Auto ISO and long exposure times with SL2-S

[NightPix]

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I recently acquired an SL2-S which I am trying to set up for an astro shoot. I have the camera set Manual,  Auto ISO, max ISO 12,500. As soon as I start to increase the exposure beyond 1/2 sec the ISO starts to drop. By 5 sec exposure the ISO is down to 320. Typically, to get any kind of image of the Milky Way for example requires at least 10-15 sec exposure at ISOs closer to 1600-3200. This doesn’t happen on my Sonys, so clearly I’m missing something in the settings somewhere, but I can’t figure it out. How can I get true auto ISO for long exposures on the SL2-S (firmware 2.0)? Any advice will be appreciated!

[michali]

I suspect the problem is that you're using AUTO ISO & that will always try & compensate either up or down as you adjust the other camera settings.

I haven't done much astro photography, but the little bit I've done has been in full manual mode incl. setting ISO manually.

This image was shot on the SL2 & Sigma 14-24mm L @ 15mm,  f2.8 - ISO400 - 24sec. exposure (please click on image for better res.)

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[beewee]

I have a feeling the reason why ISO starts dropping beyond 1/2s exposure is because the light metering on the sensor has hit a limit. For astro, the way to go is to go full manual exposure including manual ISO.

[NightPix]

On 9/6/2021 at 5:24 PM, beewee said:

Yeah, the shots I shared were from the SL2-S. I’ve been shooting with the SL2-S almost exclusively since February, trying to learn how to get the most out of the camera and lenses.

That’s interesting you mention the comparison between the a7iii and SL2-S. At least according to photonstophotos, the a7iii has a 0.1 stop noise improvement compared to the SL2-S and based on the curves shown, it looks like this advantage is fairly consistent across all ISO settings which points to something more to the sensor itself. If Leica did buy the sensor from Sony as is much speculated, it’s possible the differences come down to the bayer filter material, all else being equal. This could explain why some people feel they prefer the colors of the SL2-S as compared to the Sony equivalent. There’s a tradeoff in getting more color fidelity in that the color filters that make up the bayer array on the sensor needs to reject more out-of-band light which also means there’s less energy being transferred to the photosites on the sensor. A weaker color filter will result in higher sensitivity and less color fidelity. Taken to the extreme, you end up with a monochrome sensor with no color fidelity but much higher light sensitivity which is why some really like the monochrome cameras for black and white.

 

2 hours ago, beewee said:

I have a feeling the reason why ISO starts dropping beyond 1/2s exposure is because the light metering on the sensor has hit a limit. For astro, the way to go is to go full manual exposure including manual ISO.

Thanks! You are both right in that everything works as expected in full manual mode on the SL2-S. I guess I got used to the Sony a7iii, where the auto-ISO works even in manual mode. It’s a nice feature because you can adjust exposure to deal with star trails and ISO just takes care of itself. Guess I’m getting too lazy.

[Stuart Richardson]

I think it is probably assuming that with long exposure the priority is for lower noise, rather than shorter exposure time. So once you start getting into times below the handheld threshold, it lowers the ISO back down to limit noise as it assumes you are using a tripod and doing a long exposure, most of which are better suited by lower ISO and longer time, versus increasing the ISO way up when the times are still long enough to require a tripod. Perhaps I am not explaining it well, but that might be how I would set up Auto ISO, if I were designing the system.

But honestly, I do think the solution is just to do it manually...