The term “legacy lens” refers to old lenses produced in the film era, such as the Canon FD or Olympus Zuiko OM lenses.
If you are into astrophotography and you are on a budget, the good news is that there are many legacy lenses you could choose from to expand your equipment, particularly fast prime lenses.
Why You Should Consider Legacy Lenses?
Modern lenses are optically very well corrected and are quite sharp even wide open. Many come with a fast and reliable autofocus and image stabilization. Unfortunately, all this comes with a price :)
But if you are looking for a lens for astrophotography, you do not need autofocus, nor image stabilization: all that count are optical performances. Today, you can find in the second-hand market many decently priced legacy lenses that were at the top of the game a few decades ago. And lenses don’t age that much if they are well kept.
The Pros Of Using Legacy Lenses For Astrophotography
Price aside, legacy lenses are interesting for a number of reasons:
They provide a great handling experience;
They have a solid construction, often in metal;
They are fully manual and the aperture is set mechanically instead of relying on the camera connection;
They are optimized for manual focusing;
Most of them are cheap compared to modern standards;
Image quality can be very good;
Since they are designed for full-frame, on cropped sensor cameras you are mostly using the center of the field of view, which is the sharpest part of the image. Also, vignetting is weaker even at wide apertures;
In particular, manual focusing can be a frustrating experience, particularly with modern entry-level lenses and bridge cameras.
The focusing ring is often very narrow, the feeling is not that great, you go from focusing at the minimum distance to infinite by rotating the focusing ring of few degrees (short focus throw), some lenses lack a hard stop and the distance scale is not always there.
My old Canon FD 300 f/5.6 has a very large focusing ring which is very smooth to operate and the focus throw is 270º. This means that I can easily tweak the focus near the infinite and find the sweet spot with ease.
The other nice feature of many old lenses is the integrated, retractable, lens hood, and at night the lens hood is more useful than what you would probably think.
The lens hood prevents stray light from light pollution from reaching the lens, thus softening the contrast of your image. Also, it helps fighting dew formation and condensation on your lens and it provides a nice place where to fix an anti-dew heating strip.
The Cons Of Using Legacy Lenses For Astrophotography
While legacy lenses have a lot to offer, there are some cons to consider, when using them for astrophotography:
Optical correction is less efficient than in modern lenses;
Optical aberrations are stronger than in modern lenses;
Some lenses use full-stop aperture changes instead of the usual 1/3 of a stop of modern lenses. Stepping down the lens may thus make the lens too dark;
You need a specific adaptor to mount legacy lenses on modern digital cameras;
Be sure the adaptor you get will be able to let the lens focus to infinity. Use reputable brands such as Fotodiox, Kenko or Metabones.
They can produce somehow softer images compared to modern lens designed for high-resolution digital sensors;
Perhaps the most limiting factor on the use of legacy lens is the fact that many of them do not use low dispersion glass to reduce chromatic aberration and those fast lenses that do have it, are often for collectors and can out price modern ones.
Most likely you will find multicoated lenses.
With longer lenses, Metabones adapter can be purchased with tripod foot, or you can buy support for telephoto lenses, such this one from Andoer, also useful to better balance your gear on your tracker or equatorial mount.
If you want to have the possibility to rotate the camera, so as to have more freedom in framing your target, you can use lens rings on a vixen style plate. This solution has the plus to allows you to easily change the camera battery without removing it from the mount.
Use The Chromatic Aberration Index For The Best Image Quality
We all know that chromatic aberration, CA, in photographic lenses is highest when the lens is used at its widest aperture (lowest f-number). By stopping down the lens, CA is reduced.
When we have lenses without ED glass or other active ways to reduce chromatic aberration, the Chromatic Aberration Index, CAI, provides a criterium to set a lens aperture to suppress (or strongly minimize) the CA.
CAI is defined as the ratio between the f-number and the lens diameter, d, in inches:
CAI = f-number / d
The rule of thumb is that to suppress (or strongly reduce) CA, the CAI must be larger than 3.
So, the question is now: what aperture (f-number) should one use in order to have a CAI>=3 for a specific lens?
The f-number which corresponds to the desired CAI=3 is:
f-number = 3 * d
The image below shows how the CA around Saturn changes with the aperture.
For the image above, exposure was set to 1”, thus the image at f/8 is underexposed of more than 1 stops with respect to the image taken at f/4.5. The relation between CAI and CA holds true even if I was exposing the image at f/8 for 2 seconds, to make up for the smaller aperture used.
As an example, let's take my Olympus Zuiko OM 135 f/2.8. This lens has a diameter of 55mm or 2.165 inches. To have a CAI=3 I should set the aperture to 2.165 * 3 = 6.495
Since the aperture varies in steps, I chose the lowest f-number that is closer to the calculated one: for the example above, that would be f/5.6
You can experiment, but the rule of thumb is that if your legacy lens has no ED glass, you should prefer a lens with a small diameter, to be able to use a reasonably wide aperture to have a CAI ~3.
Which Legacy Lenses Should You Get?
Here some criteria to use when shopping for a legacy lens:
Prefer lenses in mint conditions, with no fungus or hazed glass. Check all mechanisms are smooth. Prefer lenses shipped with both front and read caps;
Because of the CAI, avoid large lenses if they do not have ED glass;
Prefer fast lenses, as they were built for the Pros, thus they will have better optical performances and handling than their slower versions;
Prefer prime lenses, as they use less glass than zoom lenses.
Read online reviews about the lens you are looking to buy and search it in Astrobin for astronomic images taken with it.
Image Gallery
I have a number of legacy lenses I have used for astro. My preferred ones are:
Olympus Zuiko OM 135 f/2.8;
Olympus Zuiko OM 200 f/4;
Olympus OM 2X-A teleconverter for the 200mm for lunar and solar photography;
Canon FD 300 f/5.6;
Olympus Zuiko OM 30 f/4.5;
All the photos in the gallery below are taken with Olympus OM-D EM-5 Mk ii or Olympus EPL-6 micro four-thirds camera with one of the lenses mentioned above. For most of the images, I was tracking the sky using my Skywatcher Star Adventurer.