MultiPro and Ai Questions re Auto Exposure (Lamp Brightness)

[mct]

Q1: The Ai switch for turning on the scanner's autoexposure (lamp brightness): is this actually changing the brightness of the lamp, or just fiddling with the gain on the CCD?

Q2: If you turn this off, how do you manually influence exposure within the scanner? (i.e. before the raw data even gets into Silverfast).

Q3: Is there any way to manually adjust the gain on the RGB sensors independently, for example to get the grey balance neutral; before profiling?

Q4: Big question: with autoexposure on, wouldn't this interfere with the correct operation of colour matching in a profiled environment, i.e. where the scanner has been profiled? I could imagine that changing lamp brightness or CCD gain could affect the scanner's reponse could it not??

Thanks,
Milt

[LSI_Muenier]

to Q1: As this is a scanner internal question, I will ask Minolta development about this.

to Q2 and Q3: SilverFast has a "Lamp" dialog under Options/Special. Here you can manually influence the scanner exposure.

to Q4: When you are scanning dark slides, then autoexposue will light them up before color matching is applied to the image data. The lightness is increased accordingly on an analog level which reduces noise.
The color corrections will only approximately be correct, as they were measured for different exposure.

[LSI_Muenier]

addendum to Q1-3:
Both, the auto-exposure and the Lamp-dialog take influence on both, exposure and gain. But it is preferred to use exposure to achieve the required result.
Only if exposure exceeds a (scanner specific) maximum time, then gain is used, otherwise not.

[mct]

I'm still not clear on this.

When you talk about autoexposure, are you referring to the switch in Options General which causes Ai to automatically adjust levels and midpoint after the scan is done? (Which is a software exposure adjsutment on the raw data).

Or is there another switch that is controlling the scanner's CCD auto gain control, which affects the data before it even gets to Ai?

Can you confirm that the lamp brightness dialog is actually changing the lamp's brightness in the scanner?

Thanks...

[LSI_Muenier]

(1) There is a button to the left of the preview with a lamp symbol on it. Its help text is "Set automatic exposure ...". This is the one I was referring to for auto exposure.
The availability of this button is scanner dependent (but I think the MultiPro has it), as its functionality is scanner internal and on an analog level.
(2) The lamp brightness dialog under Options/Special is scanner dependent, too. If present, it also works on an analog level.
(3) For all scanners SilverFast offers an image automatic that adjusts levels etc. as you mentioned in your last post, but this of course is done in the software, and not in the scanner.

I was referring to (1) and (2) in my previous post. As I said, these two take influence on the exposure time. If a certain (scanner dependent) maximum exposure time is exceeded, then gain is used addionally, otherwise not.

[mct]

That's a little clearer. Both the button you are referring to the left of the preview window, and the Lamp brightness dialog are indeed available for the MultiPro.

I'm not sure I really understand how these interplay, for example if you make adjustments in the brightness dialog, while autoexposure is switched on.

The reason I'm asking these questions is that when calibrating a scanner, it is a good idea I believe to get the scanner as close as possible to the target by adjusting the scanner's hardware gamma and grey neutrality (by playing with CCD gain), before generating a profile. This way the profile is introducing less error as it makes its corrections to get calibrated results.

At first I was looking at the Gamma setting in Options-general to do this, but this is a software gamma adjustment after the scan is performed and data received.

So I guess the right place to do this is in the 'lamp brightness' dialog, which I think is probably misnamed. Wouldn't this really be a CCD gain adjustment? (Adjusting lamp brightness individually for red, green and blue doesn't quite make sense).

Thanks...

[LSI_Muenier]

Maybe you're right that the Lamp dialog's naming could be better.
But remember that we support very different scanners which internally might implement this analog brightness differently.
For calibration I would suggest to do it without autoexposure but rather manually adjust the IT8 chart using the Lamp dialog before calibration.

[mct]

More information and more questions!

I did some further experiments on the Minolta. I turned all calibration off so I was just getting raw scanner data into Ai, and played around with Autoexposure, "Lamp Brightness" (which in this case I'm sure is probably CCD gain) and also Gamma adjustment.

Autoexposure ON seems to make sure that the highlights in the image just reach data value (255,255,255). If this is indeed happening at hardware level, one assumes the scanner must have to do a quick prescan of its own just to figure out what to set the overall gain to in order to get correct exposure. (I think it can be heard doing this before making an Ai prescan or scan).

Turning autoexposure off yielded a slighter lower 'exposure' on my test slide.

Turning up 'lamp brightness' could manually overexpose and cause clipping quite easily, even with autoexposure on. Curiously though, playing with the sliders in lamp brightness produces immediate feedback on the preview windows and the histogram. How is this possible if this is a hardware setting that takes effect during the scan? Or is this actually a software correction similar to the gamma adjustment control in Options-General?

I predict that when I put the target in there, the optimum hardware settings will be Autoexposure on, and leaving the manual controls at there default positions.

All that is left then is to do is to find the gamma adjustment to apply to the scanner's natural gamma (which I'm thinking might be near linear), to end up with a gamma slope that is closest to the gamma of the internal working space. Then after calibrating with the target, this should leave the profile having to do the least error correction on the raw data.

Have I got that right?

Cheers...

[LSI_Muenier]

Gamma is adjusted on a digital level by the IT8 calibration itself.
Please make sure to choose the gamma value in the Options/General panel according to your internRGB work space.

[mct]

That assumes the natural gamma of the scanner is 1.0? (I'm not sure if that's the case or not, it may well be).

If I set the Gamma in Options-General to be the same as the Working Space, (e.g. 1.8 for ProPhoto RGB), and the hardware gamma of the scanner is not 1.0, then that just leaves more gamma correction for the profile to do?

Also, can you confirm that for the Minolta, the "Lightness" dialog in Options-Special is a software correction and not a hardware control on the scanner?

Cheers..

[LSI_Muenier]

As I said, the Lamp dialog takes influence on the exposure of the scanner (and on gain only, if a maximum exposure time is exceeded).
So, it is working on an analog level and not on a digital one.
Is this what you mean by "hardware" vs. "software"?

[mct]

Let me rephrase the question: it seems to me that the "Lamp Brightness" dialog is controlling an adjustment that is made on the data after it is received by Ai, rather than adjusting something inside the Minolta box. If you adjust the sliders, it immediately reflects in the Preview window appearance, just as playing with the gamma adjustment does. So can you confirm that this is the case?

[LSI_Muenier]

Yes and No. (!)
As long as you are in the Lamp dialog, the effect of the exposure change is simulated by SilverFast.
But when you leave the Option dialog, a new prescan is made. This one and every further prescan and scan will use the changed exposure values, and they are applied inside the scanner, and not simulated anymore.[/u]