Macro Confusion

When I got interested in macro photography, I found web sites about it , did some reading, and got very confused.

The bug bites when you see amazingly cool photos of insects or flowers online. You go out in the yard, find a colorful beetle and point the camera at it. But even zoomed in all the way, the bug is tiny in the frame; try to get closer and you can’t focus on it. The resulting photo is a total letdown; the insect is a tiny fraction of the frame.

So, those web sites. You get a detailed lecture on Macro Optics 101, with a beautiful diagram showing an insect, a camera lens in cross-section, a sensor with the insect’s image on it, some numbers, ratios, and criss-crossing lines connecting everything. Focal length, lens-to subject distance, lens-to-sensor distance, focus distance, reproduction ratio, sensor dimensions…. your eyelids are getting heavy…

After skimming the lecture and retaining nothing, skip to the the important part: what gear should you buy? Many choices, the big 3 being macro lenses, extension tubes and “diopters”. It’s up to you, it depends on your use case, your goals, what you want to spend…

But what are the real differences in those 3 choices, the tradeoffs? To get it straight in my head I had to experiment with all 3 side-by-side. Now I can cut through some of the fog.

You might assume the best choice is an expensive macro lens. Doesn’t it magnify things and produce big images of small subjects? The answer is no, it doesn’t “magnify” anything, it’s just designed so that it can be focused close in without distortion. A 100mm macro lens is just a 100mm lens with a shorter minimum focus distance – in other words, you can put it closer to the bug. And if you do that, yes you’ll get more of him in the frame – if he doesn’t take off. But when focused on something further away, both lenses – the ordinary 100mm, and the “macro” 100mm – produce identical images.

Say you want to photograph a beetle about 1″ long, using a full frame camera, and get the beetle to fill most of the frame. The sensor is about 1- 1/4″ across, so what you really want is the beetle’s image on the sensor to be the same size as the actual beetle. That’s what a “reproduction ratio of 1:1” means; macro lenses claim to achieve it, and it is in fact the definition of “macro”. The catch is, they only deliver 1:1 at their minimum focus distance which is typically a few inches. So a 100mm macro lens, with a minimum focus distance of 9 inches, will give you a frame-filling image of the 1″ beetle when it’s 9 inches from the lens.

Another counter-intuitive fact: if a 60mm macro lens and a 100mm macro lens offer the same maximum reproduction ratio of 1:1, they’ll produce the same size image of the insect. The difference is that with the 60mm you’ll have to get closer, blocking the light and possibly causing the subject to fly away.

You can make an ordinary lens focus closer with an extension tube; and that’s all the tube does. The longer the extension tube, the closer the lens will focus. The tradeoff is that image quality goes downhill away from the center, because the tube makes the lens work outside its design parameters. Not a problem for photos of insects, where you don’t care about the corners.

You can use an extension tube on a small telephoto; you won’t be able to focus close enough for small insects, but it’s worked for me for things like butterflies and bees. The zoom and the autofocus may get weird and you have to experiment with a particular lens to see what happens.

A close-up lens – often incorrectly referred to as a “diopter” – makes an ordinary lens focus closer, just like an extension tube. (“Diopter” is actually the unit of measurement for the strength of these lenses.) It might be marketed as a “magnifier”, or a “close-up filter”, and it has a diopter value like +2. So you might think it enlarges the image somehow, but it doesn’t; it just lets the lens focus closer. That’s easy to verify: focus an ordinary lens on a close subject (like a ruler) and note the image size ; then hold a close-up lens in in front of the lens and re-focus. Again note the size: it’s the same as before. The difference is that with the close-up lens attached you could now move in closer and get a bigger image.

The bottom line is that unless you want images that are sharp corner-to-corner, a “macro lens” has little advantage over an extension tube or a close-up lens. Both the extension tube and the close-up lens will soak up a bit of light, but not much. The close-up lens, being glass, will inevitably reduce image quality somewhat, depending on its quality (and price). The extension tube only noticeably degrades the edges of the image (and here is a good post on FStoppers.com with examples of that degradation).

A way to get even larger images, without having to get closer, is to use an APS-C body with equivalent sensor resolution. In fact if you use a full frame lens and an extension tube on an APS-C body, you’ll escape the edge and corner distortion introduced by the tube – because the sensor only receives the center of the full frame lens’s image circle.