I am not a fan of using these focus test charts where the camera is focussing on a target that is at an angle to the focus plane. I am never confident just where the camera actually focussed.

Instead, with a camera that offers a live view function, I have used the technique that relies on moire interference patterns between an LCD screen and the live view screen. The advantage of this approach is that the camera is focussing on a target that is at right angles to the camera. Thus you know exactly where the camera is attempting to focus.

The following web site describes one way of doing this (it's for a Canon, but the principles will apply to a Pentax K20D):

http://www.northlight-images.co.uk/article_pages/cameras/1ds3_af_micoadjustment....

I would not let a little bit of snow stop me using my camera. My old ME Super have even ended up buried in snow several times without all this panic about weather seals. The only warning I would like to make is to ensure you thoroughly dry the equipment afterwards. Years ago I must have put my old lenses away still damp, because when I picked them up again after several years of non-use some of them were infected with fungus. I now keep my new lenses on shelves where the air can get at them.

As my Samsung GX-20 often produces faulty images when it is cold (see K20D vertical banding thread), I have been using my old unsealed Pentax DS to take to the Scottish mountains with me. I have no hesitation to use it and my unsealed lenses in the snow, besides the DA* lenses are a bit heavy to carry, what with the weight of ice-axes and crampons etc. Note however the rechargeable lithiums I use in the DS can very occasionally make the camera behave sluggishly when the temperatures a well below 0C. The GX-20's battery seems much more robust in this regard.

And a couple of pictures of the lovely Scottish snow during the last few weekends:

Hi, I thought I would paste a few more thoughts and images from my testing of the vertical banding issue I mentioned a few weeks ago. Most of these comments are based on the cold long exposure tests I did with my Samsung GX-20.

I noticed three different types of fault, although I suspect they all have the same underlying cause:

- fine vertical bands every few pixels across;
- posterisation;
- regional horizontal/vertical banding.

Below are some examples. Unfortunately my test picture lacks a good textured target so the posterisation can be hard to see, so I have included again an example from an earlier picture:








As many have discussed, the occurrence of the problems can be random and come in batches. However I believe the following contribute to the probability of a fault appearing in an image:

- Cold. As the camera drops below 10C the chances of a an issue occurring increases.
- Long exposure times. Above 1 minute exposures the regional banding occurrence increases greatly with my camera.

However all of these effects have been observed in my camera with normal exposures (e.g. 1/90s @ ISO 100).

I had a look at the individual RGB channels in Photoshop and saw the lines present in all three colour channels.

For the vertical regional banding, the position of the right hand band can change in position. Note also that the image colour in such images with regional banding can change. I have sometimes seen images where no obvious vertical bands are visible, but the colour of the image has shifted. Below are three images shot 8 minutes apart with fixed exposure and lighting conditions. The colour of the image is shifting.






Hopefully the above images help show what to look out for if you suspect your camera. I hope the people who have recently sent their K20D to Pentax will report back whether Pentax fixed the fault. With all this cold weather in the UK I have had to mothball my GX-20 for the moment. I do miss its SR though.

I don't think you can disable noise reduction on a K20D/GX-20. Dark frame subtraction is always employed in longer exposures. Note High Noise reduction was off in all tests. I also did not see any evidence that SR on/off impacts the results.

I hope these tests don't worry the paranoid, I am sure this is a relatively rare fault in an otherwise superb camera. But if you suspect the camera then you may be able to try the tests to confirm if the camera is ok or not, and if sending it back hopefully help Pentax reproduce the fault and fix it. I also suggest reading the vertical banding thread on pentaxforums.com - that is where I learned that long exposures can trigger the fault repeatedly.

Any further tests will have to wait until after the weekend - I have a date with Scottish weather. It will the the DS I will be taking on that trip - it loves the snow.

Since my initial comments a week or so ago I have been conducting tests with my Samsung GX-20 and now have a method where I can repeatedly create images showing the various faults: fine vertical banding, horizontal/vertical regional banding and posterisation.

1. Cool the camera, preferrably to 10C or less.
2. Shoot a test image that helps you see image problems, lit with tungsten light.
3. Expose for >60 seconds (I ended up at 75s).
4. Allow time between exposures to allow for the dark frame capture and to allow the electronics some time to cool down (I allowed about 7 minutes).

Since the Samsung has no PC remote control software, I built an intervalometer to get the camera to snap test pictures through the night at a regular exposure/interval.

With the above settings, the majority of pictures shot below 10C have some sort of issue, and then the probability/severity of failure decreases as the camera temperature increases. However I have had some faulty images shot above 20C. Note the PhotoME camera temperature could be different to that of the faulty circuit, and there will be some camera electronics self heating going on.

With exposures at 30 seconds, failures are much rarer. I think in some there is a hint of vertical banding but its magnitude is much lower and not troublesome in most images. However I did spot colour inconsistances at this exposure which may indicate the underlying fault is still at work.

Below is an early test image I snapped and developed by Lightroom using its defaults, using 17C and 120s exposure.



