I have a Nikon 105mm macro VR lens and I am quite happy with it. This is a true macro and gives me 1:1 magnification. If I put an extension tube with this macro lens, will the image magnification increase? I am asking this question because most of the manuals of extension tubes details only for 50mm lens.
Answer
Well, the principle of extension tubes it to shift the focussing distance range closer : just for the sake of example, instead of say [1m, infinity] you'd get [0.5m, 5m] (because you're shifting the focal plane of your lens farther away of the imaging plane - sensor or film -, but I won't dwell on the optics/maths | EDIT : actually already available here on Photo.SE).
So you can put your lens closer than its "normal" minimum focussing distance, therefore you'll get a higher than 1:1 magnification.
However, there are some points to note (possible issues... or not ?) :
- depending on the length of the tube you'll lose some light in the process, around 1 stop for ~12mm, 2 stops for ~20mm.
- the depth of field is reduced also, because you're closer to your subject (if at the same aperture), getting your subject in focus might become tricky (without specific/heavy/inconvenient gear)
- the gain of minimum focussing distance effect decreases as the focal length increases (less "efficient" on 100mm than on say 50mm)
- you risk blurring because of camera movement (even very small movement at high magnification ratio, even on a tripod)
- you risk squishing your subject (if it's an insect) because the distance between your front lens and subject becomes smaller (and see Nick Miners's comment below)
- on a low quality (low price) tube you may lose AF (although it doesn't really matter for macro...), aperture because the lens contacts are not passed through
- although no additional glass is involved, using the lens to focus closer than it was intended can degrade iamge quality
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