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I have a 500D with the 430EX II flashgun. I've taken two pictures with the camera in manual mode, one with the flash in ETTL mode and then in manual mode with the power to 1/64 and the flash exposure to -2.
I've noticed that the shot with the flash in manual mode looks like the flash is still too strong even with the settings I've picked when compared to the ETTL shot which looks natural.
Can someone give me a quick explanation on why this might be happening?
If the flash is on manual, the flash exposure control doesn't do anything. If the flash is set to 1/64 power, it will deliver 1/64 power. Set it to 1/2 power and it will deliver 1/2 power. The flash exposure control only works with TTL flash metering.
1/128 isn't the flash's minimum possible power; it's merely the minimum power you can select using the manual controls. It used to be that auto/manual flashes "bottomed out" at 1/16 when physical switches were used, even if TTL or "auto" mode could supply much less power. Even going to 1/128 vs 1/64 means an extra character on the display. At some point, interface practicality and circuit cost/complexity have to be considered.
If you are working with manual flash lighting and you need less than 1/64 power (or whatever the lowest setting is on the flash) gives you, then your only options (assuming the aperture you're using is what's needed for the shot) to lower your ISO setting to something less sensitive or to use neutral density gels (or something jury-rigged that has the same effect) to reduce the output of your flash.
There are many flashes across many brand lines that exhibit what you have discovered concerning your 430EX II. Although I don't think there is a great conspiracy amongst the flash manufacturers, the "Truth" is that the manual minimum setting is actually the minimum manual setting.
In other words, many flashes that can be controlled both manually and via TTL are capable of less power than the power used at the lowest manual setting. To use the flash at less power than the minimum setting that is available manually, the flash must be controlled through TTL. The easiest way to do this is by adjusting the flash exposure compensation downward. Note: The flash exposure compensation setting has no effect when the flash power has been set manually, it only applies when the flash is being controlled through TTL.
If you want to reduce exposure of areas illuminated by the flash and retain manual control of the flash, then either a reduction in ISO or a narrower aperture can be used. Filters that limit the amount of light passing through them could also be used. This would include either lens mounted neutral density filters or modifiers placed between the flash and the subject. You could also increase the distance between the flash and the subject. Any of these options could also affect other properties of the resulting image.
A modern flash gun like the 430EX II can output an almost infinite number of power levels between the maximum and the minimum powers within its capability. But building a user interface that would allow manual control to such a fine degree would be unwieldy and cumbersome. A good analogy might be a camera that can use stepless shutter speeds. When selecting the shutter speed manually, only 1/2 or 1/3 stops are available for selection because at some point the excessive number of choices would make the selection of any shutter speed a lengthy exercise. But when selecting the aperture and letting the camera select the shutter speed, the camera can quickly select one of a virtually infinite number of shutter speeds between each stop.
Yes, this is very annoying. The lowest power settable when in manual mode is far higher than that which the flash can achieve in ETTL. I've done some tests and the manual flash when set to 1/64 exposes around 4 stops higher than the flash can actually achieve. Really limits the use of the flash in manual mode.
Short answer: One isn't related to the other.
Long answer:
Electronic flash has a characteristic discharge curve (which you can see here).
Modern flash designs 'cut off' the discharge curve at the appropriate time to deliver anything less than their full output. Note that because the intensity of the burst of light the flash delivers is not uniform during the exposure, cutting a flash's total output by a factor of x does not cut the (time) duration of the flash by the same amount.
As you can see from the chart linked to above, a 1/32 output cuts off quite a lot of the discharge curve, so going to 1/64 is pretty admirable from a feature design perspective.
As a side note, different discharge intensities have a different peak spectral output (color balance), so cutting off the flash early necessarily affects the color of the light that has been discharged. Since no one wants yellow flash, or worse, yellow flash sometimes, these factors must also be compensated for as the flash durations shorten.
When using E-TTL, the flash does not control the duration at all. Rather the camera continuously monitors the scene exposure before and during the flash output computes the cutoff from the pre-flash reading, using the camera's ambient meter.
As a result, there really is no correlation between the manual features of a flash (eg. 1/32 or 1/64 power) and what levels E-TTL can achieve (which, from the flash's perspective, is simply outputting max power until told to stop).
If your hand was fast enough, you could arbitrarily decide to block the flash's output or, if you were really fast, yank the batteries out of your flash. If you could do something like that in ~1/20,000th of a second then you could achieve similar results to E-TTL.
It's easy to see that the light output would be a function of how quickly you could disable the flash output, and not dependent in any way on the manual feature set provided by your flash.
Here is a good article with much more gory detail on Canon flash technology.
Hope that is helpful, -Brad
