55mm diameter achromats

I have been looking for a while for a 55mm diameter 2-element achromat

(i.e., high quality close-up lens) to put on the front of my 65-200

zoom for flower shots at the 200mm close-focus setting.

 

I know Minolta used at least to make them, but have been unable to

find a part number. Otherwise, Nikon use a different filter diameter

and the ones from the filter companies are apparently designed for the

50mm focal length.

 

I might occasionally use it on the front of my 100mm f/2, but 200mm is

the length I am most interested in.

Rumor has it that diopters, achromats or not, from the "filter companies," i.e., B+W and Hoya, etc., are optimized for 50mm lenses. I've never heard any confirmation of this from those manufacturers, and I suspect that it is just that, a rumor. But for the sake of argument, let's assume that it's true. It doesn't necessarily follow that an excellent achromat designed for a 50mm lens won't also perform excellently on a longer focal length lens. I'm strongly inclined to believe that it would work very well on a longer lens.

 

In one sense some diopters are certainly matched to particular lenses. Olympus made standard "close-up" lenses in 49mm and 55mm diameters that had strengths of 2.5 diopters (f=40cm.) The reason for choosing that focal length was because standard OM system 50mm lenses have a minimum focus distance of around 40cm. By making the diopter focal length f=40cm, the diopter is able to cut in at exactly the focus distance that the lens's own focusing helicoid cuts out. The result is seamless focusing from infinity down to whatever the new minimum distance with the diopter on the lens is, with a minimum of wasted overlap.

 

One general rule with diopters is that weaker ones tend to give better results than stronger ones. But of course they give less magnification. So the trick is to choose the weakest one that will yield the required magnification. Hoya, to take one example, makes two element achromats in the diameter you require, with +3, +4, and +5 diopter strengths. And they're multi-coated. I suspect that any of them would give you a fine result. They're not inexpensive however. I've heard that they cost in the neighborhood of $100 each, and they seem to be hard to source too.

Olympus made achromat close-up lenses in 49mm and 55mm thread diameter. These were marketed as being for the <i>iS-series</i> bridge cameras, though the lenses can of course fit OM and other lenses. They are apparently optimised for use at the longer end of the cameras' zoom lenses, so one would expect them to work well with 100~200mm lenses. The 3 element life-size converter achieves 1:1 at 135mm.

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I have the 49mm +2.5 close-up lens and it works well on the 85/2 and 100/2.8 as well as 50mm. I am still looking for the 55mm fit lens, as I too want to use one on my 65-200 (and the 135/2.8), as the close-up mode of the 65-200 is disappointing, to say the least.

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All three are listed about half way down this comprehensive page on the <i>iS-series</i> cameras: <a href="http://www.star.ucl.ac.uk/~rwesson/esif/om-sif/is-series/is-series.htm">http://www.star.ucl.ac.uk/~rwesson/esif/om-sif/is-series/is-series.htm</a>

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There is a slightly out of date list of 2 element close-up lenses on this page: <a href="http://www.angelfire.com/ca/erker/closeups.html">http://www.angelfire.com/ca/erker/closeups.html</a>

Simon Evans wrote: <i>the close-up mode of the 65-200 is

disappointing, to say the least.</i>

<p>

You have a point that it might not be worth while to spend the

money. As a general zoom, my copy of this lens wags its tail and

licks its own genitals. But judging from a few shots I think the

close-up mode is actually OK. I have made proper tests but am waiting

for the processed slides to come back before I know for sure.

<p>

Mark, I think one source of the "rumour" or urban myth is John Shaw's

book. What a waste of time that book is: the few things you need to

know, such as depth of field formulae and the theory behind achromats,

are not covered. Instead there are loving colour photos of various

bits of Nikon equipment and some oversaturated insect shots. It

appears I've come away from that reading experience worse than I went

into it, and should find and read a proper photog's textbook.

<p>

I normally use a 25mm tube behind the 100/2 for flower shots, but (one

thing Shaw does get right) the 200mm length makes it easier to blur

away the background.

Add-on diopter lenses are intended to create a collimated beam of light from the subject, which your prime lens can then take in at infinity focus. As such, it doesn't seem like they would care what focal length of prime lens is it. One thing you DO need to keep in mind with them, though, is that the same diopter lens on a 200mm will give 4 times the magnification that it gives on a 50mm, including magnifying the color fringes and spherical aberration effects.....

 

:)=

Minolta makes a series of dual element anchromats in 55mm and 49mm filter threads and B&H still sells them in their filter section. I bought #0 (the weakest) in both 49mm and 55mm threads. At a price of $30 each, I don't think they are multi-coated, but they seem fine.

Thank you for your responses. I will investigate the Hoya products.

But they may be unsuitable, as more expensive glass is (was, according

to the old book I have) needed for correcting all three colours with

just two elements.

 

Canon certainly offer achromats in two series, for normal and longer

lenses, so there must be some optical reason. It sounds like Shaw was

right about this.

 

While the theory of close-up lenses is usually described in terms of

focussing the main lens at infinity, that's not how you would ever use

one. For example, a +1 diopter has f=1000mm, so on a 50mm lens gives

magnification of 0.05 at infinity, probably worse than the unaided

lens. As well, for the 65-200 lens, we want to set the focus to < 2m,

which also locks the focal length at 200mm, avoiding zoom creep.

Here is a site listing probably all of the available 2 element close up lens.

 

http://www.angelfire.com/ca/erker/closeups.html

 

Ideally you would want a diopter that would allow continuous focusing from the minimum focus of your 200mm lens without achromat to the minimum focus with achromat without too much of a gap. The Minolta "0" would seem to my calculations would allow a 200mm lens to focus 4.1 feet or less. But I am confused at all the formulas out there and am not sure of the correctness of my numbers.

 

Warren