The AF-assist spot beam (SB) works by illuminating the scene with a grid pattern of red light. This gives the camera a sufficiently contrasty area to achieve focus.

The flash has to be mounted to the camera (or connected via cable) to work at all. The SB function does not operate when the flash is electrically disconnected from the body. With a cable connected flash off-camera, you'd have to manually aim the SB correctly for the camera to 'see' it.

You could set up the shot slightly differently to get the effect I think you are after.

Remove the specimen from the arrangement and put it in a separate vase.
Setup the specimen and your camera so the image is as you would like.
Place the remainder of the arrangement behind the specimen and adjust the distance between the two vases to get the background as you'd like.

You may have to use a reflector or additional lighting to illuminate both parts of the scene properly.

A TC is worth buying if its a good one. You can expect to pay around £200 for the SMCP-F 1.7x AF teleconverter, which is the best I've ever tried.

Whilst a TC will add a bit of reach, one major point to remember is the light attenuation that occurs. A 1.4x will lose 1.4 stops of exposure and a 2x will lose 2 stops. If you combine it with a slow lens your shooting opportunities can be a bit limited.

Please forgive the selling opportunity, but if you want to try a 2x TC without much expense, I have a Takumar-A 2x for sale here. It will not allow AF, but it has the electrical contacts for metering.

I've not bought from pentaxuseruk before but the price of the manual was reasonable so I bought it.

The seller replied very swiftly to my complaint and will refund.

Maybe this is why SRS don't seem to get them any more.

Yes I did buy one, advertised as English, image showed it was English. I received one . . . in Italian. It's going back to the seller who's name just happens to be . . . pentaxuseruk.

You can download the manual from here. It was called ZX-L in the US.

Other way round surely. Coming into the warm from the cold usually causes the moisture in the warm air to condense on cold gear.

For that reason, I carry a large plastic zip-loc bag with some silica gel inside. Pop the cold gear into the bag before entering a warm environment and let the temperature equalise before removing it from the bag.

Much of the advice you've received so far is for the older, more manual than auto, bodies.

My preference however is for the later Pentax bodies.

The best of these in my opinion is the MZ-S. The MZ-6 however is a cheaper and lighter option and almost as capable as the MZ-S. Both bodies can use P-TTL flashes and have wireless flash control built-in. The MZ-6 can also use lenses having no aperture ring as the aperture can be set by the body. The MZ-S has no aperture setting facility on the body and can't use them.

These two bodies are about the closest film bodies you will find that have similar functionality to your Pentax DSLR.

Depending on your intended use or abuse of a film body, the MZ-S is much sturdier than any of the palstic bodied cameras.

That said, I still have a soft spot for the Super-A, although I don't own one any longer. The Super-A was the most refined of the ME style bodies having all manual and semi-auto exposure modes and TTL flash.

An MZ-S in good condition should cost you £200-£300. I recently picked up an MZ-6 + FA28-90 lens in excellent condition for £36.

We learn by our mistakes and expensive mistakes are quickest learned.

If your ringflash is TTL compatible then I would suggest looking for a *istD or *istDS as these are the only Pentax cameras that will provide TTL flash metering.

Any Pentax DSLR will work fine in manual.

Even if your Tamron lens is AF, you'll probably use it in MF most of the time but it'll work AF if you want it to.

Hope that helps.