[dofmaster]

I'm interested in your members' opinion.

I've been shooting with the K1 for about a year, and have the three amigos of the f2.8 zooms, plus the 28-105mm.

I've had the Pentax DFA 24-70 f2.8 for about three months (which we all know is a Tamron), and I've found its autofocus capabilities pretty poor, compared with the 70-200 and the 28-100mm. I shoot a lot of portraits, and I'm normally on the single point autofocus. With the aforementioned lenses I hit my focus around 90-95 per cent of the time, but with the 24-70 it seems to drop much lower. I've also noticed the following issues:

- At times it just hunts for focus and can't find it, zooms in and out. This happens occasionally when I'm shooting in bright sunlight, normally a backlight subject
- In some lower light situations, if I'm photographing something that doesn't have a lot of details, such as a wall, it seems like can't lock on at all, ever. I have experienced this when shooting concrete walls at about 5pm in the shade, I'm not talking about actual night shots (when it does the same). Interestingly, if I switch to live view, the zoom can lock.
-Again, in some low situations, when I'm shooting and us the back button focus, it can't hold the focus on what has just been focused on. So, I shoot a frame, and if I want to shoot a second, identical shot, and I hold down the back button AF, it then hunts again for the focus - in and out - when I think the AF should recognise that it's locked on and not have to hunt.
- Lots of missed focus. This can include shooting a portrait with spot on the eyes, but the background is what is in focus. Alternatively, I have a portrait that looks in focus but is actually a little soft. There might be an element of technique here, but I do not get this issue when I'm using my other AF zooms, the strike rate then is much higher.

I have checked the lens for front and back focus and it was basically fine. My AF setting are AF.C, Sel, 1st Frame Action in AF.C is Focus Priority, Hold AF is medium.

Anyhow, it's got to the point where I'd rather use the 28-105 than the 24-70, because I feel the focus working much better, and from an economic point of view (price!) that doesn't feel right. I'm wondering if anyone else has experienced anything similar on the 24-70mm, or if there could be an issue with my copy.

[ChatMechant]

That's a bummer. My experience with AF of the 28-105 vs 24-70 has been just the opposite. The 24-70 is fast and accurate even in low light, while the 28-105 is usually more hit-or-miss. I use the same settings, and back-button af. Are you using a filter?
Oh and fyi: the "three amigos" usually refers to the FA Limiteds, not the 2.8 zooms.

[jbinpg]

What is the firmware version of your K-1?

[Miguel]

Why would you have it on AF.c for portrait use? How does it compare with AF.s?

[PePe]

Sounds strange to me. Generally speaking I find the 24-70 to be quite OK, albeit not stellar. However, I do not recognize this kind of behaviour.

[Adam]

Could it be an AF fine adjustment issue, perhaps?

[jatrax]

the three amigos

Should be the FA Limited lenses, not these zooms. The zooms are...... I don't know, the three elephants? three anchors? three lead weights?

[C_Jones]

I use AF.C, Focusing Area (AF Point) "Spot", 1st Frame Action in AF.C "Release Priority", Hold AF "Off"", and Action in AF.C Continuous "Auto". Basically not restricting the shutter release and just acknowledging the camera AF confirmation audio signal and/or the green confirmation signal serve to affirm focus.

Testing a camera/lens combination in normal light, without glare (blowout) and not using a flat surface with no contrast features will show if the AF/lens is rendering normally or not.

[dofmaster]

I'm on 1.40 firmware. I have sometimes used a UV filter on the lens, but other times I haven't. I did check the focus and it didn't need fine adjusting.

What is the best way to test the focus performance of a lens outside of the general 'feel'? What are some objective measurements?