AstroPhotography Tips

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The selection of aperture (June 2004)

Last month we gave you an introduction on wide field imaging, touching on the basics of film, shutter speed, focal length, etc. This month we will talk in more detail about aperture. Aperture can affect astro photo’s more than you may initially think.

Firstly, if you have a fully automatic camera you won’t be able to take advantage of these notes on aperture. If this is you and you are interested in taking your own astro photo’s you may find it worth your while to buy yourself a cheap (AU$100) second hand manual film based SLR camera with lens. These are quite common in second hand stores. This will allow you to fully control the aperture and shutter speed of your exposures.

If you unsure on what aperture is, or terms such as F-stop I suggest you search the internet for some quick introductions, there are lots out there.

You may think that for astro photography the widest aperture (shortest f-stop) would be the most suitable as this would allow the most light to be received on the film/digital chip. This can be the case but often is not. We will look at two scenarios: short exposure astro photography and long exposure astro photography. These require quite different approaches. For our purposes we will assume short exposure is less than one minute and long exposure is 1 through 20 minutes of exposure.

Short exposures are quite straight forward in that the primary goal is to record as much data on the film/chip as possible in the short space of time. In this case you will be wanting to use a wide aperture, to let as much light in to the camera as possible. F/2 through F/4 is suitable for shot exposures. Anything narrower than F/4 will likely result in very little appearing on your photo.

Long exposures are quite different. Here you have several issues to contend with. Lens quality and vinyetting both affect what aperture you might use. What you often find is that a lens with aperture range F2 to F16 will actually produce the best images at an F-stop of about F/8. This is a result of lens quality and so how noticeable the effects are will depend on the quality of your lens. Shooting at F8 rather than the F2 limit of the lens will almost always produce sharper, less distorted images than at the widest F-stop of your lens.

Vinyetting is where the resulting image shows dark edges, especially in the corners. This will usually occur with lenses of focal length 35mm or wider and be worse the wider the lens and the cheaper the lens. The effect of vinyetting is emphasised in long exposure astro photography and can be very distracting. The best way to avoid vinyetting is to use a slower F-stop. This then becomes a situation of trial an error to find the best F-stop for your lens. You may find that for your particular lens you need to shoot at F/5.6 rather than F/2 to eliminate all noticeable vinyetting. Other lenses may require you to shoot at F/8 to remove all vinyetting. Try it out and see what results you end up with.