Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: Jack Flesher on October 19, 2004, 01:08:46 pm
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[font color=\'#000000\']It is the light-sensitivity rating for the sensor, much like it is used for film.
As for the best setting, this will vary between cameras. Generally speaking, as you increase ISO you get more noise from the sensor. Hence, most cameras will offer their best performance at the chip's "native" or base ISO since this generates the lowest noise.
However, some newer cameras, like the 1D MKII, actually generate slightly better color and slightly more detail at slightly higher ISO than the base ISO... So there becomes a bit of a tradeoff in finding which setting gives you the best color, most detail, and lowest noise.
Cheers,
Jack[/font]
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[font color=\'#000000\']You are adjusting the extent of signal amplification applied after the data are read from the photosites. The ability of each photosensitive site (to collect photons and create the data in the first place) is fixed by its design. Because post facto amplification amplifies noise as well as signal, the "base" ISO is usually the one where the output has the best signal-to-noise ratio. Jack's comment relates to the fact that if noise is real low (as it is in the 1DII + DigicII chip combo, for example) detail and color fidelity may not be optimal at the same "ISO" where noise is lowest...
Andy[/font]
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[font color=\'#000000\']That makes perfect sense. It should be called gain.[/font]
[font color=\'#000000\']But we have traditions to maintain.... :)
Same with bulb, unsharp mask, ....[/font]
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[font color=\'#000000\']What exactly is the ISO measurement of in regards to digital photography? Is it a setting that is generally best left in auto or is there a rule such as - use the lowest possible setting for correct exposure?[/font]
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[font color=\'#000000\']OK, but what *is* it? If it's a rating of the sensor, does that mean the sensor's ability is fixed? If so, what are you actually adjusting?[/font]
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[font color=\'#000000\']That makes perfect sense. It should be called gain.[/font]
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[font color=\'#000000\']What is often called "ISO" for digital cameras should be called "Exposure Index" (EI). It is not governed by any ISO standard. (Some trivia: a proper ISO film speed specification always combines both the ASA and DIN numbers, like "ISO 100/21", so the numbers used these days for digital are better described as "speeds in ASA units".)
This exposure index determines the combinations of aperture ratio and shutter speed that will make the subject that the light meter sees come out as a "mid-tone", after appropriate processing in the camera.
If we want to compare to real ISO film speeds, we have to look at things like the shadow noise levels, which should rise above the background noise about four stops below the mid-tones, so two things should be noted.
a) many DSLR's exceed this ISO standard for shadow handling at their lower EI settings, so that the closest thing to a true ISO speed might be a middle setting, like 400 or 800.
at least one DSLR, the Fuji S2, applies different degrees of noise filtering as the ISO setting is increased, so that the shadow noise levels do not increase nearly as much as with some other cameras. Instead, resolution probably decreases, much as it does with films of higher speeds. It seems that some Canon DSLRs do this too, though they have not started it so clearly as Fuji apparently did in an interview with Phil Askey of DPReview. In fact, the rather slow rise in measured noise levels of JPEG output as "ISO" setting increases on most digital cameras suggests that something like this is quite common: noise levels do not increase as fast as rough physics suggests it should.
With a strategy like this, it is conceivable that some or all of the ISO settings of a digital camera could roughly satisfy the ISO standard for shadow handling in films, at the cost of decreased resolution at higher speeds.[/font]