I've read those terms in a couple of lens reviews, and not even Wikipedia has an article about it.
So my questions are:
- What is back/frontfocus?
- What causes back/frontfocus?
- Is there any preventive measure to avoid getting back/frontfocus?
Front focus is when the lens and camera focus in front of your intended focus point. Your subject will look slightly out of focus and something in front of them will be razor sharp in focus.
Back focus is correspondingly when something behind them is in focus, instead of your intended subject.
As to why.. it could be mis-aligned, mis-calibrated equipment. It could be you or it could be your subject. You or your subject could have moved slightly after focusing and this would also cause front or back focus.
In terms of why it would be this way for the equipment:
There's a host of why, but most of them boil down to something being not quite calibrated, aligned, or working 'properly'.
Modern choices to fix it usually include one of three options that I'm aware of:
Send your camera and lens to the manufacturer to be 'fixed' or calibrated.
Your camera may have a global adjustment in which you can tell it to always adjust its autofocus front or backwards by a small adjustable amount.
Your camera may have a per lens adjustment in which the camera will remember that this lens always needs a certain small adjustment that you provide in order to function properly. (Usually up to some reasonable number of lenses.)
Which of these is an option, typically depends on the 'level' of camera you have. More entry levels (like my Nikon D3100) have no adjustment options. The Pentax K-x (mid level), for example, offers a global adjustment and the Pentax K-5 (mid-upper level) offer a per lens adjustment.
The focal length engraved on the lens is the distance from lens to image; this is usually stated in millimeters. The focal length is the fundamental measure of the power of the lens. A “normal” lens delivers a view that is about the equivalent of the perspective we see with our unaided eyes. A camera delivers a “normal” view when the focal length of the lens is approximately equal to the diagonal measure of the format frame. For the FX (full frame) this is about 45mm (traditionally rounded up to 50mm). For the DX (compact digital), this is about 30mm. Lenses shorter than “normal” deliver a wide-angle view. Lenses longer than “normal” deliver a magnified (telephoto) view.
When the camera is imaging an object closer than infinity, the image to lens distance is lengthened. Now we must focus. This action adjusts the distance lens-to-sensor. The term “back focus” refers to the measurement lens-to-sensor.
If the lens has a simple design, the focal length measure would be made from about the center of the lens to the image plane taken when imaging a distant target. However, modern camera lenses are complex consisting of many glass elements of different shapes and materials. Such complex construction moves the measuring point away from the center of the lens barrel. Often the measuring point is shifted forward allowing the barrel of a telephoto to be shortened making the lens more manageable. Wide-angle lenses commonly are made with the measuring point shifted rearward. This allows the lens barrel to be shifted forward providing more space between lens and sensor for a reflex mirror.
In any event, the measuring point is called the rear nodal, and distance from lens to sensor is called the back focus. Likewise, subject to lens distance is technically measured from a point called the front nodal. This distance is called “object distance”.