Which macro lens should you buy? What are the options available? I will attempt to answer these questions in a simple and succinct manner to give you an understanding of the basics principles without getting too complicated or technical.
There are 3 variables when choosing a macro lens
Ratio: All macro lenses can be used like any other lens for any subject matter, but the title “macro lens” means it also has the ability to reproduce your subject matter at a 1:1 ratio. This means the subject will be reproduced onto the sensor in perfect scale with no reduction or enlargement of size. The reason the subject appears to be enlarged is because when viewing or printing the image it is blown up due to the increase from the size it appears on a sensor (35mm or smaller) to the size it is reproduced on paper or screen which are proportionally several times larger than the sensor. Any ratio less than 1:1, such as a lens that claims a macro setting of 1:4, is merely allowing for close up work but is not producing true macro.
Focal length: some generally available or common focal lengths for macro lenses are 50mm, 80mm, 100mm and 120mm. So which is better, how do we choose? Well, as explained previously, a macro lens can resolve to a ratio of 1:1. All macro lenses are doing the same scale there is no additional magnification on longer focal lengths. So what does change? Basically the longer the focal length the less background is included in the shot. A 50 mm lens will give the same subject size as a 100mm lens but the image will contain a wider view of the surrounding background. The 100mm lens will be a “zoomed in view” so less of the background will be included in the field of view making a cleaner, simpler image.
Working distance: working length is the minimum distance the lens is from the subject where 1:1 is achieved. All macro lenses are resolving to a 1:1 ratio the distance from the subject to the lens may be closer of further depending on the lens but whether it is 10 cm or 30 cm distance the subject is still 1:1. Therefore the longer the working distances the better. A long working distance makes it less likely that you will scare away insects and butterflies for example, it also allows you to get light into the area you are trying to photograph. A short working distance may seem that you are closer to the subject matter so you getting a more magnified shot, but the ratio is still only 1:1. When using extension tubes, the working distance can become so small the lens blocks the light as it is so close to the subject matter. At minute working distances the chance of capturing live “critters” also gets prohibitively difficultly.
Extension tubes: You attach an extension tube to a standard lens and you can reduce the minimum distance at which an object is in focus. This in effect reduces your working distance. Because you are closer the object is magnified to some extent. A macro lens can achieve 1:1 but using 3 extension tubes will still not reproduce a 1:1 ratio, it is just under. Once you have attached three extension tubes you’re working distance is almost unusable and the depth of field is so shallow as to become impractical.
In summary, when choosing a macro lens it must be able to do 1:1. Working distance will be determined by the lens and is a factor you have little control over. The longer the focal length the longer the working distance in general. And a long working distance makes it easier to capture insects without disturbing them. Focal length will change the amount of background included in your shot and has nothing to do with enlargement.
My personal opinion is to buy a 100 or 105mm macro. It has sufficient zoom to make for clean, simple backgrounds and can double as a portrait lens.
By John Onderstall
Thank you for this insightful piece. Many photographers prefer to shoot in RAW because of its enhanced post processing abilities when editing with software. Does this principle hold true for macro photography as well, or is shooting in JPEG format still a viable option?