You can clearly see the horizontal regional banding across the lower third of the image. Look closely and you can also see at the bottom right a vertical region band interacting with the horizontal band.

And a 100% crop:



There you can also see the break up of detail in the shadow.

I have not had time yet to post more general images and conclusions from my test. I have a few hundred failure images now to sort through. However I do know that:

- Lower camera temperature increases fault probability.
- Long exposures will commonly result in faults.
- Sometime the fault is minor, but can result in unuseable pictures in real conditions (see my original samples images on Flikr).
- More likely to be spotted in darker exposure and tungsten lighting when the pixel's RGB data values are small in some colour channels.

Paul
Samples: link

This occurs in RAW in my case. Looks to me like a flaw in the sensor/prime engine processing.

Don't be too paranoid about this, I don't think it is that common a fault as there have been very few mentions of it until recently.

I normally shoot 100 ISO but as evening approached I switched to ISO 200 for a few pictures. Banding appeared at both speeds - I don't think it is ISO dependent.

Most of my failures were with SR on and stabilised, but I have reproduced a few with SR switched off - again it does not seem to be a factor.

My Samsung GX-20 has this fault too.

I bought my GX-20 back in May. In one of my early sessions with this camera I spotted a few images with the vertical banding, only about 3 images in a batch on one day. I thought it might be an issue with the memory card, so I used a new Sandisk 8GB and observed no re-occurrances until November.

However at the start of November, on a cold icy day in Glencoe, I found about 30% of my images had problems. Some had strong banding, others suffered severe posterisation of some colours, and some had large horizontal bands with purple discolourations. Most of these pictures are unuseable.

You can see the faulty images for yourself on FLIKR: link.

The sample images with "_Small" at the end of the name are exports from Lightroom using the default "As shot" settings from the DNG file. The images with "_Crop" in the name are 100% crops of problem areas, although I have tweaked the levels on some to make it easier to see the issues.

Two samples of these are shown below for convenience. The first has a purple horizontal band:



And this image shows how the vertical banding changes at the point a horizontal band occurs:



Clearly the camera is faulty so I sent it into Samsung to be repaired providing many of the above faulty images on a DVD. They returned it back saying no fault found, vertical lines are due to under-exposure.

I was a bit shocked by this response. I can understand it is hard to fix the camera if you cannot reproduce the fault to debug it. However I think there are enough examples of this banding on K20Ds to indicate there is a clear fault with some of these cameras, and I think it is not acceptable for a normal camera to produce some of the images I show - noise may be a fact of life but horizontal purple zones are clearly unacceptable on any camera in my opinion.

I have since found an interesting discussion on PentaxForums.com on "K20D vertical strips" that has further examples, and a suggestion on how to reproduce the fault. One user has the same horizontal regional banding I encountered.

Despite the fact that it seems cameras sent in with this issue are being returned un-repaired, my feeling is that if you have this vertical banding on your camera you should still send it in in an attempt to get it fixed. Whilst the banding may be a minor inconvenience at the moment, the fault may provide much worse images in the future as I have found and wreck important pictures. If Pentax receive enough cameras with this issue reported they may eventually take it seriously and do something about it. If you can find a way of repeating the fault on your camera then telling Pentax how to do this may help. Definitely provide RAW images illustrating the banding so they can at least look at the image data. I would also let them know you have seen identical faults reported on the Internet by other users and maybe provide them links to other users' experience - it might just persuade them to accept there is a real issue and do something about it.

I am now somewhat in a quandry. I don't feel I can trust this camera anymore. I could right off the £500 it cost and go back to my DS. I really have spent too much on the lovely lenses to change brand. I will probably continue to see if I can find a reliable way of reproducing the fault on my camera on demand, and also see if I can investigate the RAW data itself to see if I can observe what is going on. Maybe if I can analyse the fault further I may be able to convince Samsung to take the camera back again and fix it. It is a great camera when it works.

Note, PhotoME says the camera was built in February 2008 and indicates temperatures of 4-8C, however I reproduced banding today in the house at about 15C.

Hi, after monitoring this forum for a few years I decided it was now time to de-lurk. PentaxUser seems quite a positive forum and I have some comments to add the recent "K20D vertical banding" thread which I hope to post in the next day or so once I work out how to host some related image samples.

I got into photography in the early '80s when I bought an ME Super, and later moved to used LX when I started climbing/trecking in the Himalaya. But I seemed to stop using a SLR in the new millenium for some reason. It was only when on a whim I dusted off the LX and took it on a trip that I remembered the joy of using this camera. I then started reviewing Pentax on the web and discovered how things had changed since I had been away - including things like the introduction of the limited lens series. So I invested in a used DS, then more recently a Samsung GX-20, and a ridiculous number of Pentax lenses - ridiculous as I generally just stick the 43mm Ltd on the camera for most photos I take. Ah well - it is nice to know I have alternatives safe in my cupboards, although the current Pentax rebates means a few other lenses are looking attractive right now...

And since I feel photography should be more about photographs and not camera gear, I have uploaded my first image to my profile.