Macro Marvels (Part 1)

12/26/2012 9:23:25 AM

Looking for a keenly-priced macro for autumn and winter? We find eight of the best buys, starting from under $300

Macro lenses are close-up specialists, usually offering a full 1x magnification factor at their closest focus setting. It’s not always the case, though, as demonstrated by the Canon 50mm macro in this group, which only offers 0.5x magnification. The most popular focal length for macro lenses is around 100mm, especially on full-frame cameras. This enables you to keep a not overly threatening distance from bugs and other timid little creatures when you’re shooting them. So why go shorter?

This is about as close as you can get with most non-macro optics.

This is about as close as you can get with most non-macro optics.

On cameras with APS-C sensors, a 50mm macro lens has an effective focal length of around 75-80mm. That’s not only edging towards the 100mm focal length favoured by full-frame photographers, but is also ideal for portraiture. Indeed, these lenses are often referred to as ‘portrait macro’ lenses for APS-C bodies. As macro prime lenses, they should not only be super-sharp with negligible distortion, but offer a reasonably fast maximum aperture of around 172.8. This is effective for blurring messy backgrounds in portrait shots.

Prime time

The Canon 50mm, Nikon 60mm, Sigma 50mm and Sony 50mm lenses in this test group not only work on APS-C bodies but are also fully compatible with full-frame cameras. In this case, they double up as useful ‘standard’ prime lenses, closely matching the perspective of the human eye for general shooting.

One thing common to all macro lenses is that they’re designed to be ‘flat field’ lenses. At short focus distances, they aim to have little or no field curvature, so the centre, edges and corners of the frame are equally focused at the same object distance. For example, consider you’re photographing something flat like a postage stamp from head on (a stamp will practically fill the whole frame with a 1x magnification macro lens on a body with an APS-C sensor). A flat field design will ensure that the whole of the stamp is sharp in the image when the lens is focused correctly. It’s an important consideration because, even at small aperture settings, depth of field is tiny at 1x magnification.

We’re often concerned with a lens’s performance at its largest available aperture, where sharpness often drops off. But for macro lenses, where you often need to use a small aperture to eke out a little extra depth of field, good sharpness and contrast is also very desirable at around f/16. This is difficult to achieve due to the effects of diffraction (bending of light rays), which become more problematic at small apertures.

How much difference does aperture really make to depth of field in macro shooting? Let’s take the Canon 60mm lens on an APS-C body as an example. At its closest focus setting of 20cm, an aperture of f/16 will give you a depth of field of about 4mm in total. Shrink the aperture to f/5.6 and the entire depth of field is reduced to just 1.7mm.

Mind the gap

Whereas a 100mm macro lens will have a closest focus distance of about 30cm, it’s only about 20cm for a 50mm macro lens. However, this isn’t the gap between the front of the lens and the subject you’re shooting. Instead the focus distance, as always, is measured from the ‘focal plane’ (the position of the image sensor in a digital camera) and the subject. Since the image sensor is towards the rear of the camera and the lens itself cuts into the distance, the gap between the front of the lens and what you’re shooting can end up being very small.

A true macro lens enables you to fill the frame with small details.

A true macro lens enables you to fill the frame with small details.

To make matters worse, while some macro lenses in this group have internal focus mechanisms, others don’t. The front elements of the Canon 50mm, both Nikon lenses, and the Sigma and Sony lenses all extend forwards as you focus down towards the closest focus distance. For example, the Sony 50mm has a stated length of 67mm. However, it extends to 114mm at its closest focus setting.

Add on the depth of the image sensor within the attached camera body and its front element comes to just 5cm from the object you’re photographing when using the closest focus distance. You can often find that the lens casts a shadow over what you’re shooting, or at least cuts out some of the light on the subject. Illumination from a camera’s pop-up flash can also be obscured and, if you need to light the subject from an oblique angle, it’s easy to end up with deep but unwanted shadows (see Ringflash Explained).

Getting back to the use of these lenses in general photography, bear in mind that macro lenses often have a very long focus travel. This helps to enable precise focusing in critical macro work. However, it can make autofocus very slow, so a focus limiter switch can be useful, as described in Features to Look For.

Kit anatomy - Close-up options

Buying a macro lens isn't the only in-road to close-up photography. The cheapest alternative is to buy close-up filters, made by the likes of Hoya and Kood. These screw into the filter thread of a lens, their dioptre enabling closer focusing and greater magnification. They're available in different strengths and can be stacked for greater effect, although you can expect a progressive loss in image quality.

Another option is to fit extension tubes, which mount between the camera body and the attached lens. These don't contain any glass elements, but simply move the lens further away from the camera. Again, they're available in different strengths, but cheaper versions don't feature any electronic connections between the attached lens and camera body, so autofocus and exposure metering are impossible.

Yet another alternative is a reversing ring. In this case, one lens is mounted to the camera, but a second lens is reversed and attached to the primary lens via the reversing ring. It's an - unwieldy set-up, but can yield good results.

How we test lenses - Advice you can trust

Our testing procedure includes lab tests under controlled conditions, as well as 'real-world' shooting. Firstly, all lenses are fitted to mid-range cameras and used to take images of two test charts under studio lights. The results are processed using Imatest Master, so that we can quantify optical performance in terms of sharpness, chromatic aberrations and distortion. Overall quality is assessed at the centre, edge and corners of the images.

For real-world conditions, we use each of the lenses under widely varying indoor and outdoor lighting conditions. Overall handling is checked, as well as the smoothness and precision of zoom and focus rings, and the operation of all switches. We also test the speed and accuracy of autofocus systems, complete with operation of full-time manual override where available. Ratings are finally given for features, build . quality, image quality and value for money.


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