Solving Some Soft Focus Problems
I get asked questions everyday regarding photography related topics. I was asked today by someone whether they should change thier camera and/or brand and would this help improve the sharpness of their images. While this would seem to be a drastic measure to solve a problem and one I would not advise to do unless EVERY other possible solution had been explored, sometimes there is no other choice. This question got me to thinking about what problems people have with focusing.
I’m not one to use the term “Back When….” but in this case, back when we had manual focus ONLY, you rarely heard people talk about the lens being “soft”. Back focus or front focus was just bad technique, blurry images was a result of bad choice of film speed, shutter speed, aperture or it was from camera or subject movement. So what’s changed?
The cameras, we own now, are far more sophisticated, have autofocus, multiple focus points, image stabilisation, predictive AF, Face Detection, Auto ISO, Spot AF etc. So why are so many people complaining about soft photos? The technology available should make it better. To put it in a one word answer…….No. Today’s camera technology gives us a choice not a perfect solution.
To give you a better understanding of the problem we face with this technolgy let’s use a simple everyday photo opportunity. Your child is doing something cute and you want to capture it for the Grandparents. You grab the camera and aim and fire. You take several images, zoom in, change position, the child looks up and you take some more. Pretty standard stuff. You’ve captured about 15 images. You download as usual and you look at the results. 10 of the 15 images are not perectly sharp, 2 are razor sharp and 3 are OK. Sound familiar?
Notice how I didn’t mention settings, modes, ISO etc? That’s because usually most people stick the camera on Auto and blast away taking pictures with no regard to settings or anything that’s is important to achieving sharp, focused and correctly exposed image. What ISO do I need to set in order to get the shutter speed I need to be able to hand hold the camera? What aperture do I need to use to get the depth of field (or focus) for the distance I’m shooting at? These are not complicated questions but most people can’t give me the answers when I ask. Why would I ask these questions? Because they’re IMPORTANT.
There are some basic “RULES” to photography. Until you know these rules you can’t modify them to suit. Breaking these rules will just lead to disappointment.
First rule for sharper images. Use the fastest possible shutter speed. The minimum is not good enough, faster is better. Do you know what to do with either the aperture or ISO to achieve a faster shutter speed? Bumping up the ISO from 100 to 400 ISO gives you 2 stop extra of sensitivity. This means if you were working on 1/60th sec, you’re now at 1/250th sec. If you were at 1/125th sec, you now have 1/500th sec at your disposal. Faster shutter speeds capture less movement, both subject and photographer.
Question: What shutter speed can you shoot down to BEFORE the image quality starts to deteriorate? I know with each of my lenses what my limit is, shutter speed wise, to guarentee a sharp result. I also know the zone of 50% chance of success as well as where luck and a few quick prayers to the photography gods are needed.
The old rule of thumb was if you are using a 100mm lens the shutter speed had to be 1/125th sec or faster. A 200mm lens at 1/250th sec. The focal length of the lens determined the minimum shutter speed. This was fine with film but with digital sensors you have to go up again. A 10 megapixel camera with a 200mm lens you could shoot at around 1/250th sec and be OK. An 18 megapixel has almost twice as many pixels, capturing twice the detail of the 10 megapixel. It’s also renduring the smallest movements in either subject or camera. Camera & subject movement now shows up at shutter speeds we normally thought of as “SAFE”. Shoot for a high shutter speed.
For those of you relying on image stabilization remember that while the IS system is compensating for your movements up and down, left and right it’s not adjusting for forward and backwards movements. After all this is what the focusing mode is for. If you have your camera set to Single AF and you hold the shutter button down the camera will lock focus and as you move forward and backwards the focus remains at its original setting. Your subject moves in and out of focus ever so slightly. Try setting the AF to the Intelligent AF mode. This mode detects any camera to subject distance change and starts tracking the subject. So as you have your finger on the button and moving slightly forward and backwards the focus is being adjusted by the camera to compensate but wont allow the camera to fire unless focus is achieved. Another thing you need to remember is that the IS system does not stop subject movement at all. Only a fast shutter speed will freeze your subject.
Second Rule for sharper images. Don’t use all of your focusing points. Choose the ONE that will fall on the part of your subject that is the most important. In a portrait that is generally the eye closest to the camera. When you have all the focus points active in the viewfinder, the camera will choose the closest part of your subject with contrast. This could be either the hair against the forehead or against the ear. It could be the end of the nose or, worse yet, something in the background. By choosing just one AF point the camera is not going to guess what you want in focus and you as the photographer you are in no doubt what will be your point of focus.
Question: Can you change your AF point without removing your eye from the viewfinder? If you can’t then learn how to. This has to be like scratching your ear. You don’t need to see your ear to scratch the itch, you just do it without thinking. That’s how you need to be with your AF points. You should be able to set the appropriate AF point without thinking.
Third rule for sharper images. Get the Depth of Focus right. DOF is controlled by the aperture. Choosing the right aperture is critical, too wide and the DOF is too shallow, too small and you run the risk of the shutter speed being too slow. How do you know which is the right one? Take a few shots at each aperture and see which one you like the best. After a few sessions you’ll start to get a feel for the one that works for the type of photos you want to produce.
To give you an idea of the amount of room you have with DOF, here is some example from the same shoot I mentioned earlier. If you’re sitting about 1.5 meters away from your subject and you have the standard zoom on your camera. If you have the zoom set at 30mm and the aperture at f4.0 you have a DOF of 40cm. Change the zoom to 55mm at the same aperture the DOF shrinks to under 12cm. All we changed here was the zoom and the DOF shrunk dramatically. So get the focus point wrong and you have a soft image.
Here’s another scenario, with the lens at 55mm and the aperture at f5.6 and the camera is set to 100 ISO, your light meter gives you a reccomended shutter speed of 1/125th sec. At these settings the DOF is 16cm, while this may be OK for a head and shoulder shot it may not be quite enough to show what your subject is doing. So we’ll change our ISO to 400, this gives us 2 more stops of sensitivity which we can use to close the aperture down to f11 and increase our DOF to 32 cm, which will bring more of the foreground and background into focus.
It is all about balance. It’s important to get a sharp image, how you achieve this just comes through practice. Once you can achieve perfect focus each time and get more consistant results you can start bending the rules I mentioned above. How far do you push the rules? You won’t know until you’ve gone too far. You’ll know when because the results will start to go back to the way they were with more misses than hits. So apply the rules, then go bend them but don’t break them. Your photography will be better for it.