Photography Podcast

Keeping it Sharp (part 2)

Posted in General, Technique on 10-09-2011 | Comment

In part 1 of this article, we discussed methods of stabilizing your camera and some rules on choosing shutter speeds. Now we move on to look at how your aperture choice makes a difference and a few hints on using autofocus to your advantage.

Aperture Choice

Having a stable camera and choosing a safe shutter speed are the cause of most focus problems from my perspective. Choice of aperture can also, however, play a significant role in the overall sharpness of the image especially when you are shooting with consumer lenses. Each lens has a “sweet spot” which offers the best sharpness that that piece of glass can provide. This is generally found about 2 stops down from the widest aperture the lens supports so, on an f/4 lens this would be about f/8 and on an f/5.6 telephoto, about f/11.

Lens test results published by DPReview offer a great resource when trying to find the optimal aperture for your particular lens (assuming it has been reviewed). For example, the results for the popular Nikon 18-200mm f/3.5-5.6G can be found here. The interactive graphic allows you to see the sharpness characteristics at different apertures and focal lengths across different areas of the frame. If this is too much information, the test conclusions also list the best overall apertures for the lens under test.

If your particular lens is not listed on DPReview’s site, take a look at the data provided by the manufacturer since they will usually publish the same or similar information.

Autofocus

Next we have to consider how you use the camera’s focus system. In the old days, focus was entirely up to the photographer but manual focus is hardly ever used these days and instead we rely heavily upon the camera’s autofocus system. Knowing how to use it effectively is important.

Continuous vs. single vs. manual

DSLRs typically have three main autofocus modes. In manual focus mode, autofocus is completely disabled and you are responsible for setting the focus point in your images. Given how good autofocus systems are these days, it may seem odd to turn the system off but there are some occasions when this is helpful or even vital.

Shooting macros

When shooting macro, especially when using extension tubes, autofocus frequently doesn’t work at all or, if it does, it tends to hunt around trying to acquire focus. Normally, I switch to manual focus in these situations to prevent this problem. Shooting a burst of frames as you move very slowly towards or away from the target will normally result in at least one image which is sharp where you want it to be. Manual focus is also vital if you are using software to improve the depth of field of macro images by focus stacking – taking several shots of the same subject with the focus set at different depths in each one then having software such as Photoshop use the information in these frames to generate a single image with a deeper depth of field.

Action

If you are fortunate to have fast, expensive professional lenses, you will almost certainly be able to rely upon your autofocus system when shooting sports or other action. From my experience, for example, a Nikon D700 and 70-200mm f/2.8 combined will track focus faster than my own eyes can! With cheaper lenses, however, the autofocus system often can’t keep up with the pace of the action even when using continuous autofocus mode (more of which later). In these cases, switching to manual focus can be a help. In situations where you are setting up for a shot at a particular place, for example waiting for an athlete to jump a particular hurdle or a high jumper to cross the bar, using manual focus and pre-focusing on the point that you know the shot will be at will very much increase your chance of getting the image you are looking for.

Panorama and HDR shooting

When shooting panoramas, HDRs or any other special form of photography that requires multiple images to be captured of a single scene, it is usually important to prevent changes in the focus between exposures. In these cases, set the camera focus for the first shot, either manually or using autofocus, then turn the focus system to manual to ensure that no changes occur as you take the sequence of shots.

Artistic Effects

Many cameras refuse to shoot images if focus has not been acquired. While this is useful most of the time, there are occasions when you want to take an out of focus shot for artistic effect. In these cases, turn the camera to manual focus and use the focus ring to generate the level of blur you want in your scene. In manual mode, the camera shutter will fire regardless of whether or not it thinks focus has been acquired so you are free to be as creative as you want with the focus ring.

Single vs. Continuous Autofocus

In the majority of cases, you will be shooting with autofocus enabled and you will have two more modes to choose from, single or continuous autofocus. The difference here is in how often the camera checks for focus and adjusts the lens elements for you. In single mode, a camera will autofocus once when you press the shutter halfway down or when you press a dedicated “AF” button (many DSLRs allow you to choose how autofocus is activated via a menu option). Until you release the shutter or AF button and press it again, the focus will remain locked after this one check. This is fine in most cases where you are not dealing with fast movement towards the camera and has the nice side benefit of not using a lot of battery power to tweak the lens position continuously. On the other hand, if you are shooting action where your subject is moving towards or away from the lens, this poses a problem since there will be movement between the time when the autofocus locks and the point at which you push the shutter button fully to activate the shutter release. During this time, a fast moving target will likely have moved out of the plane of focus and you will end up with a blurred image.

To combat this problem, continuous autofocus can be used. In this mode, your camera will constantly check and adjust the focus as long as you have the shutter half pressed or the AF button pressed. This is the mode of choice when shooting moving targets whether they be athletes or kids running around. The downside to continuous autofocus is that it is more battery-hungry. On Nikon cameras at least, it also turns off the autofocus illuminators so it will not likely work too well in low light or when you want to use a flash.

Choice of AF point

In addition to choosing the appropriate autofocus mode, it is also important to know exactly where in the frame your camera will focus. With compact cameras, it is common to have the camera focus in the center of the field meaning that you have to focus then recompose your shot if the subject you want to be sharp is elsewhere in the frame. On DSLRs, however, most offer various choices to help you control what the camera focuses on. In addition to the “center of frame” option, you will also generally find several other modes, most of which require your input to select the initial focus point. The Nikon DSLRs I use (D90 and D700) offer the following options:

Single Point
You select the exact focus point you want by using the multiselector on the back of the camera to move a rectangle around the viewfinder. The number of possible positions varies by camera with later and higher function models offering more choices (51 for the D700 vs. 11 for the D90, for example)
Dynamic Area
This is a bit more clever than the single point option when you are using continuous autofocus mode. You still select the focus point using the multiselector and the moving rectangle but the camera will check surrounding focus points and adjust focus as needed if the subject moves away from the initial position.
Auto-area
This is a fully automatic focus mode where the camera analyses the scene and focuses on the object that it assumes is the main subject. In this case you don’t provide any input and, as a result, have a lot less control over what is to be focused.
3D-tracking
This is the most advanced autofocus mode offered by the camera. When the shutter is pressed half way, the camera analyses the scene around the chosen autofocus point and tracks the object as it moves around the frame, keeping the same subject in focus regardless of where it moves. Once again, this mode only helps when using continuous autofocus.

Each of these modes has pros and cons but, generally speaking, the advanced modes are only really useful when shooting moving subjects. For static subjects, any mode other than fully automatic should offer similar performance. In continuous autofocus, however, different people prefer different modes so experiment and see which option you prefer for the kind of shooting you do. Personally, I use either Dynamic Area or 3D-tracking for sport shooting.

Very wide apertures

If you are used to using single point autofocus, focusing on the center of the image then turning slightly to recompose your image before making the exposure, you may fall into another trap when using very wide apertures and wide or standard lenses (50mm or below). The act of rotating the camera to recompose after focusing can actually change the camera/subject distance enough that the area you had previously focused on is now no longer pin sharp. Rather than moving the camera to refocus in these cases, it is important to keep the camera static and select focus using one of the autofocus modes that allows you to position the focus using the camera’s multiselector and the focus marker in the viewfinder.

It’s taken two posts spread over exactly a year (sorry about that!) to outline some of the issues related to getting a sharp image and this may all sound rather daunting. Overall, however, the best advice I can offer is to shoot a lot, practice your camera holding technique to keep as steady as possible and try out your camera’s various autofocus modes so that you are familiar with the ones that offer the best performance for the kind of shooting you do.

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