[180pilot]

Old School, now playing with flash on new K-70:

Manual shows 12m (meters), Guide no. @ iso100, or about, 40 (ft), for onboard flash GN.

After some testing, I find using iso 400, the GN is a little less then 60, but is an easy number to use in my head. And, Iso 400 works well with my f2.5 135mm. Which I liked and used a lot for stand off portraits when on my film camera. I set flash power to, full.

So, at 20 ft., in Av, which locks shutter to 1/180, (flash sync), when flash is popped up, Guide No. divided by distance, 60/20 = f3 or f3.5

At 5 feet, 60/5 = 12 or f11. Very light or dark objects, of course require adjustment. Small f-stops are too dark for focus in dim light, so that's done wide open @ f2.5 then stop down for shot. Has it's limitations, when trying to pick instant for moving subject in dark viewfinder, so consider using, iso100, for close photos, or reduce flash power with larger f-stop, depending on depth of field effect you want. And/or keep enough ambient light on subject for focusing.

Using iso 100, and GN of 40, 40/10 ft. = f4. With this lens, I find about 15 feet distance needed for head shots, @ full power, so again using iso 400, 60/ 15 = f4. And,15 ft. works well with bashful subjects. But, I still have to grin with instant proofs, instead of the wait with film development.

[lotech]

That reminds me long time ago I tested the full manual output of the old school Metz 45CT3, I measured the GN with a Minolta flash meter @1m range and I got excess of 45 close to 46 that was amazing, where auto output tested with a gray card are also accurate, nowadays hardly any Japanese flash can produce the claimed GN, and most of them measured with 50mm lens (35mm format) instead of 35mm.