I have been asked about this several times so I decided to write an article on it. I hope someone out there finds this helpful.
Have you ever noticed that your camera’s focus seems a bit off when shooting at large apertures? I’m not talking about a completely blurry photo here. I’m talking about a situation where there is sharp focus in the photo, but its just in front of or just behind the area you focused on. An example would be like when you focus on your subject’s eyes and their nose is more in focus or maybe their ears are more in focus. If you are noticing this issue consistently happening in your photos, then perhaps you have a front focus or back focus issue. This article will show you how to test your camera and lenses for front/back focus problems.
First, what is front focus and back focus. It’s pretty straight forward really. When the auto focus on your camera focuses the lens, it establishes a focal plane in front of your camera. Anything on this focus plane will be in focus. Depending on the aperture setting of your lens and the distance from the camera, there will be a “Depth of Field” where objects in front of the focal plane and behind the focal plane will also appear to be in focus. This Depth of Field increases as the aperture on the lens get smaller in diameter. For those new to aperture, the larger the f number the smaller the aperture and vice versa.
At larger aperture values, since the depth of field is increased, any slight variance between the actual focal plane of the camera and the camera’s autofocus. will probably not even be noticeable since both will fall within the “Depth of Field” area. But, when you lower the aperture value and increase the size of the aperture opening, the Depth of Field decreases. This decrease in the Depth of Field can be pretty dramatic for lenses that will allow aperture settings of 2.8 or 1.4, or anything else in that range. With a razor thin Depth of Field, a variance between the camera’s autofocus and the actual focal plane can be very noticeable indeed. If you were taking a portrait shot of Aunt Betty’s daughter and you are certain you were focusing right on her forward eye but when you checked the photo in the computer and her nose is more in focus, well perhaps you have a problem.
Most front and back focusing problems are usually human error. If we are using a wide open aperture and we are handholding the camera, just breathing or rocking back and forth can throw off your focal plane. Let’s say you are using your nice brand new 85mm f 1.4 portrait lens and you are doing a tight head shot with the lens wide open from about 7 feet away. Sounds reasonable right? Well, at that distance on a crop sensor camera your total depth of field is only a little over an inch. So if you breathe to hard and move the camera even slightly forward or back you can throw the whole thing out of focus. In that case the front or back focus would be due to human error and not the camera/lens combo. The same would hold true if your subject moved slightly from the time you focused to the time you took the shot. A lot of us engage in the practice of using the center focusing point on the camera to achieve focus lock then recompose the shot. All this movement of the camera can lead to some forward or backward movement which could throw your focus off its mark.
But you think you hold the camera steady as any tripod and you’re still experiencing this problem? Or maybe you use a tripod and you are experiencing some issues. Or perhaps you just are curious and you want to waste an afternoon testing your lenses… Well you’re in luck because I’m going to show you a fast and easy way to check your lenses for front and back focus issues.
There are many methods to do this. You can download charts from the internet. You can shoot down at a 45 degree angle at lines of text laid out flat on a table. You can even purchase a commercially made lens calibration checker. But, you can do nearly the same thing using a few AA batteries and a flat surface. To do the battery test method you will need the following: A tripod (or beanbag), 5 AA batteries, a flat surface, and some daylight. That’s it. So gather up your materials and let’s get started.
Before we start, just a word of warning. The tripod is not a suggestion it’s a requirement. For this test to be valid you will have to be absolutely certain that your camera does not move during the testing and the only way to be 100% sure of that is to use a tripod, and a sturdy one at that. Using a flimsy tripod that wobbles or shakes under the weight of your camera will be useless. Using something like a beanbag or sand bag could be an acceptable alternative but it’s still not 100% certain. If you try to hold your camera steady on the table by hand, well… , good luck because your testing will probably show you have front and or back focus issues even if you really don’t have any issues at all. Use a Tripod. Enough said on that. And while I’m at it, this testing should be performed in daylight. Daylight is easy to find so this should be no problem :) Tungsten and fluorescent light have different color temperatures than daylight so you may see some strange results that would not be there if you did the test in daylight. Ok, so you got it right? Tripod and daylight. Ok good, now onto the testing.
