Macro Photography Tools Autofocus and aperture

Getting close to your subject, and making big pictures of small objects, offers interesting picture possibilities, like the close-up avian portrait by Joe McBroom in the next picture. It also often requires special equipment. Lens manufacturers, like eye doctors, have to deal with trade-offs when designing optical formulas.

Most DSLR lenses can’t magnify an object larger than 1/10th actual size on the sensor (written as a 1:10 ratio) without developing noticeable distortion. That’s why a telephoto lens can’t focus as close as a wide-angle or normal lens, but they can all produce an image of a subject with about the same magnification.

Some standard lenses do have limited close-up ability, but they don’t offer the results of dedicated designs. There are a variety of options that extend the ability of your DSLR to work close, varying in cost and capability. This section outlines the major macro equipment categories, their features, and how they can (sometimes) be combined.

Macro lenses are designed to allow close focusing, making objects bigger on the sensor than an average lens. Some can render a life-size image on the sensor (a 1:1 ratio), and most can magnify at least to half-size, or a 1:2 ratio. Many macro lenses look very much like a non-macro lens of the same focal length, but with a somewhat longer barrel. They come in normal to moderatetelephoto lengths (50mm, 60mm, 105mm, etc.). The most exotic (read: expensive) lenses combine macro abilities with perspective control (PC).

Depth of field at large magnification is very shallow. PC lenses can be adjusted to bend the light and shift the plane of focus. If Joe had had a PC lens for his owl picture, he could have shifted the plane to be the same as the bird’s eyes.

Of course, that would have made some in-focus portions fuzzy. The depth of field is still small. That’s why most macro lenses also offer a very small minimum aperture. The lens in the picture closes down to an exceptionally small f/45. The focal length of a macro lens doesn’t determine the magnification, but rather the working distance. A 50/60mm lens will have a very short working distance (as little as 5 inches).

A 200mm lens will have a working distance of between two and three feet. How close the subject is will define the magnification ratio, until the subject is far enough away that normal optics come into play. Dedicated macro lenses offer close focusing ability, and some also provide perspective controls. Notice the smooth out-of-focus background, due to the shallow depth of field from using a 105mm macro close to the subject.

There is a noticeable reduction in the amount of light reaching the sensor when a macro lens is focused close enough to produce more than a 1:10 magnification ratio. That requires a change in exposure, so be sure to keep an eye on your meter and check the results in the preview screen.

Diopter filters are a bit like reading glasses for your camera. They are placed in front of the lens, and magnify the image placed on the sensor. Focusing past a few feet is impossible while they are on. On the plus side, they don’t reduce the exposure like a macro lens. On the negative, they change the behavior of the light on the way to the sensor, and that causes distortion.

The better filter makers charge more, but produce products that make better images with less distortion. The filters come in different strengths and can be stacked to get the desired amount of magnification. They can be put in front of macro lenses.

Extension tubes are just that: hollow tubes that go behind the lens, allowing the lens to focus closer. As with macro lenses, the amount of light reaching the sensor is reduced. Most also defeat the communication between camera and lens, so you’ll have to manually adjust exposure and focus. They can be combined with macro lenses. (Some macros even come with a dedicated extension tube.)

Teleconverters are auxiliary lenses placed behind a lens to magnify the image. They come in set magnification factors, like 1.5x and 2x. Some are designed to have a macro function. Newer models allow communication between the lens and camera, so you retain auto-metering. They will, however, degrade the image, some more than others, and they dramatically reduce the light intensity reaching the sensor.

Some cut so much light that autofocus features may stop working. A bellows is a variable extension tube, often with PC capability, and most can focus extremely close. Some photographers use a special ring and reverse the lens on a bellows. (The front of the lens is attached to the front of the bellows.)

This may reduce distortion at very close range. Like diopter lenses and extension tubes, they reduce light levels and require increased exposure. They are bulky, rarely allow the lens and camera to communicate, and may require specialized cable releases to use features such as automatic stopping-down.