One of the hidden features of higher end digital DSLRs is the ability to micro-tune the autofocus. This stems from the fact that any mass produced widget has a perfect setting and a high and low allowable setting. You have seen and heard this referred to “plus or minus X”. But in mass production, nothing is perfect so the lenses and camera bodies both will have a plus or minus tolerance on their autofocus settings. The problem arises when the body is at the end of the acceptable tolerance and the lens is off the same way and maybe at the end of it’s acceptable tolerance. The aggregate plus or minus is far beyond acceptable can show up as a “back focusing lens” or a “front focusing lens”. This is where you set your focus point on, say the eyes and you get the nose instead when using a shallow depth of field.
People will try several lenses to get one that works well with their camera body when all the while it’s the body that is off adjustment. Meanwhile the lens manufacturer takes the hit for making “junk” lenses. Newer and higher camera bodies such as the Nikon D300 have an adjustment for this very thing but very few photographers even know about it. To use it is pretty easy and can be cheap or if you are bit more picky, somewhat pricy to use. In both cases the tools are the same, a marked ruler and a way to present it at a 45 degree angle.
The cheap way is to go to Tim Jackson’s website and download the PDF with directions and the ruler chart. The cost is how much the piece of paper costs you. Follow the directions here to actually make the adjustment.
The more expensive but professional and repeatable way is to buy a LensAlign Focus Calibration System. This fun tool comes from rawworkflow.com and while not cheap, makes a very professional and very repeatable alignment station. This allows you to check and tune the lens and body anytime something has changed or just as an annual item.
It my own case, I have both, the paper and the fancy tool. I tried the paper first since there was a Christmas delay in getting the real tool and I was impatient. I have a 50 mm 1.8 lens that I like but never was as sharp at 1.8 as everyone said it should be. I also was of the opinion that it was a “junk” lens. I did the fast check with the free tool and I was stunned at what it showed.
NOW the 50mm works like it should on my D300. So much for the junk lens. All of my lenses were off some but the 50 was the worst. Perhaps as a cheap lens, it does not get the same level of care in the final adjustments. The 50mm is so far off that it probably should be sent in for a alignment but given the cost of the lens, it’s not worth it but this provides me a way to use it effectively on my D300s.
Not all DLSRs have this feature, for example, my D90 does not but the D300 does. I would suspect that the average consumer with a D90 is not shooting tight wide open shots where a couple of mm’s one way or the other matter alot. I know when I started off shooting a 35mm film camera, I was happy just to get the background to blur out, much less everything around an eyeball 🙂 And times have not changed that much, most people are happy with a in focus picture without a worry about the depth fo field. It’s the pro’s that care about this sort of thing and even then, it’s the pro’s who are shooting wide open and need that critical focus point because it will make or break the entire picture.
I will post my results from the more expensive tool vs. the free PDF in a later post.