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January 19, 2010

Photographing The Moon

Robert forwarded me an email today from a guy that was trying to take a photograph of the moon, and wanted to calculate how many pixels the moon would strike on the sensor of his camera. The question he asked was a valid one, and I've gone through the calculations here primarily for my own edification. (I forwarded the calculations to the author of the letter, but he's a royal jackass and didn't even bother to reply. Some people are that way.)

Now, it just so happens that his camera is a piece of trash made my Panaseptic and he's using a 1.7x teleconverter which is like putting a putting a coke bottle in front of your primary lens, but that's neither here nor there.

First, you need to know something about your camera's sensor. My Canon EOS 50D has a reported Pixel Pitch of 4.7 microns. This means that the distance between each pixel is approximately 4.7 microns, or 0.0000047 meters. This is the advertised Pixel Pitch, but you can calculate it yourself for any given camera. All you have to do is divide the number of pixels by the size of the sensor.

The size of my camera's sensor is advertised as 22.3 x 14.9 mm. A normal slide of 35 mm film is 36 x 24 mm, so my camera has a "crop factor" or "multiplication factor" of 1.6, but I digress.

The maximum resolution for my camera is 4,572 x 3,168 pixels. So, to determine my pixel pitch using the vertical measurements of the sensor, we take the camera's sensor height of 14.9 mm, or .0149 meters, and divide it by the maximum vertical resolution of 3,168 to get 0.0000047, or 4.7 microns between each pixel. This matches the reported Pixel Pitch for the Canon EOS 50D.

The true focal length of the camera lens I'll use to shoot the moon is 400 mm. (Note: This is not adjusted by the "crop factor" of 1.6. This is the true focal length of the lens.)

The Plate Scale can be calculated as 206265 * pixel pitch / focal length, where the result is given in arc-seconds per pixel. (The 206265 constant is the number of arc seconds in one radian.)

So, for the Canon EOS 50D with a 400mm lens, the Plate Scale = 206265 arc-seconds * (4.7 microns /1000 microns/mm) / 400 mm = 2.43 arc-seconds/pixel.

The full moon is roughly 1,800 arc-seconds in diameter, so it would appear as 1,800 / 2.43 = 742 pixels in diameter in Canon EOS 50D images with the 400 mm lens.

Download the attached spreadsheet camera_pixel_pitch.xls for the formulas.

Suggestions on Lunar photograph:
1) Keep the camera outside in your truck, or in the garage. Otherwise, taking it from inside (72 degrees) to outside at night (20 degrees) will cause condensation on lens, sensor, etc.
2) Turn off digital zoom.
3) Put it on a tripod
4) Put sandbags on the tripod legs (or bags of rice)
5) Turn off the image stabilization
6) Set the timer to be about 5 or 10 seconds (so you don't shake the frame)
7) Set ISO to 100 - less noise in image - (moon is actually very bright if sky is clear)
8) Set aperture wide open (f/4.0 or whatever it goes to)
9) Zoom in all the way
10) Set shutter speed to somewhere between .125 - .25 secs. (I'm guessing here, but this is about where you want to be) If it has to be open any longer, then bump up the ISO to 400.
11) Experiment with the focus. Try autofocus. Try manual focus. It actually shouldn't be set to infinity to shoot the moon.

Remember, when it's cold out at night, it will drain batteries quickly. So, if you're going up in the hills for a clear shot, dress warm, take a laptop, a 6' USB cable, a red-lens flashlight, and take extra charged batteries.

Posted by Rob Kiser on January 19, 2010 at 9:54 PM


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