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As you should know by now, as you increase ISO, you also increase the amount of noise in your image. This means you can't just necessarily crank the ISO up until you get your shutter speed down to where you want it. If you do, you may end up with an unacceptable amount of ugly noise in your image. Fortunately, these days, most new cameras offer very good noise response. On my SLR, for example, I can go all the way up to ISO 400 with no perceptible increase in noise, and I regularly shoot all the way up to ISO 3200 without worrying about unacceptable noise in my final shots.
Beyond that though, I find that my images simply get too grainy and noisy, so I only use ISOs higher than 3200 when I absolutely have no other choice, and I alter my expectations to assume that the images that I get will be compromised by noise. If all I am trying to do is document something then that's not a problem, but if I am going for a high-quality fine-art level of output, then I know that I will probably be disappointed above ISO 3200. Before you go out shooting, therefore, it's a good idea to have an understanding of how high you can push ISO on your specific camera before you get a noise level that's not acceptable for the work that you are doing, and that's something you can easily determine with a few simple tests.
You can do this anywhere in your house. I had some flowers delivered. I've got them set up here just in a normal room. I have got kind of a moderate amount of lighting. What I am going to do is just take some pictures of these flowers at different ISO settings. I am going to take the same frame at all of my ISO settings, actually. So I am starting out here at ISO 100. I have got my camera set at aperture priority because I want as much depth of field as I can get. I dialed into about f/9. That's going to give me a pretty slow shutter speed at this light level.
So I am going to take that shot. It's about 3 seconds. And I am just going to crank my ISO up to 200, the next stop, and I am going to shoot the same shot again. So nothing fancy about my setup here. I am just on a tripod and again, I am just chosen an object. It doesn't have to be flowers, although these are very nice. And I am just trying to get shots at all my ISOs so that I can go look at them later and see how bad the noise gets. So here I am up to ISO 400. That's down to half a second. When I get done with this, what I am going to do is lower the light in the room.
You are going to be shooting in lots of different levels of low light. Some are going to be levels that are actually pretty reasonable for your own eyes, a little rougher for your camera; others are going to be darker than what your eyes can see, but your camera is going to be able to pick up stuff. So you want to, after you shoot this set, turn down the light in the scene and do the exact same set again. Same shots, same ISO settings, and run through all of those combinations. When you get all done, then it's time to assess the results.
This next bits pretty easy; all you got to do is get your images into your computer and look at them. So to do that, you need an image browser or an image editor that will let you see your images at 100%, and that will provide you with an EXIF display so that you can see exactly what ISO you were using with each shot. Now you might also need a printer, because to accurately assess your camera's noise response, you want to be evaluating your images in whatever form that you will ultimately be outputting them in. The thing is, an image from your camera has millions of pixels, megapixels, and if you were to take any one individual pixel and print it out, it would be invisible.
They are so small by themselves that they're almost irrelevant. So it doesn't make a lot of sense to, on my monitor, zoom into 100% and really worry about each and every pixel. What I want to do is get my test images out in the way that I will ultimately be delivering them. If that's print, then I want to print them out at the size that I expect to deliver them. If I'm going to be sizing them to a particular size and posting them on the web, then I want to look at them that way. That will give me a more accurate assessment of whether noise is really a problem in the way that I intend to output my pictures.
As far as doing the actual assessment, I tend to just open my images up, print them, or get them the sized the way that I want onscreen, and then look at each image that I shot at each ISO. Noise is a really subjective thing. What is acceptable to you may not be to somebody else. It's up to you to decide if an image is too noisy or not. Look for luminance noise--those are the bright speckly kinds of noise; chrominance noise, those are the color swatches; and stuck pixel noise, those are going to be individual little white spots.
If you decide that at a particular ISO, boy, that's just too crunchy and noisy, I don't like it, then you know that that's not an ISO that you want to go to when you're out in the field. So try to identify that upper level of ISO that you're comfortable shooting with.
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