Re: Question: Macro 'filters' - do they work?
- From: David Littlewood <david@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 26 Jul 2005 18:47:37 +0100
In article <8osFe.3443$Ow4.1432186@xxxxxxxxxxxxxxxxxx>, Bernard Saper <bsaper@xxxxxxxxxx> writes
Hi Bobby The screw on lenses you mention are known as Plus lenses and usually come in groups of three: Plus 1, Plus 2, Plus 3. They are fixed focus. The +1 lens focuses at 1 meter, +2 at 1/ 2meter, and +3 at 1/3 meter.
This could be slightly misleading.
The supplementary lenses (more commonly known as "close-up lenses") are denoted with the power of each lens in dioptres. A +1 lens - 1 dioptre - has a focal length of 1 metre, a +2 of 0.5 metre, and an N dioptre lens has a focal length of 1/N metres.
The lenses do not make your system "fixed focus" at all, they change the focussing range, making it closer. Thus, if you take a simple example of my Canon 50mm f/1.4 lens, which has a closest focus distance of 0.45m and a maximum magnification of 0.15, placing a +1 close-up lens on it will make it approximately 47.6mm focal length and have a maximum magnification of about 0.21. A +3 lens will make it a 43.5mm lens with a maximum magnification of 0.32, i.e. about twice that for the +1.
However, it is important to realise that this is just the maximum magnification; the focus mechanism of the primary lens still works. The 50mm lens with +1 supplementary will focus from approximately 1 metre to approximately 0.28 metres. Similarly the example with the +3 supplementary will range from 0.33m to 0.178m
Furthermore, these calculations will be completely different for a primary lens of different focal length, or even for one of the same focal length but a different close focus distance.
(BTW, all these distances are somewhat approximate as the formulae assume a simple lens, and that there is zero separation between the primary lens and the supplementary; neither is of course true in practice, and also some camera lenses change focal length themselves as they are focussed closer.)
Obviously they are not as good as a full-featured macro lens.
True.
However they are an excellent and inexpensive way to get into macro photography.
Also true.
Next up the line costwise as well as quality-wise would be a lens extender: a device which fits between the lens and camera. Image quality is superior to the filters because there is some image degradation anytime a filter is placed in front of a lens.
There is also image degradation caused by using a primary lens at a focal distance well away from those for which it was designed. Aberrations are controlled as well as possible over the intended range of conjugate distances, but may go well out of control outside that range. Zoom lenses and highly asymmetric designs are likely to be worse here. In some cases the results may be worse than those with close up lenses.
Finally, and not truly macro, would be to use a long lens, perhaps a 75 - 300 zoom lens. You can get fantastic "closeups" this way.
No you can't; the fact that your object distance is much larger for a long focal length does not say anything about the magnification ratio. Long lenses typically have no greater magnification ratios than short lenses, but you just get to stand further away to do it. In that sense, and that sense only, you get "close-ups" that way.
Now a long lens with a close up lens attached can indeed give some very high magnifications from a distance.
Note that a close up lens will have a greater effect (in terms of magnification achievable) on a long lens than a short one, whereas extension tubes have less effect on long lenses.
Although not truly macro, photos of flowers for example can be spectacular.
True.
To the scientist in this field, and to the stickler for accuracy, "macro" as a term applies only to pictures with a magnification ratio of 1.0 or greater (actually, in practical terms, up to 25 or 50, beyond which you need a compound microscope).
David -- David Littlewood .
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