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

My friend, a fast reply to your answer is that IR wavelength and brands depends only in the kind of colors you want to show in your images and the durability of the filter you want. The brand doesn't matter if you will use the filter a few times per year, so go for the cheapest you could get.

But to understand the 650nm, 670nm, 720nm, 850nm, 950nm, 1000nm numbers, you need to learn first how the light work.

The next figure is the representation of the wavelengths of diferent type of lights, the human eye can "see" only the waves between 400nm and 700nm, we call them, COLORS. Above 700nm is the Infrared light, that's the light produced by heat, but human eye can't see that, only feel it as radiation. (IR is btwn 1000nm-10000nm)

_{(source: scienceblogs.com)}
Well, the IR Filter function is cut the wavelength light under the XXXnm of the filter. In other words, the IR filter let pass the wavelengths (colors) over the XXnm. So the colors under the XXXnm will not be seen for the lens, but over XXXnm does. As we can see, red colors have 700nm of wavelength and in the market there are filters from 650nm to 1000nm.

Finally, if you want to do an image that shows light red color, dark blue colors and the heat over the objets, you must buy a 650nm, 680nm or 720nm filter. If you want only the heat you must buy 850nm, 950nm or 1000nm.

i.e.:

650nm-720nm

720nm-1000nm

In adition of all of those complicated things. As the IR filter let pass lower quantity of light, you will need to increase the exposure time (Lower Shutter Speed), increase the ISO value and decrease the f-stop (Higher aperture) when you take the image, in the way you use a higher XXXnm IR filter.

So my recomendation is get the 650nm for indoor (low light ambience) or get the 950nm for outdoor (day ligth) plus a tripod. If you want a versatile one use the 720nm.

I hope my explanation was useful for you.

All the best,

Nicolas Duarte from CHILE

Answer 2

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 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.

Answer 4

I take it that you are using an unmodified camera for your IR images, you need the Hoya R72 filter as you cannot use the lower cutoff filters like a 650 or 680 as these are BVR ( barely visible red ) filters and only capture IR light on modified ( either full spectrum or a 590 ( Hoya 25A) conversion with the filter on the lens cameras. The cheaper 720 nm filters may diminish the quality of your photos or introduce undesirable artefacts into the pictures, depending on how sensitive your camera is you should be able to achieve a reasonable exposure with the R72 at about 6-8 secs @f11 iso400 on a sunny day, cloudier days may require a longer exposure or higher ISO to capture the IR light. The higher nm filters are pure monochrome IR filters and would require longer exposures of minutes rather than seconds on an unmodified camera. The R72 can do monochrome and some small amount of false colour so it's the best one you can use - if you want more colours you can get an older body modified later on. Hope that helps :)

Answer 5

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