A little bit of optics 101.
A macro or close up lens is simply a lens that is at the distance between the subject being photographed and the image plane that provides the desired magnification.
That statement is deliberately vague and here is why. Any lens that is effectively at twice it's focal length from both the subject and the image plane will produce a 1 to 1 sized image. Or, in other words, an actual sized image; a 1X image. So, for example, a lens with a 100mm focal length that is placed (it's effective optical location is placed) 200 mm from the subject and 200 mm from the image plane will produce a 1X image. And if it's location is shifted closer to one of those (subject or image plane) then the other will be in focus at a greater distance. This shifting ends when the lens is at it's focal length from one which brings the focus point for the other to infinity. If you are curious the mathematical relationship for this is:
1/f = (1/Ds) + (1/Dp)
where
f = the focal length
Ds = the distance to the subject
Dp = the distance to the focal plane (image sensor)
What this means is, ANY positive power lens can be used as a macro lens. Any lens what so ever: from a cheap plastic magnifier to the largest telescope objective lens in the world. And it applies to curved mirrors too.
What does this mean in relation to the subject of this thread?
It means that EVERY lens that you presently have in your camera bag can be used as a macro lens. All you need to do is MOUNT THEM somewhat further from the focal plane (image sensor) than their mounting flange presently places them. And this can be done in a number of ways.
One way would be a simple mechanical tube with NO optics in it, just air. Put a male and a female flange on the two ends and you are in business. The length of the tube mattes. Lets say you have that 100 mm fl lens and it has a focus range that allows it to focus on subjects from 1 m (1000 mm) to infinity. The above equation tells us that it would be at 111.11 mm from the focal plane when focused on that subject at the 1 m distance. And the actual image to object size ratio would be about 1/9 or 0.111. Then if we subtract the 100 mm focal length which is where it would be when focused for infinity, we see that it has a mechanical travel range of 11.11 mm when being focused. So adding an 11 mm long extension tube would extend that mechanical travel range to 11 mm to 22.11mm which overlaps just a bit with the range without the extension tube. But with that extension tube that same 100 mm lens could then be focused to 122.11 mm from the image plane which would give us a distance of only 552.3 mm to the subject and the actual image to object size ratio would have increased to 0.221. Comparing that to the above ratio without the extension tube we see that the image size on the sensor has increased by about a factor of 2.
In other words, a simple extension tube that is about equal to the focus distance that was built into the lens has DOUBLED the size of the image we get.
And increasing the length of that simple tube would continue to increase the size of the image.
This is a much better way of increasing the image size/magnification than the stacking of lenses described in a post above. And you don't need to do the math that I used to obtain the example with a 100 mm lens. You can just adjust the focus of the lens to it's two extremes (infinity and the closest it will do) and measure how much the lens moves. That would provide a starting length for an extension tube that would take over where the lens presently stops. Then a second tube twice as long, then three times as long, and four times, etc. until you reach the magnification desired.
One down side of this is that lenses are optimized for a particular distance from the subject or a particular object to image size ratio (same things). When they are used at different distances/magnifications they may not perform as well. Once you reach the point of extending a lens by it's focal length, it may and probably will perform better if it is physically reversed to have it's camera end pointed at the subject and it's subject end pointed to the camera. This would mean mounting it to the extension tube with a filter holding thread. I would suggest that you can experiment with the lenses that you presently have before spending money on new and probably expensive ones just to have the macro capability. Some of your present lenses may provide excellent macro photos.
Another down side is the effective F opening of the lens will increase as it is moved further and further from the focal plane/camera. At twice it's focal length the effective F number will be twice the marked numbers. Of course, more light can compensate for this. And it may help keep the image sharper than it would be at the same F number.
Another way of extending a lens for macro shots is to use a bellows unit. I have one in a cabinet from the days of 35 mm film photography. It worked quite well and provided a large range of magnifications. Be wary of cheap bellows as they may not support the weight of some cameras or lenses well.
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