Misfocussing 70mm limited

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Hythloday Senior Member • Posts: 2,127
Misfocussing 70mm limited

Until last year I had no problems with front or backfocussing, perhaps apart from the DA17-70mm. This lens gives razor sharp pictures when I focus at 70mm, lock focus and zoom out to the desired focal length to take the picture. This trick was once described by Pentax in an old manual for the 80-200mm zoom lens. It only works with parfocal lenses.

My DA21mm Ltd and my DA50mm frontfocus slightly. Last week I bought a DA70mm Ltd and took some test pictures to see whether the lens worked allright. I could not find anything special. There was no frontfocussing, as far as I could see. However, I was not impressed by its sharpness. So I wanted to see which lens was sharpest: the DA17-70mm at 70mm or the DA70mm Ltd. I took a picture of a row of books, some standing a bit backwards. The picture taken with the DA17-70mm was sharper on the whole than the picture taken with the DA70mm Ltd.. Both were sharp in the centre, but the books standing slightly backwards were less sharp in the DA70mm picture.

I had taken pictures outside and the centre sharpness was OK with the AF finetune option set to zero (standard). I set this to -2 and centre sharpness improved quite a lot. The pictures were really sharp. So I tried -3 and the test pictures were less sharp.

If you have a lens that does not give the sharpness you expect, try the AF finetune option and take test pictures to see if there is any improvement.

Last week and today I took the same picture of my wife's garden gnome. In the first picture (AF finetune=0) there is a kind of halo near the beard. In the second picture (AF finetune=-2) the halo has gone.

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