I haven't quite grasped any specific dispute or argument here yet, as in specific opposing views. Its been informative for sure.

So if we seem to be agreeing that the actual lens 'DOF' remains constant, is it the case, (as MrB has said), that lens DOF scales will only be correct when applied to a sensor matching the size the lens was actually designed for?

Is this not a different question to just asking if the DOF itself will change or not?

David, looks like you've been reading some of those photography books & magazines I referred to in my post above

I don't doubt the good qualities of the 55-300 zoom as you list them, but is it really the case that such a very large focal range is a truly 'obvious choice'? For some maybe, but not for everyone. Unless an individual has already found a real need to be so far from their subject that they require such lengths, then I would find it hard to recommend.

For the less experienced, shooting at 200-300mm handheld is fairly hit & miss isn't it, even with our SR system?

So, if I understand correctly, what we are saying here is that for any single scene shot with a specific lens at a specific aperture, the DOF (the distance within the scene that appears in focus) does not change just because you take another shot with the same lens and aperture on another format.

But because to be able to see exactly the same scene on the different format we would have to move or change focal length, it is those actions that in fact alter the DOF.

Does that cover it?

There are two issues to decide on firstly - do you need to extend your available focal length range, wider or longer - and do you want better quality in and around the range you already have. ?

I would support the advice given by John above - take plenty of photos with the lens you have, at all zoom settings, and get a feel for when you are struggling to either a) 'get it all in' or b) fill the frame and get close enough.

I wouldn't spend money at this stage on upgrading to a higher quality lens that covers much of your current focal length range.

Also, especially in magazine articles and photography books, a very long zoom is often sold as the next obvious step - I'm thinking things like a 55-300 for example, which all the brands offer, sometimes in kit package. This is an enormous range to consider, and the question is what would you actually shoot at the longer end ? And even with our Pentax shake reduction, it is not easy to get steady shots at 200 - 300mm (I'm talking about those of us less experienced with such lenses) unless you're going out with a tripod regularly.

Only you can decide which way to go. If it has to be a longer zoom, then for general photographic subjects, a more modest range might be practical. I find my older 75-150 plenty, and the new 55-200 has been referred to also here.

All look in focus to me ! Are you saying you set 'catch in focus' and then waited for the insects to move and the camera to fire itself? Did you set specific focus points active?

A good read up is definitely good advice, if just for general knowledge. For critical work you might need to do some maths or use lens scales (if included on yours). But for general photography when you don't want to add too much time to your shooting technique, then the 'hyperfocal' distance approach is perhaps most practical.

Again, this distance would need calculating for exactness, but as John said above, in most situations getting something sharp at a third of the way into the scene would work in many situations.

f16 would certainly pull most of the foreground into focus at many focal lengths, but I find that there aren't many times I could really use f16 - it generally would cause too slow a shutter speed, unless you're in very bright light, or using a tripod then that won't matter. Also 'diffraction' has to be a concern at this aperture surely.

If foreground is important (eg for wideangle shots) then you could get away with a wider aperture, f11 or f8 even - you may have to pull your focus point in a bit (in front of 1/3rd) to get the critical elements of the shot all within the DOF.

We've got DOF preview on the DSLRs of course (maybe even on the other Pentax types ?). This has to be selected in the menus first (as an option instead of digital preview), but in reasonable light it provides a great way to check the exact effect you're going to get. Digital preview is more versatile for checking a lot of things, but is perhaps not so precise for checking DOF on the LCD. But, you can't have both at once!

I think that's right - that's where the real limitations kick in on something like my 'kit' 18-55 WR (I hesitate to say 'kit' as that has unfavourable connotations that don't apply in general to this particular Pentax lens)...

but I'll be photographing an indoor party today, don't know how much fluorescent lighting they'll be, lots of movement going on, the autofocus would be handy... the 18-55 range would be ideal, but I'll leave the 18-55 out of the bag in favour of the 28mm f2.8 and 135mm f2.8. I'll happily sacrifice the AF & zoom for that extra stop and the better OOF areas.

But the 18-55 is pretty OK for landscapes at f8.0.

Or maybe not - it could lead to instant strangulation if your dropped the thing !

I would certainly question the technique here .... shouldn't the camera strap be placed around the neck ?

Yep, its the SD card that gets me - a few times too many I'm afraid

Keep promising to buy an extra spare & stash it in the bag somewhere. Really feel a big d... after carefully framing up, checking focus etc etc - then the dreaded message appears after shooting