General Photography

Using Pentax DSLRs for IR photography

Posted by: karma_mechanic on Fri Nov 18, 2011 12:29

Some of you may have see the infrared (IR) images that I've posted, but this article is to give a bit of background to how it is done. I'll cover using a camera with and without modifications, and the pros and cons of each approach
A DSLR does not really lend itself to IR photography. On the other hand most of us have a selection of lenses and other equipment, plus the familiarity with the camera that means that using a Pentax (or Samsung equivalent) for IR can be very worthwhile. Foliage and grass can be very bright in IR while shadows lit only by blue sky can be very dark. Clouds stand out from the dark sky, and water can also be very dark. The effect can be quite different from a mono conversion of a colour shot.

With an unmodified camera the IR in the scene is intentionally blocked by a filter just in front of the sensor. Putting a filter on the lens to block the visible light means that the IR that gets to the sensor isn't swamped by the visible light and can be used to produce an image.

On the older bodies such as the *ist series this filter is quite weak, and it is just possible to get results in good sunshine with hand-held exposures. On the later bodies the IR-blocking filter is much stronger, so exposures tend towards many seconds even at very high ISO.

So, if you've got an older body how would you go about IR photography? First you need a suitable filter. The Hoya R72 is a good balance between IR effects and exposure, the '72' indicates that it cuts off light with a wavelength shorter than 720 nanometres (nm). That's what we'd call visible light, down to deep red. What's left goes through and is partially blocked by the IR-blocking filter, the remainder makes the image.

One problem immediately becomes apparent. The viewfinder is almost completely dark. One way to compose the image is to use a tripod, then compose without the filter and attach it when required. That's a chore, although if you've gone to the trouble of using a tripod then exposures aren't really a problem. In very bright sun I find that if I concentrate on the viewfinder image then the R72 lets enough just very deep red light through to allow the scene to be composed with the filter in place. It needs the eye to become adapted to the dark view, so may take a while. Again that's a chore.

Lenses focus IR differently to visible light, but fortunately autofocus actually works in IR when there is a filter on the front of the lens, since the AF sensors are seeing IR.

Metering also works reasonably well, for the same reason.

This shot was taken hand-held on a Samsung GX1L (*ist DL2) with the Hoya R72.

The main advantage of using an unmodified camera is that it only costs a filter and some time.

Having used the GX1L for a little while I became frustrated at the difficulties outlined above, so I decided to convert it to work properly in IR.
The IR-blocking filter was removed, and replaced by an IR-passing filter (in this case it was actually a piece cut from a Hoya R72 filter). So now the viewfinder on a converted camera gives the normal view of the scene, there is no need for a filter on the front of the lens. The filtration of all visible light is done just in front of the sensor. Since there is no longer an IR-blocking filter in the path the exposures are much more useful, typically 1/90th at f/8 and ISO 200. But this convenience comes at a price.

Exposure metering now sees visible light, but the sensor only sees IR. The proportion of visible and IR can vary in different scenes, so exposure can need one or two stops of correction compared to the meter.

Focus via the viewfinder or via AF doesn't tell us what the focus of the IR light is, and lenses vary in their behaviour in IR. I use debug mode to adjust the AF to match the IR focus, but different lenses need different amounts of adjustment. Even different ends of a zoom may need slight changes. Sticking to a small number of lenses and learning their characteristics works best here.

The advantage is speed. Once set up it is possible to use the camera in the same way as before; compose, AF, shoot. I tend to use the Sigma 10-20mm the most, almost always at 10mm. I've set the sensor position so that infinity on the lens is correct, and at 10mm there isn't much focussing required. The shot at the top of this article was on that lens on a modified K20D and a much stronger 830nm filter. Here's another with the same setup:

The disadvantage of a conversion is that the camera is now only suitable for use in IR.

I've now converted a K-7, and this avoids the focussing adjustment issue since I can use contrast detect AF in Live View. It is also very useful in composition to see the IR image on the screen since it can look very different from the visible light view.

I hope this has given a flavour of the what's involved and the two approaches. There are many more examples in my 500px stream or my own website.

Article content received from: PENTAXslr Forums,