Ok, dokay... think I have some answers

After a whole lot of "I wonder if I did..." questions and trial and error I think I have stumbled on something

* In most K10D modes the camera will always try to expose for the highlights. This is with the K10D flash setting at normal or wireless.

* When you enable the Slow Flash Sync setting on the K10D then the highlights are handled "normally"

* If you use the mode TAv then the camera will expose "normally", and not for the highlights.


So, here are a few suggestions for using all this knowledge:

{1} If you want general flash use, then set the mode to TAv and set the ISO range you want in the Fn menu. e.g. ISO100-1600. This will give you "point-and-click" style flash. The camera will very nicely balance the ambient and the flash and reflections are just blown out. Choose a shutter speed that is close to the focal length (because SR is disabled), and an appropriate aperture - the camera will pick a balancing ISO.

{2} If you want to contain the highlights then use the Av mode. Set your aperture and force the ISO. For indoor shots it is useful to turn off the flash, click the green button to see what the ambient ISO is, then override that (easy if you've changed the front wheel to do ISO in the custom menu) and choose an ISO setting one to two stops lower. So, if the camera says ISO1600, then choose somewhere between ISO400 and 800. The flash will do its work and the highlights (including reflections) will be contained as much as possible. This is amazing for wedding and fashion stuff because you retain the detail in a white dress Just shoot RAW if it's critical so you have some latitude later

It's worth mentioning that if you choose auto ISO then the camera will choose the lowest possible ISO - which usually results in a very hard and clinical flash output. But again, it's to contain the highlights in Av mode. Best to override that

{3} When shooting in a controlled environment - like a school play, party etc - where the lighting doesn't change much then use "M" on the camera and P-TTL on the flash. In this mode, you'll need to pick a suitable ISO rating that is going to give sensible results. (On my setup you hold down the OK button while moving the front wheel.) Choose an aperture and adjust the shutter speed so the K10D meter shows somewhere between -2 and -1Ev. The flash will fill in the rest and you'll get a nicely exposed shot... and again I think it will try and retain highlights


Additionally, it's worth mentioning again that you are usually best to bounce the light, or use wireless, and/or use a diffuser (if the distance allows it).

But the bottom line is - from what I see Pentax do seem to have put the right logic in there... they just haven't told you how to get to it properly

I'd appreciate it if others could try the TAv, Av, and M modes on camera to see if they experience the same results above. I'm off to try this with wireless P-TTL and see if it all holds together still...

Let's hope this helps
Matt

At the risk of appearing as though I'm talking to myself...


Overall it seems the best approach with P-TTL is to bounce the light and therefore cut the probability of direct reflections (maybe this is why I generally get excellent results in practice from the AF540 )

I'll add a couple of tips onto the sticky note...
Matt

Okay dokay...

Here's some more information and test results:
http://www.photon.me.uk/pu/k10dpttlreflect/results2.html
and http://www.photon.me.uk/pu/K10DFlashReflectionTestResults.zip

(The first shots I've lost the details for )

Most of the tests were done with a tethered AF540 - this is helpful so you can move the flash and camera independently and yet maintain full P-TTL control.

My observations:

* When there is a reflection the body will only let out the smallest amount of flash. The tests with manual control and 1/64 power illustrate that the change in exposure due to ISO rating is not a change in the metering, but a change in the effect of the smallest amount of flash that the gun can deliver. Effectively, I am too close

* Interestingly, although the manual power can be stepped down to 1/64 on the AF540, the camera can let out less than 1/64 when under P-TTL control. Would have been nice to have the lower rates manually selectable too!

* It doesn't matter where the reflection occurs, or really how large it is - anywhere in the frame will cut the exposure. (I didn't try reflecting out of frame though...)

* Most strangely the evidence points to the fact that the cut off for reflection is not metered on the preflash . For the last two results (6624 & 7625) I set the red eye reduction on, let the preflash fire and then moved the flash for the main burst. It showed that the preflash metering has nothing to do with the reflection issue It is almost as though there is some "cut off" control during the main exposure.... and I'm very, very puzzled!
EDIT: On the K10D I've just noticed that the red eye reduction works differently to the way I remembered on the *ist-D. The K10D fires THREE flashes on red-eye: 1) the red eye, 2) the preflash, 3) the main flash.
Consequently this makes the above statement untrue.


If you have any other thoughts to add to this then do let me know
Matt

Well, I'm thinking, and I'm trying to work out the above results...
There will be a difference between reflected light and incident light, especially if the scene is not uniform.

You might get away with metering the P-TTL if you enable red-eye reduction The camera will separate the preflash and main flash by about 1 second (might be enough to get your meter to read the second flash).

(Think I'll try and put some of the reflection results on the web later)
Matt

Another 10 min of messing around

This time, I had the following setup:
* CD placed reflective side facing K10D
* AF540 on a tethered lead

Now, I used the digital preview to align the flash, CD and K10D to produce a nasty reflection. Result - massive underexposure in P-TTL (like near black frame, except for the reflection).

