adrian_w1

Thanks for the replies so far. You are all making the assertion that adding a TC magically gives a lens a new focal length, and that the mathematical identities all continue to hold true. Since standard DOF equations make assumptions about the lens design I don't understand I am looking to explain the maths in terms of optics.

The explanation I've got so far goes:

At a given aperture and focal length, an observed object can move within a range such that the difference between in-focus and out-of-focus is imperceptible. Adding a 2xTC doubles the size of the image. Since the length of the out-of-focus area is proportional to the distance moved by the object, this effectively halves the distance an object can move whilst still being perceived as in focus.

So far so good. Now all I need to do is relate that to f-stops.

In Bob Atkins's review of teleconverters

(http://www.bobatkins.com/photography/reviews/tcs.html) he says "as far as DOF

goes, a 600/8 lens is identical to a 300/4 + 2x TC". I'm trying to understand

why this is.

A 300mm lens at f4 still has the same aperture diameter regardless of what is

put behind the rear element. I can see that effectively magnifying the image is

going to reduce depth of field to that of a 600mm f4, but I don't see how

anything behind the lens can increase DOF to that of a 600mm f8.

I suspect this is something to do with the teleconverter being closer to the

main lens than the film would be.

Please note I'm not trying to understand the light loss issue, that's fairly

obvious.

This is a long shot. One of my monoblocs has developed a fault. The

symptom is that it fires fine once, appears to recharge, the ready

light comes on but it won't flash again. When the test button is

pressed the "crack" sound of a flash discharge is there but no light.

Switching on and off again resets it for one more flash.

I know this isn't unique to me, because there was a Bowens Prolite on

ebay a while back with the same problem!

Would anybody who understands how monoblocs recharge care to suggest a

reason?

Your other options include the Falcon Eyes range, which are very similar to the Portaflash kit, and should be available from any Kenro supplier.

Interfit were showing yet another flash range at this year's Focus. They were also showing a very neat prototype small digital head similar to a small Style 100.

I like the Stellar range but I haven't used it outside of a demo setting. The old Pro and Colourflash range were a bit peculiar.

I've heard bad things about Prolinca head reliability from someone who runs a commercial studio for hire using a mix of Elinchrom and Prolinca. You're also locked into The Flash Centre for support and they wouldn't talk to me when I was buying. The Elinchrom heads are lovely.

Another option is a cheap Chinese import, which is what I did (see http://www.adrianw.dsl.pipex.com/photo/BY420Review.html) but I probably wouldn't do it again now the Stellar range is out.

<p>I've written some comments at

<a href="http://www.adrianw.dsl.pipex.com/photo/BY420Review.html">http://www.adrianw.dsl.pipex.com/photo/BY420Review.html</a></p>