One hundred millimeters is considered a standard or average focal length for macro lenses, and have been for ages. This allows sufficient magnification with even better working distance, since some popular subjects are alive and will react to a lens being shoved into the face (as larger subjects do too, might I add). Wider focal lengths reduce "working distance," or said another way, the lens needs to be closer to the subject to get the same magnification. Additionally, they also include a lot more background in the shot. A feature of telephoto lenses is background compression, that is, a lesser field of view of the background will show up in a photo if the subject is shown at the same magnification. However, those same background elements will be larger in the photo. The typical macro shot, however, isn't going to have a lot of background clutter that will interfere with the shot, but you can see a good demonstration of the phenomenon and how it can interfere with standard photography in this tutorial.
The lens is 360g (0.8#) and, focused at infinity, 78mm long (3.1 inches) and has 6 aperture blades and 49mm filter threads. The length grows significantly at maximum magnification. The aperture of f/4 is slow compared to today's standards. The lens bears the usual aperture ring and has a rubberized grip for easy manual focusing. The lens has a recessed front element (see photo up top) therefore doesn't require a lens hood and doesn't suffer from flare. Minimum focus distance is 45cm (just under 18 inches) producing 1:2 reproduction ratio (0.5x).
You can do macro photography without flash, but getting a fast enough shutter speed is a real challenge. Working at 100mm, then 1/100s should be a target minimum, and since when you're working close, available light is usually low. So use a flash! You don't need a high power flash, since power is proportional to distance squared. Distance is going to be very low, so even a weak flash can do the job. Just be careful if you buy a macro light... make sure you know what you're getting. A macro light is a low-power and constantly lit LED in most cases, and the output isn't usually good enough to affect shutter speed much. Make sure you're getting an automatic flash. I use a Godox TT350 for Pentax, which is about $70 or so and works as a wireless slave. Maybe we'll talk more about flash later. Or maybe not, since that's about all I know.
The top shot here is a Blue Apron vegetarian dish my wife made for lunch. This is not closest focus distance, since I wanted to include as many vegetable colors and seeds as possible without showing the plate or silverware. The focus is a little bit behind the top of the dish, as I was standing, holding the camera vertical and composing with Live View.
Our house has become infested with ladybugs in the past week. Out of nowhere, they emerged in high numbers and were walking around on the ceiling and walls and windows. Here, I used the flash and angled the lens such that the background was dark while the ladybug was crawling on the outside of the window. We have a unique underbelly view of the ladybug, and the speckled dirty window gives the image a "ladybug in space" look, doesn't it? This one, by the way, is at or very close to closest focus, so you can see what a half-life-size field of view looks like.
I didn't think the closest focus field of view was all too impressive, so I put on a powerful diopter lens, the Raynox 250. This is a multi-element set of close-up lenses that affix to the front of any lens to allow a drastically shortened close focus distance. Math tells me that this combination allows a maximum magnification ratio of about 1.5x. These images are available in the M100 Macro archive.
Finally, the rarely-blooms kuda lily. this thing looks like a spikey potato half-buried in the dirt with long spindly upward growing arms, long spikes, waxy ovoid leaves, and rarely only rarely, a cluster of blooms at the top. This is the second round of blooms, and again taken with flash at high key (bright overall image, bright background) for kicks.
Again, click any photo to see larger versions, and comments can be left on this page or under each photo if you are so inclined.