Still in the wedding ‘off season’ its time for a thorough check of all my gear. One of those jobs I do periodically is checking my lens calibration. Its always been a fairly subjective and difficult task doing it manually. Time to see if lens calibration with Reikan FoCal software is any easier than doing it manually.
A great question. Simply put while autofocus is largely accurate it only takes a tiny bit of misalignment to throw it off. Both cameras and lenses are mass produced and have a degree of acceptable tolerance in accuracy. A lens may focus in front of the target (front focus) or behind the target (rear focus). While a lot of care goes into quality control, some variance is inevitable in manufacturing and still considered acceptable. In most cases the variance within that tolerance wont make a big impact. On the extreme edges of the tolerance however, it can do.
To complicate things the variance within tolerances between camera and lens interact. In the best case a lens that tends towards back focus coupled with a lens that tends towards front focus could cancel each out. Worst case scenario is that when both camera and lens tend towards front focus or both towards rear focus. When that happens the variance of the lenses, added to the variance of the camera can make for inaccurate focussing.
Because of this interplay between camera and lenses, high end digital cameras often come with a micro autofocus function. This essentially allows the camera to compensate for the variance of a particular lens in order to achieve the best possible focus with that camera and lens combination. Note as each camera and lens is a little different the micro autofocus adjustment must be made on the particular camera with the particular lens to be used. The same adjustment could not be made against all camera and lens models of that type.
The adjustment is easy, micro autofocus adjustment can be found in the camera’s menu and has a scale of for example -20 to +20 with minus figures take the focal point towards the camera, 0 having no affect and plus figures moving the focal point further away.
The hard bit is working out what that micro adjustment should be and then gauging the results to see if that makes the focal point more accurate or less!
There are all sorts of techniques that suggest using a piece of paper at 45 degrees. While this can work to some extent, for really accurate lens calibration, you need to invest in a professional solution.
Lensalign is one such system which has a big following amongst professionals. whilst it does give you tools to accurately see if your lens is front of back focussing, the exact adjustment still requires judgement.
Reikan FoCal on the other hand is software based. It objectively works out the best micro autofocus adjustment for each lens on that particular camera and is automated.
In practice Reikan FoCal is very easy to use. You print out a target to give Reikan’s FoCal something to autofocus on and place on flat wall. Mount your camera on a tripod parallel to the target and make sure the centre AF point is in the centre of the target. Attach your camera to your laptop running Reikan’s FoCal and click on ‘connect camera’. After the camera has been acquired you hit the ‘Calibrate’ button and then the ‘setup target’ one. The software goes through a number of checks like lighting and distance to subject to make sure you are setup properly for the lens calibration. Its very hard to go wrong with a nice traffic light system of red, amber and green giving you warnings if you need to change anything to your setup.
Assuming that has gone all OK you just hit calibrate and away it goes. The software controls your camera to make a number of shots of the target at each setting to measure shot to shot consistency. It does this with different micro autofocus settings from -20 to + 20, objectively analysing the images at each setting and work out which was sharpest. For some Canon users the whole process is completely automated. A full list of Cannon cameras that support hands free operation are listed here. For Nikon users sadly we have to change the Micro autofocus setting manually, but its quite easy to do in the cameras menu.
At the end FoCal gives you a recommended Micro auto focus setting and shows you a before and after picture for comparison.
The only three caveats to be aware of are:
Yes! I was very impressed with the software. The autofocus is definitely more accurate now than it was across all my lenses. Predictably this is most noticeable at wider apertures. While before I didn’t like shooting on my 50mm F1.8g below F2.8 as I thought it was too soft, I would now happily shoot at F1.8. That’s a whole extra stop of light to play with.
For the record I had already calibrated my lens manually (or so I thought). Reikan FoCal suggested different settings which do appear to produce sharper results. Manually at best I was 3 AF points out, at worst 17! To be fair on that particular lens I only ever used it in manual mode anyway as its a specialist macro lens for close up work.
In summary Reikan FoCal is great bit of software. I’m surprised more photographers are not talking about it. When you can buy something for $39 that makes all of your lenses better its hard not to like it. Particularly if like most professional photographers you have several thousand pounds invested in lenses and it can make an already great lens better.
It’s low cost, and easy to use and gives great results. What is not to like?