Focusing Problems 1 - Front & Back Focusing

Do you have a focussing problem?

Here's how to check for a front or back focussing problem.

First of all I will start with the simple fact that the majority of focussing problems are to do with the meaty bit behind the camera more commonly known as you or I. I have met quite a few people who are convinced that their cameras are faulty because they can't focus or more to the point, have blurred images but, usually it turns out that it is the inexperience of the user. I once heard someone complaining about their camera not focussing and when questioned they corroborated their claims stating that it didn't focus whilst on a tripod. However, once it was pointed out to them that turning off their stabilisation will give better results, all was okay. The point here is that it is too easy to blame the equipment when we haven't explained away all possibilities. It is important that we check everything before returning equipment back to the manufacturer at great expense.

However, there are times when the lens or camera can be the cause of a focus issue and it happened to me not to long ago and I had to have the focus mechanism in my 100-400mm lens replaced which was a costly exercise. I knew it was faulty because if the lens had to focus from close up to something further away it would not achieve focus yet, if it was far away and had to focus in closer, it would focus okay.

So what is front and back focussing?

When the auto focus on your camera focuses the lens, it establishes a focal plane in front of your camera and anything on this focal plane 'should' be in focus.  Depending on the aperture setting of your lens and the distance from the camera, there will be a 'Depth of Field (DoF)' where objects in front of and behind the focal plane will also appear to be in focus. As we know, the DoF increases as the aperture on the lens get smaller in diameter.

At larger aperture values (small hole), since the depth of field is increased, any slight variance between the actual focal plane of the camera and the camera’s autofocus will more than likely 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 DoF 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 very narrow DoF, 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  someone and you are certain you were focusing right on their forward eye but when you checked the photo in the computer and her nose is more in focus, well perhaps you have a problem.

How to check your lens and camera?

You need 5 identical objects, I read another internet article that used batteries, so I also used batteries to check my lens. You'll need to place them in a well lit area in a diagonal line like in the illustration below, making sure that you have something to focus on such as the battery logo etc.

Now, to setup your camera and lens the use of a tripod is essential because we don't want any camera movement otherwise the test will be a waste of time. Set the lens on the widest aperture and the camera focussing onto the centre point only. Attach your camera onto the tripod, focus onto the centre battery only just slightly beyond the minimum focussing distance of your lens. Take several shots just to ensure consistency. Now remove the memory card and bring up the images on your computer.

A correctly focussing lens will have something like the image below, the battery in the centre is in focus and the batteries on either side will be slightly out of focus getting slight more out of focus as we move to the second batteries.

Click on images to enlarge

So if the centre battery was the point of focus but the 4th battery is sharper than the 3rd then the lens is back focussing.

Also if the 2nd battery is more in focus than the 3rd then the lens is front focusing.

What if none of the batteries are in focus?

Consider that your lens may not perform well at the widest aperture especially if it is of a cheaper variety (kit lens) and is a zoom lens. A zoom lenses does not perform well throughout all of its range and the bigger the range the worse it usually is. Try the setup with another lens or borrow a lens and see if you get similar results.

What can I do to resolve my lens focus problem?

Some cameras have the ability to perform adjustments within the camera. Cameras such as the Canon 1DIII, 1DIV,1DsMkIII, 5D, 7D, Nikon D3, D3x, D300, D700, Sony A900, the Pentax K20D, K-7 and K-5 have the ability to calibrate the focus for up to 20 individual lenses by using the in camera software. These tend to be cameras at the professional end of the market, consumer cameras don't usually have this feature. The only option then is to return the camera to the manufacturer with the lens to have it setup for you but this will be costly and probably isn't worth it for cheaper lenses.