I think the original thread that covers all these points will be found in the Lenses section, should anyone wish to view it.

We've been through all of this before....Yes, magnification is the key, but in a practical sense what we observe about DOF is much simpler, as I mentioned above.

I stand by the statement that a 50mm lens remains a 50mm lens, as it clearly does. The value of circle of confusion is, although not totally arbitary, nit relevant to that observation as it is a value that we select for ourselves. All I am saying is that a 50mm lens is a 50mm lens and it's characteristics do not change because of what we put behind it.

A 50mm lens is always a 50mm lens and always has the same DOF.

If you change the parameters by deciding on a different circle of confusion, that's one thing, but you haven't changed the lens you've changed your parameters for assessing the lens.

It's well established that in practical terms, for a given field of view, smaller formats have more DOF than larger formats. This is because the standard lens for a given format changes. So a 24mm lens for 110, a 50mm lens for 35mm, a 80mm lens for 6x6cm, etc.

In the case of the digital comparison you are using a 50mm lens and getting a telephoto effect. This will give more DOF than using a 75mm lens on a 35mm body.

This is why small digital compacts have so much DOF that we can forget out of focus backgrounds.

Sorry, but that's so untue in both cases. Using lenses and the seriousness or otherwise of the images has nothing whatsoever to do with the method of capture.

Would you say shooting black and white film was for serious photographers but shooting colour was for snapshots?

Professional photographers will use whatever is appropriate to their purpose. The result is what counts, not what you use.

I think this is to a large extent a moot point in that there is full compatibilty from the A series onwards. That dates back over 25 years, give or take, and I don't think we could really grumble too much at that. It's much better than Canon, perhaps on a par with Nikon and vastly better than Olympus. Olympus offer an OM adapter that has lots of restrictions as to which apertures can even be used.

Like it or not, the future is with the new digital lenses and the backwards comaptibility is just a bonus.

The irony is that go back even further and it gets better again - the screw thread lenses are, in my opinion, actually easier to use on digital than the K and M series ones.

I am enjoying the K10D very much and using it with lenses it is designed for.

Depending on the lens, there is an active market on eBay for M and K series optics. Even more so when the lens is one of the rarer ones. A 20mm f4 will always do well, a 50mm f2 will sell but for a low price.

Many of the more mundane A lenses sell for quite low prices - a 35-70mm f4 went recebtly for about £35 and it looked spotless.

In this way, at little expense if any, I have switched to A series only for my manual focus lenses.

Did the camera give the correct exposure in both instances?

It's a dreadfully awkward thing to do IMHO. Which is why I use A series or later lenses on the DSLRs.

I don't see the point in struggling when the right kit for the job is readily available.

Whatever auto setting you use, the camera will be set to Av if using an M or K series lens. The shutter speed will vary according to the light level, but the aperture will always be wide open.

In manual, you can check the exposure by using the stop down lever (why is everyone intent on using the green button?) and whilst holding that down you release the shutter.

Of course you have to set aperture ring permitted in the custom finction menu.

That should work, but it's ages since I messed with it as it's much more efficient to use an A lens on the A setting!

Stopping down the M42 lens manually could be different, because as soon as you do that whatever aperture is set is effectively the wide open aperture of the lens. The camera cannot know differently as there is no connection or stop down mechanism at all.

I haven't got a screw thread lens handy to try it, but if someone has perhaps they could check it out?