Front Focus Issue - Help Needed

Hello Everybody!

I was trying to establish what is the focusing situation of my gear; I prepared the whole focussing chart and started testing the various lenses and 2 K10 bodies.

Whilst almost all the lenses are doing well, and have an average Front Focus of 2mm +/-, I have a big issue with the FA 50mm f1.4

Both cameras show the exact same patterns with all the lenses tested, and i am more than happy to NOT correct the focus adjustment on the cameras (through the firmware, bla bla..) because most lenses are actually fine. (And a 1-2 mm FF is not a big deal.)
On the other hand the 50mm shows very hard front focus, of 4-6 mm on both cameras, up to f8. On f1.4 this is very evident, and the ff makes the lens very hard to use properly.

I called pentax and asked for their advice; they suggested bringing the lens to them, and having it cleaned. This IMO wouldn't change anything and a simple cleaning process will not change the focusing characteristics of the lens.

Does anybody have any idea on what I can do? Are there adjustments that i can do on my own? do you reckon a lens cleaning can change things?

Looking forward to your help,

Nathan

Nathan
2 things.
1) Its always the 50mm @1.4 that will show any FF/BF issue simply because of the DOF. All you lens might be FF at the same amount but without a 4mm DOF you simply wouldn't notice it....so to conclude its not necessarily the lens.

2) Are you doing your tests indoors using house lights or outdoors/daylight lighting. It makes a massive difference. K20d sensors will FF under tungsten. I've done the tests and its clear.

Do a test using both light sources. Make sure you are using the central bar of the focus test chart.

Take a look at mine. First under Tungsten:



Then in Daylight:



Let us know how you get on.

Hi and welcome to the forum, when you have a moment would you please fill in your location profile. Thanks.

Going on the above topic, are you saying that lens focus checks are best done in daylight:

Dear Norris,
THank you very much for your reply.

Just to let you know, I am not exactly a beginner, your advice is very precious, but unfortunately I was already acknowledged about the issues you mentioned.

I have tested under normal light conditions, with natural light and a good illumination.
I have also tested various lenses, including a 100mm Macro with much shallower DOF than the 50mm.


So, having established that it is the Lens's problem and it IS an issue of front focussing, the question remains.

Any advice on how to solve this?

Thank you anyway!

Ok fair enough.

Assuming it is the lens, from posts I've seen on the pentaxforums site, k10ds can be upgraded to a newer AF system at pentax and have lens correction adjusted for that lens. You would have to send both bodies as well.

OR..have chat with SRS as they have a 2ndhand 50mm (or did 2 weeks ago), and they might do a swap (for a small fee) or at least allow you to check if it is the lens or not??

Yep...for light temperature effects the focus.
(on other makes as well)

http://www.pentaxforums.com/forums/pentax-dslr-discussion/42773-front-focusing-d...

Thanks Norris..

There is no adjustment that you can do on the K10D that won't affect AF with all lenses.

Could this be the time to upgrade to the K20D? It allows correction for individual lenses.

Alternatively, I suggest a split-image focusing screen (if you don't have one already) so you can notice AF errors quickly and switch to MF.

It may be that in the real world (as opposed to test charts at close distance), AF is fine with the lens sometimes, and not at other times.

You can do the adjustment for all lenses yourself using Debug mode, but if you send it to Pentax (from what I have read) they can do the adjustment per lens up to 4x lenses

I have carried out a number of focusing tests with several of my lenses and my K10D GP.

My original results pointed to back focusing on focal lengths shorter than 28mm and front focusing above 35mm. This pattern also repeats itself with the standard 18-55mm kit lens.

I assumed that 30-35mm was deemed standard focal length on a Pentax DSLR and the AF system is optimised for that range.

My FA50 definitely FF by a similar amount to that reported by Nathan

I have made adjustments via the debug mode and yes I can get the FA50 to focus accurately, but every thing below 50mm is then thrown out of focus.

Since I tend to use the 28-35mm range the most I returned the camera to the default setting.

I know this question has been asked before, but I do not understand why focal length has such an impact on the K10's ability to focus.

Steve

In this statement I recognise the effect of DOF, but surely under controlled conditions, with QC passed lenses, the camera should be able to achieve absolute focus across the range of focal lengths?

Where did you read that?

When looking at these charts, don't forget that there will be more DOF behind the focus point than in front, simply because the area in front is closer. So don't try to equalise the DOF on either side of the zero point.

Ok you've just cost me an hour trying to find the posts. Been ages since I read this stuff

Look at:
http://forums.dpreview.com/forums/read.asp?forum=1036&message=26809326

and this one (massive thread all about version numbers etc...
http://www.pentaxforums.com/forums/pentax-dslr-discussion/25711-k10d-update-mode...

Can I ask a newbie question?

I'm familiar with the concept of back focusing and use it all the time setting up CCTV cameras ( I wish my engineers could grasp the concept ).
But how do you back focus an SLR?

where can i get a chart please any help most greatfully recieved.