Tip # 3 is: Learn to set your ISO
Over the last couple of days I have covered the use of Aperture Priority and Time Priority modes. Today we need to cover the use of the ISO setting on your camera. It’s the third leg of camera exposure and is probably far less understood than the other two legs, aperture and shutter speed. ISO is a term that comes from film photography and it represents the sensitivity of the camera sensor to light. Lower ISO settings yield the highest image quality, but need a lot of light. Higher ISO settings require less light but cause noise in an image. Noise is that grainy speckled quality you will see if you use the higher ISO settings.
There will be a button or dial on your camera to set ISO. The numbers will range from as low as 50 to as high as tens of thousands, even hundreds of thousands on some cameras. This is an incredible range made possible by camera advances. Each doubling of ISO doubles the amount of light your camera can capture with the same aperture and shutter speed. 200 ISO is twice as sensitive as 100 ISO. 400 ISO is twice as sensitive as 200 and so on.
With film, you rarely us a sensitivited of more than 800 ISO. Film had the same problem as digital cameras in that you got more and more grain as the ISO went up. Even 400 ISO was pretty grainy.
So why and when would you increase the ISO setting on your camera when it degrades image quality? The answer is simple: when you need more light. Let’s say you’re shooting a landscape and you want maximum depth of field, but it’s early morning or evening or a really cloudy day. You’ll stop down to f16 or even as much as f22 on your aperture, but your exposure time will double with each stop. At some point, the exposure time will get too long to hand hold; perhaps 1/5th or 1/10th of a second. Now you increase the ISO until the shutter speed is acceptable.
You can also open up the aperture and decrease the depth of field a bit. Some compromise between these might be the best choice. If you use a tripod and your subject is not moving, then you can leave the ISO at 100 and let the shutter speed get really slow. I’ve used shutter openings of 25 seconds or more on landscape images in low light. It won’t hurt image quality since the tripod holds the camera still.
Another example: You’re shooting a football game at night. You need a fast shutter speed to stop the motion of the players. As you increase shutter speed, the camera will open up the aperture of your lens until it’s wide open. If the shutter speed is still not fast enough, then you’ll need to increase the ISO until you can get the shutter speed you need.
With digital cameras the usual minimum ISO is 100. There are sometimes special settings to go as low as 50 ISO but I don’t really know why you would need this. At 100 ISO you will get the absolute best image quality your camera can produce. Virtually no noise will be visible in your images. It’s what I use pretty much all the time when I shoot with a tripod. At 400 to 800 ISO image quality is still good, but you will see subtle noise in the image, particularly if you zoom in to 100% magnification in your editing software. You probably won’t even see it at normal magnifications.
The amount of noise your camera makes at various ISO settings is a key performance indicator of a camera. It is rare for ISOs of 1600 or more to be clean enough for any fine art type of photography. Noise might be acceptable in certain types of images or if you just plain don’t care. I personally never use an ISO beyond 800 and I would use that only if I had no tripod and had to get the shot. I would try slowing the shutter speed and relying on a solid hand hold or the anti-shake capabilities of my lens before I would go beyond 400 ISO. Your needs will vary.
At the crazy high settings on the latest high quality cameras you can shoot hand held in virtual darkness. I know the ISOs go over 200,000. You can imagine the noise in such an image. I don’t think these high ISOs are really practical. It’s a way for camera manufacturers to compete for superiority.
That pretty much covers ISO. Remember that all three controls effect expose: aperture, shutter speed, and ISO. Adjust them according to your needs. Now you know how to do that. In practice, what I do is check a couple of exposures when I first start shooting in any particular light. If I see I don’t have enough light for what I intend, I’ll increase the ISO. I don’t mess with it again unless the light is decreasing, such as at sunset.
Aperture and shutter speed are meant to be adjusted frequently and are therefore easy to change on your camera with a simple wheel near the shutter button. ISO is a little less easy to set, usually requiring pressing a button and turning a dial. It’s not designed to be adjusted constantly but it’s not a big deal to change. You should definitely know how to change your ISO and be able to do it with a minimum of fuss. It’s not difficult to do.
I’ll mention briefly that some cameras have an auto ISO setting where you can adjust aperture and shutter speed and the camera will adjust the ISO. You can use a mode like this if you’re satisfied with your camera’s choices. Just pay attention to that ISO number to be sure your image will be clean.
Tomorrow we’ll look at how to determine whether you’re getting a correct exposure. You do that by looking at the histogram of your images. I’ll explain next time.