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This is my pet way of working it out, it seems logical, but no source for it........ If you have a lens focused at a point 1000mm in front of it, you can say it has the focusing power of one dioptre. The dioptre is the opticians' description of a lens with a focal length of 1000mm/ one metre. If the camera lens is capable of focusing to 5cm one can say it is a 20 dioptre lens assembly. 1000 / 50 = 20 dioptre. Focused at 2 metres it is a half dioptre lens. Adding the popular 6 dioptre gives it a total magnifying power of 20+6 = 26 dioptre and focuses at a point 38.5mm in front of the CU lens .... you have gained just 11.5mm. |
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With a digital camera equipped with a zoom lens the dioptre becomes very useful, particularly for the 'long' zoom variety which are not made to focus close at the full telephoto position. Some 4ft, others 2 metres. But adding a moderate powered dioptre, say just a two, makes/lets you work at 500mm from the subject and you use the zoom to get the tight framing on the subject. The more powerful dioptre makes you work closer without giving you much more magnification of the subject, and the more powerful lens the less magnification you get for the effort . As when you stack CU lenses. |
To work out the range of working distance take the maker's close distance at full zoom, say 1200mm or 0.83 dioptre, add the CU lens power to that, say 2 dioptre, giving 2.83 dioptre or 353mm. So you have a working range between 500 and 353mm when using full zoom. At wide angle it is 5cm or 20 dioptre plus 2 dioptre equals 22 or 45.5mm. 6 dioptre gives nearest working of 38.5mm. |
So Close-up lens are not that useful unless coupled with a telephoto lens or long zoom. The modern camera is vastly superior to cameras of old and even the worst can focus to around 13cm, and some to 1cm, but often only at wide-angle hence the need to be able to use the zoom relatively close to get tight framing and the CU lens facilitates this.
The advantage of working further away is no camera shadows on the subject so easier to light, Subject not scared by camera up close, You are safer if subject can sting or bite.
When you couple a CU lens with multi-element camera lens there are variations from the simple calculations above.
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