Defocusing Test - 13 May 2009

A number of digiscopers have commented that they seem to get better images when the scope is slightly defocused, thus allowing the camera to find the right focal plane. A couple of Birdforum threads have addressed this issue (1), (2). Since I sometimes have issues trying to keep focus in the field, I decided to perform some experiments under controlled conditions to determine the limits of 'my' digiscoping setup in terms of focusing/defocusing. I emphasize 'my' because I suspect that results could be completely different for other scope/camera setups. Experiment I set my Zeiss 85T*Fl, 20-60X Zoom Eyepiece, Nikon Coolpix P5100 and homemade adaptor approximately 30 feet from a resolution chart in the back yard. The eyepiece was set to 20X. The scope was focused on the center of the resolution chart so that the image was as sharp as possible. The P5100 was set to Aperture-priority (f/5.4) and the 3-second self-timer was set prior to each exposure. I then took exposures after rotating the fine-focusing knob counter-clockwise (CCW) on the scope at ~20 degree intervals and allowed the camera to adjust focus. In this direction the scope focuses to the rear of the chart (back focus). I then repeated the experiments while focusing clockwise (CW), which causes the scope to focus in front of the chart. Note - these directions might be reversed on the Swarovski scopes. The P51oo was first tested using the Macro setting, and then tested using the AF setting. Series of exposures were also taken with the optical zoom set to 1X, ~1.5X, and 2X (on my system I am unable to get a sharp focus above 2X on the Coolpix due to improper camera/eyepiece distance. I've posted the test images on my Pbase site after performing the following steps in Photoshop: Images were first cropped, then Auto Levels was applied to each image followed by an UnSharpen mask (which I normally apply to my digiscoped images). Note that Auto Levels generally results in a higher-contrast image, which results in a slightly sharper image, but this step did not improve out-of-focus images. Results The following lists the amount of defocusing required to produce an out-of-focus image: Macro - CCW defocusing: 1X - At 20 degree steps I was able to acquire focused images in Macro setting to a rotation of 360 degrees! So even though the scope image was blurry, the camera was able to still capture sharp images of the test chart. 1.5X - 180 degrees 2X - 80 degrees AF - CCW defocussing: 1X - 80 degrees 1.5X - 60 degrees 2X - 40 degrees Macro - CW defocussing: 1X - 14o degrees 1.5X - 40 degrees 2X - 40 degrees AF - CW defocussing: 1X - 60 degrees 1.5X - 40 degrees 2X - 40 degrees Results indicate that setting the Coolpix P51oo to Macro will provide a wider latitude for getting sharp images when the scope image is out of focus. This 'forgiveness' is reduced, however, when the camera optical zoom is increased. Notice also that the camera corrects backfocusing issues better than it does front focusing. The take home message here is that you can still get sharp images if your scope is focused beyond your subject, but chances are reduced dramatically if your scope is focused in front of your target. AF setting works just fine on sharply-focused subjects, but fails quicker once the subject is out of focus in either direction. So the next time I'm out in the field digiscoping, I'll be using Macro setting, and continue to focus the scope on my birds, but I'll also be purposely defocussing the scope (CCW) to see if I can get sharp(er) images!