Wednesday, 5 March 2014

Cameras - Nikon DSLRs & The B+W 093 IR filter

London Eye - Nikon D3, Nikkor 35mm f/2 - 3.8 mins, f/11, iso200

The B+W 093 infra-red filter was my first foray into IR photography. I bought the 77mm version around 11 years ago to use on my Nikon D2H & Nikkor 17-55mm f/2.8 because that was the filter size of that lens. Since this filter is so large I can get away with shooting on pretty much any lens using step-up adapters. I didn't plan this, but I would recommend it to anyone who has various sized lenses.

I mostly talk about my experience with Nikon here, but it doesn't matter what make or model DSLR you shoot with. There will be variation in how many exposure stops you lose, but that's due to the camera's internal UV/IR cut filter. How aggressive this filter is seems to depend mostly on how old the camera is. For example my 2003 Nikon D2H seems to lose about six stops, which is not too bad - the similarly aged Nikon D70 is about the same. You can get away with shooting images hand-held using wide aperture lenses, but the issue with cameras from this era is their poor high-iso performance. On the other hand the 2007 Nikon D3 has great high-iso performance. Unfortunately it's UV/IR cut filter is significantly more aggressive, taking away sixteen stops when the 093 IR filter is added!

This reduction in exposure would have been less if I had chosen the more popular Hoya R72 (similar to the Wratten 89B), but oh well. The 093 filter (otherwise called 87c or RG830) properly starts to let light through at about 800nm. The R72 however lets light through around 100nm lower, picking up some of the visible spectrum and drastically improving exposure times. As you can see by the above graph the camera's internal IR cut filter (light blue line) is blocking almost everything that the 093 (87C) filter is letting through, this is why exposure times are ruined on more modern digital cameras. For this reason I would not advise people to use these higher wavelength IR filters nowadays, there's a reason most sane people stick to the 720nm R72. If you really want to shoot higher consider getting your camera converted.

Leamington Spa - Nikon D2H, 17-55mm - 30 sec, f/8, iso200

Here is a completely unmodified infra-red image from the Nikon D2H. You can actually get the images to come out black and white from the camera if you set the white-balance on some grass. Since I always shoot RAW and I can easily modify it in software I've never bothered to do it, but if you shoot jpg files it will improve your workflow and final image quality.

Here's the previous image processed to black and white. White balance, saturation, exposure, highlight control and black level have all been manipulated to get this result so it helps to shoot RAW (to maintain image quality). Even though the IR filter is only reducing the exposure by about 6 stops it's enough to introduce significant motion to these shots. Since the Nikon D2H had no live-view and poor high-iso performance it was still too limited for shooting high quality shots without a tripod.

The next two images are taken from the same spot. Since I had to set up the camera on the tripod without the infra-red filter attached (so I could where the camera was pointing), I thought why not take a colour image first. My original plan for this was to compare the tones between the colour image and the IR shot. This in itself was interesting, but later I decided to blend the two images together in Photoshop using layer styles.

Lake District - Nikon D2H - 1/3rd sec, f/22, iso200

Here is a colour shot for reference. Note the amount of fog (and lack of detail) on the mountains in the background.

Lake District - Nikon D2H - 30 sec, f/22, iso200

This is the infra-red version of the previous shot. See how the IR filter cuts through the fog almost completely. For some reason this doesn't work with all water vapour, it does nothing to clouds for example, even though the improved dynamic range has at least made them visible.

Since all of the other settings on the camera are the same between these two shots we can see exactly how many stops we lose by adding the IR filter (about 6). This isn't too bad at all. If I had used some very wide aperture lenses and high iso settings I could have probably gotten away with shooting without a tripod. This also would have been better with a less aggressive IR filter like the colour infra-red Hoya R72 (720nm). In some ways I wish I would have tried this as I did have the 50mm f/1.4 at the time, althouhg the D2H did produce shocking large amounts of noise at high iso (at least by today's standards).

Here's a blend of the above colour and infra-red shots, overlayed in Photoshop (using layer styles). The blue tint of the fog from the colour shot looks surreal with the detail of the infra-red added in. I also like the way that the darkness of the rocks from the infra-red image adds a nice high contrast element to the scene.

Lake District - Nikon D2H, 17-55mm - 1/4th sec, f/22, iso200 (colour)
30 sec, f/18, iso200 (infra-red)

Here is an example showing some interesting aspects of water in infra-red photography. As well as coming out very dark in IR water isn't very transparent at all, even when it's very clear in colour. It reflects elements like trees & grass, but they're so noticeable because the rest of the surface is so dark and not because it's particularly mirror like. Mostly water seems to absorb light in the IR part of the spectrum, but perhaps there are also diffraction / scattering effects going on to. Of course this is not helped by the long exposure blending out the reflections over time. I'm sorry this isn't more scientific, but whatever it is doing it looks damn pretty!

Lake District - Nikon D2H, 17-55mm - 1/20th sec, f/7.1, iso200 (colour)
30 sec, f/13, iso200 (infra-red)

Here is another experiment involving a colour photo blended with the infra-red version. I felt that the tones of this blend helped balance the image and draw the eye more to the water. The colour photo on it's own feels rather flat and the water gets a little lost. That said my favourite image is still the infra-red on it's own. The tones, without the distraction of the colour, feel more interesting and surreal.

Buckingham Palace - Nikon D3, 35mm f/2 - 3.8 mins, f/8, iso200 - B+W 093 filter

Here's an image taken more recently using the same 093 filter, this time using a full-frame camera. The improved noise from the D3 does make it easier to process more vivid infra-red images, but the aggressive IR cut filter on the D3's sensor causes a couple of issues. The main one is very long exposures. This in turn often causes hot-spots because of light getting in through the viewfinder. Luckily the D3 has an answer for this. It has a built in cover for the viewfinder, otherwise you could cover it with a bit of cloth or something, but it's a nice feature to have. 

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