dedicated flashes and shutter speed

I have used Vivitar 550FD which is fully dedicated flash unit for A-series camera (at least for my AE-1P). It sets aperture and shutter speed. I guess other Canon made dedicated flashes do the same thing.

My question is whether you can override these autosetting without losing 'auto' mode. For example use a shutter speed longer than 1/60 to record the rest of the scene. (I belive that just by putting a manual aperture instead of 'A' you can override aperture.)

Another question is whether you can play with strobe intensity with other flash units like Vivitar 283 for fill-flash,etc and still use 'auto' function. (I saw a little knob looking thing on top of which the sensor is mounted.)

Some Vivitar flashes have various ranges for the f stop that you can

use in auto mode. If you change to a manual output, like on the 283,

the flash puts out that amount of power, no matter what. So no auto.

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WRT shutter speed, try it. My A-1 with 199A has a switch on the 199A

to allow slow speed sync. When I set that switch, I have control over

shutter speed. With the switch in the standard position, no matter

what I do with the shutter speed dial, it is set for 1/60. With the

A-1 and 199A the same is true with aperture. In auto mode, you don't

have control, unless you set the aperture manually.