TriX wrote:
selmslie wrote:
Probably both.
There is likely some adjustment to the reading of the physical analog signal recorded on the sensor in order to ensure linearity during the pre-ADC...
Not true. That would be an exceedingly complex operation, and the primary effect would be more noise and a vastly slower camera.
TriX wrote:
Thanks for the insights. Actually the reason I'm wondering how various manufacturers implement variable ISO control is I'm still trying to get my brain around the idea of dramatically under exposing at "native" ISO and fixing in post as opposed to changing the ISO to expose "correctly" as long as you don't blow out the highlights in either case.
"ISO Invariance" is a mythical solution in search of a problem.
Read what you write in the next paragraph! It has become clear to me over several of your posts that you have significant experience with the recording of analog data using digital systems, and the illogic to various misunderstandings is pretty obvious to you...
TriX wrote:
It seems to me that dynamic range in a data acquisition system, whether audio or imagery, is ultimately controlled by the dynamic range of the A/D (bits or resolution and linierity as aperture uncertainty isn't really applicable in single-shot acquisitions) and the noise floor of the signal. Severe under exposure would seem to "throw away" the MSBs of the A/D, diminishing dynamic range. Then multiplying the digitized value either post A/D (iF controlling ISO in that manner) or in PP also increases the noise by the amount of the multiplication. If on the other hand, you apply enough amplification to the analog signal (to control ISO) prior to the A/D to come close to saturating the MSB, you utilize the entire dynamic range of the A/D, but you're amplifying the noise also, which may restrict dynamic range as the noise may take up 1 or more of the LSBs.
It seems to me that dynamic range in a data acquis... (
show quote)
Looks like you have a good grip on basics! It might help a little to read the basis for all of this. Claude E. Shannon wrote "A Mathematical Theory of Communications", and started the Information Age, back in the late 1940's. The question then was what is the difference between data processing with analog data and digital data. Which is more efficient and which is more effective. Before WWII virtually all of these digital methods had been worked out in theory, but in practice it took a room full of electron tubes to do practical tests. Nobody was really sure where to invest money! Shannon answered that, and everything since then has centered on digital technology. It took until the late 1960's for the needed technologies to catch up (solid state electronics primarily), and digital has dominated everything since.
TriX wrote:
The reason I'm concerned about how each manufacturers control ISO, is that for example, Canon and Nikons typically behave differently in respect to dynamic range vs ISO, and I'm wondering if it's completely due to the sensor noise or whether it's due to how they control ISO. If for example, you pick something like a 5D3, the noise and dynamic range is worse then an equivalent Nikon D600 at "base" ISO of 100, whereas the Canon actually has superior dynamic range at higher ISOs above 6400 (see graph below). That would seem to suggest that using the Nikon to it's best means shooting at lower ISO, (even though underexposing) and fixing in post (as has been suggested), while with the Canon would perform better by shooting at the "correct" metered ISO with less correction in post. What are your thoughts on this?
The reason I'm concerned about how each manufactur... (
show quote)
First, the DXO measurementa are fine, but the graphs are very misleading. Rather than use the cited graph, take a look at what Bill Claff (user
bclaff on UHH) produces. This graph is specifically for the same two cameras,
http://apaflo.com/misc/uhh/chart200.jpgNote the smooth graph for the D600 with an almost straight line slope from ISO 100 to above ISO 12,800. That is "ISO Invariance". Note the bumps at ISO's lower than 3200 in the 5DIII graph. That is "ISO variance". There certainly is a lesson to learn there, and you are right about where the source of that characteristic originates. The lesson is
not that one can under expose by 3 to 6 fstops with a D600 and expect to recover in an editor the same image as a proper exposure. That would be like hiding behind the nearest tree in a forest and claiming to your companions by cell phone that you actually aren't there. The tree in this case is that a print doesn't have enough dynamic range to show the effect (as long as the image doesn't need any gamma expansion/compression). If the brightness corrections is the only needed edit, that tree will hide you. If the image would benefit from pulling up the shadows, or any other contrast manipulation, forget it!
The cause of the bumps vs straight line in the ISO graphs has to do with exactly where and how the camera adjusts ISO. Virtually all modern cameras use analog gain between the sensor and the ADC, and perhaps also adjustment of the ADC's DC comparator voltage, to change ISO. Most cameras also have some form of pseudo ISO (lower quality that may not be acceptable for every use) adjustment using digital manipulation. But for this discussion only the analog gain is of interest, because the digital gain, whether done in the camera or in an external editor, is inferior and should be avoided to the degree possible.
The lumps in the 5DIII chart (similar by the way to a Nikon D5 chart), are because designing an analog amplifier with sufficient gain and dynamic range with a low enough noise figure is clearly not yet possible. The 5DIII and D5 are designed with a good noise figure, to allow better results at high gain. The D600 (and D750 and D8xx models) don't have that great noise figure, but do have the dynamic range to deal with maximum output from these sensors. The bumps (in these and in other sensor curves, where they may be just one or two bumps and may be at higher ISO values) happen when the camera switches between different forms of ISO gain. The bumps at low ISO values are usually from switching between a digital controller on the amplifier to a digital controller for a loss device (a pad). The amplifier steps between 1x, 2x, and 4x gain. The pad provides 1/3 fstop steps between the amplifier steps.
The steps you might see in cameras like the D810, which are one or two bumps at higher ISO values are where the gain adjustments change from using amplifier gain to changing the DC voltage on the ADC.