When I first started taking pictures I never thought about the accuracy of auto-focus lenses. I just took it for granted that lenses were accurate and sharp focus was a shutter button away. Not so.
I found out over time that some combinations of camera and lens might be a tiny bit off when you use auto-focus.
How to Check Camera and Lens Accuracy
Checking your camera and lens accuracy is a very simple process, but, it can take some time. Here is how I do it.
Set up two small table-top light stands with daylight balanced bulbs. I lay them on the floor. I set an open tape measure on the floor between the light stands. If you don’t have lighting (most people don’t), find a good source of natural light or a well-lit room in your home to do this.
Next, I set up my tripod and, one-by-one, I take a picture of the tape measure from an angle of about 45 degrees focusing on the sixteen inch mark. I look at each picture just after I take it to make sure that the sixteen inch mark is the sharpest spot on the tape measure.
Done right, the fifteen and seventeen marks should start to blur and the sixteen should be very sharp.
What Are the Odds?
What are the odds that all of my lenses are in perfect focus and I don’t need to fine-tune the auto-focus accuracy? I’d say slim to none. But, what’s this?!
My Pentax KP is perfectly accurate with my Pentax 200mm f/2.8 lens, my Sigma 70-300 f/4-5.6 APO DG lens, my Pentax 50-200, my Pentax 18-55, and my Tamron 28-75mm f/2.8 lenses. Those are the assorted lenses I carry all the time when I go shooting.
What About Teleconverters?
Although I did not test teleconverter accuracy, they might change the focus point on a camera and lens combination. I have two different 2x teleconverters in my camera bag. One is an auto-focus Tamron (Bower branded) and the other is an older 2x Pentax that is manual focus. I’d venture a guess that the Pentax is, perhaps, more accurate, but, with my sixty-year-old eyes, I am not sure. I get better results with focus from the Tamron/Bower 2x converter and there is no color fringing visible in my images.
The Pentax converter is notorious for creating heavy purple fringing on my 200mm prime focus lens. The Sigma lens appears to have no fringing at all when I use the Tamron/Bower teleconverter.
There is a little adjustment wheel or slider, depending on your camera, that adjusts your viewfinder to match the focus of your lenses. To set it correctly, push the camera’s shutter button half way. Now, adjust the diopter so that the viewfinder is in sharp focus (while aiming at your subject).
If the viewfinder is adjusted right you can rely on your eyesight to focus manually when you need to do so. Auto-focus systems are often pulled away from your subject by blowing tree limbs, cattails, and other flora when you are shooting birds, in particular.
By the way, my K-50 needed some adjustment on a couple of lenses to have perfect auto-focus accuracy.
Read Your Manual
Most modern DSLR cameras have a micro-focus adjustment in the menu system somewhere. Read your owners manual to find out where and make adjustments to compensate for each camera and lens combination you shoot. The camera will hold about twenty combinations in its memory, so, you’ll only have to do this once for each lens you use. The longer the lens focal length, the more important this is.
Is it a pain? Yeah, perhaps a little. But do it, you’ll be glad you did.