AF Microadjustment: A Detailed Look

August 02, 2011

...the key is taking test shots where you can recognize focus as it shifts forward or backward from your intended subject...

AF Microadjustment is a feature seen in most recent mid-range and high-end Canon EOS digital SLRs. Introduced in 2007, it allows the photographer to shift the camera's plane of sharpest focus if he or she feels that the camera is consistently putting focus in front of or behind the intended subject. For example, if a photographer felt the camera was "back-focusing" - that is, putting the sharpest plane of focus consistently behind their actual subject - AF Microadjustment allows in-camera adjustment to move the sharpest plane of focus forward so that it's closer toward the camera.

While focus precision with any EOS camera is extremely high, AF Microadjustment is a tremendous leap forward in fine camera control for the critical photographer. We'll explore how to best utilize it in this document.

What does AF Microadjustment do?
It allows the user to command the camera to intentionally shift the sharpest focus either in front of or behind where it's factory-set. The extremely precise AF system in a digital SLR is designed to read contrast at the subject, calculate how to drive the lens to focus sharply on the subject, and confirm sharp focus once the lens has stopped. With AF Microadjustment, the user is changing the data coming from the AF system, and asking it to move the lens farther in one direction or the other whenever it has to read and calculate sharp focus.

The adjustments applied using this control are based on the depth-of-field you'd have at a lens's maximum aperture. They are not based on the lens's focal length! When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments. Don't expect radical shifts in focus with adjustments like plus 3 or minus 5.

The basic procedure:
First step: closely examine real-world images you shoot - ideally, stationary subjects using One-Shot AF mode (more on this in a moment). If most of your images are sharp and only once in a while you see a slight focus shift, STOP. AF Microadjustment will shift focus for each and every picture you take, and you probably don't need it if the AF system is generally producing sharp images, with focus where you expect it to be.

On the other hand, if there's a consistent tendency for the sharpest part of the picture to be either in front of or behind the subject, read on.

Take a series of test shots. We'll detail these in a moment, but the key is taking test shots where you can recognize focus as it shifts forward or backward from your intended subject.

Once you settle on an adjustment, shoot and examine more real-world subjects on your computer screen. AF Microadjustment is a trial-and-error procedure, so it may take a few cycles of adjust > test > evaluate to find your sweet spot.

AF Microadjustment options:
It's possible to use AF Microadjustment in either of two ways:

1. Adjust all by same amount: If you find that upon close inspection, no matter what lens you use, the camera tends to put sharpest focus in front of or behind where you need it to be (not just once in a while, but on a consistent basis), this is the setting to use. Once set, the camera will intentionally move the sharpest plane of focus either forward, toward the camera, or backward (more toward the background).

2. Adjust by lens: This is the setting to use if you find, when looking closely, that pictures taken with one lens seem to consistently be front- or back-focused, but images taken with other lenses on the same camera seem fine. Mount the lens(es) you feel are not working properly with the camera, and take test shots as described below. Up to 20 different lenses can have an adjustment applied for that specific lens model, memorized by the camera, and the camera will automatically apply your adjustments as soon as you attach that particular model of lens. You'll even see the values appear within the camera's AF Microadjustment Custom Function menu screen whenever a lens you've applied an adjustment to is mounted.

A few important points about the Adjust by Lens feature:

     • The camera can distinguish between different models of Canon EF lenses, but it cannot read serial numbers and distinguish between two samples of the exact same lens. In other words, it can tell if you have the 70-200mm f/2.8L or 70-200mm f/2.8L IS II lens attached. However, it cannot distinguish one 70-200mm f/2.8L IS II lens from another.

     • The camera recognizes an EF lens with a Canon EF tele extender attached as a separate lens. So you could, for example, have one adjustment for your 300mm f/2.8L IS lens, and another focus adjustment for your 300mm f/2.8L IS with the Canon EF 1.4x II tele extender attached. The camera would automatically apply whichever one is needed, based on whether that lens is being used with or without an extender.

