Focal Length in Reverse Lens Macro Technique

Macro, Landscapes and Seascapes are my favorite genres in photography, but as I don’t travel much, I tend to shoot more macro in my backyard. Last time, I wrote an article on high magnification macro photography on a budget, where I pointed out the fact that I use the reverse lens technique in order to achieve high magnification macro shots. The technique really works great if you give it a try and the good news is that you do not need expensive gear to yield beautiful macro shots – a cheap kit lens will do wonders!

But have you ever thought about why reverse-mounting a lens onto your camera body makes it act like a macro lens? Actually, the truth behind it is that flipping a lens around by itself doesn’t automatically mean converting it to a macro lens. Basically, reverse mounting a lens moves the lens farther away from the camera, giving it the ability to focus at closer distances, which has a similar effect as using a set of extension tubes. Both ends of the lens are not made equally. For an ideal lens, flipping it would make no difference. However, in real lenses, real compromises have to be made. The back part of the lens is typically designed so that it will project the image on a flat plane close to the lens. This allows for certain optimizations in the lens design. Since the front part of the lens is meant to focus on more distant objects, different optimizations can be made there. Simply put, a lens is normally made to take certain field-of-view and project it onto a sensor, which is relatively much smaller when compared to the entire scene. Hence, by reverse mounting a lens onto your camera body, you are simply getting the opposite projection by taking a small scene and projecting it much larger.

Focal Length

With the reverse lens macro technique, focal length is going to behave in a different manner. While in the case of a real macro lens you need a longer focal length to increase the effective focal length, thus enabling you to focus progressively closer, when using the reverse lens technique, zooming in triggers the opposite reaction: it reduces the effective focal length from your lens to the sensor. This means that a shorter focal length in the reverse lens technique would result in much higher magnification, allowing you to focus much closer to your subject, while a longer focal length would result in reduced magnification. To explain this more clearly, I have captured a couple of macro shots at different focal lengths:

From the above shots, you can see how focal length acts in an opposite manner to magnification in our reverse lens macro setup (focal length is inversely proportional to the magnification).

But have you noticed something strange in the above pictures? As my focal length decreases, resulting in greater magnification, my image gradually gets darker and darker, despite the fact that I shot all images at exactly the same exposure settings. Why is that? The reason behind it, is that the lens’ effective f-stop increases, including an increase in depth-of-field, resulting in darker images. I will talk about this in detail in my next article and for now, I hope you found the above information useful!