IR light is longer in wavelength than visible light and focuses differently. Regular photographic lenses are made for visible light photography without any regard to what happens when photographing “out of band” light like IR. Therefore each lens design and focal length, even each focal length within a zoom lens will focus IR light differently. To complicate things further, DSLR AF sensors (and our eyes as well) only “see” visible light and focus only on visible light. For these reasons we calibrate converted digital cameras to help attain proper focus. For a detailed explanation please watch the video below:
IR Front Focus/Back Focus
Without proper IR focus calibration you may experience front or back focus due to IR focus shift explained above. Here’s an illustration:
Even with calibration, there are some options and limitations as described below:
To standardize our process we calibrate digital cameras for proper IR focus to these lenses only:
Canon DSLRs – Canon 50mm 1.8 lens
Nikon DSLRs – Nikkor 18-70DX lens
With the custom lens calibration option we would calibrate your camera to the lens you supply. If a zoom is sent then unless specified otherwise the calibration will be optimized for the wide end of the zoom range. (Remember, IR light focuses differently at each focal length, even in a zoom lens.) This means that even after calibration of your camera to your zoom lens the telephoto end could still have a focus shift. There are some exceptions though, the following lenses work really well in infrared through the entire zoom range: Nikkor AF-S 18-70DX and AF-S VR 18-200DX lenses as well as the Canon EF-S 15-85mm f/3.5-5.6 IS.
Using any other lens than what we calibrated your camera to will most likely result in a certain amount of front/back focus shift or even infinity shift (inability to focus at infinity). How much shift and in which direction will depend on the lens we calibrated to and which lens you are attempting to use instead of the calibration lens.
With the advent of the Live View feature and recently, Live View Auto Focus, we can calibrate IR cameras to focus with most lenses. This is possible because in Live View mode the mirror is raised, shutter is open and your camera actually shows you a live view of what your imaging sensor “sees” when taking a photograph. In the case of IR conversion, the imaging sensor “sees” infrared light.
With this calibration despite the camera being IR converted, we calibrate the focus back to the visible range. This does sound strange and actually quite difficult to perform (how can you calibrate on visible light when the camera can only “see” infrared light?), but we have devised a method for doing just that.
We believe this is the best and most versatile method when shooting IR with modified DSLRs that have the Live View feature. Even if you have strong feelings against Live View we highly recommend using this method for IR photography.
Universal calibration can be used in these three ways:
Live View With Auto Focus (best method)
Since in live view the imaging sensor “sees” the IR image through the lens it can also auto focus it, provided your camera has the ability to auto focus in live view mode. Some cameras have more than one AF mode in Live View, be sure your camera set to use the direct (also called Live) focus mode (slower but accurate in IR) instead of the predictive (also called “Quick”) mode (faster but inaccurate in IR).
Live View With Manual Focus (for cameras that don’t have AF in Live View)
Since you see the actual IR view your imaging sensor “sees” you can manually focus IR light in Live View mode and end up with accurately focused IR images. Some cameras allow you to zoom in on an area of the live view to allow you to see greater detail and help you focus more accurately.
Lens IR Focus Marks (for cameras without Live View)
This would be the method of last resort as using Live View is faster and more accurate but for cameras that simply don’t have the Live View feature one can use the IR focus marks on lenses. Basically you’d first focus manually or with AF on your subject through the viewfinder (like you’d do with an unmodified camera). Then shift the focus collar to line up with the IR mark and shoot. Keep in mind though that lens IR focus marks are approximate.
Full Spectrum Conversion Calibration
For full spectrum conversions we calibrate the camera to focus accurately on visible light and this is our only option for this service. This is basically the same as our Universal calibration for IR conversions but is easier to perform since the camera can “see” visible light and so we include it as part of the service.
When shooting visible light with an external hot mirror filter there is no need for any special focus considerations as the camera behaves as a stock model.
When shooting IR with your full spectrum camera you can follow the Universal calibration focus instructions outlined above.
When shooting UV light the focus may or may not be off depending on the lens you use. If you use the Coastal Optics 60mm UV-Vis-IR lens then it will have the same focus in UV as well as visible and IR light.
Point & Shoot Calibration
Since point & shoot cameras come with a lens built into the camera we calibrate the camera to focus accurately with this lens only (obviously).
For full spectrum conversions with point & shoot cameras we follow our DSLR full spectrum calibration and calibrate to visible light as well. Shooting IR or UV may require to manually focus with models that allow this or simply stop down to allow focus to fall within the depth of field.
Hybrid Camera Calibration
Because of their unique design these cameras stand apart in their ease of use both as IR only and as full spectrum converted cameras. Since they are mirrorless they rely completely on the imaging sensor for all aspects of the image capture process, including focus and metering. Because of this they are able to “see” the IR focus shift real time and adjust for this on the fly while focusing. This allows you the freedom to use pretty much any micro four thirds lenses and still achieve sharp images.
All mirroless cameras like the Panasonic Lumix G series and Olympus PEN E-P series are calibrated specifically to take advantage of this unique capability.
Now that you are familiar with the focus calibration options lets proceed to the next section:
NEXT – Lens Considerations