multiple strobe pops

[Thom Bennett]

So, I'm photographing a painting and, apparently, I figured my exposure, via multiple strobe pops, incorrectly. The method I used was that after the initial pop (f8 through polarizing gels and compensating for the polarizing filter on the lens) I hit the strobes two more times for each additional f-stop i.e. two more hits would give me f11, two more f16, etc. until I got to f32. I even "overexposed" by additional pops but my chromes were under by at least a stop. Any advice on accurately figuring stuff like this? Now I have the experience of seeing the chrome and going from there but I know there must be a better way.

[Jim Michael]

If 1 pop is normal, then 2 pops for 1 stop, 4 pops for 2 stops, ... 2^n pops for n stops.

[BrianShaw]

Counting pops only works (as far as I know) if the strobe is putting out the same output and at the same distance every time. So if it is a TTL system on SLR, etc. the strobe must be used in "manual" rather than TTL and must be fully or equally 'recharged' before every pop.

My preferred way is to verify Jim's general technique with a flash meter that will integrate multiple pops, like Sekonic L-558.

[IanG]

It's not a straight "2^n pops for n stops", you need to add additional flashes with multiples exposures.

So while 1 stop is 2 flashes, 2 stops is more like 5 flashes, 3 stops 11 or 12 flashes and so on. The same is true with any method of multiple exposures.

Ian

[BrianShaw]

My preferred way is to verify Ian's general technique with a flash meter that will integrate multiple pops, like Sekonic L-558.

[Jim Michael]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?

Re Brian's comment on the light output, I would assume the flash is fully recharged between pops.

[Thom Bennett]

Thanks for the replies thus far. I'm using a Dynalite 1000K pack w/two heads in a darkened room and I am keeping the shutter open and, with the modeling lights off, popping the strobes a few seconds after the pack recycles. It seems as though my methodology was correct but, as IanG states, real world results demand a more "adjusted" approach. For my 4 stops I should be popping ~15 times rather than the 12 I was using as a base. From the look of the chrome I got back that would be about right. Probably will go to 18 pops as well.

[IanG]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?


It's a well known phenomena and borne out by many peoples experiences, including my own. There's been long threads about this on APUG.

It can be seen easily when printing, a test step of 2 second steps up to say 32 seconds (so the 2 sec is 16x individual 2 secs exposures) doesn't give the same density as one continuous exposure of 32 secs. It's to do with threshold energies.

Ian

[Vlad Soare]

Ian, why do you say that? I don't think we are necessarily in the realm of reciprocity failure. How is this different from a constant source of the same quantity of light, e.g. 30 ms of 100 units vs. 2 x 15 ms of 100 units vs. 10 x 3 ms of 100 units?
This is known as intermittency effect.
While in practice we tend not to regard it as reciprocity failure per se, technically speaking it is a form of short exposure reciprocity failure.

Because it's most often experienced in the darkroom, some people blame this on the amount of time the enlarger's light bulb takes to reach its full output. This is wrong. It's actually caused by the way the emulsion reacts to light, and not by the characteristics of the light itself (though I guess the light bulb may contribute to it, too).

[David R Munson]

Testing is necessary to determine how many extra pops will be needed for correct exposure with a given film and exposure range. The shot below was done on Provia. Eight pops on the strobes to get the exposure called for by my exposure meter. No extra pops provided for intermittency effect, and the exposure of the transparency is spot-on. Others experience will differ from mine, of course.

http://farm1.static.flickr.com/24/36322134_6a88a69044_o.jpg (http://www.flickr.com/photos/davidrmunson/36322134/)
Soy (http://www.flickr.com/photos/davidrmunson/36322134/) by David R Munson (http://www.flickr.com/people/davidrmunson/), on Flickr

[Thom Bennett]

O.K. this is getting interesting. A quick search pulled this up:

intermittency effect [‚in·tər′mit·ən·sē i‚fekt]
(graphic arts)
A reduction in the density of a photographic film when the exposing light is interrupted at a very high frequency, even though the total light exposure is held constant.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

I understand it, in principle, as IanG and Vlad have explained it in relation to the darkroom. The next question is how does one mathematically figure out the effect on film. Not a burning question as I think experience will tell the tale.

[Kirk Gittings]

I have been using multiple pops on color films to light architectural interiors for 32 years and teaching the same at my university classes for 20+ years. I routinely use 4 pops to get the aperture I want with enough DOF but often go to 16. I have never noticed this "intermittency effect". I also bracket by varying the multiple pops.

I have also done landscape and water shots on b&w with the same technique for shooting landscapes with rapidly moving clouds or water for a staccato effect. I've never noticed this effect there either.

If it exists, it is minut and not worth worrying about.

