It’s nice when you can build your own IR camera. But it has limitations. This has become clear in my test of wide-angle lenses with my Nikon D700.
If you just remove the IR cut filter, then of course you are missing a layer of glass in front of the sensor. That in turn affects the focusing. Here it gets complicated, because it strongly depends on the lens and the filter (!), with which minimum f-number you can generate a sharp image.
And please note: We are not talking about objects that are a few centimeters or even meters away – these come out sharp, even at f/4 or f/5. I am talking about 50, 100 or 1000 meters, of mountain ranges or distant buildings. These come out sharp only at f/11 or even 13, depending on the lens and filter.
This creates a problem. Primarily with old fixed focal length lenses that have to be operated manually. But also with AF lenses, for example in the 35mm or 50mm range, you have to set the f-number high to get distant objects into focus.
Professional IR retrofitters like Life Pixel or Kolari Vision replace the hot mirror with a piece of glass that lets the full spectrum through. This is a service that is worthwhile and also justifies a higher purchase price.
This replacement is the real challenge in IR conversion. The problem is dust or scratches. If dust gets between the sensor and the hot mirror replacement, you can see it very well. In fact, you can see every minimal speck of dust or scratch because the glass is so close to the sensor and therefore at the “plane of focus”. Therefore, hats off to the professionals who manage to install such a filter in an absolutely sterile environment.
What does that mean now? Does it mean that a self-converted camera that “just” has the IR cut filter removed doesn’t work or shouldn’t be bought? No. You just have to pay a little more attention. The following points should be noted:
1. strong IR filters are not a problem. Whether 720nm or 850nm, these filters work fine with a self-converted camera, usually from aperture 7.1 or 9. The problem is those that let through large portions of the visible and areas of the IR spectrum, for example my Aerochrome combo. There, on average, aperture 11 or even 13 is necessary.
2. zoom lenses work very well on average. For example, the Nikon 28-80mm – a dirt cheap but fantastic lens, goes very well. There you can easily shoot with relatively low f-stop even with false color filters.
3. in general, the wider angle and fixed, the more difficult.
4. pay attention to Liveview. Always use it if possible. But this applies to IR photography in general.
I tested the following lenses with the D700:
– Nikon 35mm f/2 AF
– Nikon 28mm f/2.8 AI-s
– NIKKOR-N Auto 24mm f/2.8
Conclusion: The 28mm and 35mm generally generate excellent sharpness and also perform well with the X1-Orange combination. With both lenses, the optimum sharpness (sharp from corner to corner) is f/11 with 720nm and f/13 with X1-Orange. The 24mm has severe corner blur at both 720nm and X1-Orange that you can’t get rid of even with a high f-number, and fringing and strange colors with X1-Orange.
I will probably test later with the Nikon D7200 converted from Life Pixel to Full Spectrum. This has a hot mirror replacement filter built in – and I’m convinced that you can “focus” distant objects there with a smaller f-number. But once you’ve used an FX camera, especially the D700, you don’t want to go back to a DX. I don’t know. Especially the X1-Orange combo just works much better with an FX.