(放狗搜寻摘要)
Every lens forms an image at a certain distance for a certain subject. In
order for a lens to be "focussed" on the subject this image must land on the
sensor. For an object at infinity, the lens forms an image at a distance of
f, where f is the focal length. For an object close enough for 1:1
magnification the lens forms an image at a distance of 2*f, double the focal
length. So making a macro lens is easy, just create the lens barrel to
place the principal plane of the lens a distance of 2*f from the sensor.
Making this lens also focus to infinity is the tricky part. You could rack
all the elements forward to bring them a distance of f from the sensor plane
. But this would result in a lens that changed its physical length
significantly when focussing. There would also be problems with vignetting
and light loss like you get with extension tubes. (This method is in fact
exactly the same as using tubes.) The other option is to change the focal
length when focussing.
Most macro lenses use the second approach. So a "100mm" macro lens will be
100mm when focussed to infinity, throwing an image 100mm behind the
principal plane, but will then shrink to 50mm at macro distances, still
throwing an image 100mm behind the principal plane, which is now 2*f.
