Tubes normally come in a set of three and they're of varying length. As a rule, using tubes equal to the focal length of the lens will give you 1:1 reproduction (true macro); so in this case 50mm of tubes would give you life sized pictures, 100mm of tubes would give you 2:1 (twice life size), 25mm would give you 1:2 (half life size and about the same as the magnification on your 70-300), etc.

At 1:1 magnification using the 50mm the front of the lens would be almost touching the subject so pretty close!

He hasn't replied yet, someone might have beaten me to it!

To be honest your current A50/1.7 would probably be better for macro work as the slower 50's tend to have a flatter field of focus. The easiest way to convert it to macro use would be with a set of extension tubes (that goes for either the A or the FA).

The K-m has a metal mount like all the other Pentax DSLRs - it's the "lightweight" kit lenses that go with it that have plastic mounts.

So yes, the Tammy will be no problem for the K-m.

PM sent

With focal lengths the important thing is percentages, not the actual length in mm.

The difference between 50mm and 60mm is 20%, the difference between 250mm and 300mm is 20%; hence the difference in the field of view is exactly the same between a 50/60 and a 250/300.

For cleaning thick dust off a lens use a good quality brush (pop down to your nearest art supplier and get a thick sable one) followed by a blow with a rocket blower or similar. If there are stubborn marks then a cotton bud soaked in lighter fluid should do the trick - don't be tempted to use water! They'll be sparkling in no time

I don't know about an original screen, but you can get a replacement with a split prism on eBay: link

Nice Father's Day gift by the way!

The scary thing is in 10 years there'll be a whole new breed of photographers who genuinely won't know!

That's true, and I don't think there's yet a satisfactory solution to shooting black & white straight from the camera. But if you shoot JPEG then you can use the contrast/saturation/etc settings to emulate pretty much any film you'd care to shake a stick at - effectively going from one brand to another and on to a third in successive shots (if you want to / can be bothered of course!).

I'm suprised nobodys introduced a dedicated mono sensor for black and white though, surely that's one of those niche markets that Cosina like to fill from time-to-time... :