So, you want to get a macro lens. Which one should you get? There are a variety of different focal length macro lenses on the market. Typically they are aroun 50mm, 65mm, 100mm, and 180mm. Why select one focal length over another?The first thing that might come to mind is working distance. With a longer focal length lens you can be further from your subject at the same magnification. If you are photographing a flower or insect at 1:1 with a 50mm lens the distance between the camera and the subject is going to be very short. You will have little room for lights and skittish subjects will probably crawl or fly away. With the 100mm lens you will have a more comfortable working distance. With the 180 you will have lots of room.
Next, you might think about depth of field. But depth of field is more related to magnification than the focal length. If your subject is being recorded at 1:1 the depth of field is going to be just about the same with any of the lenses.
There is another consideration that many might not think of. The field of view. Field of view determines what you are going to see in the background and is, I think, the most important consideration affecting the final look of your image.
With a short focal length lens, which has a wide field of view, you will get a lot of the background in your image. Here are some examples. All were taken at an aperture of f/8.
The first image was taken with a 15mm full-frame fisheye lens with a 12mm extension tube to allow it to focus as close as possible...
In this image the subject was literally touching the front element of the lens. And, as you can see, there are a lot of potentially distracting background elements. The main subject here is a bit smaller than in the other two images, but this was as large as this lens could reproduce the subject.
The next image was taken with a 50mm lens with both a 12mm and a 25mm extension tube, again at its closest focusing distance. Less background, but still distracting. You can see a chair, a white house, a brown fence, some green grass, etc.
The final image was taken with a 180mm lens set for 1:1 reproduction. With its limited field of view the background is just the green grass.
In the 50mm and 180mm versions the subject is appears almost identically. But the overall look is a lot cleaner with the longer lens.