K-5 wireless: some issues

Some of you might have seen the thread over on 'depressing reviews' about some issues when using the K-5 with wireless flash. Here is what I tried:

Set up camera 1 m from book shelf (a classic target, isn't it?), Av mode, A6.3, 100 ISO, DA*16-50 and put the wireless flash on the floor.

Distance at lens set at 1 m, no problem, exposure looks fine.
Distance at lens set at 2 m, blurred picture , exposure looks the same.
Distance at lens set at infty, even more blurred pic, exposure way too dark.


In a second attempt to get to the root of the problem I tried the following:
Set-up as was, i.e. subject at about 1 m from camera, wireless flash (was a bit further away).
Focus correct -> exposure correct (a bit on the low side perhaps)
Focus at over 2 m -> way too dark

Then I cranked up ISO
Focus at over 2 m, ISO 200 -> still too dark but already better
Focus at over 2 m, ISO 800 -> almost ok, still about 1 stop underexposed
Focus at over 2 m, ISO 3200 -> overexposed !

Focus correct, ISO 3200 -> slightly overexposed but darker than the one before

So it seems that the flash is told to give off the same amount of light regardless of ISO once the lens is set at focus distance > 2m...

Any ideas? Could you try out if you see the same behaviour, please.

Prieni

I haven't been to dpr for a while now - can you link to the thread or explain more what the apparent problem is please?

It's this one:
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=37293925&page=1

Not a lot in it though, the original post, and Ralfs, plus a whole bunch of insults

Ralf, I don't have my flash here at the office, but will give it a try soon.

Thanks for the link Tim.

I have just tried mine (AF360FGZ) and I have the same issues.

Thanks Tim for the link and the willingness to give it a try. Just had a session with some ice crystals on one of the windows with wireless flash. No problems there (but that was macro anyway).

Prieni

Ralf, during your testing, was your viewfinder covered, either by the proprietary cap or some other opaque material?

I've just tried a few quick handheld tests and it seems to confirm the bug. Focussing - whether MF or AF - beyond about 2m results in severe underexposure when using wireless PTTL, whether the subject is in focus or not.

Peter,

at some tests it was covered (by my friendly mug), in others it wasn't.
But it shouldn't make a difference when all you change is the focussing distance on the lens...

Tim, any idea of what's behind the bug? I have a theory (and I formulated that over on dpreview; just would like to know any ideas from here before repeating...)

Prieni

Nah, no clue I'm afraid! When I first read your post I thought it was related to the K5 auto-ISO flash bug, but now I don't think so.

Just had another try, this time putting the AF360 FGZ on camera as controller. And voila, the problem occurs no longer.
Edit: This was the first test, with fixed ISO.

My line of thinking:
When the pre-flash is fired the camera has to estimate the contributions from the built-in flash and the wireless flashgun to the overall exposure. For short distances that seems to work o.k.

When the lens is set to a longer distance the power of the on board pre-flash may be set higher (or not, it is not really relevant) and the camera is overestimating the contribution of it's on board flash, telling the wireless flashgun to give off a lower power flash.

If that is right then it means that it is easier for the camera to sort out the contribution from an on-camera flashgun (or the AF360 is taking care of it).

Don't know if that makes sense or not, it seems, though, as if the problem could be mitigated by a firmware update.

Prieni

I hope Pentax see the problem and can do something about it. Sadly the dpreview thread is less than clear, mainly due to the name calling, and unlikely to attract Pentax's attention.
I can't see me buying a second flash to use as a controller. At least not unless I sell my Pentax 360 and switch to a different (cheaper) make.

Oh I'm sure they will fix it, it's pretty easy to reproduce! Just a question of when.

Although from my own rather selfish point of view this doesn't bother me too much. I think it is at least 12 months since I used wireless PTTL, and that session was all close-up (< 2m) portrait stuff at a party with the flash in my left hand.

3 months, max, I hope - I haven't got a K5 yet, but I'm hoping it won't be too much more than three months.

Most of my flash photography is off-camera wireless P-TTL - I have two Samsung 540s, which I stick on convenient shelves at parties and other social get-togethers, and bounce off the ceiling, using the built-in to fire them.