How thyristor based flash works .

[dvdowns]

I have heard mention that thyristor based flash is superior to ttl flash and did a search to try to work out why . I came up with this link http://www.chem.helsinki.fi/~toomas/...q.html#working .
It mentions that the thyristor is the device that controls the flash output based on a return signal it gets from a photosensor on front of the flash , the main difference between the two being that the ttl works using the cameras light metering while the thyristor flash uses its own sensor .
The thyristor is the bit that cuts off the discharge of the capacitor , without wasting the power by shorting it out like older units used to , after it decides enough light has been reflected back to the photosensor . I would imagine then that it would be similar to "average " light metering in the camera in that it does not base its calculations on the centre of the picture but rather on whatever reflects back to it , eliminating the many links between the ttl flash and the subject ie: UV filter , lens , mirror , prism etc. or whatever else is between the subject and the cameras meter . I just wonder still how much the ambient light affects it reading since they fire even in sunlight ? Does anyone know the answer to that ?

[Wilt]

You confuse the 'light metering' of a flash with the 'power recycling'. Any photosensor does 'light metering' to tell the flash to turn off, and you are right that the flash sensor is usually simple averaging. Thyriston does 'power recycling' and recycles the surplus unused charge back into the storage capacitor.

As for the question of affect of ambient on flash sensor, that is truly dependent upon the shutter speed in use but few flash units know the shutter speed of the camera when they are not integrated into the camera's electronics. Presumably most flash units not dedicated to a particular brand will assume that a focal plane shutter with 1/60 sec. X-synch speed is in use, and would assume that in the calculation of 'enough light'. The newer flash units like used with Nikon iTTL or Canon ETTL are aware of the shutter speed in the camera.

[dvdowns]

I fully understood the light metering and power recycling , maybe I just never expressed it properly . If a shutter speed is less than the maximum synch speed of the flash then surely it makes no difference except for background exposure and a slight bit of extra exposure on the subject for slower speeds ?

[blumesan]

dv

The duration of a flash ranges from 1/1000 sec to 1/10,000 sec and the sensor built into the flash monitors and evaluates the light only during this period.* So you can see that the amount of ambient light will have little or no effect on the flash output.* Further, and assuming we are not talking about a dedicated flash either on or off camera. the light sensing mechanism in the flash needs to have some information about the aperture and the film speed in order to determine when sufficient flash output has been reached.* There are various settings on such flashes to enter these data, and usually some type of calculator to determine the effective distance range of the flash.* Shutter speed plays no role whatever in the flash exposure but, as you point out, speeds longer than the x-sync maximum can have a significant effect on the exposure of objects outside of the flash range.

Cheers/Mike

[Wilt]

<< Shutter speed plays no role whatever in the flash exposure but, as you point out, speeds longer than the x-sync maximum can have a significant effect on the exposure of objects outside of the flash range.>>

Well sort of. Many handheld flash meters now permit you to set the shutter speed into the flash unit when using it as a flash meter...for example 1/60 or 1/250. This need is caused by the fact that the amount of ambient light does matter, and is additive to what the flash produces, so that the meter needs to know if it should assume a relative slow shutter speed or a higher one in the measurement of how much/little light is contributed by the ambient lighting. Back about 30 years ago, a flash meter might assume that 1/60 was always the X-synch speed in use, and flash units still might also make this assumption!

[dvdowns]

That's the annoying thing about most cameras when you switch on flash they jump to 1/60th , then you have to find the slow-synch mode .
A flash with a speed setting ? That sounds interesting !

[blumesan]

Originally Posted by Wilt

*............. This need is caused by the fact that the amount of ambient light does matter, and is additive to what the flash produces,.........

Yes, this is certainly true and especially so in the case where the flash is used as fill and the ambient light is the principal illuminant. However I have never seen a NON-dedicated flash unit that provides for shutter speed settings. This is the main reason why dedicated flash units with TTL exposure control are much easier to use for situations requiring fill flash. With non-dedicated units the only option is to use the manual mode, setting the flash power directly based on experience or chimping.