Thyristors and How They Work?

Remember the moron's question about vacuume (Thermos) bottles - which keep hot stuff hot and cold stuff cold, and Mac asks "How does it know?"?

Well here I come again.

When a Thyristor non-dedicated flash is set in say the 3ft to 23ft range, with a fixed f stop of 2.8, I imagine the little photo cell in the front of the unit is constantly measuring ambient light and adjusting the speed of the flash's duration to compensate and all that, but how the heck does it know if I'm 3 feet from my subject or 23 feet? On my flash (Sears 987) at 100ASA, that is an f stop difference between f22 and f2.8 if I'm in manual mode!

Kim Coxon mentioned in an earlier post that I'm enjoying discovering the world of Spotmatics.

You bet Kim! THAT's even more than I can comprehend! One step up in flash technology beyond guide numbers, and I'm flummoxed!

I think Darwin would be embarassed.

Cheers

Hi Mac,
The sensor doesn't measure ambient light but integrates the light bounced back when the flash goes off. Once sufficient light is reflected for the correct exposure, it switches the flash off. The closer you are, the greater the concentration of the "returned" light. If you double the distance, you need 4 times the light as it has to cover 4 times the area.

The early guns work slightly differently than the later thyrister ones in that they grounded the capacitor when there was enough light. This "wasted" the extra charge so the guns had to have a full charge each time. With thyristers, they switch it off thus conserving the remaining charge.

Kim

Kim,

Thank's!

So that's why we don't need all kinds of extra calculations for bounce etc.! (As long as the total distance from flash to subject is within the selected range.)

I guess it also adjusts for the light absorbancy of the subject - sort of an after-flash instant coupled light meter within the flash?

Would the field of view of the flash's light sensor be very narrow (like a spot meter) or broad and average like the CDS in a Spotmatic?

Gee, all things being equal, using the flash in auto mode may be better than taking the time and effort to set up in Manual?

Hi Mac,
The OTF in the LX and Super A work very well because it measures the light in the camera. Normal guns are a bit like the meter in the Spot. They are very good for "average" subjects but can be fooled by very light or very dark subjects. The best way round this is to alter the F stop set to compensate in the same way you would slightly overexpose a very light subject such a light coloured wall and vice versa. As with all kit, it is worth having a play and doing various test exposures whilst recording the settings and then compare the results with those settings until you build up a "feel" for how your kit works in different conditions.

Kim

Thank you Kim!

This has been a super day! And I now have more neat things to try as I test out my new Macro and Tele lenses.

I think I'm single-handedly trying to keep Fuji and Kodak shares from falling - to say nothing of my local pharmacey's film processing. The clerks have the envelope filled-out before I even reach the counter.

I wish I could find an outfit like the old Canadian Photocolour, where I could shoot slides and just get then developed in strips. (I toss about 99% of my shots while I'm familiarizing myself with new gear).

Or I'll switch to B&W, and ask Don about coffee and vitamin C developing.

Oy!

Hi,
E6 processing of slides is relatively simple!

Kim

Kay,

Kim - gonna open a new thread.