With D-SLRs having ever higher resolving power, and lenses just getting better in terms of optical quality .. and digital photography giving us the possibility of looking at images at 100% resolution, we are becoming ever more aware of slight discrepancies in focusing and sharpness. Especially so with lenses that have very wide apertures.
Quite often a camera will be within spec, and a lens be within spec in terms of their individual tolerances, but combined, they might be outside of tolerance. Or perhaps a lens wasn’t quite within spec .. but for whatever reason, we might end up with a camera and lens combination which juuuuust isn’t quite as sharp as it could be.
For example, with the one lens I recently bought, the Nikon 200mm f2 VR, I frequently felt that I could touch up the focus manually a little better than the camera had focused. And checking on the camera’s LCD screen, I could see that. From the most recent shoot where I used this lens, (which I’ve shown in earlier postings), here is an image where the lens back-focused. And it appeared to do this consistently.
Now at normal web-size and even small print enlargements, you wouldn’t be able to discern that the image isn’t quite focused accurately on her nearest eye. But as soon as you go to 100%, you can see the focusing error …
… and this simply isn’t good enough. I like sharp. I really like images to be crisp.
Any softness in the image should be a deliberate choice on my part.
Now there are a few ways to address this small focusing error. You can either send your equipment in for calibration, but that means you are without your equipment for what is a small, yet seriously niggling problem. Or, you can adjust the camera yourself if you are fortunate enough to own one of the more recent models of D-SLRs that boast a feature where you can micro-adjust the auto-focus.
The Canon 50D, 5D mk2, 1D mk3, 1Ds mk3; or Nikon D300, D700, D3, D3x; and even the most recent Sony A900, and Pentax K20 have this feature.
You can either create a make-shift device, or you can take a convenient shortcut like I did, and get the LensAlign Pro kit, which allows you to do the AF micro-adjustment on your camera with confidence. They also have a smaller kit available that you can assemble yourself.
Some of the make-shift methods I’ve seen on the forums, rely on you to photograph a ruler on an incline, focusing on a certain spot. The problem here is that if your camera’s focusing sensor isn’t exactly placed where it is marked in the viewfinder, you might think you have a camera that back-focuses, when in fact it doesn’t. This will then send you off on a wild-goose chase trying to figure out where the problem actually lies.
The LensAlign system side-steps that problem by a simple yet ingenious way that you position your camera at the exact perpendicular angle to the target. This is described in detail on the LensAlign site, as well as the thorough review on the Luminous Landscape site, so I won’t go into the particular method. (There are video clips as well on how to set this up.)
You then focus on, and photograph the vertical target. The incline ruler then gives a clear visual indication of whether your camera is in fact focusing where you think it is. Here are the results I got with setting up my 200mm f2 lens.
With the AF fine-tune set to the zero default, here is what I saw:
(clicking on the image will bring up a larger version)
.. and looking at the image at 100% you can clearly see that the lens does indeed back-focus.
Then through a series of images taken at various settings of the AF Fine-Tune on the Nikon D3, I was able to get the lens to focus such that I thought it looked sharpest at the exact plane of focus. To judge the sharpness, I used the magnified view on the LCD; as well as the Live-View feature on the camera; but the most accurate way of judging the sharpness was on the computer screen.
And here is the result with the AF Fine Tune set to -10 with that specific body. (My other D3 body needed a setting of -12)
This now gives me the confidence that my lens will work as it should during a shoot, focusing with accuracy.
That is something that is hugely important to me … that I have confidence in how my gear performs. The LensAlign Pro kit helped me considerably with this, and I would highly recommend it to any serious photographer who demands the best from their equipment.