Advice Needed on Setting Optical Slave Flash

<p>As I recall from my days of manual photography, a flash setting consists of ISO, Distance, Aperture, and Power: if you know any three you can calculate the fourth. Old-time manual flashes (cheap ones with only one power setting) included a grid giving you the Aperture required for combinations of ISO and Distance. But the Sunpak PF20XD slave flash I just purchased to use manually (no hot shoe) includes only what's pictured below and the manual is no help. Anyone have any idea how to set this thing (other than pure trial and error)? Thanks in advance:<br>

<img src="https://lh6.googleusercontent.com/-cLyoNK4K7eQ/UzXkxH74ApI/AAAAAAAAjCw/ONQDgYlL-fQ/s400/DSC_0166.JPG" alt="" /></p>

<p>You have all the info ;)<br>

The lower chart is manual guide number (power) for various ISO's and the equation is GN = distance x f-stop</p>

<p>So, you can can do the calculation. at ISO 100, rearrange the equation and solve for distance or f-stop. Ie-example: 20 / 10ft = f/2</p>

<p>thats a pretty low power unit</p>

<p>Howard's response explains the lower (manual) chart. The upper (auto) guide is to allow the flash to govern it's output via it's own internal (thyristor) meter. Eg if you a have the flash set at A2 power level, and are shooting at iso 200, set the lens at f5.6</p>

<blockquote>

<p>The lower chart is manual guide number (power) for various ISO's and the equation is GN = distance x f-stop.</p>

</blockquote>

<p>Thanks so much. Of course, the numbers are Guide Numbers, not feet. You'd think the "GN" would have clued me in, but alas. I appreciate your taking the time.</p>

<p>You are right that this is a low-powered flash. Key for me was that it was small and designed to work as a slave and it's the best I could find. I will use it almost exclusively for one purpose, outdoor travel portraits against bright scenery. I'll almost never be much more than 10 feet away, and the flash set to max and the lens open to f/2.0 may work fine, particularly because there will be some assist from the camera's own flash and from ambient light. The problem, though, it occurs to me, is that if I expose for a bright background scene, if the lens is open to f/2.0 the shutter speed may be too fast for the flash (maybe too fast, period). If so, I'll have to close the lens down some, step a bit closer, and hope for a better assist from ambient light (keeping the subjects out of deep shadow). We'll see. Thanks again.</p>

<blockquote>

<p>The upper (auto) guide is to allow the flash to govern it's output via it's own internal (thyristor) meter. Eg if you a have the flash set at A2 power level, and are shooting at iso 200, set the lens at f5.6.</p>

</blockquote>

<p> Thanks, Wayne. Not sure I follow, though. The slave flash is not attached to a camera in my setup (as the camera has no shoe or connection port). If the flash is going to fire at a fixed intensity, even at a set ISO and Aperture, won't the distance matter to the exposure? Does the unattached slave have and read its own pre-flash in time to sync with the shutter following the camera's pre-flash? If so, that would be nice, and more than I bargained for with this cheap unit. But I wonder.</p>

<p>Once you set it to A1, A2 or A3 your aperture is set and thus the <em>ideal</em> distance for exposure. Most importantly though you have to work with manual exposure with your camera (M mode), pre-flash from your camera most likely going to trigger this optical slave unit prematurely. Then you will not see any contribution from the unit. Other problem could be your shutter speed which could be higher that optical units maximum synching speed, this can be determined after doing few shots indoor. Hope that helps.</p>

<p>the flash does not have t be attached to the camera. <br>

<em>"</em><em>If the flash is going to fire at a fixed intensity, ....."; </em> well the intensity is not fixed. *Sorta....* The intensity actually is fixed, but the total effective output of the flash, ie the amount of light from the flash, is not. The flash adjusts it's output via duration. It has a meter in it that sees the amount of light reflected back to it from the subject, and when that amount of light reaches the prescribed amount, it shuts off. Pretty good technology that has been in use for a long time -eg. vivitar 283's (and others). All good but; <br>

when the field of view of the flash doesn't match the field of view of the camera, then *$%(@^! ; <br>

and it's subject to the same limitation as the camera's meter in your situation of shady faces against bright background -- overall exposure for the scene is not the right exposure for the faces. <br>

That's where the manual settings via guide number become useful. You can also vary the effect of the flash by holding the flash closer or farther away from the subject, or by altering the aperture/shutter settings of the camera, or by lessening the output of the flash by diffusing it, or adjusting the output of the flash by its power setting, or........, or,,,,, etc. <br>

Balancing the output of the flash to the ambient light is commonly referred to as fill flash, if you want to research it. <br>

For your mentioned application (fill flash of faces ~10' away against bright background), the camera flash might be adaquete, you might not need the slave flash. </p>

<blockquote>

<p>The flash adjusts it's output via duration. It has a meter in it that sees the amount of light reflected back to it from the subject, and when that amount of light reaches the prescribed amount, it shuts off.</p>

</blockquote>

<p>That's what I didn't realize until doing some more research since your earlier post. I was surprised that a $40 mini-flash like this has that metering capability.</p>

<blockquote>

<p>All good but when the field of view of the flash doesn't match the field of view of the camera, then it's subject to the same limitation as the camera's meter in your situation of shady faces against bright background -- overall exposure for the scene is not the right exposure for the faces. That's where the manual settings via guide number become useful.</p>

</blockquote>

<p> Makes sense, and I will use the GN approach for exactly this reason. My method is to use Aperture priority then lock the exposure for the background, then forcing the flash on. This method does not work when the background is too bright, either because the camera's own flash is underpowered or because it is fooled by all the light and does not fix the flash intensity based on foreground objects or point of focus. Either way, what I'm after is a stronger flash that I can set manually and I can now do this, though I'll need to experiment with how much the camera's own flash (which triggers the slave) adds to the exposure.</p>

<blockquote>

<p>For your mentioned application (fill flash of faces ~10' away against bright background), the camera flash might be adaquete, you might not need the slave flash.</p>

</blockquote>

<p>As I mention, I do need the slave flash, either for the extra power or, it now seems more likely, to fire at manually set intensity, which I can find no way to get the camera's flash to do. I've long thought there should be a fill-flash setting on compact cameras, or for that matter a Scene Mode for exactly the sort of shot I'm talking about here--analogous to the night-shot flash mode, which takes a long exposure that permits the background to be lit by ambient light, or the mode that changes between first and second curtain flash. (Many more consumers would want the flash to fully illuminate foreground subjects in bright back light, I'd wager, than care about the direction tail-light streaks appear.) I would have thought that all the mode I suggest would require is to instruct the TTL to fire based on ISO, Aperture, and focus distance, ignoring pre-flash feedback. Can that really be hard?</p>

<p> It may seem that going overboard with a desire to capture this one particular shot, but at least on my family vacations, these are the shots the images that the family most wants to look at when we get home. We search for beautiful bright vistas, but in mid-day unless there is some shade for the observers, there is no real photo opportunity because the subjects are squinting into the sunlight. When there is both shade for the observers and a bright vista, there is the perfect opportunity but with a pocket camera that has a flash ill equipped for the situation, the opportunity passes.</p>