front focus - back focus

jonboy61

I guess I don't understand this issue. I get that the idea means the lens/camera combo places the point of focus behind or in front of the desired spot. Isn't the person behind the tool responsible for the image?
Seems to me that the photos I have taken in 40+ years that have been mis-focused have been my fault not that of the camera and lens.
I see a complaint that a lens is prone to front-focus and think that the camera software is directing the process not the lens... so I am missing something.

Jon

craig_d

Well, in a manual focus system, the photographer is solely responsible for focusing, so it would be silly to complain that a lens "front-focused" or "back-focused".

But the whole point of auto-focus is that the camera and/or lens take care of focusing for you. It's still up to you to choose the AF point and verify that the camera chooses the right thing to focus on, but once that is established, focus should be accurate or there's no point to having auto-focus at all. The complaints of front- or back-focusing relate to situations where the camera/lens is clearly trying to focus on the right thing but consistently mis-focuses by roughly the same amount, in the same direction, each time.

ziggy53

What Craig said is quite true. What folks may not know is that AF in a dSLR is accomplished with a different sensor than the imager, and that the image to that sensor is bounced off of 2 mirrors before reaching the sensor. Any shift in geometry of the AF system can cause a legitimate front or rear focus, but it would be true of "all" lenses if camera based AF error.

The lenses may also cause a focus shift, but a lens based AF shift is generally consistent mis-focus. In particular the cheaper consumer lenses use very inexpensive AF "shunt" motor technology and may not accurately brake or accelerate. The problems can also occur over time as internal tolerances change or problems can occur in very warm or very cold temperatures and/or very high humidity.

Better quality lenses using the more advanced motor technologies, like crescent "ultrasonic" drive motors, have much better torque, acceleration and braking qualities and allow much better control and speed for focus applications. These motors tend to allow more consistent focus over time and in different environmental concerns.

"Some" lenses, particularly zoom lenses, may also have internal shim packs to allow adjustments for best focus. Some lenses may also have the ability to identify themselves to the camera and allow the camera to store internal AF correction tables. Some newer cameras even allow user generated AF corrections (commonly called micro adjustments.)

Large apertures also lend to AF accuracy, both because they allow more light into the camera and because they allow larger angles for the beam-splitter component of the AF module. Some cameras have specialized center sensors with higher precision when used with lenses of f2.8 or greater.

AF sensors can also be of several different basic designs, unidirectional, bi-directional (or more commonly "cross type") and Area SIR with much more sensitive and accurate detectors.

Finally, some cameras and some external flash units have AF assist which can provide both additional subject illumination and some also overlay a projected pattern to help the AF sensors find the edges of a subject and create effectively more accurate AF in low-light situations.

jonboy61

Thanks to both of you. I had not thought of the viewfinder and the AF sensor as being in two parts, nor of the electro-mechanical parts not being up to the job.