M42 adapter

If you have fingernails you can shove one of them in the slot to get the same result. I use the corner of my thumbnail.
Derek, for a microscope I presume you won't need infinity focus. Consequently, you can use any of the cheaper, simple flange type adapters that have a normal K type bayonet on the rear and an m42 thread at the front. They can't get stuck !

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Both the *istDS and the K5 are incurably addicted to old glass

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Genuine Pentax adaptors are great but there are are some made in japan ones that the likes of Jessops sold that are just as good the cheap chinese ones are rubbish.

At least with a Pentax adapter you know what you're getting. With an unbranded one it's pot luck, a branded 'non-Pentax' adapter might be perfectly fine, the branding gives you the opportunity to identify it and locate reviews etc. I doubt very few good quality branded 'non-Pentax' adapters are much cheaper than the Pentax one.

Note: Some online sales show (or describe) one version of a product and ship another.

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I bought an M42 adapter a couple of years ago for a telescope attachment but tiny flecks of the black coating got onto the sensor and even behind the focus screen. A devil to remove as it was static so I would always now look at uncoated versions which I imagine the Pentax one is.