[Ole Tjugen]

I've ended up with a strange formula - for n stops, use 2^n + (n-1) pops. Or shutter triggerings. Seems to work for me...

So one stop, one extra pop, two in total: 2^1 + (1-1).
Two stops - five pops total: 2^2 + (2-1)
three stops - ten pops: 2^3 + (3-1)
four stops - nineteen pops: 2^4 + (4-1)

And so on..

[Jim Michael]

WRT darkroom and paper exposure, at least one article I read seems to confuse the integral of the intensity curve over multiple incandescent exposures with this effect. There was a paper published in 1927 which I read the abstract and described the effect somewhat, but that seemed to be associated with brief interruptions of light, if I read that right. I've heard of reciprocity failure occurring for very short flash exposures, so would tend to attribute an inconsistency to that, and don't discount anyone's experiences.

[Brian K]

I have to agree with Kirk, in real world usage I have never noticed any sort of failure using the doubling the pops = 1 stop increase method. The intermittency issues might be with very high speed flashes but most studio strobes seem to fire less than 1200th of a second and that seems to have no problems. If you start to require a huge amount of pops though, then I think that there could be an issue with breaking the inertia of the emulsion, that is the exposures are so insignificant as to create a reciprocity like failure.

I'm curious though as to why you require polarizers? Reflections on the paintings? If that's the case then it's simply a matter of re-positioning the lights and or shooting the painting from further away with a longer lens to reduce the reflection angle that the painting is picking up..

[Thom Bennett]

Ole, sounds like you've had some experience with this. My approach was going to be similar; 2 pops for initial exposure, 3 pops for next stop, 4 pops for next stop, 5 pops for next stop, 6 pops for next stop. That will take 20 pops to get to f32. I was at 12 pops originally (based on 2 pops per stop) and, by the looks of the film, I was off by about a stop and a half. I'll certainly bracket and it will be interesting to see what works. I'll repost once I get the film back.

[Brian K]

Ole, sounds like you've had some experience with this. My approach was going to be similar; 2 pops for initial exposure, 3 pops for next stop, 4 pops for next stop, 5 pops for next stop, 6 pops for next stop. That will take 20 pops to get to f32. I was at 12 pops originally (based on 2 pops per stop) and, by the looks of the film, I was off by about a stop and a half. I'll certainly bracket and it will be interesting to see what works. I'll repost once I get the film back.

I shot studio still life for 25 years using studio strobes. If the strobes are powerful and are allowed to fully recharge between pops then it's a linear progression. One of the few issues related to many multiple pops at high power is afterglow or the overheating of the flash tube affecting it's efficiency.

If Ole is having an anomaly, then the problem might lay with his strobes or methods.

F stop desired, pops required (total)
F 8 1 pop
f11 2 pops
f16 4 pops
f22 8 pops
f32 16 pops

[Thom Bennett]

Brian, whenever I shoot paintings I use the polarizing gels to cut the glare and to give the painting good saturation. The painting is highly reflective and I'm shooting it in a small studio and can't move the lights around much less move the camera back any further. I generally shoot paintings with a digital camera but this one required 8x10 and drum scanning; he's going very large with the giclee.

[Thom Bennett]

Aha! Brian, I see now how I figured the exposure incorrectly. I was giving each stop only 2 pops but, as you point out, it is a linear progression per stop. (light bulb goes off in head).

And, my base exposure was 9 pops, not 12. 12 pops was my highest bracket of pops and it was still underexposed. If I shoot at f32 I'll use 16 as my base and bracket around that.

Thanks everyone for the input.

[Brian K]

Brian, whenever I shoot paintings I use the polarizing gels to cut the glare and to give the painting good saturation. The painting is highly reflective and I'm shooting it in a small studio and can't move the lights around much less move the camera back any further. I generally shoot paintings with a digital camera but this one required 8x10 and drum scanning; he's going very large with the giclee.

Thom, I hate to say it but if you really want to do the best possible job then you need to take this particular painting to a larger space because what you're doing is going to be a compromise and will waste a good deal of your time. The longer lens and greater camera to subject distance makes a huge difference when working with reflective objects.

The use of a polarizer is for removing unwanted stray or reflected light from a scene that is lit with uncontrollable light. If you see an increase in color saturation with your paintings it's because you have too much stray light bouncing around your studio. And the light in your studio is CONTROLLABLE. In 25 years as an advertising still life photographer, I NEVER used a polarizer in a studio, and in the years I spent assisting NEVER saw any of the photographers I assisted use one in the studio either.

That said, if you have no other choice but polarizer and multi pop, then simply double the pops for each additional stop you need. Also I'm curious why would you need f32 to photograph a flat object? Are you that close to it where the camera distance to the center of the painting is far different than the camera to the edge distance? Moving further away fixes that too.

