LONG LENS + TC TEST SEQUENCE, PART 1 Question: What is the best option for a long telephoto suitable to photograph, for example, a small bird? Central image IQ is most important, edge/corner sharpness and some vignetting are not a great concern. Will a TC actually record more detail or will it degrade IQ so that just-as-good-or-better IQ can be had by simply cropping/enlarging from a photo taken with the same lens without a TC?
Long K-mount lenses I have to test: 60~250; 300; 150~450; Tamron 400 f4 (PKA mount).
TCs I have for testing: 1.4X Pentax (newest); 1.4.XL and 2XL Pentax, but these will only fit the Tamron.
Test conditions:
1) Standard USAF resolution test sheet purchased many years back from Edmund Scientific, kept flat between cardboard sheets since it was purchased so it's in excellent condition, not faded by light exposure.
2) resolution chart pinned on a wall and illuminated by a single Cactus flash at the left, aimed at about a 45 degree angle and far enough back to give near uniform illumination..
3) Camera + lens mounted on a Bogen/Manfrotto 3221 tripod with a geared head
4) K3 camera set @ X, ISO 200, triggered by cable release with 2 second shutter delay; images recorded at highest quality JPEG.
5) Center column of tripod 21 feet back from the test chart (farthest it was practical within the confines of the basement where my "studio" is located).
6) Tripod elevated so the camera + lens was at the same height as the center resolution block of the test sheet.
7) Best IQ for most lenses is about 2 stops down, so I selected f8 for all lenses without TC. When a TC was attached, the lens was left 2 stops closed (= aperture set via camera to f8, but effective aperture either f11 or f16) so the Cactus flash had to be upped in power one EV step for all except the 400mm + 2XL combination, which required a 2EV increase in flash output.
8) For each lens or lens + TC combination I refocused six times: 3 times using live view, focus peaking and enlarged 8X for almost all shots (4X for the 400 + 2XL because the slightest touch on the focus caused so much vibration it was impossible to see if focus peaking had been triggered); 3 more images were taken by using the electronic rangefinder function. For both sets of three, at least once the focus point was approached from the opposite direction (= focus twice starting @ infinity and once starting at minimum focus distance, or once from infinity and twice from near).
9) select sharpest image from each lens & lens + TC combination
10) All the images were slightly dark (white background was gray), so all were brightened about 1/2 EV
11) The central group of resolution lines, black in color, were taken to be the equivalent of a small bird (ignoring the numbers around the periphery, the block measures very close to 2X2 inches) SO, for each lens & lens + TC just this central block was cropped out for comparison.
12) For deeper comparison, I also cropped out the smaller block of resolution lines within the 2X2 block. This block, again ignoring the numbers around the periphery, measures slightly more than 1/2 inch square.
As a starting point, to give perspective on the size of the original at a distance of 21 feet, the image below was taken with a 35mm f2.8 macro mounted on the tripod. The door through which I was shooting is, of course, not illuminated by the flash. Look carefully - - the subject of interest is the black group of resolution lines at the very center of the chart.
Last edited by WPRESTO; 01-29-2016 at 12:30 PM.
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