Lens Calibration


Reikan FoCal

My initial thoughts

For some months now, I have been thinking about using some software to try and make lens calibration quicker, easier and maybe a bit more accurate.

After reading some good reviews, I purchased the pro version of FoCal this morning.

It was easy to set up and get running.

Up until now, I have always set lens calibration manually. It was a little time consuming and monotonous to be truthful but always gave a better result for the effort.

My manual method was not the most sophisticated of ways,,,,,I found a wooden fence post out and about somewhere and stood at a distance roughly to what I thought my subjects would be at (birds) I then took a number of photos at differing focus values with the same shutter speed, iso and aperture and chose the best value in terms of picture detail.

So far, out of the small selection of lenses I own I have only manually calibrated my EF500f4mk2 on a 5dmk3.

Here are the values I manually calibrated to some time ago.

500mm ~ 0 value

500mm + 1.4x tc which equates to 700mm ~ +5 value

500mm + 2x tc which equates to 1000mm ~ +10 value

Calibration values from FoCal.

500mm, the software suggested a value of +7

I couldn't quiet believe this value, as a bare 500 lens it was pretty damn sharp wide open, and the reason I never bothered to even try calibrate it. Could it be any better? I tried +7 and the results were awful. From this, I chose my manual method of calibration and came up with +3, lots better, maybe even a tad sharper than the value of 0 I'd been using for ages.

500mm with 1.4x tc

The software suggested a value of +5, this was the value I found best when I had a go at manually calibrating.

500mm with 2x tc

The software suggested a value of +1.

I found +10 to be best when I manually calibrated.

So, at present there seems to be two conflicting values. The reasons for this I am not to sure of at the moment. When I carried out the calibrations on lap top, the light was not to good. This I am sure is a factor (lighting) as AF needs good light for best focus, hopefully the sun will shine tomorrow when I shall try again.

I also have a few concerns with the interaction of the FoCal software and the camera. Once you have the focus target set up in the garden or wherever and everything is aligned properly, you bash the start button on computer and the software controls the camera, it takes a few pictures and analyses the target for sharpness. Now, from the brief bit of reading I did on the software, I thought (wrongly) that everything was automatic. I thought that the software would do its thing and give you a suggested value at the end. Its not the case for the 5dmk3 and a number of other Canon and maybe Nikon bodies as well. Some cameras, it's fully automatic, you run the test and the software will analyse and adjust the focus value in camera so you need not touch it.

With my 5dmk3, you run the test, the software will take a few shots of the aligned target, analyse them and then ask you to set the focus value in camera. You start at a value of 0, it will then ask you to change the value to say -20, take a few more shots then ask you to change it again and again and again until it has found what it thinks to be the best setting.

This I found a huge problem. A 500f4 lens isn't the lightest of things and every time I tried to set the focus value in camera, I would knock the lens out of alignment with the target on my tripod and the test had to be started all over again once re-aligned, the problem became even worse when I added teleconverters. At such true focal lengths, just knocking the camera by half a millimetre on the tripod put the target out of view of the lens. I swore quiet allot!

In the end, a solution was found, all the adjustment screws were tightened Gorilla tight on the tripod head and two house bricks were hung off the bottom of the tripod stem to hold it all tight, never had a problem after that, although I did touch the camera to set the focus values with very shy gentle movements.

As it stands at the moment, if all you want to do is set the focus, I think doing it manually will be just as accurate and maybe just as quick. There are a few other handy applications with the pro version, it will tell you the sweet spot of a lens, analyse the dust on the sensor and at what aperture it will become a problem and a few other little bits, I've yet to test those. Spent most of my testing session playing with bricks!

Im sure with good light and now the tripod is solid for the test this could be a very useful tool.


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