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Monthly Archives: June 2012
On Tuesday June 5th a very rare event occurred – the transit of Venus across the Sun. The next one will be in 105 years! I was was able to capture the beginning of the transit before the clouds rolled in. The setup was a 130mm f/8 astronomical refractor on an equatorial mount, focal length 1,040mm with a D800 as the imager. Exposure was 1/400 sec at f/8, ISO 100. A Baader solar film filter was used to reduce the bright sunlight to safe levels. Note the sunspots and clouds covering some of the images. There was a thin haze over all of the images, reducing the claity. (Click on thumbnail for larger image.)
I just bought a D3200 as a lightweight backup for the D800 on an upcoming hiking trip. Resolution at 24MP is great when used with Nikon’s best glass, but I was very disappointed when the D3200 turned out to have a bad case of backfocus right out of the box. Not only the kit lens, but all of my other Nikon lenses also exhibited the same backfocus. D3200, being an entry level camera, does not have the focus finetune function. There was not enough time to ensure that Nikon would get a readjusted unit back prior to the trip and since the D3200 is scarce, a replacement was not feasible either.
A search on “fixing back focus” yielded the site http://leongoodman.tripod.com/d70focusnew.html, which has a rather complete treatus on the whole focus issue, including how to fix it on Nikon DSLRs. After reading through the material, I decided to give fixing the problem a try. Basically, Nikon has provided a couple of mirror stops in the mirror box that control the final position of the main mirror and the secondary mirror used for focus. These stops are adjustable and control viewfinder focus and autofocus separately. As Leon explains, these stops are set during assembly to nominal positions and the camera’s computer settings complete the fine focus during final adjustment.
Checking the manual focus with a 50mm f1/4G indicated a manual front focus problem in addition to the auto backfocus issue. Following Leon’s simple procedure, I was able to adjust the manual focus first and then the autofocus. The only thing different for the D3200 is the use of the “Mirror Lockup for Cleaning” function in place of manually raising the mirror. Only small adjustments were required to bring both the manual and auto focus back to the proper positions. The adjustment cams only had to be moved about 1/20 of a turn.
Checking the other lenses showed that the auto backfocus issue had been resolved. The images below show the before and after results for the 50mm f/1.4G as well as the adjustment of the focus mirror stop. (Care must be taken not to touch the sensor assembly!). Sending the camera back to Nikon is probably the best approach, but in an emergency this approach works well.
Comparison of resolution of a D3200 vs. a D800. The cameras were set up on the same tripod, at the same location, with the same lens – Nikon 50mm f/1.4G @ f/8 ISO 100. I wanted to test the system performance showing the same subject area at the same distance. The objective of the comparison was to see how well the D3200 will perform as a back-up to the d800 in a real world situation. Since the D3200 has a finer pixel pitch, one would expect as good or better detail from the D3200. This ignores the crop factor and just looks only at the ability to render detail for a given lens. Overall a D800 image will have more image information than the D3200 (36MP vs 24MP), but the subject detail is what I am looking at here.
The test was performed by using the same tripod position, same 50mm f/1.4G at f/8, ISO 100, 1/200s raw capture. The images were processed in LR4 and PS, sharpened 60%, radius 1.0, brightness and contrast tweaked slightly to equalize images. The images were cropped to show the same subject area. Any final image detail difference would then be primarily due to the sensor resolution and AA filter. Several images were made with each camera, the best of which was used. Focus was LV and auto. D800 showed no difference in focus, but the D3200 LV was much better.
Three crops from the center are shown: D800 at 455×388 pix, D3200 at 569×485 pix and an upsized D800 crop at 569×485 pix, 1.25x enlargement, which is the same as the ratio of pixel pitch between the cameras. To me there is very little difference between the details shown, with perhaps a slight bit more with the D3200. Click on image for full sized crop.
I am very pleased with the D3200 as a lightweight backup that takes all of my lenses. I am not pleased with the back-focus issue, especially since the D3200 does not have focus fine tuning. I guess the D3200 will be going back to Nikon. Thanks for looking.