Your camera has three primary settings that are interconnected: aperture, shutter speed and ISO. You can't adjust one without adjusting the other, because each one is fundamental to the way your camera captures light, which is ultimately what it uses to make an exposure. Smaller apertures mean slower shutter speeds, or higher ISOs. Larger apertures mean faster shutter speeds, or lower ISOs. So how do you work out what settings to use?
How aperture, shutter speed and ISO are related
The reason that your camera's primary settings are so dependent on each other is simple mechanics. A small aperture allows less light to reach your camera's sensor, which means that you need a slower shutter speed. When the shutter is open for more time, more light can reach your camera's sensor through that small aperture opening, resulting in a correctly exposed image. The same is true for ISO, which increases your camera's sensitivity to light. If you're using a small aperture but you don't want to slow down your shutter speed, increasing your camera's ISO or light sensitivity will make it so you don't need to have that longer shutter speed.
So what that means for you is this: taking photographs is a process of compromise. As a photographer, you need to have a very good understanding of what will happen to your photograph whenever you change one of those settings - without that knowledge, you can't decide where you're going to make those compromises.
What are the compromises?
In general, the three primary settings on your camera have the following visual effects on your final image:
- Shutter speed. A fast shutter speed will freeze the action; a slow shutter speed will create motion blur.
- Aperture. A large aperture (small f-number) will create a shallow depth of field, a small aperture (large f-number) will create large depth of field.
- ISO. A small ISO will create an image with good digital clarity; a large ISO will create an image with lots of digital noise.
Take some time to really understand how these three primary settings affect your final image, and then you'll be in a good position to make an educated compromise when you find yourself in challenging situations.
This photographer had to make some compromises to get this shot: at ISO 6400, there is visible noise, but the high ISO allowed him to use a faster shutter speed of 1/50 and an f-stop of 4.5.That's fast enough for hand-holding the camera, and it gave him enough depth of field to keep the dog's nose and eyes in focus.33::262 by Flickr user WarzauWynn
Let's look at an example: You're taking a photograph of a meadow. In the distance are some pretty, snow-capped mountains and some trees, and in the foreground there's a little stream surrounded by some boulders. You'd like to keep as much of the scene as possible in focus, and you decide to use f/22. But when you switch to aperture priority you discover that your camera wants to use a shutter speed of 1/6th of a second. That's way too long, since you left your tripod back in the car. You might be able to hand-hold your camera at 1/30th of a second, but you have to set your aperture all the way down to f/10 if you want to use that shutter speed. You other alternative is to turn your ISO up to 2000, which will allow you to use f/22 at 1/30th, but then you may have some noise in your image. What to do?
That's where compromise comes in. As a photographer, you need to decide what's more important to you - depth of field, image quality (sans-noise) or your feet, because if you decide that you don't want to sacrifice depth of field or image quality, you may find yourself hiking back to your car to retrieve your tripod. If it's late in the day, though, you may lose the shot altogether. If you can't find something to prop your camera up on, you're going to need to make that choice - depth of field or image quality.
Here's another example: You're taking some photographs at a wedding reception, which is being held indoors in the late evening. There is no natural light coming in through the windows, and the artificial lights have been kept pretty low in order to create a certain ambiance in the room.
You want to get a shot of the bride and groom on the dance floor, but you also want to see the onlookers in the background. A closed aperture (large f-number) would bring everyone into focus, but the light is just too low to allow you to take that shot at your current ISO. Fortunately, your DSLR has good high ISO capabilities, so you turn up that setting to ISO 3200. Now you can set your f-stop to f/8 instead of f/4. You're still going to get some blur on the onlookers, but there won't be so much blur that you can't recognize them. The compromise, of course, is that you're going to get some digital noise in your image, which wouldn't have been there at the lower ISO. In this case, you may find that noise acceptable if it means you get the depth of field you want.
Now of course, photographers have many other tools they can use to get the right shot. You may want to use a bounced flash in a low-light situation, or the aforementioned tripod, depending on your tolerance for motion blur in the scene. Filters can also make it possible to turn down your shutter speed to motion-blur levels even on very bright days, but for now you should just stick with settings rather than mucking around with add-ons.
I know I'm always ending these articles with the word "experiment," but it really is the best way to get a feel for these concepts and for the way that your camera's settings are going to impact your final image. During your experiments, make sure you take multiple exposures of the same subject or setting using different aperture/shutter/ISO combinations. Comparing these different exposures on your monitor can really help you develop an eye for what your camera's settings do visually to your photographs. Don’t forget to pay attention to each image's EXIF settings in case you can't remember exactly what you did in each situation. Eventually you'll develop such a good eye for this stuff that you'll understand these things intuitively, and that's a good goal to shoot for.
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