Macro lens choice

Macro lenses come in a variety of different focal lengths but how do you choose the best focal length for any given macro situation?

Macro comparison pictures showing the effect of focal length.

Let's get the obvious (wrong) answer out of the way first: focal length alone does not determine depth-of-field in macro or any other type of photography, so anybody who chooses a short focal-length macro lens in the hope of achieving more sharpness is likely to be disappointed. Actually that needs clarifying... depth-of-field is determined by the reproduction ratio and any lens that records an object at life-size (the definition of true macro photography) will exhibit the same depth-of-field characteristics.
The biggest effect of focal length is on perspective: this is because perspective depends on the distance from which the object was photographed. In other words, a 100mm lens will cover the same image field as a 50mm lens when the latter is positioned twice as far away from the object as the former. But when the camera is twice as far away, the relative distances of different parts of a three-dimensional object vary less than they would close-up. This in turn affects the way in which the object is rendered even though the overall image field is the same.
It's not a macro situation but let's take a human head as an example of a very familiar object that is often photographed. The tip of an adult's nose might be about 3cm forward of the head whereas the adult's ears might be 12cm to the rear. If the person is photographed at close range, from 60cm away, then the nose is 5% further forward and the ears are 20% further away. Now suppose that a much longer focal length were used and the human head filled the viewfinder from further away, say 3m (equivalent to using a 250mm lens setting rather than 50mm)... in this case the nose is a mere 1% closer and the ears are just 4% further away. In essence, the longer focal length makes the distances more similar and creates a "flatter" rendering.
Rendering, therefore, is one reason to choose a specific focal length. Similarly, a short focal-length used closer to the object will include more background detail than a longer focal-length used from further away. This difference is very obvious in the example picture shown above, which compares Sigma's 105mm Macro with Nikon's 40mm Micro. Both pictures were taken at f/5.6.
But perhaps one of the most important reasons for choosing a longer focal-length (in particular) is to put more distance between the camera and the object being photographed. This might be done for safety or it might be to allow lighting to be placed in the intervening distance. Certainly it is the case that short focal-length macro lenses can be very hard to use if artificial lighting is required: often the only solution is to use a compact ring-flash.
I am not a macro specialist but in my experience the 105mm focal length is ideal for a lot of general-purpose close-up photography. That said, in my opinion Nikon's 40mm lens gives the nicer rendering of the freesia photographed here because it better separates the front petals from the rear petals. Nevertheless, the only true macro lens I own is a Micro-Nikkor 105mm and nice though Nikon's 40mm lens is, I wouldn't swap it for my 105mm!
Interestingly, Sigma's lens recorded a visually sharper image than was achieved by Nikon's lens - and this is despite the fact that both lenses were used at peak sharpness and both resolved an excellent 0.35 cycles-per-pixel at this aperture in technical testing. It is a fact that sometimes numbers don't tell the whole story and lenses have to be used in the field to find out how they really perform. I touched on this in my previous ‘blog entry and it's a topic that I'll expand upon again at more length in the future.

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