I’m feeling a little guilty about skipping a couple weeks of Friday Foto Talk. My excuse is that I was mostly away from the internet, camping in the desert. I think I’m about ready to collate all of these into an e-book (or two!). Looking back I’ve poured a lot of my knowledge and experience into these Friday posts.
Last time we looked into a fairly subtle topic (subjective vs. objective approaches), so this Friday let’s get back to basics. Achieving good focus, and the larger issue of getting sharp photos, should be one of the first things you get good at, from a technical point of view, when learning photography. This post will focus on focus! It won’t go into the other things you need to do to get sharp images, which I’ve discussed in past posts.
WHAT IS FOCUS
The best way to understand this is to play with lenses (free of cameras, eyeglasses or binoculars) and a blank wall or white sheet of paper, with a strong directional light source. You probably did this in high school science class, drawing light ray diagrams like the one below.
Light rays (which can also be understood as waves) travel roughly parallel with each other as they travel from where they were reflected off the subject to your camera lens. They are bent inwards by the lens, coming together into a focal point. From the center of your lens to the focal point is the focal length, usually expressed in millimeters. Just behind the focal point sits your sensor (or film), the focal plane where an image is formed. By changing that distance between sensor and lens you bring the subject into focus.
It’s important to realize that once you have a subject in focus, it is sitting in a “plane of focus” (which corresponds to the focal plane inside the camera). Things above, below and to the side of your subject that are the same distance from your lens also sit in that plane, and so are in focus as well. Things that are off the plane of focus, either closer or further from your lens, are technically not in focus. But hang on! They only get blurry gradually as the distance from the plane increases.
What this means for a photographer is that, depending on your depth of field, much of the image (even all of it in many cases) can appear to be sharp & in focus. This is despite only a small part of the image being smack dab on the focal plane. It’s a case of having a sufficient depth of field. If you go for shallow depth of field, only what is on or very nearly on the focal plane will be in focus, with the rest of the image being blurry.
GETTING FOCUSED IMAGES
Now that we’ve done a little optics 101, let’s get into some practical tips on how to achieve good focus. Most of what follows applies to whatever DSLR you may be using. It’s even mostly applicable to mirrorless cameras. But since I use a Canon, there are a few things that you’ll need to translate to your camera’s specific controls. Which leads to the first point:
- Know your camera. You should be able to work the controls that affect focus (and exposure) without looking, and really without thinking. Most DSLRs allow you to change which buttons control focus and exposure. The default setup that most people use is where shutter button controls both auto-focus and exposure. A half-press of the shutter button starts autofocus and also forces the camera to take a meter reading, fixing exposure. Full press takes the picture.
- Be flexible in how you use auto-focus. There are several ways to go about shooting with autofocus. As you get better as a photographer you’ll realize that where you focus is usually not the composition you want to shoot. There are three basic ways to approach this using the viewfinder (see below for further options using LiveView).
- You can point the center of the frame at your subject, half-press the shutter button to get focus, then move the camera to the composition you actually want.
- It can be easier and more accurate to frame the composition you want first, then change the autofocus point to the one that covers your subject. On Canon DSLRs, there’s a little button on the top-right that you press with your thumb. Then you work the joystick on the camera back to change the AF point.
- A third option is to just focus where you want the focal plane to be, for examples 2/3 into the frame for a landscape where you don’t have important elements that are very close to you. Then switch your lens to manual focus and shoot away, concentrating on composition and exposure without worrying about focus. This can be a quick and easy way to go if you’re doing several shots of the same general scene.
- Depth of field and focus go hand in hand. The diagram above shows depth of field in the simplest way. And it really is simple in concept. But the devil is in the details as they say. How adept you are at working depth of field and focus directly affects how many good shots you get, especially in dynamic, rapidly changing circumstances.
