Back Focusing

It has been said there lens`s are back focusing or FF if you look at Dofmaster Dept of field charts it clearly shows point of Focus is never centre.

55mm lens f1.2 is 49% Front Focus 51% BF

55mm lens f16 is 34% Front Focus 66% BF

55mm len f22 is 29% Front Focus 71% BF

This is at 10ft some of the longer lenses are close but the shorter are farther out

I think what you are saying is the there is more DOF behind a given focusing point than in front.

This isn't what people are discussing when they refer to back focusing or front focusing. They are referring to the point focused on being inaccurate.

What you have been reading confirms the "rule of thumb" that to get maximum DOF in a scene, you can focus one third of the way in. Coupled with a small aperture this will usually keep all of a subject within the DOF available.

Thanks for that explanation, but some of the charts on another thread that I have seen indicated to me, the focus point was on centre of the chart, but was showing more in focus at the rear of the chart which could be normal, but I thought it was seen as being front focusing (can`t find the thread) mabe my memory is to blame.

DoF is rarely equal either side of the focus point, if you'd like a focus test chart I've found the following gives very good consistent results:

http://focustestchart.com/chart.html

or in pdf format:

http://focustestchart.com/focus21.pdf

Regards,

John

To do an autofocus test you use the lens wide open.

Make sure you use a tripod; S/R off; and use natural light. You will get misleading results if you do the test in your kitchen in the evening

And ensure the camera is using the central AF point!

The thing about BF/FF testing is that it establishes the point at which where the camera/lens combination is *actually* focused, as opposed to where the camera *thinks* it's focused.

You establish where it thinks it's focused by:

- using the centre AF point only
- ensuring that you get green-hexagon focus confirmation
- ensuring that the camera doesn't move between focusing and taking the picture

Then you find out where it's actually focused by noting the point of greatest sharpness, which the test chart makes a lot easier. The greater the depth of field, the harder this is to do, so you need as little DOF as possible - having the lens wide open, as Peter says, will ensure that the range of sharpness is minimised, and only the point of actual focus will be sharp.

This image shows the sort of result you get - with no significant BF or FF in this one. Perhaps it shows a trace of FF, but that could easily be down to slightly inaccurate camera positioning.

Has anyone else found that the AF focus point shifts to the front or rear depending on whether you start focusing from the infinity setting or from the minimum focus setting on the lens.

Greg

I believe this is entirely normal.

It is interesting that I have never read any were to use the lens wide open , when checking for focus,this would help me to get it right,maybe.

When I cleaned a 600mm mirror lens and reasembled it, having moved all the mirrors and lenses I was worried about the focus, but checked it out on a corregated iron shed shooting a dove and found it was good but that was f8,fixed when I do the smaller lenses I will do them wide open and probably on the shed at an angle like before,it seems to work ok.

thanks for that.

You normally have to use the lens wide open to see the actual difference. If you can still see it when you stop down a stop or two you would be better trying it there as well. With some lenses there is a focus shift when you stop down, so it makes sense to try and do it at whatever aperture you use the most. Although that would probably apply more to MF because the AF works before the lens stops down.... thus missing any focus shift.

An alternative is to use a focusing rack and take one shot then move closer by 10mm take another then back to -10mm and take another then see which is the sharper. 0mm, +10mm or -10mm

I've got a stereo Novoflex focussing rack which which actually has mm's forward and back engraved on it. Same as this one but worth £100 less.

Three AA batteries stood up and staggered would also do.