I recently acquired a “focus confirm” adapter for my Zeiss Mirotar 500mm which comfortably won my previous test on mirror lenses.
Tests with “Focus Confirm Adapter” on the Zeiss Mirotar 500mm f8.
The “Focus Confirm” adapter was from eBay (China) and has a programmable chip that enables me to fool the Canon 5D mk3 into thinking that the lens is f5.6 which can thus provide the flashing focus point and green light to confirm focus. I have spent some time adjusting and calibrating it. Early signs are promising, though it is not as accurate as live view – it may make it more viable for viewfinder use out and about. Trying to hand hold a 500mm lens and use live view is really very difficult, as the image on the back of the camera is simply not stable enough, particularly if you magnify by 5x or 10x to get precise focus, so live view is restricted to tripod use only (I’ll experiment with a monopod later). I went for a walk on Christmas day morning and took some shots in the park of static and moving subjects. My success rate was around 1 in 3 shots sharp when hand held, far better than I have achieved just by eye alone. Here are some examples. Note the “doughnut rings” of the out of focus highlights behind the people.
Above is the Shoreham Power Station chimney. Click to view larger. The detail recovery is remarkable.
Setting up the adapter was straightforward, involving selecting particular combinations of apertures and firing the shutter (f64 f57 f64) to trigger “setup mode” on the adapter then choosing
f2.2 (fire shutter) to enable maximum aperture to be set (this needs to be f5.6)
f2.5 (fire shutter) to enable the focus adjustment in a range 0 to 30 ( different apertures correspond to the numbers 1-30 – a table is provided)
F2.0 to set the focal length – so that the EXIF data records this as 500mm – useful when searching in Lightroom for example.
I spent over an hour on setup with most time experimenting with the focus adjustment. What is clear is that if you approach focus from near to far, and then from far to near the focus confirms in different places (close of course) but there is a range where the focus light stays on. I’m well used to using focus confirm with my manual focus Zeiss Distagon 21mm – but in that case errors are minimal due to the wide angle lens having large depth of field even at f2.8 compared to the very small depth of field of the 500mm f8. This problem is exaggerated at close distances. In the field I tried approaching focus from both directions and then oscillating back and forth until I could get the focus light to stay on.
It is certainly better to have focus confirm with a manual lens. In the case of the Zeiss 500mm f8 it provides a benefit, but does not entirely solve the problem, due to the inherent issue of a long telephoto lens and the tolerances of the focusing system. It will certainly mean that I will use the lens more often though. At £24 it is worth a try.
My original Mirror article and comparison test explains about the advantages and disadvantages of mirror lenses in detail, and how with a digital SLR and post processing techniques, many of the inherent faults can be overcome.