     • Operational reliability is guaranteed with Canon EF lenses only. While users are free to try the procedure with third-party lenses, there's no assurance of proper interaction between body and lens. A third-party lens can also be mistaken for a Canon EF lens, if the latter is subsequently attached.

     • Even if you perform the Adjust by Lens procedure, if you attach a different sample of the exact same lens model (example: a newspaper shooter owns a particular lens, but borrows the same model lens from his or her paper's loaner equipment for an event), the AF Microadjustment can be temporarily turned off (but held in memory) by simply selecting "Disable". You can re-apply your settings later when you attach the exact lens you made the adjustments for by re-selecting "Adjust by Lens" within C.Fn III-7.

     • You can always take more test shots in the future, and change any previous AF Microadjustment for a specific lens model. In this case, the previous adjustment amount for that particular lens is cleared completely from the camera's memory.

     • Any AF Microadjustments you apply always stay within the camera body, not with the lens. So if you make adjustments for one particular lens in an EOS 7D, there is no change in the lens's characteristics if that lens is subsequently mounted on (for example) an EOS 5D, or even another EOS 7D body. If you have more than one Canon EOS body which allows AF Microadjustment, you should evaluate each body separately, and not immediately conclude AF Microadjustment values from one body will automatically be what's needed for another.

Setting AF Microadjustment:
The basic procedure: you take a test shot with Microadjustment intentionally shifted, examine it closely on your computer monitor, and repeat until you find an adjustment value that provides the best results. There's no way to tell by just looking at the camera's menu which of the settings on the +20 to -20 scale will work best, without shooting controlled test pictures and evaluating them.

Two keys to getting maximum use out of this feature - proper evaluation of actual image files you've shot with no adjustment applied, and proper test procedures in calculating the amount of AF Microadjustment needed. We strongly urge new EOS camera owners not to start manipulating this feature the moment they first take the camera out of the box. Like changing the ignition timing on your car, it's best to leave the factory settings alone unless and until you find a distinct need to shift them. Shoot full-resolution images as you normally would: RAW or JPEG, of real-life subjects - not on-line test charts. Examine those images on your computer screen at 100% view, and carefully ask yourself if you're consistently seeing soft images - but images with *something* else in the picture, even if it's grass or pavement behind your subject, that's tack-sharp. If this is happening most or all of the time, you may be a candidate for AF Microadjustment.

Remember: for AF Microadjustment to work, there has to be some issue with your AF system's ability to render the intended plane of sharpest focus truly sharp. Whether you use AF or focus manually, sharp focus can only be in one of three places: either right on your intended subject, in front of that subject, or behind it. In other words, if focus is the problem, something in the picture usually looks tack-sharp. Here's the point: if you're seeing images where *nothing* looks critically sharp, you probably need to examine other possible issues. This includes camera or subject movement from too slow a shutter speed, the amount of electronic sharpening being applied to your files, Picture Style settings if you're shooting JPEG images, and so on. AF Microadjustment cannot account for these problems.

The AF Microadjustment scale:
Don't let the scale you see on the AF Microadjustment screen confuse you. The "plus" settings add distance, and are labeled Backward on the scale you see within the camera's Custom Function menu. With "plus" settings, you're telling the camera to shift focus to a plane farther away from where it normally does. Set this way, the camera will tend to focus a bit more toward the background. This would be the direction to choose if the camera tends to continually focus in front of your intended subject(s).

The "minus" settings are labeled Forward on the scale within the AF Microadjustment scale. This moves the plane of focus closer to the camera. If your camera consistently seems to put focus behind your intended subject, start your test shots with settings on the Minus side of the scale. It's called "minus" because you're subtracting a bit from the distance the camera would normally put its sharpest focus upon.

