Steps

1. Set your camera to aperture priority mode. This differs from maker to maker (read your manual), but here are some hints for a few common types of digital camera:
   • Most Nikon digital SLRs: You have a mode dial. Turn this to "A". Once you've done this, spinning your front control dial (on your right-hand grip, very close to the power button) will adjust your aperture. (If you have a camera without a front control dial, then your rear control dial will adjust your aperture instead.)
     
     
     

     The mode dial on a Nikon camera (actually an F55 film camera), set to A.
     
   • High-end Nikon digital SLRs: Hold down the "MODE" dial while turning the rear control dial until you see "A" in your top LCD. Your front control dial will then adjust your aperture.
     
      
     The Nikon D2H's LCD reading "A", meaning it's in aperture-priority mode.
   • Nearly all Canon SLRs (and some Canon point-and-shoots): Turn your mode dial to "Av". Your main control dial (next to the shutter button) will then adjust your aperture.
     
     

     The mode dial on a Canon digital SLR, set to "Av".
   • Many point-and-shoot digital cameras do have an aperture-priority mode, but you may have to work through menus to activate and adjust it. This is just a simple way to tell the computer and parts already present to work together, which shouldn't cost much extra, but shows the manufacturer has taken care to help you make the most of even an inexpensive camera.
2. Memorise some basic terminology. You'll need it to make sense of the rest of this article:
   • f/ numbers are your apertures. This is represented as a fraction of your lens' focal length. A smaller aperture is a larger f/number; f/32 is a much smaller aperture than f/5.6.
     
     

     f/ numbers are a ratio of the diameter of your aperture compared to your lens. Hence, a larger number is a smaller aperture.
   • Stopping down means to use a smaller aperture than your lens' largest (smallest f/ number).
   • Wide open is the opposite of being stopped down.
   • Depth of field is, formally, "the range of object distances within which objects are imaged with acceptable sharpness". There is only one distance at which objects will be in perfect focus; the depth of field covers the subject-matter that is outside your intended area of perfect focus, but that are still close enough to being in focus so that to the viewer, all that gets captured within the depth of field appears deliberately within focus.
3. Test your lens. All lenses are different and are better shot at different apertures for optimal performance. Get out and shoot something with lots of fine texture at different apertures and compare the shots to figure out how your lens behaves at various apertures. Here are some hints as to what to look for:
   • Nearly all lenses have lower contrast and are less sharp at their widest aperture, especially towards the corners of your image. This is especially true on 35mm and digital camera lenses. On sharpness, this is a totally separate issue from depth of field; this will happen even with a flat subject. Consequently, if you're going to have detail in the corners of your pictures that you want to keep sharp, then you'll want to use a smaller aperture. For flat subjects, f/8 is typically the sharpest aperture.
   • Most lenses will have some noticeable amount of light fall-off wide open. Light falloff is where the edges of the picture are slightly darker than the centre of the picture. This can be a good thing for many photographs, especially portraits; it draws attention towards the centre of the photograph, which is why many people add falloff in post production. But it's still good to know what you're getting. Falloff is usually invisible after about f/8.
     
     

     Falloff, which is darkening towards the edges of a picture, is common when shooting lenses wide-open.
   • All lenses will be softer across the frame if you stop down far enough. This is an inherent physical limitation of camera lenses; forcing light through a smaller hole causes light rays to interfere with each other.
     
     

     The left shot (cropped from a boring test shot at f/8) is sharper than the second shot (shot at f/32), if you look too hard. The softening is caused by diffraction at f/32.
   • Zoom lenses can vary depending on how far in or out they are zoomed. Test for the above things at a few different zoom settings.
4. Get out and shoot.
5. Control your depth of field. It's as simple as this: a smaller aperture means more depth of field, a larger aperture means less. A larger aperture also means more background blur (which is a related, but not identical, issue to depth of field). Here are some examples:
   • Use a small aperture to force more depth of field.
     
     

     The left shot was done at f/2, the right shot was made at f/16. Note how the letters furthest from the camera are brought into focus by stopping down.
   • Remember that depth of field becomes shallower the closer you get. If you're doing macro photography, for example, you might want to stop down far more than you would for a landscape. Insect photographers often go way down to f/16 or smaller, and have to nuke their subjects with lots of artificial lighting.
     
     

     Depth of field is shallower the closer you are to your subject. This shot was made at f/6.3, and depth of field is perhaps a millimeter or two.
   • 

     Large apertures force backgrounds to be thrown out of focus; this is great for portraits, as in this shot made at f/2.
     Use a large aperture to force a shallow depth of field. This is great for portraits (much better than the silly automatic portrait scene modes), for example; use the largest aperture you have, lock your focus on the eyes, recompose and you'll find the background is thrown out of focus and is, consequently, made less distracting.
     
