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Lenses for shooting stars

Charles Clark , Jun 17, 2010; 05:10 p.m.

I camped in the desert a few weeks ago and made my first attempt at photographing the starry sky. I had my Canon 5D with a 24-105 L lens. I experimented a bit with changing the aperture from 4 to 5.6 (huge reduction in the number of stars captured) and with changing the ISO from 1600 to 800 (again, a huge reduction). I kept a fixed 30-second exposure to try to minimize the star trails.
The picture in the link below is an example of the ISO 1600, f4.
http://www.cgclarkphotos.com/page9/files/page9-1015-full.html
So I got to thinking that shooting with a wider aperture lens would be even better. I am thinking about renting the 35mm f1.4 for the next time I get to go ouot in the desert.

Thoughts?

Responses


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jason linch , Jun 17, 2010; 05:47 p.m.

i like the pic alot, i shot the moon the other night for the first time after seeing advice on what to shoot on and really liked my result, but anyway i really like your pic, makes me want to go camping. i miss the stars, i live in NYC and it is difficult but attainable here
thanks for the pic
sorry i have no advice on lens, i used the 55-250mm on the moon, i dont have a 'quality' lens yet...soon enough

Nathan Gardner , Jun 17, 2010; 05:53 p.m.

Apparantly it has something to do with lens diameter. The larger the front glass the better Don't ask me why, that's just the way it is, something to do with telescopes and surface area and calculus and kryptonite. I'm not arguing with it, I just don't understand all the mumbo jumbo.

http://photo.net/learn/astro/star-streak

Down in the comments Vince Farnsworth writes this:

Concerning astrophotography and the recording of stars, there seems to be some confusion about f-ratios, aperture, focal lengths, etc. For point sources such as stars, it is the focal length, not the physical aperture, that determines the limits of what will be recorded on film. This is because the amount of background sky included in the picture varies with focal length and thus the amount of magnitude-limiting sky fog goes up as the focal length decreases. Longer lenses include less of the sky and therefore less of the sky fog. Since the stars are points, their light is not spread out as focal length (magnification) increases. This effect results in an increase in the ratio of starlight (point source) to skylight (non-point source) as focal length increases, and fainter stars are recorded before being limited by the sky fog. This light-source ratio is not affected by the f-ratio or physical aperture of the lens. For example, a 50mm lens at a dark site has a limiting photographic magnitude of about 11.5. A 500mm lens has a limiting magnitude of about 16. The magnitude scale is a way of estimating the brightness of an object, with each successive magnitude number being about 2.5 times brighter than the next one (magnitude 1 is 2.5X brighter than magnitude 2). The f-ratio does determine how fast the sky fog limit is reached. Exposures longer than that needed to reach the sky fog limit will not record fainter stars.
For star trails, you should pick an f-ratio that will give you a decent star exposure for the faintest stars you want to record. For a given exposure time, too low a ratio will cause a fast sky fog build up with little contrast between stars and sky. Too high an f-ratio will result in fewer stars against a darker background. The f-ratio you choose will depend on the local sky conditions and the focal length of the lens for the reasons stated above.
In visual astronomy, the physical aperture determines the limiting magnitudes of stars. At the same magnification, a 10" diameter scope will display objects at four times the brightness of a 5" scope at the same magnification. This is probably where the confusion arose.

Scott Ferris , Jun 17, 2010; 07:34 p.m.

Hi there Charles,

I was doing the same the other day. Here is a 35mm (EXIF shows 31 but it is not) at f2.8, 30 seconds and 800 iso. One shot no enhancements, the orange is a helpful street lamp about half a mile away. LR3 gets me an image with zero noise at that too.

Charles Clark , Jun 17, 2010; 10:31 p.m.

Thanks for the responses so far. I have problems with the idea that aperture doesn't matter. That might be true of time didn't matter. However, if I shoot wide open for 30 seconds I clearly do not get the same star photos as I get if I shoot stopped down.

Given that I want to minimize the star movement I capture, I need to limit my exposure time to 30 seconds or less. Given that constraint, I want let as much light in as possible to capture as many stars as possible. To me, that implies that I can record more if I can shoot with a fast lens.

Is my logic wrong?

Charles Clark , Jun 17, 2010; 10:33 p.m.

Scott, I also use LR3. I upgraded from LR2 last weekend. I got LR2 for Christmas and have really enjoyed using it. I am hooked on the podcasts from killerlightroomtips.com

BTW, I really like the way the light in your photo highlights just the edges of the tree limbs.

Nathan Gardner , Jun 17, 2010; 10:56 p.m.

I think aperture does matter. I know when I do star trails, a star is only in one spot for so long until it has moved across the sky. If I want the trails to be bright, I use a wide aperture so that the pixels that record each star get as much light as possible before they move on to the next series of pixels. If the stars didn't move you could stop down and eventually get enough light, but since they do move you have to gather all the light you can in the short period of time that the star is in one spot. Stopping down doesn't give the pixels enough light to record the stars before they move on to the next pixels, thus creating very dim trails.

Nathan Gardner , Jun 17, 2010; 10:59 p.m.

Sorry, I know you don't want trails, but I was just explaining my logic that aperture does matter. I would go with a fast lens and use a high ISO. There was an article in Outdoor Photographer a few months back about this same thing. I'll see if I can find a link.

Nathan Gardner , Jun 17, 2010; 11:10 p.m.

Here's the link. Its about shooting the moon, but there's a photo in there of Yosemite Falls with the Big Dipper. He shot it with no star trails and the specs were: ISO 800, 30 sec shutter, f/4. There may be some helpful stuff in there, but that photo and those specs should help you know what's possible.

http://www.outdoorphotographer.com/how-to/shooting/shoot-the-moon.html

Scott Ferris , Jun 17, 2010; 11:18 p.m.

Aperture is very important, that is why telescopes get bigger and bigger, it is all about aperture. The difference for point light sources is absolute aperture size, not comparative lens speed. So a 50mm f1 would have an aperture opening 50mm in diameter, a 200 f2.8 is slower than the 50 by 3 f stops so you would think it would be worse at recording dimmer stars, but its aperture opening is 71.4mm, so for point light sources, the 71mm aperture lens beats the 50mm aperture lens even though for our common understanding the 50 f1 is faster.

The sky glow issue in these situations is just light pollution, obviously you get to a point, especially where pollution is bad, where the whole sky just becomes white, think about it, the longer you leave your shutter open the more time the non white objects have to burn the image to light. Where we are doing this, 30 second exposures are fine for wide field and constellation images, they are just on the border of subject motion.

A 35mm f1.4 (25mm aperture) will give you better images than a 35mm f4 (8.75mm aperture). But a 50mm f1.8 (27.7mm aperture) will get you better ones still if it can cover the field of view you are after.

Hope this helps, Scott.


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