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I’ll warn you right up front; this post is not for everyone. Some cameras have the ability to make a fine tune adjustment to autofocus. For those with a camera model that allows for fine tune adjustments, checking focus accuracy can be important, especially when shooting macro images where pinpoint focus is more vital. For the average shooter, the millimeter adjustments might not be as vital.
First off, here is a list of camera models known to have the ability to make micro-adjustments. This list is by no means complete, so check your camera’s instruction manual to see if this ability is available to you:
I was sent a device from datacolor which aids in checking focus. You can certainly make your own device as the principle involved is simple: Focus camera on a known flat object and then measure front or back to where the camera is actually focusing. The time and know-how involved in making an accurate device would likely be comparable to the MSRP of $59USD for the datacolor LensCal.
Get a price for the Datacolor DC SLC100 SpyderLensCal Lens Calibration System on Amazon.
To check camera and lens autofocus (this test is best ran with each lens/camera combination you use) you will either need two tripods or a level surface. The LensCal comes with a tripod mount hole in it and it contains a bubble level.
Insure both the camera and LensCal are level, exactly perpendicular and at the same height. This is very important as being off to one side or the other, or pointing the camera slightly to the side, will cause an error in results. In my case, I used a table and bubble level. Turn off vibration reduction/image stabilization capabilities.
Lighting is important but not super critical, meaning a studio setup is not required. Make sure the target is light from in front. I chose a sunny day when the sun was slightly off of noon to give ample lighting.
datacolor notes the distance from lens to target should be around 5 to 10 times the focal length. The lens should also be zoomed all the way out, if it is a zoom lens. For instance, a 100mm lens should be 500mm-1000mm away. Some camera manufacturers might have other recommendations. The aperture of the lens should also be opened all the way (lowest number possible at that focal length) and it is best to use aperture priority mode. An open aperture will insure a shallow depth of field, making differences in focal points easier to spot. To help with viewing the image after shooting, I highly suggest using the lowest ISO you can achieve. There should be plenty of light available, so a low ISO won’t be a problem.
With autofocus turned on, choose a focus point at the middle of the target area. It’s important to not let the camera select a point from the ruler, which is why manual focus point selection is suggested. Now take a shot.
I suggest importing the photo into your computer for checking. Some people use the screen on their camera, which provides all the accuracy they are looking for, but I would not recommend its use. In the shot above, it appears my camera is back focusing; it is focusing too far to the rear. I note this by checking to see if the 0 is the sharpest point on the image. Here’s a 1:1 zoom of the above image (click for full size).
Now, I don’t have the best eyesight in the world, but I can see that the area behind 0 is more in focus than the area in front of 0. Is it huge? No, but it is noticeable. I selected a point about four steps back from 0 as what I thought was in focus. I then made the adjustment to my camera. Canon has an easy to understand symbol these days (it used to say Forward and Backward and confused some people). In this case, I adjust my focus backward by 4 millimeters, because it was back focusing.
I then reshot the target to check focus.
Is the change huge? Nope. And hopefully it won’t be for you too. At times I would use a 3:1 zoom in Lightroom to check, but at that zoom it becomes difficult to really tell. 1:1 works best.
Just to see how far out of whack things can be, here are two shots adjusted to -20 (pulling focus closer to the camera, creating more front focus) and +20 (just the opposite).
The biggest question I know many of you will ask; is the price worth the result? If you have multiple cameras and lens combinations, this device might serve you well (especially in a camera like the Canon 7D which allows for adjustments to be saved for each lens used). If you shoot macro, this device will also serve you well because at that range, millimeters often mater. Lastly, if you are a pro and tight focus matters to you, this device will help, especially if you rent lenses or upgrade camera bodies (it folds flat for easy storage).
For the average user, I would say the result might not justify the cost. In that case, an alternative method might be to shoot a flat, contrasting target and step through all 40 of the adjustments available (if your camera goes to -20 and +20). Then review in a computer and make your best guess which is most in focus. This is certainly more time consuming and requires a steady hand as you make about 120 presses of buttons on through your camera’s menus.
Check out the Datacolor DC SLC100 SpyderLensCal Lens Calibration System on Amazon.