To me that is a contradiction.
A electrical version of distance-based flash autoexposure was devised by Canon. As it does not control the aperture, but the charging voltage at a dedicated flash, and thus its light output. There is no need for interference at the diaphragm apparatus. Instead only a holding device at the filterring and and a stud at the fucusing barrel is needed. You then add a ring to the front of the lens which is coupled to that stud. Basically one could use the same ring on all lenses of same lens-speeded and same barrel-rotation/distance ratio. The ring then controls a potentiometer depending on the barrel twisting and thus on the distance the lens is focused on.
I was not implying that Canon aTTL controlled the aperture! Yes, it was varied flash output, as all TTL flashes ever have been. As I do not have direct experience with aTTL, I only related what I had read more than a decade about it incorporating distance and that method failing.
I just took the lazy way out, and here is the Wikipedia explanation of Canon aTTL
"
Advanced-Through The Lens (
A-TTL) is a
through-the-lens (TTL) metering system that was expanded to support flashes. A sensor inside the camera reads the amount of light being reflected off the film during the exposure. When the sensor determines that the exposure is sufficient, it terminates the flash pulse. A-TTL, first seen on the
T90 (which predates the EOS family), is a flash exposure system that adds a brief preflash during exposure metering when the camera is in the programmed exposure (P) mode. The amount of light returned from this preflash is used to determine an appropriate tradeoff between aperture and shutter speed in P mode. Depending on the specific flash unit and whether or not it is set to bounce, the preflash might be infrared or it might be the normal white flash. In an A-TTL system the sensor that reads the preflash return is located on the flash unit; this caused some issues especially when using filters as the filter would cover the lens (but not the sensor outside the lens) thus causing inaccurate settings. Some early Canon EOS cameras also used the A-TTL preflash in non-programmed exposure modes to detect "out of range" conditions; the "out of range" warning feature was dropped on later models, reportedly due to patent conflicts."
Now in spite of my prior NOT understanding aTTL another article states:
https://www.cameraexperts.us/canon-eos-flash-photography/attl-advanced-ttl.html
"The original purpose of the A-TTL preflash in those modes (Av, Tv, M) was to provide information to the flash out of range warning light in early EOS cameras... Another use of Distance was that the camera sets the smaller of the two apertures (P mode auto aperture selection for ambient vs. aperture for flash), particularly if the distance to the subject is determined to be fairly close...A-TTL simply ends up setting a pretty small aperture most of the time, to assure wide depth of field, which isn't always what you want."
And then the Canon 430EZ (ATTL flash) manual states:
- "If the camera was set to FULL AUTO (green) or "P" mode, the A-TTL program selects an aperture determined by the preflash metering, but also affected by the ambient light meter reading, whichever was the smaller. In general, the program is biased towards maximizing Depth of Field, rather than balancing background and foreground exposure. As a result, photographs shot in A-TTL tend to be well focussed, but with dark backgrounds. If the camera is set to Av, Tv, or M, the A-TTL program"s
Having just found the above descriptions, and with no direct experience of my own with aTTL, I am now as confused as you!
In distance-based flash autoexposure you got a manual flash and a flash-autoexposure shutter. At that shutter you set the film-speed of your film and the GN of your flash. That is all. From then on the shutter contols automatically the aperture based on your focusing.
A plain mechanical thing. Copal leaf shutters come to my mind with this feature.
The fact that it was in a leaf shutter lens makes it somewhat more understandable that 'shutter controls automatically the aperture based on your focusing.' And the flash was simply fixed output.But I cannot imagine any electronically controlled Copal. Even a Seiko shutter has electronic regulated shutter speed, but a mechnically dialed in aperture selection.
The advantage of a distance based flash autoexposure I described in one my posts above.[/QUOTE]