[fozzy]

Hi there

I wonder if any of you nice folk can help me with a problem.

I recently purchased a Pentax K20D and have been using it with a Sigma 17-70mm lens.

The problem is that sometimes the pics are completely blurred. This has happened using the camera on autofocus and manual focus modes. Shake reduction is off (on the camera and lens). It seems to happen more often (but possibly not exclusively) when using a wide aperture (F4) but the speeds I'm shooting at shouldn't cause blur (500th, 750th).

I've attached two pics, taken within seconds of eachother, that illustrate the problem. Both were taken with autofocus. The first is sharp, the second is completely blurred. The first was 1/250 at F6.7; the second 1/750, F4.

I have other similar examples, including where the shutter speed and aperture are identical for each pic but one is sharp and the other blurred.

I don't know if this is significant but the file sizes of the problem pics are often smaller. In the attached pics, the first was 10.6mb, the second 9.7.

I've just tried out the Pentax kit lens to see if the problem is repeated. It was fine on the ten autofocus pics I took (the Sigma can be fine at this quantity too). However, I've come back with two very blurry pics on manual focus. Of course, it's possible my eyesight is the problem for these but I'm not sure that's the case!

If anyone can make any suggestions I'd be really grateful. I showed the pics to my local camera shop and they were very puzzled.

Of course, I'm hoping it's human error (i.e. me being stupid!)

Thanks for your time

Andrew

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[labnut]

Which Sigma 17-70 lens are you using. The newer F2.8 lens has built in Optical Shake. With this lens you must select either OS on the lens or SR on the body but you must not enable both at the same time.
Peter

[imtheguy]

That picture should never happen with the cam set to AF.S focus mode. I have a couple lenses that sometimes go to hunt and stop way short focus causing me to repress the shutter button. But the cam knows its not focused so the indicator light is blinking at the bottom of the viewer. Another quick 1/2 press and the lense tries again and all is fine. Its as if they gave up too soon and usually when the subject is less than obviously defined against the background.

So, try it in AF.S mode if not already if the post by labnut does not apply.

Stage 2, if none of the above helps, please supply more info: tripod used?, f-stop, focal length, iso shutter speed.

[fozzy]

Thanks Labnut and imtheguy,

In response to your comments:

Labnut - Yes, I'm using a Sigma lens with optical stabiliser. When I took those pictures, the optical stabiliser on the lens and the shake reduction on the camera were both switched off.

imtheguy - Yes, both pics were taken on AF.S mode. No tripod was used. The f-stop of the sharp pic was F6.7 (250th) and the blurred pic F4 (750th). The ISO setting for both was 200. The lens was fully extended - 70mm.

I've had the same problem with manual focus. Attached are two pictures taken two minutes apart, the first (Kings Sutton 1) is in focus, the second (Kings Sutton 2) is completely blurred! Each was taken at 500th/second at F4, ISO 200, shake reduction on camera off. Both were hand-held at 70mm. That day I ended up with more blurred shots than sharp shots!

Sorry to lumber you all with pics of British trains!

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[Ash]

Something's fishy regarding those f/4 shots.
I'm wondering whether the lens is the problem here.
It should at least have central sharpness but clearly doesn't despite every attempt to focus correctly.
Check more of your wide open results and see if they're all consistently blurry. Then check all of your stopped down results to see if they're consistently sharp.
If this is the case, then the lens may be at fault - manually open the aperture of the lens when dismounted and look through the lens. Does that look OK?
Hope this helps.

[jamesm007]

Seems you may have some problems and all are with the lens really but one. First the Sigma 17-70mm at 70mm is kinda soft, stop down to f/5.6 and all is well. It will snap into sharpness with just this one stop! But of course your pics are too blurry even for a soft lens. You also may have a FF/BF problem. The K20D can handle this situation with AF fine tunning. Try this, use live view to focus at f/4 and manually. That is set the lens to manual focus, turn on live view, have the lens at f/4 zoom in the LCD if need be but take a few pics using live view. If they improve you know its a FF/BF problem or...

