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What does "gain" setting on QHY 128c actually mean?

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#1 nathanm

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Posted 16 September 2020 - 03:46 PM

I am familiar, of course, with gain for DSLR cameras (where it is folded into the concept of ISO) and gain on scientific cameras, where it is a multiplier on the number of electrons in a pixel well.  

 

I now have a QHY 128c camera which has "gain" numbers which run from 0 to 4000 (see https://www.qhyccd.c...=94&id=12&cut=1 .

 

It can't be a multiplier if the value 0 is a legal value.  

 

There is a graph which relates the "gain" number to  "system gain" which has units electrons/ADU.  At gain 0, the system gain is 4.5 electrons per ADU, at 4000 it looks from the plot like it is about 0.2 .   For what it's worth, the plot seems to be of the function   

 

system gain =   -0.001075* gain + 4.5

 

A typical use of ISO is to all the photographer to pick a different exposure.   If you know the exposure at ISO 100, then you can easily calculate the equivalent exposure at ISO 50,  ISO 400, or any other value.    By equivalent exposure, I mean something that gives the same pixel values (ADU).

 

That does not seem to be the case here.  if we have a scene in brightness B1 and gain G1, exposed for t1 seconds another at B2, exposed at t2 seconds with gain G2  the two shorts are equal if and only if exposure time t2 is 

 

t2 =  (B1*t1*(4.5 - 0.001075*G2))/(B2 (4.5 - 0.001075*G1)) 

 

That's not particularly intuitive or useful.    Why not just make the gain look like ISO numbers?   Since the same sensor is in Nikon DSLRs it is clearly possible to make that happen.

 

I am surprised that there isn't more about this in the QHY manual.   Don't you need to know where to set the gain to take good pictures?     Since many people have QHY cameras and I see some great pictures out there I have wondered if there is some secret of setting gain that I have just missed it.


Edited by nathanm, 16 September 2020 - 06:06 PM.


#2 nathanm

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Posted 16 September 2020 - 06:30 PM

I have plotted the function above here:

 

 

gallery_334946_14430_131210.jpg

 

The plot graphs the ratio  t2/t1, so to get t2 you look up the ratio and multiply by t1.

 

As expected, if G1 = G2 then the ratio is 1.0, because t2 = t1.

 

For other values of G1, G2,  the ratio varies from 0.04  (if gain G1 = 0, G2 = 4000) to 22.4, if G1=4000, G2 = 0)

 

==============

 
This crazy formula makes little intuitive sense to me, but the QHY site seems clear that that this what is going on.   
 
This means a QHY 128c user can scale exposure times using the formula - if you have an exposure at G1, you can get the "same" exposure at a different combination of time and exposure.
 
That does NOT mean the two photos will be exactly the same.   In conventional photography it is like saying you can get the "same" exposure for ISO 100 at 1/100 sec and ISO 1000 at 1/1000 sec (assuming same subject and f-stop etc).  The shot at ISO 1000 will have approximately the same pixel values, but it will have more noise, and less dynamic range than the shot at ISO 100. 
 
Nonetheless it is useful to know the point of approximate sameness.
 
A good example is skyfog - i.e. brightness from the sky due to airglow and light pollution.  if you get a certain skyfog pixel value at one gain, this will let you tell you the equivalent pixel value at a different gain.

Edited by nathanm, 16 September 2020 - 06:44 PM.


#3 nathanm

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Posted 17 September 2020 - 10:16 PM

Just to flesh this out a little more.....

 

The lowest gain is gain = 0.    Photographers would ask what gain gives you the same exposure with 1/2 the time.     That's what happens with ISO 200 versus 100.   So what about QHY?

 

That gain is 2093.    Intuitive, right?

 

Continuing farther the gain settings in full stops are  0, 2093, 3139, 3663, 3924.   We can't go above that because the gain is only defined to 4000.

 

If we do it in 1/2 stops we have:   0,1226, 2093, 2706, 3140, 3446, 3663, 3816, 3924, 4000

 

These numbers really don't make much sense to me - I can't figure out what QHY was thinking when they defined gain in such a strange way.


Edited by nathanm, 18 September 2020 - 01:11 PM.



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