How to use the ISO setting option?

[Ray]

All digital images can show objectionable noise, whatever the ISO, if the scene has a higher dynamice range than the sensor. Usually, raising the black point gets rid of it. Black shadows can be nice. But sometimes you might want that detail in the shadows, in which case you need to take 2 different exposures, one for the shadows and one for the highlights, and blend them in PS.

Raising ISO will always result in more noise in the shadows and lower mid-tones, for the same scene. In other words, the signal-to-noise ratio is worse, the higher the ISO. For this reason, there seems little point in using a higher ISO than necessary, unless you want to deliberately create a noisy image for some artistic effect.

The usual reasons for using a higher ISO setting is that it permits use of a higher shutter speed at the same aperture (useful if you are using a long telephoto lens or photographing a quick moving subject which you want to freeze), or it permits use of a smaller aperture at the same shutter speed when increased depth of field is needed.

Sometimes there is a conflict between less noise on the one hand and a potentially sharper image on the other hand. When I got my first DSLR (the D60) I used to err on the side of less noise, until I realised this was crazy. I now prefer sharper images   .

[Jonathan Wienke]

"ISO sensitivity is done by changing the polarization of the sensor cells (CCD or CMOS), and or amplification of the analog signal."

ISO adjustment is done by changing the gain of the analog signal from the sensor photosite before digitization or by math tricks after digitization. Polarization has absolutely nothing to do with ISO. Some general rules of ISO selection:

1. It is always preferable to increase ISO to achieve ideal exposure than to underexpose and increase exposure levels in post-processing.

2. If increasing exposure time is an acceptable option, (i.e. it will not cause an unacceptable degree of motion blur) decreasing the ISO setting and increasing exposure time to achieve ideal exposure will yield better results than the equivalent higher ISO exposure. Use your camera's Bulb mode if necessary. The only exception to this principle is when the lower ISO setting captures less dynamic range (like the ISO 50 setting on the 1Ds) and clipping would result.

Digital Exposure And Metering Strategies

[1hart1]

In a past topic we arrrived at a subject that I felt I need help with. The ISO setting on digital cameras. A poster replied:

"ISO sensitivity is done by changing the polarization of the sensor cells (CCD or CMOS), and or amplification of the analog signal."

This is interesting as it seems to change the band width of the sensor.
Just how have you found this useful?
When do you change the ISO settings?
What conditions have you found changing the ISO setting a benifit?

Thanks all,
Stephen

[BJL]

"ISO sensitivity is done by changing the polarization of the sensor cells (CCD or CMOS), and or amplification of the analog signal."

I have not heard of the polarization change idea; the two methods I know of are
a) adjusting the amplification applied to the analog output of the sensor before the signal is converted to digital
 compensating up in the digital domain within the camera

The latter gains nothing that could not be achieved later in post-processing by "digital exposure compensation" so is just a convenience for getting ready to use JPEGs ina low light situation. Ihave heard it aid that it is used for soe of teh very high ISO settings, and one gues is that the special high ISO "boost" settings of digital cameras are often doing this.

Option (a) does potentially reduce the effects of noise introduced in the analog phase and A/D conversion, so might be better than digital exposure compensation later.

Naively, I am happy to use somewhat higher than minimum ISO settings on my E-1 DSLR (but mostly avoiding the extreme "boost" ones") in situations where the need to reduce motion blur by increasing shutter speed is evena bit significant. Many DSLRs have no significant noise on prints of normal contrast scenes at up to several stops above their minimum ISO setting, so I often see almost no practical penalty in going a bit above minimum ISO.