Lens Review: Opteka 500mm f/8 Mirror Lens

What kind of lens can you get for $95? How about a 500mm f/8 manual focus lens, that's compact, lightweight, and produces darn good images? If that sounds too good to be true, then read on -- it's not.

A couple of weeks ago we had a rare opportunity to photograph a transit of the planet Venus across the face of the sun (see my original post here). I had planned to have my Canon 5D Mk II on a tracking telescope to photograph it, but wanted a backup using my Canon T2i camera. I decided to risk $95 on the Opteka 500mm f/8 mirror lens sold by Amazon . It worked out just fine, and I decided to do a full review of this lens, especially since there's so much misinformation about it on the internet, and so few sample photos.

Mirror or catadioptric lenses have been around for quite some time now -- at various times, Minolta/Sony, Vivitar, Nikon, and others have offered versions of this same design. Basically, a mirror lens uses specially curved mirrors to "fold" a 500mm focal length light path into a very compact shape, by bouncing the light around inside the lens. The basics are illustrated in the image below:


Light enters the lens through a shaped glass "corrector" element, bounces off a parabolic primary mirror at the back of the lens to a central secondary mirror in the center of the corrector, then out the back of the lens through a hole in the primary mirror. The design is the same basic one used in Schmidt-Cassegrain telescopes, and it's a good design for folding a long focal length into a short physical length instrument or lens.

There's one big downside to this design: a central obstruction (the secondary mirror in the front corrector) reduces contrast in the formed image. The greater (by percent of the area of the corrector) the obstruction, the greater the reduction in contrast. It's a given that a catadioptric lens will have lower contrast than a "normal" lens of the same focal length, and it's simply a design trade-off.

The Opteka 500mm f/8 uses this same design. When you first open the box, it might be hard to believe that a 500mm lens can be so small and light -- it's only about 4 inches long and less than 3 inches wide, and weighs less than 0.8 pounds (360 grams). That's the benefit of the mirror design.

The fit and finish of the lens are amazingly good, especially for the $95 price. The lens is mostly metal, with a bit of plastic up front and a plastic lens cap. The rubberized focus ring has been changed since previous versions, and is no longer the knurled rubber we're used to on lenses -- but the textured rubber surface used now is easy to grip and I had no problems with it slipping. Focus action is smooth and well-damped, and *amazingly* long -- it turns about 300 degrees from beyond infinity focus to the stated close-focus distance of 1.72 meters (about 5 feet).

The lens itself has a t-thread mount on the backside, and is mounted to various cameras by use of an included t-mount adapter, which you specify for your camera brand when ordering. Screw the lens onto the t-mount adapter, then mount the adapter to your camera's bayonet mount in the usual way.


So how does it perform? Above is an image of the 2012 Venus Transit of June 5th -- the reason I got this lens in the first place. With a home-made solar filter attached to the front of the lens, images of the sun and little Venus crossing it were sharp and very well-defined. Contrast of the images was a little low, as expected from the design of the lens, but a little bump of contrast in post-processing produced very nice images.

Focusing a manual-focus lens at f/8 can be challenging (see below for some tips to make this as easy as possible)...the best way to go about it is to find a high-contrast edge to look at, and then gently move the focus ring back and forth while you look for the highest contrast on that edge. Using the edge of the bright sun against the nearly black sky, I had no problems getting good focus using this lens on my Canon T2i DSLR.

That f/8 aperture, by the way, is fixed -- it can't be changed. There's no aperture ring to open up or stop down, it's f/8 or nothing. The lens does come with two neutral-density filters which screw onto the *back* of the lens -- one a 2X ND filter, the other a 4X ND filter -- so you can get effective apertures of f/11, f/16, or f/22. Keep in mind, however, that these don't give you ACTUAL apertures in those ranges, and the depth of field of the lens doesn't change.

And speaking of depth of field...there isn't any. That's simply a function of focal length and aperture, and at 500mm depth of field is for all intents and purposes non-existent. Calculations show that at 10 feet, depth of field is a total of 0.03 feet -- about 1/3rd of an inch or 12mm. Things don't get much better at 100 feet of distance, giving you only about 2 feet in front of and behind your subject (3.65 feet, or about 1 meter, of total depth of field). If you want a lens that will give you very narrow depth of field and blurred backgrounds/foregrounds, this will do. If you're looking for everything in an image to be in focus, this isn't it. The image below (taken about 5 feet away) gives you an idea of the very shallow depth of field with this lens.

I used the edge of the orange to focus on, and there the image is sharp and in focus -- but the front curved surface of the orange less than 1 inch closer is already getting out of focus.
Another unique feature of mirror lenses is the donut-like shape of out of focus highlights; the central obstruction of the secondary mirror gives such highlights a bright ring with a dark center. Some people like the odd and unique quality of these, some people hate them. You can make up your own mind. The image below shows what these look like, taken at a distance of about 30 feet.



If you read the reviews on Amazon from some people who've purchased this lens, unfortunately you'll get a lot of misinformation. One big "complaint" there is that the lens isn't hand-holdable -- poppycock. Every shot (except the Venus transit image) in this review was hand-held. The same "rule of thumb" that applies to every other telephoto lens applies to this one: keep your shutter speed at about 1/focal length and you won't have shake issues. That means you'll want to shoot at between 1/500th and 1/800th of a second to avoid camera shake ruining images. If you do so, the lens is *very* hand-holdable, and as you can see from the sample images here, you can get very good results. Here are a couple more sample images, taken at our town's annual parade in late May right after I got the lens:



In conclusion, for $95 you get a 500mm lens with very good build quality, that is capable of some very good images -- as long as you understand what it's good at and what it's not. I have the Sigma 50-500mm "Bigma" zoom lens, and use it often; however, at 500mm the Bigma is about two feet long, and weighs over 3 pounds -- and operates at f/6.3. Hand holdable it's not. This tiny, lightweight lens is an inexpensive alternative that, when used within its capabilities, produces some very good images. If you don't have a lot to spend on a long lens, it's certainly worth a look.

Adjusting the Lens Orientation on the Camera

When you get the lens and mount it on your camera using the supplied t-mount adapter, the top (focus mark) of the lens may not line up with the top of your camera. This won't affect operation or focus of the lens, but if you'd like to line it up correctly, it's easy to do so.

The t-mount adapter has three set screws recessed into its side -- one of them is shown in the image above. Using a small jeweler's flat screwdriver, you can gently loosen the three set screws (only a little bit!), and turn the inside ring of the adapter until the top of the lens lines up with the top of your camera. Once it lines up, tighten up the set screws again, and you're done.