F-Stop Blues

[Daniel Browning]

<span style="color: #888888;"]

It appears that this hasn't been discussed here yet, but I can't imagine getting a more informed interpretation anywhere else...

<span style="color: #888888;"]I've been complaining about it here since the forums first started back in February 2009 and before that I was complaining on other forums. I was first alerted to the issue in 2008 by Peter Ruevski:


<span style="color: #888888;"]http://www.pages.drexel.edu/~par24/rawhistogram/CanonRawScaling/CanonRawScaling.html


<span style="color: #888888;"]I'm sure there were more advanced users who knew what Canon was doing even before 2008.


<span style="color: #888888;"]

<span style="color: #888888;"]In a nutshell it claims that lenses with more aperture than about f/2 are useless on digital because the light comes at such angles that the sensor doesn't record it;


<span style="color: #888888;"]

<span style="color: #888888;"]Well, they don't claim that it's always useless; just that it's often less useful than you'd expect.


<span style="color: #888888;"]

<span style="color: #888888;"]but it does seem that they are on to something here


<span style="color: #888888;"]

<span style="color: #888888;"]Yes, they are. Unfortunately, there is a significant error in DxO's calculations. They seem to have forgotten about the difference between the simple (but wrong) f-number and the *real* (or "effective") f-number.


<span style="color: #888888;"]The f-number we all know and love is f/D. We generally assume that there is a one-to-one relationship between f-number and light intensity. But in reality, that is only a very good approximation -- it becomes less and less accurate as you increase the value of D for a given f. To get the effective f-number, you need to use the more accurate 0.5NA. For narrow f-numbers, it is the same as f/D, but for wide f-numbers, it differs. Using 0.5NA, you can see that an f/1.2 lens has an effective f-number of only f/1.3.


<span style="color: #888888;"]The loss from sensor angle of response is separate from and in addition to the loss from "effective" f-number, but DxO incorrectly assumes they are the one and the same.


<span style="color: #888888;"]In other words, the light losses from sensor AOR that DxO calculated are off by about 30% -- things are slightly better than they put it.


<span style="color: #888888;"]

<span style="color: #888888;"]camera manufactures hide this fact by raising the effictive ISO to offset this loss of light.


<span style="color: #888888;"]

<span style="color: #888888;"]The problem isn't really that they are hiding it. I'm fine with that, actually. The problem is that they are doing such a fantastically stupid job of hiding it. I mean, really, the method they chose is totally braindamaged. They are using digital gain, which accomplishes three things:


<span style="color: #888888;"]

• <span style="color: #888888;"]
• Clips highlights as much as a stop! &gt;
  
  
  <span style="color: #888888;"]
• Increases noise significantly in cases where it would have been otherwise possible to increase exposure.
  
  
  <span style="color: #888888;"]
• Increases quantization error (posterization, "banding", etc.)
  
  
  <span style="color: #888888;"]
• Slows down the camera with unnecessary processing steps (probably inconsequential, but still).
  
  
  <span style="color: #888888;"]

<span style="color: #888888;"]Downsides of doing it the right way, with metadata or exposure:


<span style="color: #888888;"]

• <span style="color: #888888;"]
• None.
  
  
  <span style="color: #888888;"]

<span style="color: #888888;"]So as I see it, there are four separate issues:


<span style="color: #888888;"]

• <span style="color: #888888;"]
• Sensor angle of response
  
  
  <span style="color: #888888;"]
• Compensation for difference between approximate f-number and effective f-number.
  
  
  <span style="color: #888888;"]
• Metadata compensation vs digital gain compensation.
  
  
  <span style="color: #888888;"]
• Ability of the AE meter to compensate without using stop-down metering.
  
  
  <span style="color: #888888;"]

[Jon Ruyle]

<span style="color:#888888;"]Downsides of doing it the right way, with metadata or exposure:


<span style="color:#888888;"]

• None.

You're overlooking the obvious: if they did it the right way, we couldn't have all this fun reading your post about how stupid they are

[Daniel Browning]

You're overlooking the obvious: if they did it the right way, we couldn't have all this fun reading your post about how stupid they are

Not to mention how bored I would be without having things to complain about.

[Dr Croubie]

One thing that sprang to my mind when reading this:


- so camera sets aperture, say f/1.2, and ISO, say 100


- camera reads exposuring info from metering sensor and sets speed, say 1/250s.


- camera says to itself "f/1.2 will make my photo darker than it should be (ie, more than 2 stops darker than f/2.4), so i

[Daniel Browning]

which photo ends up better?

The second one, because it will have less clipped highlights, less quantization error, and will take up less slightly space on the computer due to a smaller raw file size. Any other differences will be due entirely to the raw converter mishandling things. (Many raw converters, e.g. Adobe, tend to foul up the colors when you use exposure compensation. You can mostly work around it by using a 0 black point.)

[wickerprints]

If it were truly "useless" in the sense that the light from marginal rays at f-numbers faster than f/2 are lost at the sensor, you wouldn

[Colin500]

In response to some of the posts here (including my own): Of course this effect is gradual, it