Camera Adjusts to Slave Setting Mystery

<p>For travel, I use a Panasonic LF1 compact camera (an excellent, <em>small c</em>ompact). A frequent image I want to capture is of my family standing in the shade against a bright scenic background. So I purchased a small Sunpak optical slave flash; there is no hotshoe on the LF1. In testing the flash I noticed something remarkable. When the slave is set to low power the image capture is darker than if the same scene is captured by the camera's internal flash alone. I thought I was imagining this, so tested it many times.<br>

<br /> So, I get it that the slave may be firing a pre-flash pulse that the camera is reading (though even that surprises me), but what seems utterly impossible is that the camera knows that I've set the slave to low power. Put another way, I get it that the camera would adjust to the fact that the slave is firing and level the exposure to what would be captured without the slave. But how does the camera know that the slave is set to underexpose? I am baffled.</p>

<p>The low power setting of the slave is irrelevent. <br>

The camera fires a preflash of its own internal flash to measure exposure. The shutter is still closed at this point.<br>

The optical Sunpack optical slave flash sees the preflash, recognizes it as a command to fire, and fires. <br>

The camera meter sees the scene illuminated by this flash, and calculates exposure accordingly.<br>

Then, having calculated the exposure, the camera adjusts the aperture and shutter speed to this calculated exposure, and opens the shutter to make the exposure. But by this time the light from the optical flash is over and gone - the scene brightness has changed (dimmed). Thus the calculated exposure is incorrect for the brightess of the scene.<br>

Fixable? -- not easy. There are, or were (I believe) some third party optical slave triggers that can either ignore the first preflash from the camera, or be adjusted to wait a designated amount of time before actually triggering the flash. But it's a kludge solution. </p>

<p>Side note, how do you like the LF1? How's the auto-focus?</p>

<p>Thanks, Wayne. You are right I think about what's happening. I was fooled for a reason that I did not state when I originally posted (and why I'm not certain you are right). If I set the power of the slave to high the exposure is significantly brighter than when I illuminate using the camera's flash alone (though the image is not blown). The explanation I take it is that although the slave is firing on the pre-flash, when the slave is set to high its pulse is long enough to cover at least some of the exposure. Does that sound right? In any case, there is an easy solution. The Sunpak flash is designed for the very purpose I'm putting it, to work with a shoe-less compact. (Comes with a tripod socket bracket, in fact.) And it has an "ignore pre-flash" setting. I didn't use it because I thought that referred only to the anti-red-eye preflash, which I don't use. But that may have been my mistake. I'll change the slave's setting when I get home and report the results.</p>

<p>As for the LF1 it is a gem. It has a built in EVF and a large sensor for a compact (1/1.7) but is 25% lighter and 20% thinner than Panasonic's already small pocket travel zoom. The focal length range of the LF1 is limited, 28mm to 200mm (in 35mm film equiv.), and I do miss the wider end of other travel cameras (which go to 24mm), but I seldom miss reach above 200mm. The auto-focus is quick and accurate. And the camera is a steal, less than $300 now (though I paid much more) as Panasonic customers are all turning to its new 24mm to 720mm travel zoom, which is larger, as I mentioned. With the LF1, I now shoot RAW and batch convert using SilkyPix (Panasonic's native converter) and couldn't be happier with the results. Won't take it whale watching (because the zoom is too short), but I use it for everything else even though I own other, better cameras. It's hard to describe just how small the LF1 is and how it disappears into a pocket. Feels like cheating to have so much image quality in a camera smaller than my cell phone.</p>

<p>You called it, by the way. Changing the setting on the slave solves the problem, and the mystery.</p>