High Speed Sync Query

Ok all you Pentax Flash experts! I've got something here I want to confirm. I've been getting to grips recently with the AF 540FGZ gun (what a great piece of equipment!), and as I'm interested in writing some 'mini guides' to practical flash use with Pentax DSLRs, I've been checking how things work with different combinations of lenses and flashgun modes.

Now, we're lucky of course to have a gun that offers P-TTL, TTL, Auto & manual flash modes - wow, we can really have it all ... but I've noticed an interesting thing ... tell me what you think ...

In theory, P-TTL is not compatable as a flash mode for anything other than autofocus lenses - and mostly thats going to be the later DA type, or third party AF equivelants. (It says so in the manual). So, for A-series lenses, if we want auto-flash on a digital SLR then we should use Auto Flash mode ('A' on the LCD display). Here we tell the gun the ISO & aperture. It needs to be told that because the lens / camera does not send the info through the contacts, nor does the flash get distance and focus info - sound right so far?

Now P-TTL just doesn't work with 'a' lenses - in practice I have found that some lenses will activate P-TTL and you can get a decent exposure, but there's very little consistency - try any bounce and you'll soon see!

BUT - and this is the question really ... my tests have shown very consistent and repeatable performance with HSS , in P-TTL mode with 'a' series lenses ... in manual exposure mode with HSS set on the flashgun you set a speed higher than 180th (this then controls the 'background' lighting) and let the flash provide the 'foreground' lighting using HSS. You get the normal P-TTL flash exposure compensation adjustment (+1 / -3 stops), and this seems to work perfectly! I didn't think it should??? But I've had 100% perfect performance and exposures using HSS in P-TTL with 'a' series lenses ....

Is this how it should be , ie designed to be (because the manual doesn't suggest that), or is it a fortunate and explainable freak of the technology that just gives us lucky Pentax people another great choice for flash shooting?

I can't test this out right now because my 360 flash is in storage pending a move, but it may be worth your while picking up MattMatic's excellent guide to using Pentax flashes. It doesn't seem to cover the issue you raised but does give an overview of the capabilities of the equipment. You can buy it at MattMatic.co.uk (I will expect a small commission).

Good luck with your latest project.

Al

It's an interesting observation... AFAIK, when you select HSS the flash should use regular flash below 1/180s, and HSS above that.

Will test when I get half a mo'!

Matt

From what I've seen Matt, in HS mode the flash indicator ('HS') cycles on and off as you change shutter speeds above and below 180th, so it appears to activate automatically as required - the switch remaining in HS position does not matter I seems.

What I'm interested in confirming really, is the apparent good performance of HSS using P-TTL mode with 'a' series lenses. In theory (I thought) any P-TTL operation should not work with 'a' lenses ?? And it certainly doesn't, not consistently, in normal mode (ie not HSS) - but for some mysterious reason, exposures and compensation adjustments in HSS mode seems to work perfectly with 'a' lenses! Great I say It will make for some great wide aperture outdoor portraits