When you do this, the lens has a field of view similar to a 127.5mm lens on the 35mm camera, so you are not comparing like with like. We would expect a 127.5mm lens to have less DOF, but of course the relevance here is not actually the focal length but the magnification.

The circle of confusion is a useful design objective, but as we can define what we want it to be, so it isn't a constant value. It's a convention. When using a DOF scale or table I might take the DOF at f8, focus accordingly, but I would set the stop at f11. This is so the quality level is more critical and bigger prints can be satisfactorily made.

Some manufacturers are more accurate than others when defining their DOF scales and tables, and some more generous with their expectations, it depends how you look at it.

Don't foget that in an ultimate sense there is no DOF. The only sharp point in an image is the plane you are focused on.

The standard specification for the L&CPU (Lancashire & Cheshire Photographic Union) to which our club is affiliated, may be typical and relates more to the mount size than the image size.

Mounts should be 40x50cm, which means that an A3 print would be ideal for the purpose.

As a club we adhere to that so that anything we want to send to the L&CPU Annual Competition is already the correct size and doesn't need reprinting.

Having said that, sometimes we have an A4 competition, as we are doing tomorrow night.

It's not the circle of confusion that causes your second observation, but the difference in magnification. They are different images so it doesn't have a relevance to DOF in the context of differences through format.

Wherever did you get that idea?

It is true that converters are designed with a purpose in mind, which is why Pentax made -S and -L versions for shorter and longer focal lengths.

It's also true that cheap converters are usually worse than just blowing up the image more.

I totally agree.

If you look back at the old competitions in the forum, lots of pictures are now missing because people have not maintained their images on their hosting services. It's a real shame.

The Gallery is much easier.

A 1.4x converter multiplies the focal length of any lens by 1.4x and there is a light loss of 1 stop.

A 2x converter multiplies 2x and there is a light loss of 2 stops

A 3x converter etc. with 3x and 3 stops loss.

This will apply to all lenses and if it's not happening then I would look at the communication between camera and lens and coverter. Are the contacts clean?

I would never use a 3x converter - every one I have ever tried has been dreadful.

We'll leave it to the sellers and buyers to check it out for themselves. As I've said elsewhere, it's always a good idea to keep up to speed with the changing terms and conditions on eBay and PayPal.

Paul, it's not a problem, it will all be forgotten by the morning!

Just enjoy your photography as many of us do and join in when inspired to...

This is a thing that's often misquoted, but smaller formats have more DOF for a given field of view and that's because the focal length of the lens to achieve that field of view is shorter.

An extreme example is that a Minox with its 15mm standard lens has such an immense FOV that it doesn't need to be focused even at f3.5, having DOF from 9 inches to infinity. The circle of confusion is minute to make a reasonable sized print.

A Rolleiflex TLR (6x6cm) uses an 80mm lens and has relatively little DOF by comaprison, but also has a larger circle of confusion as less emlargement is needed.

So an APS-C DSLR will have more DOF than a full frame one. We know this is true - look how difficult it is to get an out of focus background with a compact digital camera with its tiny sensor.

Yes, the Quick Shift is a brilliant idea. It took me a while to realise it, but now when I use older lenses I miss it.