DA 18-55 soft?

I've noticed that my DA 18-55 kit lens seems out of focus when set at infinity, but only when used near to the long end (50-55).

I've been practising wide angle and keeping focus set to infinity at 18mm with F8 - F11. This has been giving me nice sharp images from foreground to background, but when I zoom in to 55mm the image is soft until I adjust the focus ring back slightly.

All this is done in manual mode, subjects are further away then than the max 7m indicated in the lens body.

Anyone else seen this, any explanations?

thanks.

Zoom lenses should keep in focus as you zoom, but may not always be designed to do so perfectly and may need some refocusing. A lens which has to be refocused after a zoom change is called a varifocal. However, th 18-55mm is definitely a zoom and any refocusing would be small.

I think a more likely explanation is in the poor accuracy of manual focusing with wide angle lenses. It is not easy to focus a wide angle precisely - if you doubt that try focusing several times on an object and see how much the distance setting on the lens varies with each attempt.

When you zoom in to 55mm there is less DOF and the focusing error becomes more noticeable. For better focusing accuracy try it the other way round - focus at 55mm and then zoom out towatds 18mm.

Alternatively, let the AF system do the job for you.

Hope that helps!

I've had the same problem.
I use manual focus to set mine on infinity for widefield astro shots but keep getting soft images, even at the wide end of the scale.
I've now just bought a sigma f2.8 28mm manual focus (PKA) to do the job and given up with the kit zoom.
I'm also looking at replacing the kit zoom with something like the sigma 24-135 f2.8/4 if I can find one, I know that lens has some issues but I think it'll do a better job all round.

I've for one of the Sigmas as well as one of the kit zooms and can confirm that the Sigma is a much, much better lens, although it doesn't go anything like as wide as the kit lens, which limits its usefulness.

I have the Sigma and it lives on my istD I find it a prefect all rounder length even if it does get a bit heavy. There is a lot of glass out front, possible concerns of wear/play may cause problems. It responds very well and quick in AF.

I just got a DA18-55 as they are so cheep now (Park cameras) but I have found last week there is strong Vignette, certainly at the left corners even at F13. I ran a batch of frames to put together as a panarama only to find the sky colours so variable that PS could not put it together but that may be another topic

Regards
Alan

There is also another factor. The wider the angle of view the less the depth of focus. There could be two reasons here. If the lens is put together slightly out, any problem might not be apparent at the long end but it could affect the image at the wide end. It would be worth trying another 18-55 to see if the same problem is there. If not it is the lens. It ia also the main reason I prefer to use MF lenses for the wides. The "slack" in the lens when manually focussing AF zooms will make the problem more critical.

The other reason is operator error. Because the depth of field is greater at wide angles, it may be more difficult to "see" the correct point of focus and this combined with the lesser depth of focus could be part of the problem.

i'm getting rid of my 18-55, replacing it with pentax 50mm/1.4. the 18-55 is okay but feels pedestrian--that in-bed, not-in-love feeling.

The correct way to focus a zoom lens is to zoom in fully, focus up and re-frame.

The logic behind this is that there is less DOF at the long end of a zoom, so focusing would be more accurate.

A word of caution though that this depends on a zoom being a true zoom and holding its focus throughout the range. Some "zooms" may not and some small degreee of refocusing could be necessary.

Please could you explain this a little more as I don't quite follow?

Thanks

Davem

Hi Davem,
Look at John's post above yours.

Kim

Davem, as John says, critical focus can only be achieved by focusing at the tight end of the zoom and this applies to both manual and automatic focusing. Then zoom out to the desired framing. However, since DOF is greater at wide angles, usually any 'error' is compensated for but not always.

Getting back to the original subject of this thread it seems to me that there may be a wide variance of quality with the 18-55s. Some people like myself are very happy with their's whilst others seem very disappointed. Because of their complex construction and, in some cases covering wide to tele, zooms are never going to be as good as primes. In some very un-Geoffrey Crawley like tests I conducted, the difference between the 18-55 (at 40mm) and the 40mm DA prime was quite marked especially with zoom at maximum aperture but only in a side by side critical comparison.

I the good old film days magazine reviewers placed great emphasis on the quality of glass in front of a camera. Now it seems body features are more important - a grave error IMO.

Lenses and bodies were tested in some ways more rigorously than they are now.

When did we last see a camera's shutter speeds tested? Or are we to assume that all designs now deliver 1/4000 sec when the shutter indication says so? I doubt it.

Lenses are tested quite well in AP, although very often the pictures shown as examples seem to bear little relation to what is in the text.

It looks as though we have to test for ourselves these days. Is it fit for purpose, that is our own purpose.

I've had two and both have been rubbish. That's not rubbish compared to other more expensive zooms, but when directly compared to other consumer zooms such as the FA 28-200 and Cosina/Pheonix 19-35mm.

With both my 18-55's centre resolution was passable but edge/corner sharpness was very poor.

I'm generally using the 18-55 at the widest end, 18mm. occasionally 30-35, hardly ever at anything greater.

It gets soft at the max zoom even when using auto focus. I'm also disappointed with the 50 -200 kit lens, it's not long enough for wildlife and not sharp enough for landscapes.

I suppose it may be a good portrait lens then!

I would really like some wide manual primes: 10, 12. or 15mm (or whatever is available), for landscape images, and a 300 or greater for wildlife.

Anyone got some for cheap (as in ridiculously cheap or even free)?