Test shots for AF Microadjustment:
Canon suggests using the procedures below to perform test shots, to see which of the AF Microadjustment settings is best for you. Always remember: you can return to the factory default AF settings in two ways, either by setting AF Microadjustment Custom Function back to option "zero" (this doesn't clear any of your Microadjustments from memory, it just ignores them until you re-activate the "Adjust all by same amount" or "Adjust by lens" options on the C.Fn menu), or alternatively by going into one of these AF Microadjustments, and dialing your setting back to zero on the +/- 20 step scale. This last action will clear your previous settings from the camera's memory.

To change an AF Microadjustment, you need to take test shots of a 3-dimensional subject, where you can precisely check whether focus is exactly where you want, or if it's occurring behind or in front of your subject. Simply shooting squarely into a flat wall won't tell you that; shooting at an angle might. Keep the following points in mind when taking these test shots:

     • Shoot test pictures using the lens(es) you normally use, and at the distances you typically use them. In other words, if you're seeing a consistent focus shift when you shoot group pictures at weddings, don't take test shots of a ruler on your desk with a macro lens.

     • Use Av mode, and always shoot the test shots with the lens aperture wide-open - regardless of whether you normally stop the aperture down or not. You'll be much better able to see any subtle focus changes wide-open, than with the lens stopped down.

     • If you're using a zoom lens, zoom to its maximum telephoto focal length for test pictures. AF Microadjustment can only apply one correction to a zoom lens, so you cannot have one adjustment for the lens's wide-angle setting and another one for the same lens when it's zoomed to telephoto.

     •Even if you're a full-time sports shooter, do not use AI Servo AF mode for these test shots. Shoot a completely stationary subject, with the camera set to One-Shot AF. There are far too many other variables involved in focus-tracking with AI Servo AF to get conclusive results for AF Microadjustment taking test shots this way.

     •If at all possible, use a tripod to keep the camera absolutely positioned on one part of your intended subject, and also eliminate any potential camera movement from entering the mix.

     •Manually select only the Center AF point (regardless of whether this is how you typically use the camera), and be certain that any AF point expansion is completely disabled. Do not, under any circumstances, use Automatic AF point selection mode - this can definitely lead to unpredictable results.

     •Be sure the center AF point is solidly upon part of your subject with sufficient detail, and that there's adequate detail in front of and behind your target to assess whether focus is indeed occurring in front of or behind what the center AF point is seeing.

Since the values for each step on the AF Microadjustment scale are so fine (again, only 1/8th of the forward or backward depth-of-field!), it's best to start your test shots with major adjustments like plus or minus 20, and then work your way back to finer values if necessary. If you feel your camera focuses behind where it should, for example, you might take two initial test shots at minus 20 and minus 10, and see how your test subject appears when closely examined. If you start with very fine increments (such as plus 3 or plus 5, for example), the changes may be so subtle that you'll have trouble detecting them.

When you get test shots that seem to place the focus dead-on, note the adjustment value in place and be sure the camera is set there on the +/- 20-step scale. AF Microadjustment is complete. Go out and shoot some real subjects!

Summary:
This terrific Canon feature will surely be a benefit to very critical users, whose only option if they encountered a tendency for focus error in the past was to send equipment to a service technician for evaluation and adjustment. Especially since AF Microadjustment can be performed for individual lenses if needed, it's a great step forward in allowing users to fine-tune their cameras to their exact needs.

But remember: AF Microadjustment isn't a cure-all for any and all images that simply don't look sharp. It can only cope with shifting your plane of sharpest focus closer to or farther from the camera. And of course, it works within a specific range - if you need even greater range of AF shifting for proper sharp focus, you'll probably have to have your equipment examined by a service technician. However, used appropriately, it's a feature that opens a new range of possibilities for the working pro photographer or serious, dedicated amateur.

 

Canon EOS cameras with AF Microadjustment (as of spring, 2011):
 EOS-1D Mark IV  C.Fn III-7
 EOS-1D Mark III  C.Fn III-7
 EOS-1Ds Mark III  C.Fn III-7
 EOS 5D Mark II  C.Fn III-8
 EOS 7D  C.Fn III-5
 EOS 50D  C.Fn III-7

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