Many modern cameras, particularly full frame or relatively high end models, have the ability to fine tune the autofocus for each lens. Calibrating autofocus is not hard, but it does require a couple of items. You’ll need a precision target for calibrating and it greatly helps if you can tether to a high resolution laptop (I tether to a Mac Book Pro). A great tripod and head are very helpful since the whole calibration requires that you aim the camera precisely and that it stays absolutely rigid during the calibration process.
I undertake calibrating autofocus by using the SpyderLENSCAL. This device has a bubble for leveling, as well as an attachment for a tripod. You can just as easily use it on a table, so long as you are exactly level. Using a single central focal point, focus on the small square beside the ruler. Go to the monitor and examine whether the focus is exactly on “0” or if it is in front or behind. Front focusing and back focusing can both be adjusted. With a device like this one, it is easy to see what needs to be done.
Maybe I should also state what I think is obvious. This is essential. Calibrating autofocus is how we get the sharpest images from those expensive camera bodies and lenses. They all have a tolerance range, and you won’t be sharp unless you dial it in. Your camera would not have the feature of fine tuning this function if the engineers at Canon or Nikon thought it was unnecessary.
How much to change the focus in the plus or minus direction is a matter of experimentation. You might want to start with a wildly strong adjustment like +20 and examine the next test shot. Then adjust a more reasonable amount, if any adjustment is needed. Because you have to estimate and test the result, that’s why it helps to be tethered to a good screen. You can’t see well enough on the back of a camera to be accurate and it takes too long to make multiple trips to download to a computer. Once you have done a lens or two, you will have the hang of it and will dial it in quickly.
I have some very fast lenses, f 1.2, and they have tiny depth of field when they are wide open. All tests are done with each lens wide open, the widest (smallest number) aperture available on that lens. The camera knows which lens is attached, and it will give you specific adjustment choices. If you have a zoom attached, it will ask you to calibrate the lens at the widest and at the most telephoto setting. With both ends of the lens calibrated, you’ll have sharp images.
B&H Photo sells the SpyderLENSCAL. It comes with simple instructions, but you also need to know that you should set the camera close to the card when you do the tests. You’ll want to be fairly close to the closest distance at which autofocus works for that particular lens.
Finally, this is most essential with macro and telephoto lenses, as well as exceptionally fast lenses. If you’re lucky enough to have an f 1.2, then that really narrow depth of field needs to be precise. If you are shooting wildlife with your 600 mm lens, then this is absolutely a must. A $65 device that can make a $10,000 lens accurate is clearly worth it. With really wide lenses, it is less likely to be critical in “real life” since there is so much depth of field. However, if you look at the ends of the scale on the photo at below left, you’ll see that the 6″ is clearer in front than in back. A minor adjustment evened that out. But with my 14 mm lens almost everything is sharp.
This whole process is very lens specific. If you own two 50 mm lenses, even if they are both 1.2, your camera can tell them apart and will apply the right calibration to each. My those Japanese engineers are pretty much magic. but only pretty much or this would never need to be done.
If you’re a Spokane resident, I can calibrate your lenses for $50 each, or 3 for $100. I’ll get cheaper each if you have a bunch. You can buy your own SpyderLENSCAL for $65, but it does require tethering or ridiculous time and patience. That little piece cost me over $4000, since that cheesy Mac was $3995. Maybe worth having someone else do it for you. Of course, your camera must support calibration. High end Nikon and Canon do that. These pictures may make it seem like the differences are minor, but they really were significant. I can see more when magnified on the Mac and some lenses were desperately in need though I had all name brand and excellent optic lenses.
We wish you sharp images. Really sharp. We want those eyes in focus in our portraits, every feather in focus and every blade of grass sharp!