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12-10-2010, 11:08 AM   #1
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S25 close up lens

I found a "ASAHI PENTAX SMC CLOSE-UP LENS S25" (text from the rim) on a Finnish auction site for 12. This is basically a single lens that can be fixed to a 49mm filter thread akin to the Raynox 150 or the Cosina 100mm 1:3.5 "macro adapter". This produces the sharpest image of a window when held to about 25cm from the opposite wall. Since this is apparently Pentax made and even SMC coated I thought this would be similar to a Raynox 150 and work nicely with the DAL 55-300. The Cosina achromat (49mm thread and around 28-29cm in focal length by the above test) with a 58-49 step-down ring actually works nicely. The S25 doesn't but produces blurry, low-contrast images akin to severe flaring.

This is no big deal by any means, but it would be interesting to know why the Cosina achromat and the S25 are this different as one would expect the latter to be similar, only better :-)

12-12-2010, 01:40 PM   #2
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Bump.
12-12-2010, 06:38 PM   #3
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The S25 ('S' for 'standard lens) is, according to this site a single-element closeup filter, and thus not an achromat. If I've understood correctly, achromatic close-up filters are superior to non-achromats in image quality, regardless of coating.

Another aspect could be that it's meant to be used on focal lengths ~50mm. How does it do on your 50/1.4?
(I suppose the old 50 and the S25 would be an optimal combination, given the S25s age.)
12-12-2010, 10:39 PM   #4
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Single element close up filters always produce pretty bad results. The Raynox and other more expensive close up filters use two lens elements in a way that one compensates for the errors of the other.

Did you see the Pentax information for the S25? It suggests that the "S" indicates combination with standard (50mm) lenses. However, I guess that the difference between the "S" and "T" variants is just the dioptre strength.

12-13-2010, 02:11 AM   #5
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With the M-50 1.4 there is no apparent problem so I guess it does what it is supposed to do. Given the similar diopter I was hoping for similar results as with the Cosina (Promaster) add-on with the 55-300; with a step-up ring this would have been convenient to have in the bag. Promaster used to sell the the add-on as spares, but these wouldn't seem to be available any more. These were somewhat of a deal for US$20 as the quality seems to be decent. There is always the Raynox, of course; reading about this led into trying the Cosina add-on with the 55-300 and the subsequent wish to having something similar that could stay in the bag. I have a used Pentax FA 100mm macro in transit from an Ebay seller and might sell the Promaster in which case the add-on should go with it, I suppose.
12-14-2010, 04:22 AM   #6
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Hee ... I found another way to maybe waste some money before actually getting the Raynox: a 58mm Heliopan NL4 (= +4 diopter, nahlinse) this time from ebay.de, again for €12. The brand at least would seem trustworthy? :-)

Also I came to think about a difference with the Raynox and/or the Cosina in comparison with the S25: with the former the diameter of the actual glass element(s) is much less than the 49mm thread diameter while the S25 is pretty close; it might be that this simply eliminates some light that would have to travel at a low angle trough a lot of glass and/or get reflected which might be especially bad with a longer focal length such with the 55-300 closer to 300?
12-14-2010, 04:58 AM   #7
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Hi Jolepp

I was going to buy that s25 but I was little late. Could You think about selling?

LeGu
12-14-2010, 06:08 PM   #8
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I have several auxiliary "close up lenses" but only one of them (Nikon 6T) is a chromatically corrected doublet.

The general formula is Magnification=Diopter Strength x Focal Length of Main lens (mms)/ 1,000.

So, the bigger the diopter or the longer the focal length of the main lens, the greater the magnification, but -of course- so also are the greater the problems, such as shallower depth of field and poorer resolution.

My observations using these add-on lenses are that: good hand-holding technique is most important (and can be improved with practice); the quality of the main lens is most significant; colour corrected doublets (such as the Nikon 6T) produce sharper images than simple lenses; the weaker the diopter, the sharper the image; and the smaller the aperture used, the sharper the image.

From this, it would appear that -for the same magnification- using a weaker diopter (such as a +3) with a longer focal length main lens (such as a 100mm) will produce a sharper, cleaner image than using a +6 diopter and a 50mm standard lens.

On the other hand, I have had quite "acceptable" results even when stacking a 55mm (good quality) standard lens with a good quality 2xTC and a (doubtful) +3 diopter simple auxiliary close-up lens.

These auxiliary lenses also work with zooms: for example, I have combined the Nikon 6T (+2.9 diopters) with a SMC Takumar M42 4.5/ 85-210 zoom and obtained very clear, sharp images comparable to dedicated "Macro" lenses (of which I also have several).

BTW, another "gizmo" that produces very sharp images is the 2x Teleplus Macro MC7 focusing tele-extender, used with a good quality 50mm prime lens: resulting in some really gratifying results, and so convenient to use.


Last edited by Banjo; 12-14-2010 at 06:32 PM.
12-16-2010, 08:32 AM   #9
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LeGu: did you get the PM?
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