Focus accuracy is tackled by a device from Michael Tapes Design that tests camera and lens pairings, and is now available at nearly half the price of the original. Kevin Carter tests the Mk II version of the Lens Align.
Author: Kevin Carter
07 Mar 2012 Tags: Software
Focus systems on cameras with interchangeable lenses aren’t as precise as we imagine them to be, and manufacturing tolerances between both camera bodies and lenses, even when brand new, can give rise to ill-suited pairings, resulting in focus inaccuracies. But focus accuracy can be difficult to assess precisely and consistently, and the more lenses and bodies you have, the more complicated and confusing it can get. While sending a number of bodies and lenses back to the maker’s service department is one answer, there are other more cost-effective and less wearisome solutions.
Over the last year or two, US-based Michael Tapes Design has produced three focus-calibration targets under the Lens Align brand. The original “pro” version is highly regarded, but as it’s supplied ready built, and boasts a rugged, stainless steel depth-of-field ruler, at $149.99 (around £94) plus shipping it’s the most expensive of the variants to date. A second budget model was made for a while before rationalising the design with a third. This new model, the Lens Align Mk II, the subject of this review, offers most of the benefits of the original pro version, yet it’s nearly half the price at $79.95 (£50) and packs flat for portability for use in the field (while also reducing shipping costs).
Even though the Lens Align Mk II requires self-assembly, no tools are needed and it only takes a couple of minutes at best. Despite that, the design maintains the accuracy of the original pro version, which is now no longer in production. The flatpack design reduces the cost of manufacturing but the Mk II lacks the metal focus display ruler and drops the clever magnetic tilting feature of the ruler, which was used to alter the scale’s visibility and thereby accommodate different focal lengths, shooting distances and depth-of-field. For all that, the screen-printing on the MK II’s polystyrene ruler is to a very high standard, and there is a choice of graphics (on the reverse side) to lessen the effects of chromatism.
The 24cm long ruler can’t be tilted, but it’s placed at the same 20˚ mid-setting as the pro version, and is optimised for moderate wide-to-short telephoto lenses. If wide-angle or super-telephoto lenses are to be calibrated, an optional 60cm ruler will be available, although there wasn’t one to be had at the time of testing. The Mk II also lacks the “enumerator” of the pro model. This panel allowed autofocus adjustment and distance settings to be recorded, but if you’re shooting tethered then it’s unnecessary. It’s also worth noting that the focus distance is often recorded in the metadata, although it will take something like Adobe Bridge to display it easily.
A highly detailed user manual is available online giving precise instructions on lighting and distance. Needless to say, the best way to set up the target and camera is to use a tripod for each. The maker suggests a minimum focus distance equivalent to 25× the focal length – half that recommended by Canon – but the results from my own tests at those longer distances have been inconclusive with a rival calibration product, not to mention frustrating. A handy online distance calculator is available at the Lens Align website but the maker’s “8ft per 100mm” edict is easy to recall and there’s some margin for error too. This is an excellent starting point, but it also makes sense to test at a distance at which the lens is typically used.
Like the original, the Mk II uses a novel yet clever alignment system that negates the usual bubble levels (and a shortcoming of rival products), while assuring the focus target is parallel to the camera’s image plane. The Mk II uses what’s called the True Parallel Alignment (TPA) system: essentially two sets of sighting holes, one to the front and another to the rear of two parallel plates, with targets consisting of concentric rings surrounding a red-coloured bull’s eye. The sighting hole closest to the ruler is for the close focusing required with macro lenses, the other more central of the two is used for everything else.
At the prescribed distance, you carefully align the camera’s central AF point with the front sighting hole, making sure the bull’s eye on the rear plate is centred. Next, sight the camera lens from the rear plate, adjusting the Lens Align so the lens of the test camera appears in the very centre of the sighting hole. This back-sighting method is pretty efficient, but one final check and adjustment using the camera in live view mode is required. It sounds far more complicated than it is, but is relatively straightforward in practice.
Until you’re entirely familiar with the process, though, I recommend starting with one lens and camera combination. And, if possible, shoot tethered, otherwise you’ll be back and forth between computer and camera. Not only is visual assessment less problematic with a larger screen (at 100 per cent pixels), but any focus correction made using the camera’s AF fine-tuning option is better seen in as close to real time as possible.
Despite that, due to their imaging characteristics, the on-screen results vary with the lens used. After trying to establish the focus accuracy of my Canon EF 50mm f/1.2 L and 1Ds Mk III with the rival Datacolor Lens Cal (BJP #7792) I decided to double-check the results with the Lens Align. My 50mm f/1.2 is a little soft wide-open even in the centre of the frame, but focus accuracy is difficult to assess thanks to quite high levels of longitudinal chromatic aberration and some spherical aberration at maximum aperture.
As there have been various reports about back-focus issues with this lens I wasn’t completely sure if this was the result of inaccuracy with our 1Ds Mk III. Even after checking with the rival Lens Cal, I could not draw any firm conclusions, due to aforementioned aberrations at maximum aperture.
However, the Lens Align’s graphics do well to counter the masking effects, and after several test photos and surprisingly consistent results, it was clear the 50mm was focusing accurately. I then set the camera’s AF micro-adjustment to +5 and then to +10 repeating the process. While the enforced back-focus was obvious, it was tricky at first to discern the difference between the +5 and +10 settings (bear in mind the range of adjustments varies over ±20 steps).
Converting the image to mono helped remove the distracting chromatism, making it a little easier to assess. However, fine-tuning the 50mm f/1.2 accurately to just ±1 or 2 steps is just too much of a stretch. I checked several of my lenses over a four-hour period, so if you are planning to check and calibrate a number of lenses it would be best to set aside an afternoon or morning, as it’s a time-consuming process.
The Lens Align Mk II is certainly a well-designed tool, and at just £50 plus shipping it’s a must-have for many professionals. However, the results are somewhat dependant on the imaging characteristics of the lenses to be tested. My copy of the 50mm f/1.2 is simply a bit soft wide open. Other lenses with better optical performance wide open are easier to assess.
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