Why do they make DSLRs' ISO settings go so high? I own three different Nikon cameras and every one of them, once you pass a pathetic 320 ISO, the noise in my photos is lousy and — all 3 cameras cost over a thousand pounds each.
The short answer is: because it's useful. I'd much rather have to deal with a bit of noise at 3200 and to still get my shot, rather than not getting my shot at all.
The real question is: what is the maximum amount of noise you can deal with for your intended photo and, given that, can you still get your shot?
ISO 320 is really low for any camera made in the last 5 years (personally, I considered ISO 800 on a 20D the absolute highest I could go - and that was back in 2006.)
I'm wondering if you are underexposing your shots and trying to increase the exposure in post. This will exacerbate the noise. Can you add some more info about your shooting to your question, including a sample shot?
On DSLRs, ISO is essentially done by increasing the gain on the sensor, or if you will, amplifying the signal. In some respects, this is not unlike turning the volume up on a speaker: nearly all speakers do well enough in 'normal' ranges, but when you turn up the volume, cheaper speakers begin to distort and color the sound. Better speakers simply get louder without distortion. Even better speakers can fill an arena with sound, even when full of people, and still not distort.
This is similar to camera sensors: most do well enough when there is plenty of light and no need to amplify beyond the 'native' ISO of the sensor. But when challenged, such as in low light scenes, they quickly exhibit noise, effectively making them useless. However, much better (and more expensive) cameras, with better sensors, exhibit little noise in such conditions.
As an example, my earlier Canon 40D was useful to about 1000 ISO, but beyond that the noise was too much to provide really marketable images. It was an expensive camera when purchased new. My much newer Canon 5DMkIII has a much better sensor, and happily yields good images a ISOs far beyond 3200+ (and often as far as 12,000). It is a more expensive camera, but a worthwhile expense in my opinion.
Why do cameras have high ISO settings even though they produce less than useful results at those ISOs? An image is really in the eye of the beholder, and I suppose its the same reason cars have speedometers marked with top speeds beyond the capability of the car or why some guitar amps go to 11.
The first picture, taken by Niepce in 1829 required a shutter speed of 8 hours. Since then, light sensitive materials have gained incredible sensitivity. Now we can freeze a hummingbird’s wing in flight. The photo industry designated The International Standards Organization of Geneva Switzerland (ISO) as the official regulator to devise testing method and assign sensitivity to light values.
Should we desire to image in low light situations or when occasions dictate a super high shutter we up the ISO. High ISO’s come with a price. If we are imaging with film, we get “grain”. This is a lack of uniformity called “grain”. It is caused by a clumping of the light sensitive silver salts we have piled on to get a speed gain.
Digital cameras image work by swapping light energy for an electronic signal. Because the amount of light playing on the imaging chip is incredibly weak, the resulting charge must be amplified to get a decent picture. The amount amplification applied remains moderate when the ISO is low. When we reset the ISO to a higher value the software strengthens the amplification. Sorry to report that the iamge signal is intermixed with both a good and a corrupt signal. When the amplification is upped, both the good and the bad signal are boosted. This affords an opportunity for the bad signal to show itself. We call this image degration static or in photo jargon “noise”. Noise is seen as a granularity akin to grain seen in film.
Photo engineers dream about making systems that produce a faithful image. So far no cigar! Imaging chips evolve. Maybe the next generation will be free of “noise”. Maybe you can study and take up the quest. Maybe the faithful image is right around the corner.
High ISO settings with lots of noise can be useful in some situations. The obvious case is where you get a noisy picture instead of no picture.
However, my main point here is that noise can be traded off with resolution. A high ISO picture at the full 12 Mpix may be quite noisy. Filter that same picture down to 1 Mpix, and you may not notice it at all. 1 Mpix pictures can be quite useful. You usually don't even need that if the purpose is to embed the picture in a web page. Note that 1000 x 1000 is 1 Mpix. That's bigger than you need for a web page most of the time.
So high ISO can be quite useful when you only need small pictures. In that case, it gives you better f-stop and shutter speed combinations than you would otherwise not have access to.
Take a look at tests I've done to see the noise at various ISO settings of my camera. While there is a lot of noise at high ISO in the original picture, the versions filtered down to 700 kpix are all pretty much the same.