What should I look for in an infrared filter?

Question

I'm looking to get myself an infrared filter to put in front of my lens. The cheaper ones all have nm counts (like 720nm, 950nm, 1000nm, and the way I understand this, the thicker, the better), but there also are more expensive filters, like hoya r72 ect, and it is never explained, why they are 3+ times more expensive than the brand-less filters.

Is this only because of the brand, or is there something more to infrared filters?

Answer 1

The two factors to consider in an infrared filter are; the cutoff wavelength (when the filter starts to block light, which are the numbers you've listed in your question) and the sensitivity of your camera to IR.

This question summarises the looks achieved by the different wavelength filters but you need to keep in mind that higher wavelengths require either a more sensitive camera or a significantly longer exposure to produce a usable image.

All digital sensors are naturally sensitive to infrared (and UV), and are actually too sensitive for normal visible-light photography. As a result all manufacturers put an IR cutoff filter in front of the sensor which (unless you opt to have your camera permanently converted) substantially reduces the amount of infrared radiation that hits the sensor.

The easiest way to test how sensitive your camera is to infrared is to point an infrared remote control at it and take a picture of the IR emitter on the control while pressing a button on it. The brighter the spot you see in the picture, the more sensitive the camera is to IR, and the higher wavelength of filter you can use with reasonable exposure settings.

Answer 2

The more expensive brands you see are analogous to L vs non-L glass; both do the same thing, one just does it better, the question of course being how much better and how much does it matter?

Most making a living by photography would err on the side of quality, however I suspect that you'll find the difference to be negligible. More expensive filters may be less prone to artefacts, but considering you can use unexposed film as an IR filter with acceptable results the difference you'll see between the two won't be overly obvious.

It really depends on where you want it to cut off visible light. Thicker is not neccesarily better if it doesnt give you the result you're after.

edit: see here

Answer 3

Expensive filters are manufactured with high-quality optical glass. The radiation rejection up-to particular wavelength say 720 nm and (infrared) transmission above a particular wavelength say 720 nm is also high as compared to the cheap IR filter. The better optical glass won't cause any problem with auto-focus which could be a problem with low-end IR filter. Costs other than better glass goes for better ring and sealing material.