I published this over on the Coin Community Coin Photography Forum, and thought it might be of interest here....
Downsizing an image 50% eliminates most of the camera sensor demosaicing aberrations, but results in half the effective magnification. 100% crops of the original image show demosaicing/interpolation blurriness. Using a 2x teleconverter (2xTC), then downsizing the image 50%, the final image has the same effective magnification, but half the field of view. It is essentially a crop from the center of the image without the 2xTC, but the downsizing eliminates most of the sensor aberrations. Fortuitously for coin photographers, the corners of this image extend to the radius of a coin, so checking corner sharpness on the magnified/downsized image can tell us how good a lens is for shooting coins.
Take a look at the image below to explain this concept:
Using a 2xTC this way is similar to using a microscope objective to magnify the aerial image from a lens, just with lower magnification. However, the microscope objective only lets you view the very center of the image in great detail. The corners of the image are outside the image circle produced by the objective.
Here are some images to show the new testing method using a 75ARD1 at f4. Each image was rendered from a focus stack of 5 source images.
Overall image, without 2xTC:
Above image, 50% Center Crop:
Overall image, with 2xTC:
The above two images are the ones to be compared, so I will take crops from them, and magnify the crops 200% to make it easier to see differences.
Center crops animation from images with and without 2xTC:
Upper left corner crops animation:
The animations show improved sharpness on the 2xTC shots, consistent with removal of the demosaicing aberrations. The center improvement is more obvious than the corner, perhaps indicating that the corner is already degrading, even at 50% of the APS-C radius. There is also some distortion due to the stacking process.
I'm hoping the improved resolution offered by this technique will help me to better distinguish between lenses for coin photography. Often lens differences are subtle, and I believe by removing the basic demosaicing blurriness the lens differences will be easier to evaluate.
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