Any Help WIth A New Flash?

McGregNi wrote in post #17880531
Well the 'range' indication is the flash telling us what is the maximum and minimum distance that it can correctly expose a subject given the ISO, Aperture and Zoom settings .... these settings are the user telling it what the camera has set. In 'Auto' Wireless mode there is no communication of this information at all between the camera and flash .... it has to be manually input.

In Manual Wireless mode then the indication changes to a single distance unit .... eg '4m', not a range, and this takes account also of the actual manual power setting on the flash (1/16, 1/64 etc). So the indications are responding exactly to the ISO, Aperture and Zoom inputs from the user.

Of course, in Auto mode (which uses the light sensor on the flash to judge the final output based on reflected light) it is up to the user to either set the exact same settings on the camera, or 'trick' the flash into outputting a different amount of light in order to control the exposure as needed.

I just saw your post come in msowsun .... I think Peterson uses it for practical flash and subject placement, or power tweaks if the distances are fixed. I accept though that someone has to find a particular workflow approach to build it into to make it helpful, but if you can do that then it is a very accurate and efficient way to get your flash exposures spot on first time.

I was just really making the point that I find it interesting that my 'auto-thryster' mode on my Pentax unit can still give the range and distance indications even off-camera .... precisely because the calculations are performed by the flash based on the directly input settings, and do not depend on any camera to flash communication.

They aren't giving you distance indicators. They are giving you limits. Of course that doesn't change on or off camera.

msowsun wrote in post #17880564
The reason your Pentax can do it is because in addition to TTL metering it also has external flash auto-thryster sensor mode. The Canon 680EX II and 600EX-RT also have an external flash auto-thryster mode in addition to the modern TTL auto flash.

All older film era auto flash units worked this way and most had some sort of an ISO/aperture/distance scale.

I wasn't sure about the Canons, but I had assumed the Pentax design was similar to comparable Canon models, so thanks for confirming that. The AF-540FGZ is now a discontinued model which was updated in 2013 with a new design (and two GN options '360FGZII / 540FGZII'), and these have lost the auto sensor to be replaced with a front LED light !! This serves as a continuous light for video, a secondary flash and catchlight emitter. Most users report that they miss the old Auto Mode.

I went back to re-read the OP, and I think a bit of 'primer' on flash might help other flash novices with a general understanding Manual and Auto photosensor modes....

1. At a given FL setting (establishing Angle of coverage) and ISO, at Full power the light can reach out to a certain maximum distance

   a) in Manual, the user has to depend upon Guide Number arithmetic to help computed what f/stop to use, based on flash-to-subject distance
   b) in Auto mode, the user predetermins the f/stop and then the flash (if it knows the f/stop) uses fractional light output to provide the 'proper exposure' as measured by the photosensor

2. The number of levels of fractional power control of the flash determines the Minimum flash distance within its range of distances

   a) if a flash has only 4 levels of control, its minimum is 1/8 power, so if the flash could reach 40' Maximum, it can reduce power to have 'proper exposure' to a 5' Minimum distance
   b) if a flash has 8 levels of control, its minimum is 1/128 power, so if the flash could reach 40' Maximum, it can reduce power to have 'proper exposure' to (about) 0.3'
   c) some flash units have Auto power control to achieve a lower level than the minimum Manual fractional power setting, e.g. M/128 (8 levels) vs. Auto with 9 levels of control (1/256).

• At 50mm coverage, the 580EX has Guide Number = 138 at ISO 100, its flash scale display will show a 69' Max at f/2, or show 8.6' Max at f/16
  
• at ISO 400 its Guide Number changes to GN=276, the flash scale display will show a 138' Max at f/2, or show 17' Max at f/16
  
• at ISO 1600 its Guide Number changes to GN=552. the flash scale display will show a 276' Max at f/2, or show 34.5' Max at f/16
  
• the Minimum distances are computed and displayed, based upon the flash's ability to control to 1/256 of full power output
  

gonzogolf wrote in post #17880570
They aren't giving you distance indicators. They are giving you limits. Of course that doesn't change on or off camera.

I'm using the term 'range' to mean the limits .... everything within this range should be exposed correctly. A range indication applies to an automatic exposure mode (eg TTL & 'Auto-Thyrister'). The other display indication is a single distance, and that applies to Manual mode where the power is also fixed by the user, therefore giving one single distance where correct exposure would occur.

The main point I was making about Auto mode was its ability to offer the 'range' indication even off-camera ..... neither TTL or manual can do this because they depend on the communication through the hot-shoe to 'tell' the flash the ISO and aperture that is set on the camera. Auto mode does not need that communication as the figures are given to the flash by the user. The exception for TTL and manual off-camera flash would be when using a TTL cable connecting the flash and camera, in which case the flash 'thinks' its on the camera anyway...... but these days the cool thing is all about 'wireless' isn't it?

Wilt, Nice explanation thanks! I'd just add that with modern TTL units, like the type that Bryan Peterson shows in his book, and my Pentax one I've illustrated, that the manual mode arithmatic you mention is carried out by the flash unit itself, and the results displayed for us on the flash panel ..... so your distance figure automatically updates as you change the camera ISO or aperture setting, or adjust the flash head zoom. That is the convenience of the modern 'connected' flashgun type, it acts as your personal calculator in real time.

McGregNi wrote in post #17880617
Wilt, Nice explanation thanks! I'd just add that with modern TTL units, like the type that Bryan Peterson shows in his book, and my Pentax one I've illustrated, that the manual mode arithmatic you mention is carried out by the flash unit itself, and the results displayed for us on the flash panel ..... so your distance figure automatically updates as you change the camera ISO or aperture setting, or adjust the flash head zoom. That is the convenience of the modern 'connected' flashgun type, it acts as your personal calculator in real time.

^ I believe that capability is reflected within the past paragaph of Post 93.

Yes of course, you did cover it. Sounds like the OP may now have been persuaded of the merits of a radio manual system however .....

I always feel that this choice is very dependent on specific intended flash photography situations, as both TTL and manual are suited to different ways of working.