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10-13-2011, 12:04 PM   #1
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Simple flash question

How much does flash compensate for? If my meter reading says I'm 1.5 stops underexposed and I turn on the flash, does the system know to compensate for exactly 1.5 stops of underexposure?

10-13-2011, 12:25 PM   #2
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In the old days there was a sensor in the camera that measured the exposure in real-time (TTL), but now, everything is pre-computed and the settings are adjusted to obtain the proper exposures based on that.

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10-13-2011, 05:20 PM   #3

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I assume that you are referring to the built-in flash. When you turn on the built-in flash, it more or less assumes that IT is the primary light source and ignores ambient light. IOW, it doesn't try to figure out how much light to add to the ambient; it simply calculates how much light IT needs to provide to give a proper exposure.

On current Pentax dslrs, this is done by triggering a low-power pre-flash, before the real exposure, with the mirror still down. Normally, the lag between the pre-flash and the exposure flash is so short that you can't perceive it and you only see one flash. Anyway, the light sensors in the viewfinder read the light returning from the subject and determine who much light is needed. Then, the camera flips the mirror up, opens the shutter and fires the flash. Based on what was calculated during the pre-flash, the main flash duration is cut off after enough light has bounced off the subject.

If you are only 1.5 stops underexposed, be careful using flash. That is close enough that you might get a "ghost" image. This is caused when the ambient light is almost enough to give a proper exposure. The flash provides the main exposure, but the ambient may be enough to cause the ghost image.

To avoid a ghost image, try using a faster shutter speed. The duration of the flash is only about 1/1,200 second at its longest. At its shortest, it can be as brief as 1/30,000 second. A pentax dslr will trigger the flash at any shutter speed up to 1/180 second. In basic flash photography, the shutter speed is pretty much irrelevant, as far as the exposure is concerned, as long as you stay under the fastest x-sync speed (1/180).

If your ambient exposure was 1/15 at f/3.5 (which is why you wanted flash), you can crank the shutter up to 1/180 and the flash will provide all the light you need. At that shutter speed, the ambient light will be so underexposed that there is no chance of a ghost image.

Am I making sense?

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