Originally posted by Lowell Goudge What is the guide number of the flash?
Note that at ISO 400 the AF540FTZ has a guide number in excess of 12 meters at F8, therefore at your ISO you may be simply too close to the subject, even with bounce to control exposure with the camera.
Learn to do manual flash calculations to start your tests. Try again at ISO 100
I have no issues with my AF 540 on the K5. There must be something you are doing wrong, somewhere.
What is the metering, spot or matrix, and what is the subject to background ratio
Lowell, I apologize for writing such a long and information-heavy post (and no images) that certain important details are easily missed on first read. Please allow me to re-explain the core idea of the test methodology:
Using
exactly the same conditions (ISO, aperture, distance), I took several shots where the
only difference is that the flash is pointed
forward in one shot and
very slightly tilted in the other shots - so slightly that the flash is practically pointing to the same direction. The purpose of this is to provide an apple-vs-apple comparison with absolutely identical conditions.
Please notice that I did in fact repeat the test using several very different combinations of ISO and aperture, including one set at ISO80.
In
all tests, the forward shot was exposed nicely with a hefty highlight-clipping margin. This proves that the flash was indeed capable of exposing correctly in the given conditions. I also took shots at all available manual power levels to assess the power used in the P-TTL shots and to verify that the flash was indeed within working range (BTW, these shots also demonstrated that P-TTL mode is capable of firing at a weaker level than the lowest available manual level of 1/64, something around 1/128 ~ 1/256).
Given these fixed identical conditions, one would expect a similar (good) result in the other (i.e. slightly tilted) shots. However, huge overexposure was observed, which indicates a bug somewhere in the camera/flash logic. More precisely, whenever the flash thinks it is bouncing (the distance scale disappears) it behaves badly. Whenever the distance scale is displayed (forward shot and slight downward “macro-tilt”), the exposure is nice.
Suspecting that the slight upward tilt may be “illegal” in some way, I also did shoot at a normal 60 degrees tilt and still got overexposure. In fact, the tilt and off-ceiling bounce should have made it
even harder to over-expose (requiring more power to illuminate the scene), thus further
amplifying the indication that there is a bug here. Furthermore, as noted, I also shot wireless with the flash off-shoe held in my left hand, close to the camera and pointing forward, and it exposed nicely.
So, to summarize the essence, we have identical shots in identical conditions - except one is forward and one slightly tilted - that demonstrate a terrible gap in exposure (overexposure in the latter) instead of similar results. And this is repeated in all ISO, aperture and distance combinations tested. I call this a bug.
It would still be interesting to know the flash parameters, and I apologize for not including them in the first place.
According to the flash user manual, the guide numbers of the flash, with flash zoom at 50mm, are (@ISO100):
1/1 46m
1/2 32.5m
1/4 23m
1/8 16.3m
1/16 11.5m
1/32 8.1m
1/64 5.8m
(as expected, each stop gains sqrt(2) more distance)
Here are again the sets I shot, along with the appropriate guide numbers and the corresponding flash power:
1. ISO400 f/8, 3.5m: 3.5 * 8 / sqrt(4) = 15.2 ~1/8
2. ISO800 f/4, 3.5m: 3.5 * 4 / sqrt(8) = 5 ~1/64
3. ISO800 f/4, 2m: 2 * 4 / sqrt(8) = 2.8 ~1/256
4. ISO80 f/8, 3.5m: 3.5 * 8 / sqrt(0.8) = 31.3 ~1/2
3. ISO400 f/4, 2.5m: 2.5 * 4 / sqrt(4) = 5 ~1/64
The conditions for series 3 are possibly a little off, but even here the same (mis-)behavior was observed.
Lowell (and others), if you own a 540 and a K-5, then by all means please repeat my controlled test procedure and contribute your results - Shoot the same stuff on a tripod with fixed identical parameters - forward and tilted - and report your findings.
Metering mode was Matrix (not that it matters in P-TTL, and anyway I shot on manual (X) and the exposure was pitch black without flash).
I shot a green-painted wall with a canvas print hung on it and a matte-finished brown upright piano along the wall, so there is no subject-background ratio - It’s all a pretty flat subject. No shiny parts either.
Sorry again for not posting images yet. I’ll do it back home.