Author: Dave Wilson

For 9 hours a day, I'm a professional software engineer for a major IC manufacturer but the rest of the time I'm a fanatical photographer who loves shooting all kinds of images and mucking with them in Photoshop. Although I shot film for 20 years, I'm now totally digital (though if Brian & Antonio have their way, I'll be breaking out the 35mm gear again shortly and buying some FP4). I shoot Nikon DSLRs but also love my Canon and Fuji P&Ss so have a foot in several camps so to speak. On the web and Twitter, I'm mostly known for my HDR images some of which you can find on my photoblog at http://www.davewilsonphotography.com and my Flickr account (DaveWilsonPhotography). Should you be interested in reading more of my hints and tips, you can also find me at http://blog.webartz.net.

Keeping it Sharp (part 2)

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.

Keeping it Sharp (part 1)

We’ve been promising it for a while but this post is the first in an occasional series devoted to photography technique. We would really appreciate your suggestions on further articles that you would like to read in this series.

Aside from a few isolated creative cases, photographers strive to ensure that their images are as sharp as possible in the areas that are supposed to be sharp. When an image falls short, we are often quick to blame poor equipment but, if we are going to be honest with ourselves, we should realize that the vast majority of focus problems are caused not by a cheap kit lens but by “user error”. This article and the one which follows, aim to help you work through the kinds of problem which can reduce sharpness in your pictures and which can be fixed without spending thousands on new lenses.

Before delving in, we should remember that your definition of whether or not a photo is sharp will vary a lot depending upon what you are going to do with it. If you print nothing larger than 6″x4″ or use only low resolution versions of your images on a web site, you will be a lot less likely to notice an unsharp image than if you are producing 36″x24″ prints. The larger your output, the more careful you must be to nail the focus exactly where you want it.

Stabilizing The Camera

Most problems that appear as poor focus are likely due not to lens focusing at all but are caused by camera movement during the exposure (or “camera shake”). A slight movement of the camera is all it takes to blur those eyelashes in an otherwise perfect portrait or turn the leaves on the trees in your landscape into green mush. The problem of camera movement is magnified with longer lenses where a small angle change in the camera translates into a large positional change in the image you are capturing. Conversely, a significantly larger movement is required to produce equivalent levels of image blur when using a wide angle lens.

The most obvious way to prevent camera movement is to lock the camera down hard using a sturdy tripod. While this is the best option, carrying a tripod is unlikely to be practical in many situations but there are several other things you can do to reduce camera shake. The first of these is to hold your camera properly – be your own tripod and support the weight of the camera in a way that is comfortable for you and as stable as possible. The position of your right hand is generally fixed by the camera controls – grip the right side of the camera tightly. Your left hand, however, has a lot more choice and this tends to be where the problem lies.

I was recently at a sporting event and watched an amateur photographer hold a 300mm lens with his hand above the lens barrel. Aside from the fact that this must have been rather uncomfortable, it was also a very unstable way to support a heavy lens (or any lens, for that matter)! Holding your left hand underneath the lens offers a far more stable platform and tucking your elbows in against your sides helps even more. While doing this, keep your feet apart and, if you have the option, brace yourself against something solid – a wall, a tree, a fencepost – to further reduce the likelihood of you moving during the critical hundredth of a second or so that the shutter will be open.

One of the masters of the genre produced a video a while ago that describes what I am talking about here. Joe McNally is a left-eyed shooter so his camera grip technique won’t work for most of us but do watch his video since it contains great information even for those of us who clamp our right eye to the viewfinder.

Even when you are in the fortunate position of being able to shoot from a tripod there are a couple more things you can do to ensure ultimate camera stability. Firstly, to minimise your need to touch the camera and possibly jolt it, use a cable release or, if you don’t have one, set the self timer so that your hand is away from the camera when the shutter fires. Secondly, if your camera has the option, use Mirror Lock Up or Exposure Delay (a Nikon feature which delays the shutter release for a second after the mirror is raised) to reduce vibration caused by the DSLR’s mirror moving. This may sound nitpicky but it can make a noticeable difference if you are producing really large prints.

Shutter Speed Choice

Having fixed problems associated with stabilizing the camera, another question often comes to mind. Just how slow a shutter speed can I use and still get sharp images when I’m hand-holding a camera? I know one professional photographer who claims to be able to get decently sharp, handheld exposures with a wide angle lens when shooting with a half second shutter speed but, for the rest of us, there is a useful rule of thumb. Don’t try to handhold a shot if the shutter speed will be any slower than 1/(focal length). For example, if I’m shooting with a 24mm lens, I should be able to handhold it successfully at shutter speeds shorter than 1/24 second. If, however, I change to a 300mm, I must keep my shutter speed shorter than 1/300 to ensure sharpness. Different individuals obviously have different abilities here but this makes a pretty good starting point for most people.

I still use this rule of thumb today but I can now bend it if I’m shooting with a Vibration Reduction (VR) or Image Stabilization (IS) lens. These lenses include special optics that sense vibration or movement and cancel it out automatically. Most claim something between 2 and 4 stops of improvement over a non stabilized lens of similar focal length, so, using one of these and assuming a conservative 2 stop improvement, I can shoot at a shutter speed 4 times slower than the rule of thumb would dictate and expect to get a shot of similar sharpness. Taking the 300mm example again, without VR, I should keep my shutter speed shorter than 1/300. With the 2 stop improvement I get with VR, however, I can move this limit to 1/75 second (1/150 is 1 stop down from 1/300, 1/75 is 2 stops down) and still expect a sharp shot. I must admit to having been skeptical of this but, on trying out my Nikon 70-300mm VR lens at the 300mm end, I was delighted to find that I can indeed get images whose sharpness I am happy with when hand-holding at 1/30 second.

The previous rules do make one assumption that you also need to consider. Regardless of how well you handhold and how fast your shutter speed is, if your subject is moving and the camera is fixed, the longer the shutter speed, the less sharp the subject will be. In these cases, you would need to move the camera during the exposure (pan) to track the subject position but that’s a subject for another article.

In part 2 we’ll consider how your choice of aperture can affect overall sharpness and discuss the use of your camera’s autofocus system to ensure that you get the sharpest focus in exactly the right place.

Branching Out

Up until now, we’ve used the PhotoNetCast site mainly to announce new shows and offer links and other material that are not easy to provide in the audio stream. Moving on, however, we’re really keen to start posting occasional technical articles and reviews too. This brings up the question, however, of what our listeners are interested in reading about. We have a few articles lined up already but please let us know what you would like to read here by leaving us some suggestions in the comments. Just to get the ball rolling, here are a few high level categories that may spark some ideas:

Basic photography concepts – camera controls, exposure, focusing, white balance.
Advanced exposure control – when to ignore the meter and using histograms to best effect.
Rules of composition – using them and breaking them.
Photo correction and post-processing techniques.
Camera raw adjustments.
Lightroom workflow.
Night and low light photography.
Photography book reviews.
Software package reviews.

Let us know your thoughts and suggestions!