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January 3, 2008

Photographing Meteors

Weather permitting, I'd like to drive up into south Park County tonight and try to take some photos of the night sky, with the goal of trying to capture a photo of a shooting star. I searched for information on the internet, and they all say that you need an SLR camera on a tripod with a remote shutter release, and to set the camera to use Manual Focus and set the focus to infinity. So, all of this is intuitively obvious to the casual observer. What is less obvious is the settings for the Aperture, Shutter Speed, and Film Speed.

There are many things to consider. Initially, I figured I'd just use a very long bulb exposure. You can, in theory, keep the shutter open for a very long time - like say for 30 minutes or so. The thought being that the odds are if a shooting star passes during this time frame then you're bound to catch it. There are many problems with this however.

1) You'd have to be in a very very dark location. Like, for instance, in the middle of the ocean or in south Park County. Otherwise, there's too might light pollution from the cities and the photo will be over exposed.
2) You would have to set the camera film speed to 50 or 100 and set the aperture at 40 to prevent from over exposing in any event.
3) If you had the camera set up to record a correct exposure over 30 minutes, the stars would all appear as streaks because they appear to travel a good deal over that amount of time. So, all of your stars would look like curved lines.
4) If a shooting star did occur during this time, I'm not sure that it would even show up. If you set up a camera to shoot a long exposure in low light, it's possible to walk through the picture while the shutter is open and not appear in the resulting photo. I know because I've done it.

So, if you don't want your stars to appear as arcs (curved streaks), then really, what you need to do is take a fairly short exposure. This website seems to be on the right path. They give a formula for computing how long the exposure can be before the stars begin to smear.

The simple formula for 35mm format is 600 / (Focal Length) = Maximum Exposure Time. So, for example, if you're shooting with a 24mm lens the math would be: 600/24mm = 25 seconds.

I'm going to be shooting with a Canon EF-S 10-22mm 1:3.5-4.5 USM lens. This is a wide-angle telescopic zoom lens that goes from 10mm to 22mm. So, according to this formula, I'd be able to keep the shutter open for 60 seconds at 10mm or 27 seconds at 22mm. Now, we can argue all day about the fact that my camera's sensor isn't the size of a 35mm frame of film and that I need to take the 1.6 multiplication factor into account but my suggestion is that the lens is made for the Canon EOS 40D frame and it will not work on a Canon Film camera and so the lens is effectively equal to a 10mm - 22mm lens on a 35mm film camera. But lets not go there. Let's just not.

What about the aperture?

Reducing the amount of light entering the lens by increasing the aperture will increase the depth of field, but this is not a consideration when shooting meteors. With the lens I'll be using, the DOF Calcuator says the hyperfocal distance of the lens set at 22mm and f/4.5 is 18.7 ft. So, everything that's 18.7 feet or further away from the lens will be in focus. (At 10mm and f/4.0, the hyperfocal distance is 4.35 feet.)

I could, in theory, close down the lens diaphragm by increasing the aperture to say f/22. Then, the DOF Calcuator says the hyperfocal distance of the lens set at 22mm and f/22.0 is 3.77 feet. (At 10mm and f/22.0, the hyperfocal distance is 0.8 feet.)

So, stopping down the lens to f/22.0 doesn't really buy us anything. The odds of a meteor coming within 18.7 feet of me are fairly slim, I would think. So, I'll set my aperture wide open, at either f/4.0 or f/4.5 (depending on the focal length of the lens).

All that's left is the film speed.

Now, all I have to do is choose a film speed. I don't know off the top of my head what the film speed should be. I'll experiment on this to see what gives the proper exposure. It depends on how bright the moon is, how cloudy it is, how much light pollution I'll have from Fairplay, etc.

As the film speed increases, though, the amount of noise in the photo increases. So, ISO 400 is better than ISO 1600, as there will be less noise in the picture.

There is a setting on the EOS 40D for noise reduction and night shooting, so I'll play with that before I go out.

Finally, there is the problem of the lens freezing up. So, I'll try moving the lens into my truck today to get it adjusted to the temperatures slowly so that the condensation hopefully won't freeze on the lens like it did last time. :)

Posted by Rob Kiser on January 3, 2008 at 11:12 AM


I'm going to be shooting with a Canon EF-S 10-22mm 1:3.5-4.5 USM lens. This is a wide-angle telescopic zoom lens

Don't get too close to those meteors with that wide angle lens.

Posted by: Robert R. on January 3, 2008 at 4:08 PM

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