The Reflex Lens

August 11, 2015 by rodsager

Hand-held 1/500th at f8.0 Canon 5D Mk II at 400 ISO uncropped

In the 1980s, reflex lenses became very popular among amateur photographers. Reflex is a bit of a slang term for catadioptric lenses that use a Cassegrain design originally invented for telescopes. First off, forgive some of my slightly soft images here. I hand-held some of the photos under overcast skies and a slight breeze. Reflex or “mirror” lenses are not known for extreme sharpness anyway. The idea behind the lens was to create a long focal length telephoto lens that was small and compact. In this regard the design is nothing short of brilliant. I still own my 1980s Tamron SP 500/8. Tamron made one of the best 35mm cat lenses. The Tamron lens is very small as you can see held in my hand in the photo. This is a 500mm lens!

Tamron SP 500mm f/8

Reflex lenses have some interesting characteristics that can be both derogatory and beneficial depending on your perspective. First the aforementioned lack of absolute resolution. I am a bit of a sharpness freak, but this Tamron lens is not “soft” by comparison to other cats in fact the opposite is true; it is soft by nature. It just won’t resolve as well as a traditional lens design. Another issue is that these types of lenses have no aperture diaphragm. The lens is always ‘wide open’. There is no stopping down for extra depth or sharpness. That said this 500mm lens is 1/4 the length and likely 1/2 the weight of a similar 500mm traditional lens.

Central Obstruction

The lens achieves this amazing feat using a Cassegrain style design originally created to make large astronomical telescopes more compact and sealed. There is a large central mirror that reflects light back towards a smaller central mirror and then back again to the rear of the lens, through a corrector and to the focal plane. Essentially the required 500mm of light travel is bounced around inside the smaller assembly rather than taking a straight path. The central secondary mirror creates an obstruction that shows itself as doughnut shaped out of focus highlights. Some find this strange bokeh disturbing, but it can be used effectively as well.

Out of focus highlights with some motion 1/250th at f/8

Most reflex lenses made by the major players were 500mm f/8 models. There have been others such as a 300mm f/6.3 or 1000mm f/11 but the 500mm was the clear favorite.

Since the photographer doesn’t have the ability to stop down the lens, either neutral density filters or shutter speed adjustments must be used to match exposure. A reflex lens used in aperture priority automatic can be effective for speedy use. It is important to pay attention to the shutter speed as these lenses are hard to hold steady.

Flower at 7 foot focus 1/250th @ f/8 Canon 5D Mk II at 400 ISO uncropped

The Tamron lens is particularly good at close focus. The lens will focus to less than 6 feet (1.8m). This may not seem “close” but remember this is a 500mm lens! That close focus delivers a 1:3 macro ratio! The purple flower shot was taken at a distance of about 7 feet (2.2m) that flower is smaller than an open rose.

The bokeh effect can be used as an effective tool for a unique look but it can in fact be a distraction as well. The purple flower shot shows the negative bokeh well on the out of focus leaf in the lower right corner.

Another issue with catadioptric lenses is that the photographic contrast is a bit soft. Modern software allows this to be easily corrected however so I find it less of an issue than it was in the ‘film days’ The picture below of my cat was not adjusted for contrast and you can see it is a bit flat. It is however reasonably sharp.

These mirror lenses are fun and since they can be found priced reasonably on ebay, at cool camera shops like Seawood, or at camera shows such as PhotoFair, why not get one today! I would recommend sticking to the better known brands. Many “knock-off lens makers built crappy versions of these lenses. Tamron, Tokina, Sigma, Zeiss, and camera maker lenses (Nikon, Canon, Minolta, Pentax, etc.) are good choices here. I think the Tamron is by far the best bang for the buck, it challenges or bests the camera maker’s version and typically can be had for much less money.