Astrophotography places great demands
on the optical performance of a lens. Most subjects in normal photography
are soft targets. For example a holiday shot of a landscape is not going
to show up optical aberations around the edge of the photograph. The
same cannot be said for astrophotography. Stars are pin point objects
covering the whole field of view. Even the best lenses will have a degradation
in performance towards the outer edges of the image. To minimise the
exposure time and opportunity for tracking errors, it is desirable to
use 'fast' lenses, ie. lenses with a low f/no. The downside is that
these are often the most expensive lenses, as the lens performance has
to be optimised over a larger lens area.
Lenses with multi-layer optical coatings help minimise the problem of internal reflections from brighter objects. Internal reflections within the lens can cause 'double' images. Shown opposite is the excellent SMC Pentax coating (Super Multi Coated).
It is also best to remove filters from the front of lenses when you are about to use them, to avoid double images from bright objects.
1.
28 mm LensesThe 28mm lens is regarded as a wide angle lens. This is ideal for shots of the Milky Way or large constellations. Shown opposite is the standard f/no, f2.8. This is significantly 'slower' than a standard 50mm lens. It is one of the easiest lenses for beginners to start with, but due to the large field of view, wide angle lenses do not give the sharpest images of stars in the corners of the pictures.
2.
50 mm LensesThe standard 35mm SLR lens is the 50mm. Each manufacturer provides these in a variety of speeds. The cheapest usually starts at f/2. More common is the f/1.7 and rarer and more expensive is the f/1.4. Rarer still is the f/1.2, these usually cost a small fortune. Shown opposite are three 50mm lenses, clockwise from lower right; f2, f1.7 and f1.4. The light gathering capability is inversely proportional to the f/no., so the f/1.4 onlys need half the exposure time of an f/2 lens and the f/1.2 only needs half the time of the f/1.4. You would be forgiven for thinking the f/1.2 lens is the best. The f/1.2 has a lot of glass in it, with the iris wide open at f/1.2 the lens struggles to give sharp star images over the whole field of view, so it is best avoided.
3.
85 mm LensesThe 85mm is regarded as a portait lens, as it gives life-like perspective on photographs of peoples faces. These lenses tend to be fast to allow the photographer to have a shallow depth of focus and blur the background behind the subject. The low f/no makes them excellent lenses for astrophotography. They are generally f/1.8, f/1.9 and f/2. Beware of some of the 'soft focus' variants, these should to avoided. I have a Pentax SMC Takumar f/1.9. This is a fairly old lens but it is of high quality and produces very sharp images.
The 135mm is a very common telephoto lens. They are generally available in four f/no's; f/4, f/3.5, f/2.8 and f/2.5. Once again, at maximum aperture the f/2.5 is likely to be outperformed for image sharpness by the slower f/3.5. The images below show an f3.5 and f2.5 lens together.
The most common 200mm lens is the f/4. It can't match the speed of the smaller lenses or the light gathering of the larger 300mm f/4, so is tends not to be the first choice 'out of the bag' on both these counts.
6.
300 mm LensesThe 300 mm lens usually comes in f/4 or f/2.8 speeds. The f/2.8 lens is usually outwith most peoples budgets, as they cost the same as a good telescope! The f/4 is the more common and can still provide stunning images.
My first 300 f/4 lens was a Pentacon, which I bought second hand. More recently I bought a Pentax SMC Takumar. The Pentax easily outperforms the Pentacon, being much sharper with less chromatic aberation.
Beyond 300mm the tracking accuracy required becomes prohibitive on longer exposures. However Lunar eclipses can be tackled with a 300mm f/4 lens and 2X converter. This gives a 600mm f/8 lens. The brightness of the moon, even in eclipse, only requires relatively short exposures of under 1 minute.
The diameter of the objective lens is 75 mm and so it has excellent light gathering performance.
The most common 500mm lens is a f8 mirror lens. These have a short barrel and are relatively light and easy to handle. The high magnification of a 500mm focal length puts great demands on tracking accuracy if they are to be used on subjects other than the moon.
The biggest lens in my collection is a Pentax SMC 500mm f4.5 telephoto lens. This has an 111mm diameter objective and weighs in at 3.4kg. It is at the limit for lens based astrophotography. Beyond this you enter the realm of the telescope.
The choice of lens depends on the subject matter. If the subject is an extended object such as a nebula, then a low f/no is desirable. On the other hand if you are trying to record as many stars as possible then the larger the aperture the better.
The graph below compares the attributes of some of the common lenses.
