DIGITAL CORNER
By Bob Dean
The changing face of ISO!
The newest digital cameras are touting
ISO numbers that stretch the imagination. Back in film days the very idea of shooting
a film with an ISO (or ASA for the seasoned photographers in the club) exceeding
10,000 was unheard of. Now a good many digital SLRS exceed that by staggering
amounts. How can we properly utilize this new tool? Recent magazine articles have
addressed the use of ISO as a creative tool in ways we've not seen before. The
ability to get decent saturation in very low light is the first thing that comes
to mind, secondly we can crank up shutter speed and freeze action with reasonable
depth of field in ways we only dreamed of in years past. OK, what are the drawbacks
to this great tool? As with everything in this field, there are tradeoffs. As
I have mentioned before in articles and in the classes I teach, ISO does not change
the sensitivity of the sensor, it changes the overall "system gain".
Let's
get into the details! The digital sensor, by its very design, has a system gain
which simply put defines the amount of electrical current generated by a given
amount of light striking each pixel (photons to electrons). The engineers designing
these sensors call this parameter "native sensitivity". The electrical
circuits that takes these extremely tiny signals and boosts them to a usable level
are called amplifiers. The signal coming from the amplifier (at this point it's
an analog signal, related more or less linearly to the amount of light striking
the sensor) is routed to another circuit called an analog to digital converter.
This circuit takes the signal and converts it into a digital form. The number
of bits
that you see referenced in a lot of literature comes from the design
of this circuit. The mor ebits, teh more information (light level) conveyed to
the processor. The ISO setting DSLRs controls the gain of the amplifier. If the
ISO is set to the "native sensitivity", the camera is pretty close in
sensitivity to the equivalent ISO of film. This is essential the optimum setting
for gain. As ISO is increased, the gain goes up and along with it, any noise in
the input signal as well as noise generated by the amplifier itself. If the ISO
is reduced below the native sensitivity, the signal gets attenuated a bit and
some information is lost, impacting the saturation of the image. The software
in the image processor can do quite a bit to alleviate the negative impacts of
noise and to some degree the reduced information, so deviating from
native
sensitivity is manageable.
So how can we translate this
into something usable? Well first you need to find the ISO for your camera that
corresponds to the native sensitivity. You can use that as a starting point for
your shooting and increase or decrease as the situation dictates, but at least
you'll know that starting point. How to find out? Good question. It took a while
but the "conventional wisdom is that Canon native sensitivity is at ISO 100
while Nikon tends to be 200. I'll bet some internet searching can produce better
as well as conflicting numbers but such is life in the digital age. I searched
on "native sensitivity + Canon + 7D and got 342 hits on Google. Many had
additional links and a good many had wrong information!
Set your camera to its native value, move the ISO up and down, and see for yourself the variability in images. Hint- You probably won't see a lot until you get to the extremes. Once you know, you can then use ISO as a creative control with more understanding.