons converted to electrons.

[*] Well, another useful measure of noise is electrons/√Hz; but this affects only the technological part: how many ADC do you need.

When you take all this into account, it turns out that everything but the full-well is absolutely trivial: there are only two relevant pieces of data: angle of view, and entry pupil (forgetting for a moment about the lens transparency!).  This means that if you compare 8×10in film with a ⅔" sensor, if your lenses have similar optical quality [**], and the same angle of view and entry pupil, you can get identical images if your sensors are of similar quality [***] and the full wells do not matter.

[**] One should keep in mind that it is progressively harder and harder to make a lens with given optical quality, given angle of view, and given entry pupil when the focal length goes down.  And: at some moment the Abbe's sine law will strike back, and this will not be physically possible.

[***] My back-of-envelop calculations show that it is not possible to match 8×10in film by a ⅔" sensor (about ¹⁄₂₇ difference in linear size).  With QE=1, digital would match the best film at about ¹⁄₁₆ difference of formfactor.  And AFAIK, current QE of digital cameras (considered as systems) is about 0.12 nowadays. So today it is ⅙ difference in formfactor: digital-APC ≈ MF-film. [And what am I talking about here?  Essentially, it was the discussion of the effective-QE of film; in my estimates, it is about ¹⁄₂₅₆. But this topic is completely orthogonal to the discussion of digital-to-digital comparison.]

And since full-well is determined by technology, not by physics, it is natural to expect that this should not matter — at least when one compares bodies of different generations.

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