Macro on the Cheap, Part II

Reversing a wide-angle lens with a reverse mount adapter is a pretty neat and inexpensive way of creating a manual focus, manual exposure macro setup without buying more lenses. The flip side to that is creating an auto-focus, auto exposure macro setup which is more convenient, albeit at the expense of speed.

Typically this is done by reversing a 50mm lens and attaching it to an existing auto exposure and/or auto focus lens. As with mounting the lens directly to the camera body in reverse, the wider the angle, the greater the magnification. However, this technique is prone to vignetting which may increase as the angle gets wider.

What we need to do this is known as a reverse coupler, a simple device with two male filter threads which allows you to attach two lenses front to front. The reversed lens behaves just like a closeup lens that screws onto your filter threads. A multi-element, multi-coated, high-diopter, closeup lens.

It works like this; if you add a magnifying (convex) lens in front of your main lens, it shortens the minimum focusing distance so that in order to achieve focus, you need to move closer to the subject. When you do, the subject is magnified in the process. Since a normal lens gathers light from the scene and reduces it to fit onto the camera’s film frame (or sensor) flipping it around has the opposite effect…it becomes a magnifier!

To figure out the diopter value, divide 1000 by the lens’ focal length. So, a 50mm lens would have a diopter of 20. Compare that to a stack of three typical closeup lenses which would give you a total diopter of 8, and degrade image quality with significant chromatic and spherical aberration.

Since this technique often produces vignetting effects, you might want to test your intended “diopter” lens first by “freelensing”; hold the reversed lens up to your main lens with the aperture wide open to see if the image circle is reduced significantly. Setting the “diopter” lens to its shortest focus distance will enlarge the image circle, but reduce the working distance. Also, make sure all filters are removed from the facing filter threads to reduce the lens-to-lens distance as much as possible.

The Nikon BR-6 Auto Diaphragm Ring helps here as well. Even though you won’t be using the aperture of the “diopter” lens, you can use the BR-6 to set it to its widest position, and add a filter to protect the rear element.