Modern digital SLR cameras are capable of producing images with
very fine grain (noise) at high ISO settings. This is one area
where they are far superior to a high-megapixel digital compact
with a tiny sensor. However, as I said they are "capable"
- this is not the same thing as they always do! There is a definite
technique to be followed, and it is very rarely (if ever) written
1. It reduces the chances of camera shake
First, what we mean by ISO and why it is important ?
Film is sold according to it's ISO (previously known as ASA) speed
rating. The higher the ISO rating, the more sensitive the emulsion
is to light - so for the same aperture setting, a higher shutter-speed
results. For every doubling of ISO, the available shutter-speed
also doubles. In other words, if working at 200 ISO and a hypothetical
shutter-speed of say 1/125sec, then (for the same aperture setting)
a shutter speed of 1/250 sec will be available at 400 ISO and
Here are some reasons for wanting a higher shutter speed:
2. It reduces the chances of subject motion blur
3. It permits smaller apertures to be used if desired (for increased
depth of field)
4. It can reduce the necessity for carrying a tripod where this
is not practical
5. It helps freeze motion in action images
6. It permits shooting in lower light levels
7. There is less need for "fast" lenses (lenses with large
maximum apertures) which are more expensive and bulkier to carry
than their "slower" equivalents.
High shutter speeds are terrifically desirable when working at high
magnification - such as with long telephoto lenses or in extreme
close-up. However, there's no such thing as a free lunch and the
downside is increased grain. Grain makes images look coarse and
So how does this translate to digital cameras ? Digital camera
sensors are sensitive to light - just as film is, and the more
the signal from the individual pixels on the sensor is amplified,
the more sensitive the camera becomes to light. The sensor is
grainless - that is a property of film emulsion particles, but
as the digital signal is amplified, digital noise starts to creep
into images and it is this which is comparable to film grain.
The camera manufacturers have kept things simple for us for once
by making digital camera sensitivity settings equivalent to film
emulsion sensitivity at each ISO level.
If you would like to read more about noise
and sensors , this is an excellent article by Vincent Bockaert.
Canon cameras use a CMOS sensor, which produces lower noise at
higher ISO settings than the CCD sensors employed by most other
manufacturers. I feel that this is a huge benefit - and one reason
that I shoot with Canon.
Technique for working at high ISO.
If I am shooting static subjects such as plants in good light
I will usually shoot at 200 ISO. This produces very low-noise
images of maximum quality. Although my camera goes down to 100
ISO, I never usually bother to use this setting as it offers negligible
improvement in noise over 200 ISO and you sacrifice some shutter
speed that would be better employed fighting against camera shake
and motion blur.
However, subjects such as small birds move constantly - making
micro movements that even shutter speeds as high as 1/250 sec
can result in image softening. I am therefore experimenting with
a technique which is a bit unconventional - is to use high ISO
settings, even in good light for the gains to be made in very
high shutter speed.
I rarely used to shoot above 200 ISO as I found the level of noise
objectionable at 400 and above .However, I didn't know then what
I am going to tell you now !
Below is an image of a great-spotted woodpecker taken with a
Canon 1D mkII, a 500mm f4 lens plus 1.4x converter (700mm). It
was taken on a very dull February day.