Mirror lenses: worth it?

A few weeks ago I attempted to take some photos of the moon with my 300mm lens. The results were pretty good, but I wanted more, more, more! At 300mm, the moon is still quite small in the frame, and the photo I just linked was cropped a fair bit to “zoom in” some more.

The trouble is, telephoto lenses are expensive. I looked at various options and eventually got an excellent deal on an Opteka 500mm f/8 mirror lens.

Using a lens with a focal length of 500mm (or 1000mm with supplied teleconverter) was never going to be easy. At such a huge zoom, the tripod needs to be rock-steady, otherwise the image will move all over the place.

The Opteka 500mm lens was manual focus, like most (all?) mirror lenses. In principle this didn’t bother me, since I frequently use manual-focus Canon FD lenses. But in touching the barrel of lens, no matter how lightly, you send the moon ricocheting wildly around the viewfinder. You have to wait ten seconds for it to stop wobbling to see if your focus adjustment was any good.

Which brings me onto my next point. The focus ring is extremely sensitive. Even for an infinitely-distant object such as the moon, moving the focus ring just a millimetre sends the image to a blur. It’s a bit like using a rangefinder – when not perfectly focussed, there are two ghost images that slide past each other as you adjust the focus. I never managed to make them line up properly with this lens because it was too sensitive.

After an hour with the lens and camera on a tripod, I hadn’t managed to get a well-focussed picture of the moon. And in cases where I had got the focus not too far off, the general image sharpness was terrible, and there was considerable chromatic aberration. The following image is the best I got out of the mirror lens in an hour of shooting.

The moon

That could have been a really nice image, as the moon almost fills the frame.

I won’t give up just yet, but I’m reasonably confident that I won’t be able to improve on that picture. I might do better to buy a Canon 2× teleconverter to make my 300mm lens into a 600mm. I guess the L-series glass is a little out of my league for the time being :(

So, to answer the question in the title…

No. Mirror lenses are not worth it.

  1. Did you try using a lower ISO sensitivity, longer exposure, enabling mirror lock-up (if your cam has it) and a cable/IR shutter release?

    I managed http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs385.snc4/44870_678058235632_193107414_41286266_899812_n.jpg with my kit 55-200mm lens, braced against a telegraph pole.

  2. Yes, I used mirror lock-up and a cable release. I left it at least 10 seconds after locking the mirror up before I exposed the picture.

    I experimented with longer exposures but I was also nervous about any tiny vibrations at that focal length.

    The problem seems to be the lack of focus and sharpness on the lens itself. I’ve managed photos of a similar standard to yours with my 70-300mm lens.

  3. Heavenly bodies | Jonathan Gazeley - pingback on December 8, 2010 at 9:58 pm
  4. Jonathan's blog » Long telephoto lenses - pingback on December 16, 2010 at 8:53 pm
  5. The moon is remarkably bright. I’m sure I’ve used 1/200 at a narrow aperture:

    http://www.flickr.com/search/?q=moon&w=86617506%40N00

    I would say yours is well overexposed.

  6. There is one type of mirror lens that works well, a Newtonian Telescope, a Skywatcher 130 is (relatively) cheap to buy and comes with a really heavy duty tripod, which is another essential, and gives you a 650mm f4 lens when used as a prime. On the lens cover there is a cap covering a hole, fit the lens cover and remove the cap and you have an f8 lens with all the advantages of stopping down brings in contrast and sharpness.

    You can, of course, fit an eyepiece and use that to project it’s image onto your sensor, then you have more than enough magnification for the Planets, Nebulae etc.

    I find that 1/125th of a second at f8 and 100 ISO is almost always the correct exposure for the Moon, even a crescent Moon, just because part of the Moon is covered in shadow the part you want to expose for is exactly the same exposure as the full Moon, the shadow doesn’t make the exposed area any less bright, which is kinda counter intuitive until you think about it.

    Chris

    • Since I wrote this article, I actually purchased a Cassegrain-Maksutov mirror telescope, rather than a Newtonian. The quality is excellent, so perhaps I should be more specific in my conclusion of this article: not all mirror lenses are bad, but the cheap, generic mirror lenses for cameras probably are!

  7. Forgot to add a link to my examples the first is shot with the telescope used as a prime and is not cropped.

    http://www.flickr.com/photos/chris-judge/3886998486/in/photostream/

    Adding an eyepiece and projecting onto the sensor gives more magnification

    http://www.flickr.com/photos/chris-judge/3886998426/in/photostream/

    The sharpness probably has more to do with the heavy duty tripod avoiding any trace of blur than the lens quality.

    Chris

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