Vivitar 3500 Thyristor Flash

I have two Vivitar 3500 flashes (one is a 35-D, but they are identical as far as I can tell). For the longest time I could not figure out how these things actually worked. There are four settings on back; red, blue, orange, and manual. From the 'calculator' up top you can tell the three color settings are two stops apart, and manual is full power. But until recently I did not know that there is a photo resistor up front which actually adjusts the power of the flash based on the amount of light the sensor sees. The more less light, the more resistance, and the stronger the flash. Thanks to this discussion on the Strobist Flickr discussion group I was able to get a better understanding on how this flash works. I decided to set up an experiment to see how this sensor works in practice. This will apply to all Vivitar thyristor flashes.

My next plan is to remove the photo resistor and replace it with a potentiometer giving the flash full manual control. I'll post a write up once that's done.

The idea here was to test all four settings with ambient light, a headlamp aiming at the sensor, and the sensor taped over with three pieces of electrical tape. The headlamp idea didn't work since it didn't produce enough light, and the pictures actually came out brighter since the headlamp was blocking the reflected light from the flash. I decided a better way was to take my second flash, snoot it, and aim it directly at the sensor.

All pictures were taken at 1/160s, F6.3, and ISO 200. The one thing I forgot to set manually was white balance, but I'm not going to normalize them in Lightroom since the differences are very slight. I took three pictures of each shot and used JPEGs straight from my Canon 20D. All the pictures are below. Simply open them in a different window or download them and compare them with a program like Lightroom or FastStone.

As expected all 3 tests on manual produced identical results. The settings function as expected too, with red being the weakest and manual the strongest. Even with the sensor covered you can see this pattern is followed, although it is very slight and you really have to stare at the images to notice. The red setting with a covered sensor is much stronger than the orange setting with ambient light and a flash hitting the sensor since the sensor itself governs when it comes to flash output.

The one deviation from what I would expect is comparing the ambient and flash tests with the red setting. The flash test is slightly brighter than the ambient test. This happened for all three shots of each. Only thing I can think of is that the second flash is adding light to the shot. The blue and orange settings work as they should and the ambient shots are brighter.

Even though this flash doesn't have a full manual mode making it less than ideal for off camera flash, it still provides some power adjustment and isn't so bad to start out with. My hope is replacing the photo resistor with a potentiometer will make it much more useful.

Red with ambient

Blue with ambient

Orange with ambient

Manual with ambient

Red with flash

Blue with flash

Orange with flash

Manual with flash

Red with tape

Blue with tape

Orange with tape

Manual with tape

more photography on my blog

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