A Little Off Code, Computers, Photography and Guns

Infrared Photos

I'd have to say that my favorite style of photography is good black and white infrared photography, occasionally false-color infrared photography is good but black and white is by far my favorite.

I finally got a chance this summer to take some decent IR photos on my parent's ranch in Wyoming. I did all of the photos with my Nikon E-Series 28mm f/2.8, which on my Nikon D50 is a bit closer to a 42.7mm lens than a 28mm lens. How did I figure that you ask?

Given that I can find the angle of view of the camera using the equation^[1]:

Where alpha is the angle of view, f is the focal length of the lens and d is the diagonal length of the sensor of the camera in question. Given that my camera's sensor is 24.7mm x 15.5mm^[2], we can substitute this in for d as well as the focal length of my lens, 28mm and solve for alpha:

Now that we've found the angle of view that my camera sees through the 28mm lens we can solve for the focal length f in lets say Nikon's D3X which happens to have a full-frame sensor. The Nikon D3X's sensor is 35.9mm x 24mm^[3].

Substituting in the diagonal length of the D3X's sensor and the angle of view my D50 has we can find what focal length of lens the D3X would need to have to have the same angle of view as my D50.

And now we can figure out how much cropping (magnification if you will) my camera's sensor causes when using a lens meant for a 35mm//full-frame camera.

This happens to be pretty close to the ratio that most people give for calculating the focal length of a lens meant for a 35mm//full-frame camera on Nikon DX format cameras. The general rule given is that you should just multiply the focal length by 1.5 which... as you can see is ever-so-slightly off. I'm just a little OCD about that sort of thing and it seemed too simple to be completely correct so I figured I'd just calculate it myself.

Anyway I suppose I should have warned you at the beginning of the post that this would have more to do with photography math than infrared photos but it probably didn't hurt you to learn something new.

Also I almost forgot to mention what filter I used. I used a Hoya R72 Infrared bandpass filter. This particular filter passes light wavelengths 720nm and greater. Since my camera was designed with visible light photography in mind it has a low pass filter installed over the sensor that blocks out a good deal of infrared and ultraviolet light so even in broad daylight I still have to do long exposures to get infrared photos exposed properly.

The photos in that album were all done at f/16 since I found that using extremely long exposures at smaller f-numbers just blows out the photo and I'm not sure why just yet. However it helps to use a smaller aperture anyway since infrared light also focus' at a different point than visible light does, my 28mm lens has an infrared focusing mark on it for correcting but, it's not perfect and I can't compose or focus the shots while the filter is on the camera since being human my eyes are only sensitive to light of wavelengths between about 380nm and 750nm^[4]. As well as making the photos not-blown-out with the aperture so small it also aids in any focusing deficiency since the depth of field is much deeper. Most of the photos were between 2 and 5 second exposures depending on the cloud-cover. You can see detailed shooting info by looking at the EXIF data Picasa shows in the More Info section of each photo, the aperture is erroneous there so ignore it, my Nikon E-Series 28mm is a non-cpu lens so it can't tell the camera what it's current aperture is.

1. http://en.wikipedia.org/wiki/Angle_of_view [↩]
2. http://www.dpreview.com/reviews/specs/Nikon/nikon_d50.asp [↩]
3. http://www.dpreview.com/reviews/specs/Nikon/nikon_d3x.asp [↩]
4. http://en.wikipedia.org/wiki/Visible_light [↩]