accounting for ambient light for multi pop flashesgreenspun.com : LUSENET : Large format photography : One Thread
When shooting LF with multiple pops of a flash, it is easy to determine with a spot flash meter, in multi mode, exactly how many pops of flash it takes to illuminate the subject sufficeintly for a given f stop. To prevent picture blur, I prefer to leave the lens open throughout all the pops. With recharge time of 7 seconds per flash, and 5 pops, that is an additional 27 seconds of ambient light the film will see not accounted for in the flash meter readings. How does one calculate the amount to cut back the multiple pops to account for all the ambient light hitting the film? Is there some value that can be assigned for an ambient EV reading of the existing light x the seconds open?
I realize I can use a black cloth over the lens between pops, but I prefer to leave the lens open. Thank you all in advance.
-- Bill Glickman (email@example.com), February 11, 2001
Bill, I've done a few multi-pop exposures...and left the lens open. What I did, after figuring the number of pops needed for a specific f stop, was to meter the ambient light and determine the exposure time for the ambient light at that f stop. If the time I would need to get in all the pops was more than a stop under that which I was trying for... (e.g. if I needed a 30 second open shutter for f45 to get all my flashes in, but the metered exposure indicated, say 60 seconds at f45)I figured ambient impact was minimal and ignored it. I'm sure some would say, for critical color balance, if the ambient light was within a stop, that you should consider color balancing either the flash or ambient sources so that even the small impact wouldn't cause unwanted color balance problems. Most of my work has been in B&W so a little 'ambient' wasn't a big deal. Fred
-- Fred Leif (firstname.lastname@example.org), February 11, 2001.
Some flash meters have a mode that will read the ambient as well as the flash. You might want to see if the instructions for your flash mention that.
-- Ken Burns (email@example.com), February 11, 2001.
Get flashes that recycle faster.
-- Ellis Vener (firstname.lastname@example.org), February 12, 2001.
I usually do multi-pop shots with my 72mm or 90mm in Prontor shutter so that I can assign a shutter speed that, when multiplied by the number of pops, will end up the right exposure for ambient light, ie. ambient might be f/32 at 8 seconds and I need 16 pops, so I would do 1/2 sec. x 16 pops at f/32. But this does not answer your question. If I am using a lens without Prontor shutter, I do a subject brightness range calculation between the ambient light (like that outside of a window) and the flash exposure at full exposure (all pops combined) and process accordingly (b&w only, of course). With color, the only solutions that I know of are use of a self-cocking shutter, covering the lens as you stated and risking vibration, getting faster recycling, as Ellis suggested, or adding flash power, and lots of it.
-- Rob Tucher (email@example.com), February 12, 2001.
1. shoot at night 2. turn the lights off 3. if 1 and 2 aren't possible, shoot polaroids until it looks right and bracket your exposure (after you have taken ellis' advice and) 4. get flashes with a shorter recycle 5. a minolta V meter will be helpful
-- adam friedberg (firstname.lastname@example.org), February 12, 2001.
If you meter the ambient light on the brightest object and keep its exposure (f/stop & time) below a Zone I exposure, the ambient light will have virtually no effect.
Check out Ansel Adams's The Negative under Pre-exposure to understand the concept, though he didn't relate it to flash.
-- Charlie Strack (email@example.com), February 12, 2001.
Another thing you can do is while the flash is cycling up, cover the lens with a black sheet of mount board so it cuts down on the ambient collection. The same practice works well when shooting things like fireworks between the pops...
-- Scott Walton (firstname.lastname@example.org), February 13, 2001.
A dark baseball cap works well for this also...Seriously though, we shoot almost all of our studio stuff based around multiple pops. On location though, if for whatever reason we have to do multiple pops, we usually figure out just how many we need to do, meter the ambient at our desired f-stop, time how long it's going to take our wimpy Speedotrons to recycle and just try to "beat" them with the exposure. This is where Polaroid is really handy. If we can't beat the ambient, then we'll cover the lens with whatever we can find (a ballcap, rosco foil, whatever) but make sure you don't actually touch (shake) it while the strobes recycle. Sometimes, even if it means having to gel the ambient sources, having this light leak into the shot will even work in your favor. We shoot mainly b&w/color transp. on everything and haven't suffered too terribly on location this way.
-- DK Thompson (email@example.com), February 13, 2001.