Moon Photography Question

[CGW]

Hi, I'm wondering if anyone knows how long I can keep my camera aimed at the moon (1300mm lens) before its movement will begin to affect the sharpness.

thanks!

 

[sfs]

Hi, I'm wondering if anyone knows how long I can keep my camera aimed at the moon (1300mm lens) before its movement will begin to affect the sharpness.

I don't know, but it should be possible to estimate it. The motion of the moon is mostly just the rotation of the Earth, which is 2pi/day, or 7.3x10^-5 radians/sec. The angle of view of your 1300 mm lens is approximately L/1300 radians, where L is the size of your sensor or film (say 36 mm horizontally if you're using a 35mm film camera). 1 mm of your image therefore corresponds to 1/1300 radians. The speed of the moon's image on your film is then 7.3x10^-5 radians/sec x 1300 mm/rad = .095 mm/sec. How much motion is required to affect the sharpness? A typical limit on the circle of confusion for 35mm is 0.03 mm; if you use that, the maximum shutter time would be about a third of a second. Unless I've made a mistake in the math, which is not unlikely.

Your post suggests two follow-up questions.
1) where did you get a 1300 mm lens, and how big is it?
2) Do you really want to take a long exposure of something as brightly lit as the moon?

 

[CGW]

Nice mathwork! To answer your two questions--

1) The lens is a 600-1300mm made for Canon digitals. It's still in the mail, bought it on ebay. It's not given great reviews from what I read, but I found one for just over a hundred so I'm taking a chance.... Good way to learn a bit.

2) Good point, I didn't even think about that. So it probably won't be a problem to keep it under 1/3 second. I assume I can get away with a little more with stars?

Thanks

 

[cristianna]

I don't know, but it should be possible to estimate it. The motion of the moon is mostly just the rotation of the Earth, which is 2pi/day, or 7.3x10^-5 radians/sec. The angle of view of your 1300 mm lens is approximately L/1300 radians, where L is the size of your sensor or film (say 36 mm horizontally if you're using a 35mm film camera). 1 mm of your image therefore corresponds to 1/1300 radians. The speed of the moon's image on your film is then 7.3x10^-5 radians/sec x 1300 mm/rad = .095 mm/sec. How much motion is required to affect the sharpness? A typical limit on the circle of confusion for 35mm is 0.03 mm; if you use that, the maximum shutter time would be about a third of a second. Unless I've made a mistake in the math, which is not unlikely.

Your post suggests two follow-up questions.
1) where did you get a 1300 mm lens, and how big is it?
2) Do you really want to take a long exposure of something as brightly lit as the moon?

Sorry to distract from the op... but are you some scientific mathmetician or something? I would've never known any of that info in a million years!!!!

Normally I just bracket shoot, but with this newfound knowledge that may not be necessary anymore. First I need to understand the above info.

 

[sfs]

2) Good point, I didn't even think about that. So it probably won't be a problem to keep it under 1/3 second. I assume I can get away with a little more with stars?

It's a sunlit landscape, so a reasonably fast shutter should be possible, even if the maximum aperture of your lens if small (and you'll probably want to close down a stop or two, since sharpness usually improves that way). The attached photo was taken at f/5.6 and 1/320 sec, for example. (I took it while standing around bored last week. It's a highly cropped image taken with a 300 mm lens, handheld but with an image-stabilized lens -- not as good as a tripod, which you will certainly need.

A long lens won't help you with stars, since they are point sources at any achievable magnification, except for the blurring ("twinkling") caused by the atmosphere. I don't know what kind of exposure they would need. Pretty long, I would guess.

 

[sfs]

Sorry to distract from the op... but are you some scientific mathmetician or something?

Something like that. I used to be a physicist.