infrared filter

Your K20d has an IR blocking filter in front of the sensor. You can take IR shots but you will need MUCH longer exposures. Best thing is to keep experimenting!

All digital cameras are very sensitive to IR so the manufacturers put a filter in front of the sensor to remove some or all of the IR. It can cause colour shifts and loss of contrast.

The *istDS is not too bad in this respect and will work quite well with IR, but the K10D filtered quite aggressively and wasn't so good. The K20D I have not tried.

The technique is to put the camera on a tripod, compose your image, refocusing manually to the IR marker if there is one on your lens. Then put on the filter and let the camera calculate the new exposure. You won't see through the viewfinder at this stage because the filter removes all the visible light.

When the (possibly quite long) exposure is done you will be able to view the very red image on the camera screen. This then needs converting to black and white either in camera if it does it or in Photoshop or similar.

That's it in a nutshell, but there are plenty of books on IR imaging. It can look fantastic with the right sort of subject.

Basically, the filter you have bought only allows IR through, but the K20d (in common with most DSLRs) has a filter designed to block IR. Therefore the two cancel each other out.

Because you now have hardly any light entering the camera you need to use a very long exposure. Just keep experimenting with longer and longer exposures until you start to see something.

Generally, internal reflections within a lens can cause flare and loss of contrast. Because the sensor is sensitive to IR light any IR that is being scattered will affect the sensor even though we can't see it. This causes colour shifts and lack of contrast, so it is filtered out by a "hot filter" in front of the sensor.

Leica found this to be quite a problem in their rangefinder digital R8.

Canon also filter out IR very aggressively, so Canon cameras are unsuitable for IR photography.

It is possible to have the hot filter removed by a repairer and replaced by clear glass, thus creating an IR sensitive camera. Fuji actually market IR versions of their S5 and S9600 cameras.

Susan

When you say that you have a Cokin Infrared filter, is it a screw-in circular filter or a square slide-in type. I ask this because to get good infrared images you need to stop all unfiltered light from reaching the lens. A Cokin square filter may allow daylight to reach the lens down the side; if this is so then you may want to apply some black tape at the edges.

To take infrared photographs on your Pentax K20D, I would use a tripod and take a trial exposure in Manual setting, at 1/15 sec at f8 using 400 ISO. This assumes good sunlight. Be prepared to alter this to 1/8, 1/4 or 1/2 at f8, with ISO settings of 400, 800 and 1600. The higher ISOs will produce plenty of noise and this makes the images look very soft.

I have not tried infrared on my K20D but have taken plenty on my istDL2.

Best of luck with the experiments.

There's an old thread here that might help.

There again...

My attempts at this involved 20 or 30 second exposures with apertures in the f5.6-9.5 range (and 100 ISO). As the guys on here pointed out, it's an uphill struggle given the effectiveness of the sensors' IR filtration. It's worth a try though and I'll have another go if the sun ever comes out again.