- Focal length matters. You probably already know about how aperture affects your depth of field (how much of the field of view is in focus). What many novices don’t appreciate enough is how big an influence focal length is on depth of field. The shorter the focal length (wider-angle of view), the more depth of field you have. As you zoom in to longer focal lengths, you lose depth of field and need to stop down in aperture (higher f/ numbers) to maintain depth of field. With some very wide-angle lenses, everything will be in focus for any apertures above f/5.6 or f/8.
- Lens matters. In a similar way to focal length, each lens has its own focus characteristics. While it’s often subtle, some lenses tend to give better depth of field than others. And of course some are sharper than others, but that’s really separate from focus. Learn how your lenses render subjects in terms of focus and depth of field.
- Lens calibration. Some lenses arrive to your door with their focus needing to be calibrated with your camera’s auto-focus system. A lens may actually focus slightly in front or in back of the focal plane, where your camera says it is focused. Most DSLRs have the ability to calibrate the auto-focus for quite a long list of lenses. So check out your owner’s manual and Google to see how to check focus for new lenses. I’ve only had to calibrate a couple of mine. Most good lenses, especially when they come from the same company that makes your camera, seem to be spot on in focus. But all it takes is one to mess up a lot of pictures, so it’s a good idea to check each lens.
- Know when to switch to manual focus. When light is dim, or when contrast is low (such as in foggy conditions), it’s time to think about manual focus. Sometimes what you’re shooting is dim or low-contrast, making your camera search for autofocus. Sometimes I point your camera in another direction, at a subject that is about as far away as my intended subject. Then I turn off autofocus and switch back to shoot my intended composition. Or if everything is pretty dim and/or low-contrast, I will go to manual focus. When I’m working close-up, especially with a macro lens, I almost always switch to manual focus, often setting the distance and moving the camera back and forth until I get good focus.
- Manual focus is often better. For some shooting manual focus is actually easier and more precise, especially with macro as mentioned above but also with landscapes. Your camera has ways it will tell you when something is in focus. Let’s say you change the switch on your lens to MF (manual focus). If you point the center of the frame (or your selected AF point) at your subject and then rotate the focus ring, a green light is visible in the viewfinder to let you know you’ve achieved focus. Also if you have it enabled, an audible beep sounds as well. I have a couple lenses that are manual focus only. For those I use the focus confirmation light nearly all the time, unless I’m using LiveView (see below). I don’t like beeps so I never have that enabled.
- Using LiveView to focus. When you switch to LiveView, where the image is displayed on the LCD screen on the camera back, you can do everything that you normally do, including focus. The ability to magnify the image makes LiveView a good way to achieve precise focus. There is a little white square that shows which part of the image you will magnify, and you can move that white square around. Normally the white square also is where your exposure is read from too. Once you have your subject magnified, you then turn the focus ring slowly to get perfect focus. Then you can move it around to check out how much of the rest of the scene is in focus. By the way, you can also use autofocus with LiveView. In that case the white square becomes your focal point, and lights up green when focus is achieved.
- Use the depth of field (DOF) preview button. If you’re using LiveView in the manner above, the DOF preview button comes in handy. It will show you what is in focus in front or behind your focal plane. Some cameras don’t have one, so for them you’ll need to shoot and review to zero in on your shot. When you press the DOF preview button your lens stops down to the aperture you have set. This allows you to see exactly how much of the frame is in focus, and how blurry the rest is. You don’t have to be in LiveView; the button works through the viewfinder too. But with LiveView’s magnifying abilities you can see a lot better. Remember: whether you’re looking through the viewfinder or on LiveView, what you’re seeing is the view at the largest aperture your lens has (f/4 or f/2.8, for example). It isn’t showing you the scene at the aperture you have set, and what the picture will be captured at. If you’re at f/11 for example, you’re seeing more blurriness than the picture will have, unless you press the DOF preview button.
Whew! That’s enough for now. Practice makes perfect, so play with all the different ways to get your camera to focus where you want. Use manual focus and LiveView, auto-focus points and the DOF preview button. Change composition while fixing focus (and exposure) where it needs to be to get the focus and depth of field right for your images. Have a great weekend and happy shooting!