     Remember that opening the aperture like this will cause faster shutter speeds to be chosen. In bright daylight, make sure you aren't causing your camera to max out its fastest shutter speed (typically 1/4000 on digital SLRs). Keep your ISO low to avoid this.
   • Remember that you won't see any of this through your viewfinder (or on your screen as you're composing. Modern cameras meter with the lens at its widest aperture, and only stop down the lens to its selected aperture at the moment of exposure. What's more, viewfinders on modern digital SLRs don't even show the true depth of field even if you're shooting the lens wide-open with faster lenses (meaning ones with a larger maximum aperture).
     
     Many SLRs have a depth-of-field preview button on the front of the camera. If you've ever hit a button on your camera and then wondered why your viewfinder went dark, that's the one. Unfortunately, because it darkens the viewfinder, it's very difficult to gauge your depth of field this way (though it might give you some indication of how far out-of-focus distant backgrounds are, which is not the same thing). A better option on digital cameras is to simply take the picture, then play it back and zoom in on your LCD to see if the background is adequately sharp (or blurred) enough.
6. Control your shutter speeds. Using a larger aperture means that you can use faster shutter speeds (or a lower ISO with the same shutter speed); conversely, a smaller aperture will force a longer shutter speed, or require you to kick up your ISO to grab the same one. This has several practical applications:
   • Grab the fastest shutter speed you can. If, for example, you're hand-holding your camera or trying to freeze motion in poor light, set your aperture to the largest one your lens has. Crank up the ISO as far as you dare, too (exactly how high is tolerably noisy is something with which you'll have to experiment for yourself). The camera will then grab the fastest shutter speed you can use.
   • Grab the slowest shutter speed you can. This is great if, for example, you want to blur motion (think of those dreamy flowing water pictures). Set your ISO to its lowest setting, stop down to f/16 (or smaller, if you're willing to defy the laws of physics, or at least if you're fine with diffraction kicking in). The camera will then grab the longest shutter speed that the situation permits (though typically modern cameras won't time out exposures longer than 30 seconds).
7. 7
   Shoot for sharpness. As mentioned earlier, nearly all lenses are sharpest stopped down a little. If you've made your own tests as suggested, then use this aperture for any shot for which you think it'll give you an adequate depth of field and shutter speeds. For those of you shooting stills from a tripod, then use this aperture all the time.
   
   If you're too lazy to have made your own tests (and really, shooting test subjects like walls is boring), then there's plenty of wisdom embodied in the old saying: f/8 and be there. f/8 typically gives sufficient depth of field for most still subjects and it's where 35mm and digital SLR lenses are typically at their sharpest (or close to it).
   
   

   This shot was taken at f/9.5, forcing a 15 second exposure, because this is where the lens used seemed to be at its sharpest; as it was shot on a tripod and as it was a mostly still subject, the longer exposure time didn't matter.
   

   Tips

   • When not actively using your camera keep it ready for whatever may present itself by leaving it in fully-automated program mode, or perhaps in aperture-priority mode with a reasonable default aperture such as f/8.
   • Don't worry too much about the results of your tests. Those tests will tell you how to get the sharpest results on a flat subject in ideal, tripod-mounted situations, not about the shot that real-world conditions will necessarily permit. In particular:
     • If you really need a lot of depth of field, don't worry about using smaller apertures, even those at which diffraction is obvious. The defocus caused by part of your subject being out of the depth of field is a very, complex thing that's impossible to correct; it's an extraordinarily complex phenomenon that differs from lens to lens, and even on the same lens depending on aperture, subject distance and focal length.
       
       Diffraction, on the other hand, is a relatively simple phenomenon. A simple "unsharp mask" in your favourite photo editor will often work fine.
       
       

       A simple "unsharp mask" in your favourite photo editor might be sufficient to plaster over the effects of diffraction, as done here; the difference between the f/8 and f/32 shot is now invisible. If you need the small apertures, use them.
     • Don't hesitate to shoot your lens wide-open if you need it. For example, if you're hand-holding and can't stop down a little without getting camera shake, or you want to freeze motion, then shoot wide open; a little visible fuzziness in the corners is a lot less ugly than the effects of camera shake or a blurry moving subject. The lower contrast is trivial to correct in software, too.
       
       

       The real world is not a boring test subject. Use any of your apertures if that's what you need.
   • Stopping down to small apertures like f/16 will, with many lenses, turn bright points of light into "sun stars". These will either have the same number of points as your lens has aperture blades (if you have an even number of them), or twice as many (if you have an odd number of aperture blades).
     
     

     Stopping down to small apertures will give you these little sunstars.

   
   

   Warnings

   • Using a small aperture (high f/number) can also bring unwanted things into focus, such as dust on the sensor or dirt or damage on the lens. You may need to clean the sensor or lens, or go back and edit each image afterwards. If the lens has a big scratch avoid pointing it toward the sun, which would cause flare. (Overzealous cleaning often causes more problems over time than a few specks of dust. If you have a cheap filter over the lens, preferably coated or multi-coated to prevent flare, clean that all you like. Sensor dust shouldn't be much of a problem if you change lenses in calm, relatively dust-free places.)

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