Manually focusing does not guarantee a focused pic. The sensor still has to be in alignment with the OVF and AF module.

Now you should know the K10D hated trees and brush and the K20D will barely tolerate it when using AF. Your scenes will have the camera focusing all over the place. And my bodies would do the same with my older Sigma (I know you have the new) in such a scene if you don't nail AF as told above by looking and re-focusing if needed on a higher contrast spot.

Stopping down always helps to hide DOF or focus problems.

[SpecialK]

I think it's a lens thing, too, but not a BF/FF issue as really nothing is sharp in the blurry shouts.

The blurry shots logically have a smaller file size because some of the resolution is lost.

But why turn off SR when it can only help, even at fast shutter speeds?

[Ole]

Thanks Labnut and imtheguy,

In response to your comments:

Labnut - Yes, I'm using a Sigma lens with optical stabiliser. When I took those pictures, the optical stabiliser on the lens and the shake reduction on the camera were both switched off.

imtheguy - Yes, both pics were taken on AF.S mode. No tripod was used. The f-stop of the sharp pic was F6.7 (250th) and the blurred pic F4 (750th). The ISO setting for both was 200. The lens was fully extended - 70mm.

I've had the same problem with manual focus. Attached are two pictures taken two minutes apart, the first (Kings Sutton 1) is in focus, the second (Kings Sutton 2) is completely blurred! Each was taken at 500th/second at F4, ISO 200, shake reduction on camera off. Both were hand-held at 70mm. That day I ended up with more blurred shots than sharp shots!

Sorry to lumber you all with pics of British trains!

I enjoy train photos so do not excuse anything.

Regarding the test where the two photos were manual focus:

Did you change the manual focus between the shots? If not then were down to:
- Camera shake, or
- Faulty camera - the sensor is not held firmly in place by the shake reduction mechanism (even with SR off the sensor is held in place by the SR magnets)
- Faulty lens - the moving lens elements are not held in place even when the lens SR is turned off

If you use a tripod next time you can rule out camera shake.

If you - with camera on a a tripod - get same result with two different lenses, the camera is at fault. If the non-stabilized lens gives consistently great results, then the stabilized lens is at fault.

I suspect the stabilized lens is at fault since in your other test your Pentax lens produced consistently sharp photos (in autofocus mode).

Regarding the file size: blurred photos have less detail and therefore the compressed files will be smaller.

[JohnBee]

It seems to happen more often (but possibly not exclusively) when using a wide aperture (F4) but the speeds I'm shooting at shouldn't cause blur (500th, 750th).

If I had to guess, I'd say it looks as though the camera is taking pictures during AF.

[simico]

Something's fishy regarding those f/4 shots.
I'm wondering whether the lens is the problem here.

I'm wondering the same. If this happens consistently only with f/4 on the 17-70 lens then it's a lens issue.

[mickeyobe]

I don't think any of the pictures are critically sharp.
All of them are sharper in the foreground.
I am sorry I don.t have an answer.

Have you tried setting the lenses at infinity as marked on the lens?

Mickey

[fozzy]

Hi all

Thanks for all the helpful suggestions on the causes of my blurred pictures. I've tested the two lenses over the last two days and it seems the views of posters are right: its a lens problem rather than a camera problem.

The Pentax 'kit' lens has given consistently good results on autofocus (indeed, better than I'd expected from having read a review).

The Sigma performs well at smaller apertures but is very 'hit and miss' at widest aperture (F4), (which is the setting the completely blurred pics were taken at).

On holiday next week, so I'll take the camera with me and just avoid F4! Then I'll hand the camera and lens to Sigma, who'll have a look at what's going on. I'll let you know what they find.

I know this is an international forum but perhaps I can just say thanks here to the staff at Station Cameras in Reading who spent two hours today comparing the results from the different lenses. They truly know the meaning of customer service.

[Ole]

fozzy, good luck with the Sigma lens repair. Please let us know what they find!

Feel free to join the railroad social group and post your photos:

Railroad Photography - PentaxForums.com

[fozzy]

Thanks Ole, I'll check it out!