These days, exploring discussions and comments from photography learners, I read constantly the «problem» when focusing with fast lenses, namely lenses with a maximum apertures of f/1.8, 1.4, and the rare f/1.2. I read of someone who even return an expensive f/1.2 lens because he feels he can’t «focus» any subject with it. Those are regular issues when you don’t understand the correlation between lens apertures and the DoF.
Less experienced photographers fight with the situation of «focus» when they set their aperture to the broadest their lens allows. They have heard the advantages or the «fast» prime lens, and the well-know «nifty-fifty» 50mm of f/1.8 or f/1.4, and run immediately to buy a new one. Then, the frustration comes almost as fast as the lens, when they shoot and shoot at such apertures and see that the major of the frame soft and blur, with only a tiny portion of it—not necessarily the one that they want—in focus. The problem is worst when they have been pursuing a mobile subject, clearly, becoming out-of-focus in a fraction of a second.
Nobody tells them that their «problem» is not probably about the auto-focus mechanism of the camera nor a defective lens. Neither their capability to focus properly or the shutter speed (at bigger apertures, it’s highly probable that their shutter speed is settled at a tiny portion of a second). Their problem is the DoF.
Let’s first discard the other common problems.
The default auto-focus mode of many cameras—from point and shoot, to the semi-professionals—is the single servo or single focus mode. In this mode, the camera will try to focus and locks it after half-pressing the shutter/release button. No matter you move the camera, or the subject moves from you, the focus remains the same. This is a problematic setting because is a frequent source of lost-of-focus images in a fast-moving environments. You have to continuously release and press the button to catch the subject. On the contrary, the continuous focus mode or continuous servo mode has the convenience of focusing the subjects all the time while you press the button. But, if any subject interfere with your scene, the focus change instantly causing a missing shot. Although these are rare occasions, there is a better alternative. Nowadays, the back-button auto-focus setting is available for almost all of DSLR and mirror-less cameras. With this setting you can benefit of all the previous setting: pressing once the back button button (configured to active the auto-focus) to lock the focus and recompose, or maintain the button down to continuously re-focus. Below, there are some links to this technique from Nikon and Canon, and also from the Tony & Chelsea YouTube channel.
The depth of field (DoF) is a central subject of discussion and opinions under the different photography genres. If you are a landscape photographer, you have to adjust your mindset whenever you want to shoot a portrait or vice versa. Most of the time, when you read an article or discussion about of the DoF, it concentrates in the f-stops. As we know, lower f-stops brings a shallower depth of field, higher f-stops means bigger depth of field.
Nevertheless, limiting the discussion of the DoF to the lens aperture misses the other factors that contribute to the matter: focal length and subject distance. As recommended, wide or semi-wide-angle lenses (less than 50mm, but in the market 40mm or below) bring long DoF, and it’s the contrary with telephoto or semi-telephoto lenses (technically more than 50mm, but in practice 75mm or more). For landscape photos, it is recommended wide-angle lenses for the obvious great angle of vision, but also for the less obvious strong DoF. On the other side, telephoto lenses are recommended for portraits and wildlife. This seems contradictory, but have a clear explanation. In portraits, you want to flattering your subjects, and separate them from all background distractions. They should fill most of the frame. With a telephoto lens you achieve perfectly that (your subject doesn’t look like a balloon), and you get a nice blur of the backgrounds. In wildlife, with a telephoto lens, as well as allowing you to catch your subjects without disturbing them, you separate the subjects from all the natural distractions around, trees branches, other animals, etc.
Managing the aperture is probably the main resource you have to deal with the DoF. But you can reach a good DoF when you take a distance from your subject, and also when you use a wide-angle lens. Shooting with lenses of 35mm, 28mm or less, a little far from your subject, will get you a high DoF. On the contrary, a telephoto lenses near your subject will give you a very shallow DoF. You might combine those resources to get what you want. For example, approach a subject with a wide-angle lens, or take distance using a telephoto, open or close your aperture, etc. The decision in that case is the same: how blurriness or DoF you want.
While take distance or came near your subject is a matter of the perspective selections and the environment constraints, getting the proper focal length and max aperture is about the lens you have. For example, you can’t be in the field, where the players are running, to get a shot. Many times you are very far away from the bird’s nest you want to shoot, and can’t even think about to be near it. Your solution is to get a good zoom lens with the bigger aperture capabilities. With them, you extend your opportunities to make fast decisions for a shot, always controlling the final quality you want for your image. But all those benefits don’t came free. A fast zoom lens can cost you one or even two full frame camera bodies.
Also, when considering the distance from the subject, you have to look at the movements in the scene. In short distances from the subject, the movements are more noticeable than with long distances. If you want to show movement, you might to be near the subject. Or on the contrary, get distance to freeze a little the movements. This is an important tip, besides the adjustments of the shutter speed to manage the notion of movements in your pictures.
What if you want to use a cheaper fixed lens? To me and other street photographers a 50mm fixed lens in a crop body (equivalent to 75mm field-of-view in a full frame) and a lower f-stop of 1.8 (equivalent DoF of a f/2.8 in a full frame sensor camera) is a bless. It should allow you to get good approximation without running toward a distant scene, but also can get a decent coverage of a street or building, just walking away several meters. (Although I would prefer the 35mm f/1.8 for this purpose.) Another reasons of this lens is the increased possibilities to play with the DoF as wish, and shoot without flash under relative low light condition (as in interiors or shadows zones).
Remember, fast lenses requires you to pay attention of the DoF you get when use the maximum aperture. You have to manage also the focal length and the subject distance to reach all in focus or on the contrary, to distinguish your subject of a blurry background. Special attention must be taken to these factors and make coherent decisions. It would be a contradiction to open the lens and take distance for a more depth of field, one cancels the other. Or, taking the decision to increase the focal length for a shallow depth of field, but closing the aperture.
Pay attention to this:
- Get closer or take distance. If you want more DoF get distance. For portraits, is better to get closer and get the subject distant from the backdrop or the background. Remember the effect of distance regarding motion. If you get closer, open also the lens; if you get far from the subject, close also the aperture.
- Use a telephoto or a wide-angle lens. If you want great DoF, use a very little focal length. This lens also will get you a bigger angle of view. For portraits use a telephoto wide open, with a distant background. If the selection of lenses is primarily for the DoF consideration, maintain the lens open or close accordingly with your decision.
- Open or close your aperture. Use f/2.8 or less when you are near a subject (taking a detail of it) and want to get a very shallow DoF. Otherwise, you can achieve a decent sharp subject, but good DoF, from f/2.8 to f/4. For a strong DoF, try f/8 or more.
Dealing with DoF is a combination of factors. All of them are correlatives. You have to try alternatives and test a combination that works for you.