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Arriving at the best exposure for a photo is part science and part art. In Foundations of Photography: Exposure, Ben Long helps photographers expand their artistic options by giving them a deep understanding of shutter speed, aperture, ISO, and all other critical exposure practices. This course covers the basic exposure controls provided by all digital SLR cameras, as well as most advanced point-and-shoot models. Learn how to master a camera's metering modes, how to use exposure compensation and bracketing, and much more. By the end of the course, you'll know how to develop an "exposure strategy" that will allow you to effectively employ your exposure knowledge in any shooting situation.
The ability to change ISO on a shot-by -shot basis will get you out of a lot of situations where your shutter speed would otherwise be too slow for hand-held shooting. But you'll pay a noise penalty as you increase ISO, so you don't want it to go any higher than you have to. Therefore, before you go out shooting and wantonly raising your ISO, you want an idea of how much noise you'll suffer in your images as ISO increases, and you can easily figure this out by taking some test shots. Grab your camera and find a low-light situation--just go out at night.
Put your camera in program mode, set the ISO to its lowest setting-- usually 100 or 200--and shoot a scene. Now raise your ISO by one stop--that is, one doubling. So if you are at 100, you go to 200. Shoot the same scene, same framing. Now work your way through each of your full-stop ISO increments, so that's going to be 100, 200, 400, 800, 1600. Your camera might even go higher, 3200, 6400, 12,500. It might even go more than that.
Shoot the same shot at each one of the ISO's. Now, find the brighter situation. There will be times when you might raise ISO in brighter lights simply because you need some more aperture or shutter-speed latitude. Maybe you want to close down the aperture very far or shoot with an extremely fast shutter speed. So it's good to have an idea of how your camera will fair in brighter light as well. So find a bright scene and shoot an image at each one of your ISO settings. Now you are ready to take those images into your image editor and evaluate the noise.
Okay, these are our low-light, high ISO images. Let's see what we have got here. This was shot at ISO 200. Now at ISO 200 in this situation, I had to do a 20-second exposure, and at 20 seconds I have got a soft image. I was on a tripod, and if you look, you'll see that the horizon is sharp, so I wasn't getting camera shake, but the boats were bobbing around in the water and so in 20 seconds they are blurred out. And meanwhile fortunately, the earth continued to turn while I was shooting, so the moon and the stars are a little smeared out. But we are not worried about sharpness here; we are worried about noise. And at ISO 200, I have got very little noise to speak off; even if I zoom in to 100% here, there is no problem with noise at ISO 200.
This particular camera can also go down to ISO 100, which would have been a 40- second exposure, so it would have been even blurrier. So if 200 is safe, with ISO 100 certainly would be. Let's go on up to 100. Again, a 10-second exposure, so we are getting a little bit of motion shake, but overall still very, very clean. Not going to worry about noise with this image. Moving on to ISO 800. All right, now we are staring to get somewhere noise-wise, and it's not a great place that we are getting to. This image is a little bit chunkier. You can start to see some kind of bands happening here.
There is a dark band here, a light band here. If I zoom in, you can see that not only do we have an increase in speckling-- that is, an increase in luminance noise-- we also have these green and magenta patterns; that's chrominance noise. Now, normally we don't mind luminance noise because it just looks like film grain. It can actually be kind of attractive and atmospheric. Chrominance noise though, is--I don't know. It looks a little more digital. It's not that pretty, and is really hard to remove. That said, we are looking at this image at 100%, which means we are looking at individual pixels.
In an image with 10 mega pixels or 8 mega pixels or more, an individual pixel is tiny. If you were to print it, it would be invisible. So I am not going to worry too much about what's happening here with individual pixels. Nevertheless, there is enough noise here that I am probably going to want to do a test print of this image to see if 800 is actually usable, because these color splotches and these bands might actually be visible. So let's go on up now to ISO 1600, and now things are really starting to get kind of chunky. Here's 800, 1600, 800, 1600.
The bands are becoming more visible. You can see lots of colored patterns here. We are on the verge of possibly an image that's getting frustratingly noisy. But again, evaluating on-screen it's difficult to tell how things are going to show up in print. If your goal is to print 4 x 6, this image might be fine. This is a case where we are going to need to do a test print. Moving on to 3200, and now we are into full-on noise land. We have lost detail here, and here. I know there wasn't a lot of detail on the moon, but the image is just starting to break up, and it's starting to become dominated by these magenta pixels.
This is bad chrominance noise, bad luminance noise too. So 3200, this noise is probably going to show up in print. So 3200 is probably beyond where we want to go with this camera--at least in very low light like this. Again, I would want to do prints at my chosen output size and evaluate my noise there also, and try to come up with an upper limit. What I am going to say about this camera right now is 1600 is definitely a usable ISO. But if you can, you want to avoid going beyond 800 on this camera. Now this is purely just for low-light situations. I would want to do this same test in brighter light.
You may think, well, why would I be cranking up the ISO in bright light? Well, as we have discussed, there will be times when you want to buy yourself more shutter speed or aperture latitude, and so you'll increase the ISO. So do these experiments with you camera and see what you can find. Remember, don't get hung up on individual pixels at 100%. Take some time to do some prints. Or evaluate your image in whatever way you are ultimately going to be outputting it. If that's a 640 x 480 image that you're going to e-mail to someone, then resize some images to that size and see how your noise holds up, and try to figure out what your upper usable noise limit is on your camera.
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