SHARPNESS TESTING

From Art Morris Birds as Art

Back issues of all BAA Bulletins can be found in the Bulletin Archives which may be accessed from the home page at http://www.birdsasart.com/. If you photograph birds then you should be getting his emails! He is THE bird guru.

SHARPNESS TESTING

I receive several e-mails each week from folks who state that this camera does not focus properly or that that lens is not sharp. In 99% of the cases I am sure that operator error rather than equipment malfunction is the cause. That said, on rare occasion, folks using quality equipment from Canon of Nikon do experience real problems with focusing accuracy.
If you are handholding, please do not complain about unsharp images. I was walking around in Sabine Woods late yesterday afternoon. It was cloudy dark. I was working at ISO 800 with shutter speeds of about 1/60th second and even slower at times. There were two folks walking around handholding 500 f/4 lenses; they might as well have gone fishing as there was no way that they could create a sharp image without a tripod in those conditions. It is best to work on a tripod at all times when using your longest lens (unless you are handholding for flight or for action).

Another factor to consider is that most of the folks who write complaining of unsharp image are using cameras with 1.6X or 1.5X multiplier effects. They need to realize that these cameras multiply vibrations and movement caused by operator error by the square of the equivalent focal length!

If you are using a tripod you need to check and note the shutter speeds of your unsharp images. I had one guy on an IPT complaining that none of his images were sharp. I checked his set-up and saw that he was working in near darkness at ISO 100 with shutter speeds in the 1/8 to 1/15 second range. Making sharp images with a long lens at such slow shutter speeds is simply not possible for most folks. When working at long effective focal lengths I am confident that I can make sharp images down to 1/60 second as long as I have time to lock the tripod head and provided that the bird does not move during the exposure. With the prime lenses alone, I can usually get down to 1/30 second. With the relatively new Canon 800mm lens and its new 4-stop IS system, I have made some sharp images at shutter speeds as slow as 1/6 sec.

Below are two simple tests to determine if you have faulty equipment.

#1: Tape a sheet of newspaper to a sunlit outdoor wall on a relatively still day. Make sure that it is as taped down as flat as possible. (A magazine cover with fine print or a plastic or cardboard test chart are of course better options.) Mount your telephoto rig on a tripod. Lock down the tripod and the lens collar. Make sure that you are beyond the minimum focusing distance of your lens and that the distance range switch (if your lens has one) is set to full. Make a few images at the wide open aperture using both One-Shot or AI Servo with Canon gear or Single (S) or Continuous (C) with Nikon. Then do the same thing at f/8. As long as the wall is sunlit you will have more than enough shutter speed to know that your focusing issues are not caused by too-slow shutter speeds. Now download your images and check them for accurate focusing. If all of the images are sharp, then you can be sure that your unsharp images were being caused by operator error.

#2: Stand well off a somewhat busy road with the sun angled so that the approaching vehicles are coming right down sun angle. Use the wide open aperture and choose an ISO that results in shutter speeds greater than 1/2000 sec. Choose AI Servo (Canon) or Continuous (Nikon) and select the center AF sensor. As the cars approach, place the central sensor on the license plate and hold the shutter button down once focus is acquired. Even this simple task requires some practice so be sure to take lots of images. Download the images and sort them into two groups: sharp on the license plate and unsharp on the license plate. Now using an application that allows you to see the position of the active focusing sensor, in this case the central sensor, note the position of the sensor in the unsharp images. If the sensor is consistently on the license plate and the images are unsharp, then you likely have equipment problems. You can repeat this test with various camera bodies and various lenses in an effort to determine the cause of the problem. When you are pretty sure that you have an equipment problem it is best to send the gear to the manufacturer along with a CD of the test images.

In most cases, folks will learn that their equipment is perfectly fine but that their poor sharpness techniques are the cause of the unsharp images. Do also realize the importance of subject movement. While working at the Cozad Ranch probably well more than half of my images were unsharp. This was due in most cases to subject movement. In other cases, the unsharp images were a result of the AF system being unable to maintain sharp focus on the tiny songbirds as they leaped off of their perches. It is important to understand and to realize the limitations of our equipment.