Paul C. Buff, Inc. Technical Forum

A couple of things to consider. Is there a reason you need to shoot in midday sun? If not, then shooting at dusk may be a better option, as you bring the intensity of the sun down, without bringing down the flash. You also get less specular highlights on the water. Overcast days would be similar.

A custom white balance would be recommended with the filter, and it should not affect exposure greatly. However, if it were me, I would do an actual custom profile when using the filter. Kelvin/color temperature is just one part of color rendition. Adjusting the color temperature will definitely help, but some colors may get muted or shifted differently than others.

Your daylight meter readings sound reasonable. However, you say you are placing the filter over the flash rather than over the meter. If you are in a dark room, this is fine. If you metered outside, or in brighter light, the meter will see unrestricted ambient, and only restricted flash. This can cause a discrepancy in your calculations.

What is your target aperture and target shutter speed? Also, what light and modifier are you using?

I put the ND filter over the flash "later" when I found the flash wasn't filling in well and maybe mid-way into this post. That's how I came up with the GN=20 or so. I was using a GN=80 prior (Way too dark.).

Ideally, I'd like minimum of 1 second @ f/5.6 since that seems the sharpest spot on my lens and water seems to look best around there without going still and sparkly. The current flash power seems to be holding me back with the low GN=20 for now.

I do have the ability to maybe stack 4 PCB Ultras (Two 1200, Two 1800) but don't know how I can come up with a working GN since I can only read one at a time with the meter with the ND in front. I don't think it would be GN=20 times 4 or GN=~80? (Example: Two 1200's = 2x20=40 and maybe the 1800's would by GN=25 or the pair at GN=50? Add the two pair GN=90?). I seem to recall 4 strobes (Same power & distance) make for one stop more exposure, but don't remember.

The mod is the 86" PCM (as a shoot through) with the silver backing cover. Would be nice to get it back to around 8-10 feet but seems my gear isn't allowing me to get there. I'm measuring at the front of the PCM umbrella too since the flash is buried inside.

Some cave shooter gave me a couple of old Sylvania #33 bulbs which are the exact same thing as the Meggaflash #330 2-second burn bulbs, just how to come up with a working GN without going broke. He said they just did open flash on bulb with them in the caves, but still a bit of guesswork. Might be fun just to watch them go off to see what they do, but don't know if I can meter them right with their long burn time.

Mack

It seems the "sunny 16 rule" matches with what you are getting with the meter in the sun. So, as a start, 1/125 f/16 ISO100.

1/125 to 1s is 7 stops; So, your flash needs to meter to match the new ambient setting, f/256 @ 1s. You are at f/8 with the current set up, so that is 9 stops. Using four heads will gain you about 2 stops. An extreme silver PLM will gain you about 4 stops (this may be a little off as focusing will change a bit when 4 lights are ganged). This is 6 of your 9 stops.

Moving from 8' to 4' will get you two more stops, or to 8 of your 9 stops. This will give a 2:1 ratio in lighting with ambient.

Moving the light to about 3 feet, or sacrificing and using a 1/2 shutter speed will close the extra stop of difference.

Now you have your ratio set, you can use a 10 stop ND filter to bring all down from f/256 to f/5.6.

To be honest, I think this is exceeding the limits of your equipment, and pushing (at best) the limits of the equipment suggested. Even the most powerful light we have made, A Zeus Bi-tube head and 2 2500Ws power packs, would only provide one more stop than the four lights you have (give or take).

I would reiterate the idea of waiting until dusk, finding a shady area, or shoot on an overcast day.