Digital Camera Question

Hi all,

As you may or may not know, I have a Canon 350 D and am looking to learn more about the workings of it. My question deals with the combination of the f number and exposure times.

What is a good f number for taking astro pics or does it depend on the type of object you are capturing, i.e. moon, planets, deep sky, wide field shots etc.

Also what is the maximum exposure time for the different f numbers before you start getting star trails etc? What are good exposure times for different objects like the moon, planets, general sky shots etc before they get overexposed?

I appreciate that there are a few questions in there but any help would be useful.

Thanks

Neill

Hi all,

As you may or may not know, I have a Canon 350 D and am looking to learn more about the workings of it. My question deals with the combination of the f number and exposure times.

What is a good f number for taking astro pics or does it depend on the type of object you are capturing, i.e. moon, planets, deep sky, wide field shots etc.

Also what is the maximum exposure time for the different f numbers before you start getting star trails etc? What are good exposure times for different objects like the moon, planets, general sky shots etc before they get overexposed?

I appreciate that there are a few questions in there but any help would be useful.

Thanks

Neill

neill,
the F number will determine how wide a field of view you get with your camera, the smaller the F number the wider the field of view you get i.e. F1 is wider than F4. which you use depends on your subject and your equipment. with F1 the planets will be very small as you have a wide field of view, if you use a telescope and a barlow lens giving you F30 or F40 for example you will get a much larger planet but much less sky around it. the maximum exposure time will also depend on your equipment and the subject. as all objects move across the sky the exposure time will be limited without tracking. unless your camera is mounted on a telescope either at the prime focus or parallel to the scope on a mounting bracket the exposure will blur due to the earths rotation ( assuming your scope has a drive and is equatorially mounted) and the image will also rotate if on an ALTAZ mount. my best advice would be to get a book on astrophotography. this link www.covingtoninnovations.com/astro/ is to an updated version of a book i got years ago which was helpful.
stephen.

Are you using a scope or a lens?

If you are using the lens then you will want to use the lower f numbers. They control the apperture of the lens and the lower number effectivly means that more light can get in. So for constallation shots you will want the lowest f number, or perhaps the second lowest as lens performance often isnt as good at the extrems. For the kit lens with the 350D its sharpest around f8 I think.

Moon shots with a telephoto lens you can get away with higher f numbers without needing to slow the shutter speed down too much.

The only time I use f/22 for anything at night is when I'm taking lightning pics.

Similarly lenses perform best away from the extreme ends of their angle. So although the 350D kit lens can open to 18mm you probably want to stick around 20something for wide shots.

I've given up onthe 350D kit lens and placed an order for a sigma 18-50 replacement today. There is also a highly recommended 50mm fixed lens from canon (CANON EF 50mm f/1.4 USM) since that goes down to f/1.4 it might be a good bet for constellation shots.

As for exposure I'd recommend that you experiment and bracket. There are forumule you can use to see how long an exposure to use on a given target at a given declination. Objects close to the celestial equator move faster than near the poles. But since its a digital and you dont have to pay for mistakes with film processing experimentation will probably be best. For astronomy shots (assuming you dont have a telescope mount) you'll need a solid tripod and a shutter release cable. Lightweight tripods will suffer from vibrations; though to reduce this you can set the mirror lock to on which will reduce some of the vibration.

The final peice of the equation is the ISO rating. You'll probably be aware that the higher the ISO the more sensitive the image. However with the added sensitivity comes noise. As a rule you should try to use the lowest you can; the 350D performs reasonably well up to 400ISO.

All three things, Exposure, Aperture and ISO can be traded against each other depending on circumstances for astronomical photos. You dont need to worry about depth of field for example which you would with taking normal photos.

If you want to get to learn about your camera I'd recommend experimenting more with daylight shots. See how the apperture affects depth of field, see how the ISO rating changes the grainyness of the shot, see what shutter speeds you need to use to get a good image when you use different ISO's and apertures.

I'm not sure if this answers your question or whether you were looking for a more detailed explanation of apperture. Just post again if you need to know something more specific about the camera. I use it mainly for regular photography so perhaps someone like KeithG can give more practical advice on astrophotography.

Hi,
f numbers one or two stops slower should give sharper images, but you get less
light (this may be good in an urban area - to reduce sky fog).

As a rule of thumb a standard 50mm lens can be exposed for up to 12-20sec (for an equatorial star) before the star trailing becomes too objectional. However, I have found that for digital rather than film one can only get away with half these values.

John

Since we are on the topic of f ratios and exposures.

Last year I bought myself an SLR to take some serious astropics. I took the hassle out of learning about focal ratios and exposures by just buying an automatic SLR. But, soon I realised that i had to know some limited information on ratio and exposures. If you want to take a shot that would mimic eyesight, set your lens at 50mm and reduce the focal ratio to the lowest common ( should be f2. The low focal ratio should yeild a well focused image especially if you're under light polluted skies. Reducing the ratio kind of, in a way, acts as a filter against hazes and sky fogs.

In other news, I have set down plans to establish an asteroid and comet search. I am currently creating a computer program that will plot and update information on all known discovered asteroids and comets that I tobtained from Minor Planet Societyin Harvard. Next step is to purchase a new telescope and a ccd. I hope to have everything up and running by January at the earliest, thats if we dont all get squashed by an asteroid in he meantime. If anyone is interested please reply to this post.


Clear Skies, Philip