This thread and Matt's website should give you a general idea about the capabilities of the different types of currently available flash guns, that support the Pentax P-TTL flash control system:
https://www.pentaxforums.com/forums/camera-studio-accessories/43215-pentax-p-...omparison.html
For most flashes you will find the following basic numbers given:
GN = Guide Number = light output ("power") of the flash at a given ISO (sensor sensitivity, usually ISO 100, but some manufacturers cheat and use ISO 200, to make the number appear bigger) and at a given distance (either meters or feet).
The GN is of great practical value, to first anticipate the max. reach of the flash gun in any automatic mode and secondly, to calculate the correct aperture for a perfect exposure, when using a fully manual flash.
EXAMPLE: GN = 60 (metric). If you want to shoot an subject 5 meters away, just compute GN/distance = aperture, here 60/5 = 12 - This would be an extremely powerful flash and you would need to close down the aperture far (to f/12) to achieve a correct exposure at 5 meters distance.
GN = 30 (metric). Same shot, will give you 30/5 =6, which means an aperture setting of f/6 (or use the standard f-number of f/5.6).
This would be a fairly standard flash gun with a medium power.
What you see immediately:
– dividing the Guide Number through the shooting distance (use same measures for GN and distance, aka either feet or meters), will give you the required f-number
– doubling the the GN will allow you closing down the aperture two more f-stops
- halving the GN will force you to open the aperture by two more f-stops
THe Guide number is dependent on:
– real light output of the flash
- the setting of the flash reflector.
The flash reflector in older flash guns is usually a fixed thing and will illuminate the field of view of a 35mm lens on a 35mm film camera (roughly equivalent to using a 24mm lens on the Pentax DSLRs). If you use a wider lens, the wider field of view will not be fully illuminated, you get dark borders/corners.
Thus older flashes will give you a GN, which refers to a reflector with roughly 67 deg. illumination angle.
In newer flash guns, the reflector is often moveable and thus the illumination agnle will change, for instance, between 72 degs to 20 degs. What you recognize immediately: if the reflector is in its wide setting, the light will be distributed over a larger area and thus be less bright on each point . If the reflector is in its narrow setting, the illumination angle will be small and the brightness will increase. The manufacturers usually will not state the illumination angle, but refer to these angles as equivalent for lenses with a certain focal length. So you will find flash guns with reflector settings reaching from 28mm to 80mm or 24mm to 105mm, which are simply the focal lengthes they will optimally illuminate, when these lenses are used on a 35mm film camera.
That can be translated into Guide Numbers. Thus a flash gun with GN 30 at its widest setting may reach 45 at its longest setting. The GN varies with the illmuniation angle.
Cunning manufacturers nowadays state the GN for the longest setting and even well reputed manufacturers see the need to to so, too, because otherwise their flash guns will look less powerful.
So, when comparing Guide Numbers be careful and look, whether the numbers stated
– are given for the same ISO
– are given for a comparable illumination angle/reflector setting
Another parameter to look for in flash guns is the choice of operation modes:
– P-TTL = full flash control by the Pentax camera (equivalent to Canon E-TTL or Nikon iTTL)
– TTL = the older fully automatic flash control system for Pentax film cameras and only a handful of old Pentax DSRs - no current modell supports this old mode!
– "Auto" or Auto-thyristor mode. An automatic flash control mode, which the flash itself handles. Therfor it has a photo diode, which measures the flash output during the flash duration and stops the output, when there is enough light for the scene.
A very reliable mode with many flash guns and also available on many old flash guns of the pre-P-TTL era.
– "Manual" - the flash will fire off a "certain amount of light" without any control. YOu have to calculate the correct aperture setting from GN and distance (see above). SOme flash guns offer the possibilty to choose fractions of the full output, like 1/2, 1/4, 1/8 well down to 1/64 or even less. This can be important, if you shoot at very near distances or need to use a very wide aperture, when a higher output would lead to overexposure.
Shutter speed does not play any role for the exposure of the image, when using flash guns. It will only control the level of the ambient light. This is, because the flash duration is so short (between 1/600s and 1/30000s, the higher the output the longer and vice versa), that the flash must fire during the fully open shutter time. The shortest fully open shutter speed is usually the so-called (and as such on the program dial marked) x-synch time (1/180s for many Pentax cameras). Any shorter shutter speed will give you a black shadow in the image, as the shutter, while travelling over the image area will block the flash light somewhere. Longer expsoure times are fine, because at any time from the x-synch and longer, the shutter will completely free the image area for a long enough time to record the full flash with the sensor.
-- I know the shutter thing is very short and a bit confusing, but just have a look into the Wikipedia and you sure will find, how a shutter works and understand , why it can block the flash light at short shutter speeds).--
Anything else? Bevore buying and using an old flash gun, make sure, its trigger voltage is safe for the DSLR. Pentax gave for some cameras a limit of around 30volts, I prefer to stay below 12volts. You do not know, what the trigger voltage is: do a Search here and you'll find lots of threads with info.
If you have more specific questions now, keep on asking!
Ben
Originally posted by jolee1990 
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