[Entropy]

As most of you may have figured out, I'm not a big fan of the AF540FGZ, and plan on selling it if a K10D firmware update doesn't fix the underexposure issues it has within the next few months.

Today I tried an experiment - I was out in an environment (The town park) where it was difficult to get an angle on subjects (members of a flock of ornery geese) that wouldn't result in some shadows. Not only difficult, but impossible if you didn't want to wade through river water or have your subject(s) play "bite the guy with the camera".

This sounds like just the situation that nature photographers buy the "Better Beamer" flash extender for. I figured that while I don't have a Better Beamer, I'd try seeing if a full power zoomed pop from my AF540 would have any noticeable fill effect.

So I put the camera in P mode to meter for the daylight, and set the flash to manual full power mode. I snapped a picture of one of my rather uncooperative subjects.

No noticeable fill effect at all, but wait... Something's strange... I'm not hearing that distinctive "charging from a full power pop" whine from the flash. In fact, I'm hearing almost nothing at all, as if it had dialed itself back. But I had it set on manual mode in full power?

I looked at the flash and found it was back in P-TTL mode. I couldn't even put it back into manual mode, only P-TTL and "SB" spotbeam-only mode. A few seconds later I could again.

I took another picture, and it was back in P-TTL mode AGAIN.

I eventually figured out the following:
If the camera is in any automatic exposure mode (P, Av, Tv, whatever), pressing the shutter button halfway would cause the AF-540 to revert to P-TTL mode. A and M modes would be unavailable until the metering information disappeared from the top LCD.

I realize that using a manual flash pop when the camera is performing automatic metering is a very uncommon situation, but shouldn't the decision to do something a little crazy/unusual be up to the photographer and not the camera? Is there any way (other than putting a shim between the flash and the camera that insulates some of the control pins) to make the AF-540 STAY in manual mode regardless of what you're doing on the camera? Kind of annoying to have to go to manual mode and do the green button thing with an uncooperative subject.

[blende8]

You have to set your camera to M.
This is said in the manual.

[fwbigd]

I'm not a big fan of the AF540FGZ, and plan on selling it if a K10D firmware update doesn't fix the underexposure issues it has within the next few months.

The problem is not your Pentax AF540FGZ, it is the reflective metering system of ALL single lens reflex (SLR) camera system. Most if not all SLRs have their metering system set to 14 percent gray. Not 18 percent as most people think. So if even if you were metering off an 18 percent gray card you would not get a perfect exposure.

With that said, the problem with the under exposure is that your meter is receiving the greatest amount of light from something brighter/lighter (white) than the 14 percent gray. Therefore, the meter is turning the flash off so that it yields the brighter/lighter reflection as the 14 percent gray. In other words the meter is turning white to gray by under exposing your subject. The opposite would be true if the meter was reading off black. It would overexpose the picture because it would be trying to see the black as 14 percent gray.

To compensate for this you simply dial in more exposure/less exposure by adjusting the exposure wheel on the back of the flash. The other method would be to adjust the flash using the Fn button on the back of the K10D.

Good article on meters here: Meters Don't See 18% Gray by Thom Hogan

[Entropy]

That would explain a slight underexposure, but not the fact that the flash never even touches the right half of the histogram when in P-TTL mode unless you dial in a significant amount of exposure compensation. Even with +1 exposure compensation dialed in, the flash still underexposes significantly. (ufraw in automatic exposure adjustment mode will typically increase exposure by an additional stop).

14% vs. 18% grey would explain some underexposure, but it doesn't explain an average of well over one stop underexposure, typically more. I'm not complaining about slight underexposure here, I'm talking massive underexposure which is typically on the order of at least 1 EV (if you're lucky) and typically 2 EV or more.

As to the M mode limitation in the manual - I must have missed that, such a limitation should not be there in my opinion. Yet another reason to sell this thing and purchase a more versatile (and somehow cheaper despite that!) flash setup such as a Vivitar 285HV mated with an RF trigger system.

[fwbigd]

That would explain a slight underexposure, but not the fact that the flash never even touches the right half of the histogram when in P-TTL mode unless you dial in a significant amount of exposure compensation. Even with +1 exposure compensation dialed in, the flash still underexposes significantly. (ufraw in automatic exposure adjustment mode will typically increase exposure by an additional stop).

14% vs. 18% grey would explain some underexposure, but it doesn't explain an average of well over one stop underexposure, typically more. I'm not complaining about slight underexposure here, I'm talking massive underexposure which is typically on the order of at least 1 EV (if you're lucky) and typically 2 EV or more.

As to the M mode limitation in the manual - I must have missed that, such a limitation should not be there in my opinion. Yet another reason to sell this thing and purchase a more versatile (and somehow cheaper despite that!) flash setup such as a Vivitar 285HV mated with an RF trigger system.

Very interesting problem. Is the flash still under warranty? If so you might consider returning it to Pentax Imaging in Colorado for repair. My first AF540 had the opposite problem it would over expose everything. Pentax replace the flash and I haven’t had a problem since.

[wlachan]

That would explain a slight underexposure, but not the fact that the flash never even touches the right half of the histogram when in P-TTL mode unless you dial in a significant amount of exposure compensation. Even with +1 exposure compensation dialed in, the flash still underexposes significantly. (ufraw in automatic exposure adjustment mode will typically increase exposure by an additional stop).

14% vs. 18% grey would explain some underexposure, but it doesn't explain an average of well over one stop underexposure, typically more. I'm not complaining about slight underexposure here, I'm talking massive underexposure which is typically on the order of at least 1 EV (if you're lucky) and typically 2 EV or more.

As to the M mode limitation in the manual - I must have missed that, such a limitation should not be there in my opinion. Yet another reason to sell this thing and purchase a more versatile (and somehow cheaper despite that!) flash setup such as a Vivitar 285HV mated with an RF trigger system.

Not much help from me as I don't have the 540 anymore for the same reason. Other than the massive unpredictable underexpose from time to time, that particular flash seems to be designed to behave in particular fashion which I have never seen in the past. I don't remember the detail anymore but some modes would be auto reset if not careful and could be rather frustrating when you need to work fast.

[Entropy]

Very interesting problem. Is the flash still under warranty? If so you might consider returning it to Pentax Imaging in Colorado for repair. My first AF540 had the opposite problem it would over expose everything. Pentax replace the flash and I haven’t had a problem since.

Yes, I may give that a try. Unfortunately it's 2 weeks out of B&H's 2-week return limit. Given how many people have the exact same problem, I don't have much faith in that though. You're the first person to report a case of it overexposing.

Edit: Were your problems with a K10D or another body? From all I've seen, the severe underexposure only occurs with when a 540 is used with a K10D.

[fwbigd]

Yes, I may give that a try. Unfortunately it's 2 weeks out of B&H's 2-week return limit. Given how many people have the exact same problem, I don't have much faith in that though. You're the first person to report a case of it overexposing.

Edit: Were your problems with a K10D or another body? From all I've seen, the severe underexposure only occurs with when a 540 is used with a K10D.

No my problems were with the K10D body.

I returned the flash directly to Pentax Imaging I did not return it to the store from which I bought it. They (Pentax) replaced the flash free and I haven't had a problem since and I have two 54Os. My problem has been with the 360 and it's weak battery door.