Can RD be used to select best bracketed exposure.

Submitted by paulo on

Forums: 

I have RD Exposure and use it to determine which of my bracketed exposures are the best in terms of ETTR. I have a Sigma DP2M and shoot at ISO 100. I believe It's telling me which of the 3 exposures is closest ETTR. I'm now considering using higher ISOs for B&W as I've noticed the camera works well at ISO 800 for B&W. After doing some tests it appears that I can't use RD for determining the best bracket at ISO 800 (or any higher than 100).

Here is what I'm finding. If I expose at f4.0, 1/60 sec at ISO 100 and also expose at f4.0, 1/500 at ISO 800 (these are equivalent) then RD shows just .5% pixels as over exposed for the ISO 100 image, yet the ISO 800 image shows as being considerably underexposed.

I'm not saying anything is wrong. I'm guessing that my expectation that I can use RD to assess which exposure is best at various ISOs is incorrect and that it cannot be used in this way….and I should just use RD to calibrate my camera. Exposures at ISO 100 have been basically ETTR. Just looking for confirmation of this, or if there is a way to judge which bracketed exposure is best.

Thanks,
Paul

Deart Paul,

Submitted by Iliah Borg on

Dear Paul,

Sigma Merrill series cameras do not have hardware-implemented ISO settings, so from the point of view of the raw data only one ISO is available, ISO 100. The higher ISO values are resulting from multiplication in a raw converter. Taking shots at the same combination of shutter speed and aperture, while varying ISO, you will see that the raw data does not change with ISO changes.

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Best regards,
Iliah Borg

Thanks Iliah. I suspected

Submitted by paulo on

Thanks Iliah. I suspected this might be true, but I had never seen it mentioned or discussed on any forums. I did take the same shot at various ISOs and noticed that RD gave the same histogram. That would seem to imply that I can't use RD to help select which bracket exposure is optimum (in ETTR terms). I find that this is not easy in SPP. I usually just adjust exposure, reduce sharpening and noise correction and do the rest of my editing in PS CS6.

Dear Paul,

Submitted by Iliah Borg on

Dear Paul,

But why would you need to use ISO 800 if it does not affect the raw levels?

However, if you wish you can simulate higher ISO settings in RawDigger, for ISO 200 set the Auto OE offset to -1EV, for ISO 400 - to -2EV, etc. This feature is available in RawDigger 1.03, see figure_08.png When combined with OvExp warning it allows to see what areas of the image are getting clipped.

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Best regards,
Iliah Borg

Thanks! I just downloaded

Submitted by paulo on

Thanks! I just downloaded Rels 1.03 and it does just what I need.
This helps to explain why the Sigma DP Merrills do not perform nearly as well at higher ISOs. At ISO 800 I'm underexposing the sensor by 3 stops. While for color I only use 100, for black and white ISO 800 produces good results, particularly because of the sensitivity of the blue channel.
Since the camera has such a poor histogram, I'm now bracketing more often. RawDigger now provides the best way to select which of the 3 brackets to keep, not just for ISO 100, but also for higher ISOs. For my Canon 5DII, I've loaded Magic Lantern firmware and verified (with RawDigger) that the camera's histogram is accurate.

paul

Dear Paolo,

Submitted by Iliah Borg on

Dear Paolo,

> For my Canon 5DII, I've loaded Magic Lantern firmware and verified (with RawDigger) that the camera's histogram is accurate.

Yes it sure seems so. The guys did a very good job.

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Best regards,
Iliah Borg

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