You will have to position 5 AA batteries on a diagonal at approximately 45 degrees to the sight line of the camera. Check out the photo below.
Position the batteries as shown in the photo. Make sure that you have some nice text facing the camera to focus on . Next, you will need to set your camera to use only the center focus point. All DSLR cameras have focus points. Some more than others. But they all have the ability to select an individual focus point. Read your owner’s manual to learn how to set your camera to use only the center focus point and leave it that way for the duration of the testing. If you skip this step you may as well just not do the testing. Once again, this is not a suggestion. Center point focusing is a must. If you are doing this in daylight (and you should be) the camera’s ISO setting shouldn’t be a problem. You can probably leave it set to 100 or so but if need be, set the ISO high enough so you can have a shutter speed of at least 1/125th second or faster. Remember we are trying to eliminate any possible camera shake issues here. Next, make sure the lens you want to test is mounted to the camera. Set the aperture setting to as wide open as the lens will allow (the smallest f number). Remember from above, this will limit the Depth of Field so it will be easier for you to see any problems with focusing.
Once you are all set you are ready. With the camera on the tripod, frame the shot as you see in the photo below.
You want to be far enough away from the batteries so you are just a bit further away than the lens’ minimum focusing distance. If you are too close the lens won’t be able to focus. But you also want to be close enough that the batteries mostly fill the frame. This may take some trial and error to get the positioning right but one or two tries should be enough. Once you have this set up, you will want to focus your camera on the center battery. To minimize the chance of camera shake you will probably want to use the camera’s short self timer mode (yep, get the manual out again). Focus the camera and push the shutter button, wait a few seconds, the timer will do its thing, and the camera will take a photo. Take a few photos to make sure you are getting consistent results then pop out the memory card and open the files in your photo processing program.
This photo shows what correct focus looks like.
When focusing on the center battery it should look like this. Center battery in focus and the rest out of focus to some degree. If your photo looks like this you are good. Time to break down the testing setup or time to test another lens.
This photo shows back focus.
This photo shows front focus.
If nothing is in focus, well I’ll address that at the end of the article.
If you have either front or back focus you either have a problem with the lens (bad but not so bad), a problem with the camera (bad but not the worst), or a problem with both camera and the lens (Yep, really bad stuff here). If you have more than one lens you should try this with all your lenses. If only one lens is causing a problem then more than likely you probably have a problem with just the lens. If all your lenses are causing a problem then more than likely your camera’s sensor is not aligned properly.
So what do you do? Some cameras have the ability to perform micro focusing adjustments in the camera software. Cameras like the Canon 1DIII, 1DIV,1DsMkIII, 5D, 7D, Nikon D3, D3x, D300, D700, Sony A900, and Pentax K20D, K-7, K-5 have the ability to calibrate the focus for up to 20 individual lenses by using the in camera software. This is not a complete list so check your camera’s manual. If you camera does not support this feature then you really have no alternative other than sending your camera back to the manufacturer to be calibrated. Contact the manufacturer to see if you need to send the lenses in as well. Some require that you send in the lenses so they can set the calibration for each lens you will be using.
(As noted in the comments below, here are a few things you should also consider:
(1) A camera/body pair nicely focusing at the tested distance may still behave slightly off at another distance.
(2) For the zoom lenses one should either choose the mid focal length or the most used focal length for testing. In a problematic condition, you may have good focus at, say, 24mm and back or front focus at 90mm.)
Ok, now what about that issue of nothing being in focus. If nothing is in focus, it’s just possible that your lens is not sharp wide open. If the center battery is more in focus than the rest then it is possible that all is well but your lens just isn’t sharp wide open. If this is a cheaper lens then it’s possible that this is the best the lens can do. If it’s a more expensive lens you may want to return the lens for another copy or try another copy of the same lens to see if there is any improvement. If you think your lens is bad you should send it in for repair.