can back focus be fixed?

Nathan gave us the link but I can't find it now. It explains a lot about how different people get different reactions to lenses.

Mike

I had the same problem on my 16-50 DA*, it can only be fixed by adjusting the focus of the CAMERA and not the lens. You either need to have the camera and lens calibrated by a Pentax service center, or you can do it yourself. I downloaded a small application from the web, plugged my camera into the computer, ran the debug application and accessed a new submenu on my K10D which allowed me to adjust the focus of the lens through the software. Now it focuses crisp and clear. I recommend searching the web for this application : GX10-K10D Debug Mode Control and follow the instructions (AT YOUR OWN RISK I might add).

But if all other lenses focus fine and this one doesn't isn't there the risk getting this one right screws up the others? With the K10 you only have one adjustment I believe.

I can't see why Pentax can't adjust the lens but that's probably my ignorance.

Mike

You do have a point there, in my experience all my lenses were out to varying degrees so I made an average adjustment for all my lenses with consideration for the one I used the most. The adjustments are so fine you bascially are adjusting mm at a time which is only really visible when shooting close up photography.

Tolerances in manufacturing are inevitable - nothing in this world is 100% accurate at any price.

However, you should expect that a lens will work fine on your camera body and usually they do.

In cases where they don't, then recollimating the lens will no doubt put it right. The K20D goes a step further by enabling user adjustment for up to 20 lenses. This is an interesting development and I wonder why it has been put in there. On the face of it there is no need for it, but clearly there must be.

It's been included in some form (hideen on the K10D) sice SR was introduced and we have a sensor that moves. Maybe this is where the need for extra adjustment is coming from?

This is the link that Nathan posted http://tinyurl.com/cwmjgo.

The problem arises when the camera and lens are are opposite ends of their tolerances ranges. In an ideal world, the tolerances of a lens would be tight enough so that even at those extremes the camera/lens would remain within specification. This seems to be a recognised problem by all manufacturers, which is why individual lens 'tuning' is an option is becoming a standard feature.

Having accepted it is a problem by adding the feature, it would be nice if the manufacturers included a decent tutorial on how to tune a lens with their cameras.

Quite right any adjustment made on the K10 is a compromise as it effects all lenses. I made a slight adjustment to my K10, which improved 3 of my most comonly used lenses but now produces a slight front focusing with my FA50.

Steve

Thanks Steve both for the link - which I think could be made a sticky somewhere and for confirming that my thoughts on the K10 adjustment were correct.

Mike

Correct. So on my K10 I measured many of the lenses in my collection that had a focal length of around 50mm and then calculated an average setting:

Lens / K10D Focus setting
Samsung 35mm F2.0 / +150
Pentax DA35mm F2.8 / +160
Pentax FA50mm F1.7 / +90
Pentax FA50mm F1.4 / +100
pentax DA*16-50 @ 50mm F2.8 / +100
Pentax DA18-55II @50mm F5.6 / +0 !!
Sigma 18-125 @50mm F5.6 / +210 !!
Pentax DA18-50 @50mm F5.6 / +120
Pentax DA40mm F2.8 / +100
Pentax FA28-105 @50mm F4.0 / +150
Pentax FA28-80 @50mm F4.0 / +100

A full average came out at +116 and dropping the two extremes gave an average of 118.88. You can only set the K10 in steps of 10 so I set it to 120. After that I was very happy with its performance.

With the K20D you can set an individual value for one of each type of lens up to a maximum of 16. The setting values are in a different range. My lens collection had evolved and I am using these measured settings:

Lens / K20 AF adj
Samsung 35mm f2.0 / -3
Pentax DA 35mm f2.8 / -3
Pentax FA 50mm f1.7 / -6
Pentax FA 50mm f1.4 / -5
Pentax DA* 16-50mm f2.8 (old) @ 50mm / -6
Pentax DA 18-55mm II (1) @ 50mm f5.6 / -10
Pentax DA 18-55mm II (2) @ 50mm f5.6 / -10
Pentax DA 40mm f2.8 / -5

Pentax DA 18-250mm @ 18mm f3.5 / -4 (left @ 0 see below)
Pentax DA 18-250mm @ 50mm f4.5 / +5 (so left @ 0 AF adj!)

Samsung 100mm f2.8 macro / 0
Pentax DA* 16-50mm f2.8 @ (New) @ 50mm / 0


I expect to soon have to repeat the process on a K-7!


Glyn

PS - How do you do tabs in these posts? It also seems to convert multiple spaces into a single space

Thanks for posting the link to the article again Steve. It is a very good read and explains quite a lot.

I have two lenses at f2.8 and one at f1.4 and have not noticed any problems. But the 16-50 could be slightly out as the article suggests.

So this kind of answers my question. If the lens I have just had sent back is within specification then it will come back exactly as it is. There is an off chance it is actually out of specification and they could improve the focussing. I hope they communicate what they do to the lens.

I have the k10 so no point having my camera adjusted. All this may go away if I purchase the k20 or k7 but that is not happening any time soon

I am curious though, has anyone had any problems suddenly after upgrading to a new body or noticed a lens works perfectly on one body but not the spare?

Yup, the 10 times more invested is what got me started

That's one of the first things i did after i got my k20.It's quite a few pictures to take if you've got a few lenses. All seems fine

It's a fine link that Nathan posted about front and back focusing but it doesn't explain what effects that tolerance in the lens.

In a way the AF unit in the camera works like our eyes, it looks through the lens and confirms focus that way.
So if the Lens is out with -10 (don't know what would be out but play along) and the AF unit is out by +2, according to the read you will get -8 "focus unit" out of perfect.
What I say is that since the AF looks through the lens and simply focus the lens to +2 focus since the AF unit is out by that.

I know that the first is true in real life but none still haven't explained how it comes and why the second one isn't true.

as your all probalbly aware and feed up of hearing about my sigma 150-500mm which is back at sigma again for recalabration as the could't get it right in the first place, sigma now have my k20 to focus the lens to the body, this makes the af adjustment in body usless, if they cant 0 calibrate a lens(or near), lens has been with sigma 6 weeks

kris here are all the back focus issues https://www.pentaxuser.com/forum/search/back-focus
link

jack.

I am surprised more people don't comment about that.

Thanks

I want something brand new so I can blame Pentax if it is not sharp