Algorithm to set ISO and shutter speed for low light

[Einst_Stein]

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To simplified the question, assume it is S3 on tripods shooting in low light: Aperture is fixed (say, already the widest), the object is still. The choice is to set lower shutter speed or higher ISO.

What is the best practice to decide when to lower the shutter speed and when to raise ISO?

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[Carlos cruz]

I usually go for lowest iso possible, preferably native/base iso. As to shutter speed don’t know how ‘still’ is the object. If it’s still nature no wind any shutter speed is ok. 

[Einst_Stein]

13 minutes ago, Carlos cruz said:

I usually go for lowest iso possible, preferably native/base iso. As to shutter speed don’t know how ‘still’ is the object. If it’s still nature no wind any shutter speed is ok. 

Do you mean highest ISO? or lowest shutter speed?

Edited August 20 by Einst_Stein

[Carlos cruz]

2 minutes ago, Einst_Stein said:

Do you mean highest ISO? or lowest shutter speed?

If shutter speed is no issue I’d go with lowest possible iso setting. Don’t know what’s base/native iso for S3 but I am guessing something between 200-500, and since you have your camera on tripod there’s not much difference between 1/30 and few seconds.  
 

[Einst_Stein]

I don't know what tripod do you use, but S3 on my tripod and "still" in real world has never made shutter speed none issue yet. 

[BernardC]

6 hours ago, Einst_Stein said:

What is the best practice to decide when to lower the shutter speed and when to raise ISO?

You need to find the highest EI that you are comfortable with. That number will be different for different cameras, and for different photographic styles. For instance, if you find that the S3 is clean until EI 1600 (I haven't used an S3, don't trust that number), then set the camera to 1600 and adjust the exposure time accordingly.

[Einst_Stein]

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4 hours ago, BernardC said:

You need to find the highest EI that you are comfortable with. That number will be different for different cameras, and for different photographic styles. For instance, if you find that the S3 is clean until EI 1600 (I haven't used an S3, don't trust that number), then set the camera to 1600 and adjust the exposure time accordingly.

You mean, after ISO 1600 or whatever your highest tolerable ISO, you'd let it go with slower shutter and tolerate the potential bluring instead of raising ISO? 

Edited August 20 by Einst_Stein

[BernardC]

5 minutes ago, Einst_Stein said:

You mean, after ISO 1600 or whatever your highest tolerable ISO, you'd let it go with slower shutter and tolerate the potential bluring instead of raising ISO? 

Exactly. 

[Carlos cruz]

5 hours ago, Einst_Stein said:

I don't know what tripod do you use, but S3 on my tripod and "still" in real world has never made shutter speed none issue yet. 

Any reasonable tripod should do, I am using an old tilt-all it’s good enough for me. Just some hygiene for long exposure shots -no walking, no trams/trains within earshot, when possible raise the mirror before exposing, use cable release. 

[Stuart Richardson]

I would say that in most cases it is better to have a lower ISO. A very slight blur, but with the best color, DR and lack of noise is usually better than a noisy, compressed, desaturated file that is tack sharp. With a tripod, you will likely find better results with extending the shutter time into a few seconds, rather than sticking at 1/4th or 1/15th. With an exposure of 2-4 seconds, the time of the blur will be negligible in comparison to the exposure, and therefore it will not register. Often photos with exposure time of several seconds can be sharper than shorter exposures because any incidental vibration is tuned out. Of course, this does not apply to things like high winds, or jolting the tripod, but mirror-slap is a non-issue in longer exposures. So I would say instead of shooting wide open for 1/4th or 1/15th etc, stop down to f8 and extend the exposure to a few seconds, and you will get significantly sharper results -- both because the lens will be at a more optimal aperture with more DOF, and because the incidental vibration is not a factor. 

[Einst_Stein]

With M I usually take the opposite approach, I leave ISO to raise freely while tightly control the shutter speed, of course this is due to the hand held. That was a big switch when I switched from film and SLR. 

With S, I was treating it like a bigger M, even shooting outdoors with tripods, the still landscape has never been real still. My mindset tells me when slow speed vibration is a concern, raise aperture first, then deal with shutter speed or ISO. I end up at short second shutter speed, which might make tripod and camera instability more visible. 

I guess be brave in long exposure should be the right direction, and, of course, smaller aperture. 

[Joakim]

Aperture and shutter speed determines how much light hits the sensor, ISO affects the light when it’s read from the sensor so I try to decrease the shutter speed (longer exposure) as much as possible before increasing ISO to get the best image quality.

Edited August 26 by Joakim

[Einst_Stein]

1 hour ago, Joakim said:

Aperture and shutter speed determines how much light hits the sensor, ISO affects the light when it’s read from the sensor so I try to decrease the shutter speed (longer exposure) as much as possible before increasing ISO to get the best image quality.

For perfect stable shots this should be the right strategy.

[davidmknoble]

At the end of the day, the minimum shutter speed necessary to get the motion stopped (or blurred) the way you want, defines the ISO to use.  If that minimum shutter speed allows the minimum ISO, then there is a the best dynamic range.

I don’t think there is any other way to determine what you want for maximum “image quality” given the original post parameters.