If you're an aspiring nature shooter who is discouraged by the high cost of supertelephoto lenses (which can run into the thousands of dollars), here's a low-cost alternative: the Pro Optic 500mm f/6.3 mirror lens.
Instead of a grand or more for all-glass superteles, this lens can be had for a mere $140, plus a T-mount (around $15) to attach it to your digital SLR (mounts are available for Canon EOS, Nikon AI, Pentax K, and Olympus Four Thirds system DSLRs). At a fixed f/6.3 aperture, the viewfinder might be a tad dark but you could use live view on newer cameras to see clearly.
It ain't heavy...use a tripod. Use a tripod or monopod for stability when shooting with this lens, and stick to shutter speeds of at least 1/500 sec. when using the Pro Optic 500mm f/6.3 mirror lens. I tested this one on a Canon EOS 40D.
Want to double your optical reach? Two choices: The Pro-Optic 1000mm f/11 (also requires a T-Mount to fit your camera), or add a Pro Optic 1.4x or 2x tele extender, which will extend your 500mm lens to 640mm and 1,000mm, respectively.
In the field
Can the Pro Optic 500mm f/6.3 mirror lens deliver? I got a hold of one, and put it through its paces, using a Canon EOS 40D camera. It can be used on cameras with APS-D or 35mm-szied sensors. Factoring the size of the 40D's sensor, the lens has an effective focal length of 750mm! The lens is tightly constructed, a good thing--in fact, when you turn the focus ring fast, a bit of air hisses out of the lens.
Mmmm, donut Bokeh! Out-of-focus specular highlights, such as the leaves in the tree in the foreground, look like little donuts, reflecting (literally) the lens's array of optics and mirrors. The long focal length also leads to some interesting compression of perspective. The base of the bridge in the distance is about a quarter mile away!
Setup: I screwed on the T-mount, then mounted the lens on my camera. Since the lens is manual-focus and has a fixed aperture, I set the camera to manual exposure mode. I wanted the best grain possible, so I turned the ISO to 100 and shot RAW images, deliberately underexposing by 2 stops so I could get the fastest shutter speed possible as well. Later, in Photoshop, I used the RAW editing tool to get back those 2 stops, following the procedure explained last week by Bryan Peterson.
Focusing: Depth of field is so shallow that you have to be really careful focusing. Use live view if you have it to confirm focus, or zoom in on your preview image to make sure you got it. However, when focus was good and the camera was stable, I was impressed by the quality of the images. I was also pleased with the fact that you could focus as close as approximately 3 feet.
Image quality: On a flat plane subject I found image quality to be very good. While I didn't have an all-glass equivalent tele lens to compare to, independent testers such have found the quality difference between better-quality mirror lenses and much more expensive glass lenses to be minimal.
Center detail: Here's a center detail of the above image, shown at ISO 100, 100 percent. It is acceptably sharp, with no fringing or distortion.
Corner detail: Quality in the corners--where sharpness tends to deteriorate, looks very good. Hardly any noticeable deterioration in quality in this 100 percent detail.
It's a bird! It's a plane! It's Supertele! With a 750mm (equivalent) focal length, I was able to get this close encounter between real and man-made bird (even though the plane and bird were probably close to a mile apart). I shot this hand-held at 1/1500 sec.
Conclusion: A fun lens!
Yes, there are some challenges with this lens: the aperture is not adjustable, it's not autofocus, and some people don't like donut-shaped specular highlights. It is faster than the other indie mirror lens on the market, the Samyang 500mm f/8 and worth the few extra dollars. As you might expect from a sub-$200 500mm lens, the quality is not going to rival the quality and flexibility of an all-glass, autofocus, variable-aperture 500mm lens that costs thousands of dollars. But for the price, it delivers quality that should please most casual users.
But beyond the technical performance, for serious amateurs and hobbyists, this is a great lens. It's fun to experiment with, because the results are visually radically different from shorter focal length lenses and it provides the kind of perspective-mushing look at the world that makes DSLRs so much fun to use.
Besides, I think those out-of-focus donuts are cool.