Sounds like a bit of 'bounce' creeping through. Most mechanical switches 'bounce' a bit at times - ie make a number of very fast sucessive connections at first when operated. This is normally compensated for electronically (it just needs a capacitor across the switch really) - so it sounds like there is a slight fault there - more likely in the 'debounce' circuit than in the button itself.

I have the Cosina. It has the 'A' Setting. It is actually quite a good performer, but feels somewhat plastiky. I don't use it anymore and would happily part from it. Only problem is it is in Rome and I am currently in England and I don't go back there until the 22nd...

This should do the job...

One other to consider - Fuji F200 EXR - large sensor (1/1.7) decent zoom range (28-140 equiv.) Manual control. Great reviews. No flash mount but a lot cheaper than the other options (about £160). Nice and small too.

28 on to 100 will give you nearly 4:1. However you can mount a 28mm directly reversed onto the body with a kmount to 49mm thread adaptor and that gives about 2:1 (not 1:2 as I mistakenly said in my post) - at least with my smc m 28 2.8

To get 2:1 you would need 100mm of tubes with a 100mm 1:1 lens I think, so you would need two sets.

Another alternative is a 2xconverter.

Bellows will get you closer. Mine go to about 150mm. That actually gives me about 3:1 with a standard 50mm lens. They can be had quite cheaply on ebay.

Pentax also made a helical extension tube - which is a sort of mini bellows.

Reversing one lens on another is possible but fiddly - to get 1:2 you need to reverse a lens of half the focal lenth on the front - so a 50mm on a 100mm lens would do it. But if you have a 50mm all you need is a thread adaptor to link the two together.

Another possibility is simply reversing a wide angle lens on to the camera body. That seems to give about 1:2 with a 28mmm lens. For that you needa K mount ro 49mm thread adapter

A further possibliy is a close up lens on the front

The trouble with simple answers is that they can oversimplify. While any lens of f1/1.4 is certainly fast, a 50mm f1/2.8 would be regarded by many as quite slow, and conversely a 400mm f1/4.0 would be regarded as pretty fast... Whether a lens is 'slow' or 'fast' is itself a relative question depending on wht it is compared with. So a fast lens is one with a larger maximum aperture than whatever you are considering to be normal... So the various 50 mm f1/2.0 lenses are relatively slow as 50mm primes, but pretty fast compared to an average kit zoom...

When you actualy look at the lens from the front, with the aperture wide open, the actual aperture is the diameter of the 'hole' you can see. With standard and telephoto prime lenses this is often the same as the diameter of the front element. The f number of the lens is the focal length of the lens divided by this diameter.

Thus my nice litle SMC M 135 3.5 has a front element about 38mm in diameter 135/38 = more or less 3.5, My slightly scruffy SMC M 28 2.8 is harder for me to measure, because the hole is smaller than the diameter of the front element, but it sure looks to be about 10mm. (28/10=2.8 ). My diminutive SMC M 40 2.8 is easy to measure at 14mm - 40/14 = about 2.8. The fastest lens I have with me at present is my SMC M 50/1.7 which has maximum aperture of 33mm. 50/33=1.7 (more or less). I would quote my SMC 17 f4 fisheye but that would only invite envy...

PS to see the aperture wide open on a lens without an aperture ring, just slide the sprung lever on the rear of the lens clockwise.

Well.. in absolute terms the largest apertures are on big long telephotos. The f number is a relative aperture (which is much more useful to know when calculating exposure) and is the ratio of the focal length to the actual aperture as seen when looking through the front of the lens. Thus a 50 mm f2.0 lens will appear to have an aperture of 25mm when looked at from the front.

[geek mode on] Strictly speaking the aperture is f1:2.0, but the 1: is usually ommited. However this sometimes creates confusion when a 50mm f1:2 is listed on ebay as a 50mm f1.2 - the former been common as muck and cheap as chips, the latter being very 'fast glass' and not quite a hens tooth but still quite expensive. [geek mode off]

So for example the Pentax 500mm f4.5 has an apparent aperture from the front of about 110mm. This is, of course bigger than the actual lens mount, but the lens 'concentrates' the light down so it can get through that space.

According to Wikepedia, the fastest lens made was a Carl Zeiss 50mm f0.7

[edit - beaten to it by Tim]

There is a review of the Sigma HSM II version of this on dpreviewhere and of the Tamron here. According to these the Tamron wins on image quality.