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I've seen on the CHDK page, that you can change the ISO values, and put lower ones. Now let's suppose I want to shoot a waterfall in the daytime, and I also want a long exposure time (e.g., 10 seconds).

I know I can use ND filters, but what about using a very low ISO (like 15)? Is there a limit for the lowest ISO in each camera?

In theory, the light sensitivity would be very low, so the shutter time will need to be longer. Don't name aperrture sizes, because I can only have f/11 as my minimum.

If all of this is true, the best of all is the reduction of noise!

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In my case, the lowest ISO available with the original firmware is 80. When I change it to a lower one with CHDK loaded, the photos are just the same as if they have been taken with ISO 80. Don't know why! – tomm89 Oct 20 '10 at 17:08
No matter how low you set the ISO, the absolute lower-level is when you start overflowing the sensor's photosites. At that point, there's no way to reduce the gain any further. – Fake Name Oct 25 at 3:33

5 Answers 5

up vote 8 down vote accepted

I don't really know much about CHDK, outside of that it lets you program P&S cameras. When it comes to ISO, though, the story is rather tricky. It may be possible to program ISO down to 15 with CHDK, but it may not really do you any good. There are "real" ISO settings, and there are "artificial" ISO settings. A real ISO setting would be something like ISO 100, which is usually the base ISO for many cameras (some have a base ISO of 200). This is an analog setting, adjusting the ISO to any of the "real" native settings will usually work by changing the analog readout of the sensor.

In contrast, an artificial ISO setting is either achieved by making other "behind the scenes" changes to other settings on the camera, or by "digitally enhancing" the nearest real ISO setting. In the case of "behind the scenes" ISO, rather than actually changing the ISO when choosing, say, ISO 50, the camera may actually reduce the exposure time at ISO 100 instead. In the case of "digitally enhancing" ISO, the camera may use ISO 100 when you choose ISO 50, then apply a digital filter to the resulting image to make it appear as though it was shot at ISO 50.

Generally speaking, using artificial ISO settings is undesirable. You generally don't really know what is going on behind the scenes, and if the camera is changing settings to make it appear as though you are really using ISO 50, it may adversely affect your image, preventing you from capturing the scene you really wanted to capture. Most of the time, it is best to use "real" ISO settings that change the analog readout from the sensor. If you need to lengthen your exposure times beyond what the native base ISO setting allows (usually ISO 100, sometimes ISO 200), then filtration is usually the best option.

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You also risk highlight clipping with these fake low ISOs. – Paul McMillan Oct 20 '10 at 6:35
Yep, essentially fake ISO settings overexpose (at the base ISO) and use a tone curve to bring the image down to the correct exposure, clipping highlights. Generally you don't really use IQ other than that, since there is more precision in the higher bits anyway (hence the movement to ETTR). If anything, the shadows may have less noise due to allowing more light in, unless their processing is really bad. – Eruditass Oct 20 '10 at 18:35

This doesn't add anything new beyond jrista's answer, but to summarize: probably what is happening when you select those ISOs is that the camera overexposes and then divides the numbers by 2 (or whatever). So you lose IQ fairly rapidly.

I would suggest looking up on CHDK website exactly how it achieves those low ISOs. I'm certainly curious, so do report back if you can! :)

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It is possible, but there is a limit to the lowest ISO your camera supports based on the sensitivity of the sensor.

Every 1/2 ISO level requires 2x the light. So ISO 15 would roughly be 2.5 stops darker compared to ISO 100.

So while lower ISO would help (in theory), lower-iso + an ND filter would give you a lot more options to work with.

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The best way would be to shoot with intervalometer to your lowest true ISO (probably 100), expose them right and then later blend them in average mode. CHDK has an average blend for the RAW's or just do them later in PC. The result will be pretty much of a long exposure shot.

BTW: Blending many shots cancels noise because it's arbitrary (random), the more shots blended the more noise gone.

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No, with the CHDK, you cannot use a lower ISO instead of an ND filter.

To quote from the CHDK 1.3.0 user manual:

... ISO override gives full control of ISO settings, but does not usually extend the range of available values.

If, however, you are using a camera that has extended ISO settings (i.e., settings that are not accomplished through hardware, but through image processing), you might be able to use the lower settings instead of an ND filter, and the noise will be less, but you will lose dynamic range. And you probably won't be able to go down far enough to do a 10-second exposure in the daytime, as the most common setting will simply give you an additional stop.

I have a 5DMkII with extended ISO settings. The native iso range is 100-6400. I have one "Low" extension (iso 50) and two "High" extensions (iso 12800 and 25600).

The high ISO settings are done by underexposing at iso 6400, and then the image is processed to "push" the exposure higher (like adjusting exposure in post). This will probably add more noise than actually having those settings on the sensor, and in the case of the 5DMkII can cause banding.

The low ISO setting is done by overexposing at iso 100, and then the image is processed to "pull" the exposure lower. Something you can also do yourself in post. This can reduce the noise in the dark areas, but will reduce the overall dynamic range of the image and possibly lose detail in the highlights.

On some Canon dSLRs, partial stops are also done by digital push/pull, which is why you sometimes hear the advice to use ISO multiples of 160 if you want to reduce noise (i.e., the -1/3EVs are pulled, the +1/3EVs are pushed), but this is dependent on your camera model. IIRC, Nikon dSLRs actually use gain across the sensor so there's no "stairstep".

There's no such thing as a free lunch. You have to pick your tradeoffs. The iso 50 setting on my 5DMkII is something I'd only use if that extra stop on the shutter speed makes losing the dynamic range worth it. Typically, it doesn't.

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