Because I could move the camera without moving the flash, I found the following points:

1. The effect of reflection appears to be the same all over the viewfinder.

2. The size of the reflection is absolutely irrelevant. If I zoomed out to 24mm so the reflected part was a tiny dot in the bottom left corner (that's about 2mm square on the K10D LCD), then the massive underexposure still occurs.

3. The underexposure is directly related to the ISO setting. (Note: even though the without-flash-shot at the same aperture and shutter speed is waaay underexposed.)


So, it seems to me that the flash metering is working purely on the peak light level - without any decision regarding its positioning, or relative size with respect to the whole image
And the ISO rating, mysteriously, has a great effect.
(If you get some person in a group photo with a shiny brooch or badge - that will throw the whole exposure completely off).

On the one hand, it's nice to be able to say to the camera "please don't blow out the highlights" (Nikon have this setting for their newer flash systems, as far as I understand). That's a real boon for weddings (where it's so easy to blow the detail out of the dress completely). However, for general shots, there's something curiously amiss with reflected items

Might try and put up some of the images later...
Matt

Ok, some more tests (but nothing extra to post right now)...

Carefully set up a CD in the middle of the frame so that it will reflect the light. It appears to be down to reflections, for sure. Haven't yet worked out if it's just in the centre area or not, but the compensation when there is a reflection seems overly aggressive from what I can see.

The "A" mode on flash remains a better shot overall. However, the K10D on P-TTL seems to wind back the flash too hard when there's a reflection - even with flash compensation there are still only a couple of pixels blown out. I can't believe the metering is that sensitive to pin-lights.

The "Slow Sync" flash settings are all perfect (as they were with M-mode on camera with P-TTL flash). "HS" makes no difference (as I'd expect at low light levels). Tried red eye reduction mode, trailing curtain sync - no difference (again, as expected).

The flash compensation within the K10D flash menu seems to work reasonably, though I've not done any exhaustive testing on it - it may well behave the same as the "flash-compensation-on-AF540" that I did yesterday.


My current thoughts to pin down are:
* Is the flash metering on reflective lights down to pin-points of light?
* Is the metering just in one area in the viewfinder, or matrix?
* Could the flash be controlled in discrete steps (like 1Ev steps) that produces these rather "stepped" results?

Thoughts, thoughts, thoughts.... and not too many answers
Matt

I'll try and suss out what's going on with the images - probably down to the upper/lower case of the filenames (not case sensitive on PC, but is on the server)

Hefty,
It is not always inconsistent Especially when there is a reflective patch in the viewfinder do things go AWOL.
What I did was work out a setup where the P-TTL was definitely underexposing every time. Other scenes will expose just fine.

When I've used the AF540 for events for fill in flash I found that it will consistently expose to keep highlight detail. That was very, very helpful when you're talking about a wedding dress! (Think it was on the HS setting.)

It's just in some circumstances the flash goes completely potty - and I am seriously curious to work out what it is. I did try on the *ist-D many times, but it is easier with the K10D because you can instantly adjust the ISO rating



There are other modes of flash - slow speed sync and HS - that I need to give a try as well. Unfortunately I don't have a flash meter (But that would've been a good idea!)

Matt

Ok folks...

I've spent a little while setting up a reasonably controlled experiment for the dreaded P-TTL on the K10D and AF540-FGZ. I've tabulated the results into an HTML page complete with histograms for easy comparison. However, I've not yet completed my analysis.

What comes out straight away are the following points:
* Underexposure is related to ambient light + ISO + aperture.
* The flash compensation on the gun works as expected at ISO100, but as you increase the ISO it has less and less effect.
* Using "A" mode is pretty much consistent - the P-TTL is the dud. (Also shows that the flash gun has plenty of power at all ISO ratings for the test scene)
* Using "M" mode with P-TTL flash gives excellent results AFAICS
* Camera exposure compensation has odd effects


It's almost as though the flash gun thinks the camera is always in "Auto ISO" mode, even when it's not.

I'm happy to discuss the results with anyone that can be bothered. Hopefully I can get this mess back to Pentax and help them sort it out once and for all!!!!

A slow, slow URL link is here: http://www.photon.me.uk/pu/k10dpttl/results.html
(It's slow because there a couple of hundred images to download )

And a prezip'd package: http://www.photon.me.uk/pu/K10DPTTL.zip

(Better to download the zip file, unzip to a directory and open the "results.html" file )

Matt

Very, very nicely indeed. A couple of weekend's ago I had a go of a 31mm Ltd and 77mm Ltd and did a little comparison with the 24-90. Actually the 24-90 held its own - gave me new respect for it
http://www.ephotozine.com/u16488/gallery/810170
Check the two versions - V1 is 31mm Ltd, V2 is 24-90mm. Obviously this doesn't test the whole "feel" of the lens, but I was surprised all the same at how good the 24-90 is. Bokeh and rendition at 90mm is lovely for portraiture too. This was at 90mm: http://www.ephotozine.com/u16488/gallery/731379

It's been one of those "sell the 24-90.... no, keep it... no sell,... no keep" kind of years As much as I liked the DA*16-50 and DA*50-135, the 24-90 still has a good place
Matt

Clarky: answer here = http://www.bdimitrov.de/kmp/technology/lens_terms.html

Matt