I frequently photograph wading birds at a Rookery early in the morning or late in the evening and I end up photographing with a wide open aperture with my 600mm lens to maintain a good shutter speed. I also like to intentionally only have one point in an image in focus such as the image of my cat and this is accomplished in the same way, long focal length, wide aperture. A combination of a wide aperture and long focal length I'm using means that only a very narrow portion of the image is in focus. Due to variances in the manufacture of lenses and camera bodies, it is not uncommon to have the point where you've focused not be sharp. Particularly with the resolution of newer cameras improper focus is now more noticeable. Fortunately the camera makers have stepped up and allow you to adjust this to match your equipment.
The autofocus mechanism used by DSLR camera's involves sensors in the mirror box. The autofocus system detects the difference in the waveforms of one sensor to the next and is able to control the point of focus based upon this difference by controlling the autofocus motor in the lens. Very small variances in either the sensor positions or the autofocus motor can cause radical differences in the focus point. To fine tune the focus point both Canon and Nikon have a custom function (AF Fine-Tune Adjustment for Nikon and AF Microadjustment for Canon). The adjustment works by the photographer entering a value in to change the point that is in focus. The trick itsto accurately determine the value to enter to make things better not worse.
The best procedure is to use a target with a flat surface parallel to the plane of the sensor in the camera and with a ruler like scale projecting at a 45 degree angle on one side of the target. Several commercial systems are available.(Lens Align, Datacolor SpyderLensCal) as well as some DIY systems on the web. There are several things the photographer must do to get good results: 1. Place the target away from the lens at approximately the distance you usually use it at. 2. Insure the target is well lit. 3. Have the camera on a tripod and practice good shooting technique 4. Make sure the target is truly parallel to the lens plane.
The Lens Align system has a sighting mechanism to make the process of aligning the target easier. Another approach is to place a small mirror over the center of the target and use that to sight through the viewfinder until the lens is centered to find the right point.
Once the target is set up and aligned, you simply photograph the target, note the point on the scale where it is in focus and begin the process of setting the adjustment until the result is the same consistently. Note that the focus mechanism in live view mode is not the same and you must shoot in normal mode for the checks.
Several factors limit the accuracy of the adjustment 1. The adjustment can change with temperature. 2. The adjustment may vary with distance 3. The adjustment may very with the focal length of a zoom lens.
This is why it is recommended to test at the distance you most often use and it's best to test a zoom lens at its longest focal length as that the most critical setting due to the depth of field.
Another option, which I've chosen is Reikan FoCal Automatic Lens Calibration Software. With this system the photographer attaches the camera to a computer using this software. A well lit user printed chart is used with the camera place on a tripod. The target is again placed at the normal shooting distance from the camera. When activated the software initiates a series of shots and changes the focus adjustment between each shot. For Canon this is totally automated, for Nikon the photographer must enter the focus values manually. The system is able to measure the quality of the focus and with a series of shots determines the best setting.
I'm generally pleased with the results from FoCal. However, there are couple of things to note. 1. Lighting is super critical, without good lighting the tests are unreliable. 2. There's a certain amount of frustration involved with getting the software, camera, and target set up. 3. Setting up to test a long lens with the distances required means setting a laptop with the software loaded outside and mounting the target in such a way that it is flat and does not move in the wind. 4. You're limited in the number of camera bodies you're licensed to use. For example it would be nice for me to be able to test my tour customers lenses as part of each tour, unfortunately it would require the purchase of one or more copies of the software for each tour.
For my D800 the values ranged from -12 to +7 on a range from -20 to +20. So in some cases things were quite a bit off. For several of the lenses I used FoCal and then validated with a DIY chart, results were virtually identical.
There are several side benefits with FoCal. One is the ability to test for the aperture with the optimal sharpness for each of your lenses. The following is an example of one of my lenses.
I was able to diagnose one of my lenses that needed repair due to very poor sharpness on this test (I ended up replacing it). The feature that convinced my to purchase the software is the ability to test the accuracy of each sensor. Some D800's have a sensor alignment problem effecting focus on the left side sensors, fortunately mine was not one. One feature I've yet to test is the ability to check for dirt on a sensor. For those users comfortable with this approach this is a very nice option. However unless you're comfortable with tethering your camera and working with it via the computer it may not be for you.
Regardless of the approach, careful setting of the focus tuning adjustment can make the difference in many critical situations. Even with the limitations of the systems, you can increase the percentage of images that are sharp.