I've just picked up a RMC Tokina Doubler for significantly less than the Pentax doubler T6-2x I've been seeking and I've been looking for reviews to justify the purchase
Something I saw on photo.net was the comment that enlargement reduces resolution by the factor of enlargement (obviously) whereas a teleconverter might only lose 20% of the resolution. I thought about this statement and realised that, indeed, as the doubler is exposed to the resolution coming out of the back of the lens, not looking at the image on the film/sensor, there are different physical effects in play. If the aerial resolution of the lens is miles better than the sensor (as is normally the case with a decent prime), then the degradation from the doubler may well be less than that due to cropping after the image has been captured.
If the doubler has the same resolution as the prime in use, then the resolution of the combination will be 1/sqrt(2) of the prime alone. If the prime lens' aerial resolution, for example, is 200lp/mm, then the combination will still deliver ~140lp/mm, way beyond that which the sensor or film can discern.
Only once the resolution of the doubler itself starts approaching the resolution of the sensor will there be any real concerns. So I thought I'd post something to the forum along this line, as I don't think I've seen anything like this written down here, just statements along the lines of TC's degrade the image just as much as cropping, so you might as well crop. Either way, something immediately obvious is that distant focussing is far more accurate with the Doubler. As I shoot film, I'd rather just use the Doubler, than have to move the enlarger head and crop at the time I come to print, so its a bit of a moot point for me, I just thought I'd share a thought.
Last edited by whojammyflip; 09-27-2010 at 03:04 AM.