[Thom Bennett]

Brian,

Thanks for the advice but the lighting setup that I use works great for photographing paintings. There is no glare and the colors are rich and saturated. With the polarizing gels on the lights and a polarizing filter on the lens all glare is eliminated. Rarely, and depending upon the painting, I will adjust the polarizing filter on the lens to allow just a hint of glare in order to give the paint an edge.

I know f32 is overkill with a flat subject but I was a bit paranoid since this is the first time I've shot an 8x10 chrome in forever. I will probably only go to f22 on the reshoot.

[Sevo]

It's not a straight "2^n pops for n stops", you need to add additional flashes with multiples exposures.

So while 1 stop is 2 flashes, 2 stops is more like 5 flashes, 3 stops 11 or 12 flashes and so on. The same is true with any method of multiple exposures.


... but only past the threshold where you hit reciprocity issues, which you will not encounter that often. The 2^n thing works quite well into the 4-8 pops (2-4 for colour transparencies as the colour balance gets affected first). It probably will not hold up past four stops/sixteen pops - but by that time you'll usually have that much issues with stray ambient light and vibration/movements that reciprocity is among your least worries.

[Jim Michael]

Not to be overly pedantic, but the (2^stops) relationship is nonlinear. You can see for yourself by plotting number of pops vs. number of f-stops (http://www.wolframalpha.com/input/?i=Plot+%7B2%5En+n%7D%2C+n%3D1..5).

[W K Longcor]

Adding my two cents --- intermittency effect WAS a problem way back when --- most modern films do not show the problems as much as in the old days. When you get up to the 16+ pops -- you MAY experience a little problem. At that point, maybe 17 or 18 pops would be more correct, but the percent of total exposure is so small that most of us would not see the differrence.
As for the Polarizer -- use caution. Back in the late 1960's - I was starting out in the business. I was asked to make a 4x5 transparency of an oil painting. I used polarized lighting and camera filter. The artist HATED the photograph. A wise and talented artist, who always did his own photo copies, explained to me that painters ( and other artists) paint -- visually allowing for ambient reflection in the room. When a polarizer is used, it makes the painting way too saturated looking. Ever since -- I left the polarizers in the drawer. Not sure if this has anything to do with your project -- but, hope it is of some help.

[Vaughn]

Adding my two cents --- ...

Thank you for that insight. Very interesting!

Vaughn

[Thom Bennett]

Actually, way back in the day a studio-mate of mine used a method whereby he could control the contrast of a painting that had a lot of detail in the lower register. He would get everything set up and put a white foam board in front of the painting and expose that 2 stops (if I remember correctly) under his metered exposure then move the board and expose for the painting. Basically flashing the exposure to render more details in the blacks while not blowing the highlights.

[Brian K]

Actually, way back in the day a studio-mate of mine used a method whereby he could control the contrast of a painting that had a lot of detail in the lower register. He would get everything set up and put a white foam board in front of the painting and expose that 2 stops (if I remember correctly) under his metered exposure then move the board and expose for the painting. Basically flashing the exposure to render more details in the blacks while not blowing the highlights.

The problem with that is that it's not necessarily even in that the card is closer to the bottom than the top so that the fill affect will be greater on the bottom. Plus while it may fill in the bottom shadows of a highly textured painting, it will also add exposure to all the other tones especially in a painting that is perfectly smooth. You're not actually affecting the contrast of the scene this way, you're changing the exposure to a larger extent.

When I would shoot high contrast studio scenes I would over expose by about a quarter to a half stop and then pull process by a quarter to a half stop. You also could pre-flash the film with a zone I flash exposure.

[Thom Bennett]

Brian,

Yes, I do think he would overexpose the exposure for the painting and pull process. So many things I've forgotten!

[Thom Bennett]

Brian, just to clarify I did not mean a fill card. I meant a card that would be placed in front of the entire painting essentially pre-flashing the film.

[Thom Bennett]

Got the film back and the exposures look great. However, one side of the tranny is soft so apparently I was not square to the painting. Damn! Third time will be a charm. Thanks to everyone for help with the multiple pops. Once I get the next batch back I will post a scan of the results.

[Two23]

I shoot at night a lot in the winter, lighting up freight trains with either a pile of monolights or a pile of Nikon SB-28 flash. Usually they are moving and I can only take one pop. However, sometime I shoot other "stuff" such as canyons, country churches, etc. My monolights have enough power to only need one pop, but sometimes I take the SB-28 flash because I hike in a ways. Those often do need more than one full power pop. I've been following the formula of 1 stop = one pop, 2 stops = two pops, 3 stops = four pops, etc. This has worked for me, however I've only been using a DSLR for this so far. It's my intention to begin shooting night shots with my 4x5 this year. I'll see how it goes.


Kent in SD