Paul C. Buff, Inc. Technical Forum

This is my first time posting....We frequently do interiors for interior decorators and color balance is extremely important. We use the room incandesant lights and a few strobes to fill in the dark areas, but the bluish color cast causes white balance problems. Does anyone have 3200 degree kelvin filters for the strobes. I've used gels in the past and they wash out. The best thing so far is tail light lenses for cars / trucks. We need to balance the lighting with the room lights.

You would be looking for CTO (Color Tempreature-Orange) gels. There is not a "3200K" filter. Filters shift XXXK, so a gel that drops our strobes to 3200K would drop an incadescent light source even lower. We do sell 1/4 and 1/8 CTO gels we call warming filters, however LEE and ROSCO are well known brands that make a full line of gels with CTO gels in various strengths. These can be gotten easily at better stocked photo stores or stage lighting vendors. I believe each brand's website also has a calculator that tells you which specific gel you need. You will need the original color temperature of the strobe (5600K at full power from our lights, shifting 80K or so cooler per stop, Einstein will stay with in 50K), as well you will need to knwo the temperture to which you want to shift (3200K, in this case).

OK, four questions, all concerning this "washing out" thing...

What exactly do you man by "wash out"? Is the gel fading and losing color? Is it burning through? Or is it just overexposure? Because proper digital exposure of tungsten balanced scenes is a back art...

What brand of gels were you using?

How did you mount them to the flash?

What sort of modifiers (reflector, barn doors, soft box, umbrellas, etc) do you use?

A gel over the end of a 7 inch reflector will overheat and then fade or melt. If you're mounting the gel a couple of inches away in a 12 inch frame, it will last longer. If you give it its own cooling fan, it will last even longer, and computer fans are cheap. If you build a holder that sandwiches the gel between two 12 inch square pieces of tempered glass, it will last longer still. A gel that takes strobe down to 3200K dissipates literally over half the strobes energy in the chemical bonds of the filter's dyes. That tears the dye molecules apart.

There are soft boxes and umbrellas that have tungsten balanced reflector panels. Or plaster the soft box's front diffuser panel with CTO gel, you can get it in large sizes. That shouldn't "wash out".

I think your taillight lens idea will overheat the flash. And taillights are not warming "spectral tilt" filters.

The main purpose of the strobe if to fill in the shadows. Areas behind furniture, plants, flower pots tend to go too dark and decorators don't like it. The strobes use Rosco gels and they are taped to the front of the reflector. We do not have room for umbrellas, light boxes, reflectors as the space usually is very small. We use a bare bulb and the modeling light is not on, so there is not a heating problem. Consequently, the gel is not fading.

The washing out means the gel does not seem to be doing anything to color correct to 3200 kelvin to match the room lights. ( we still seem to get a bluish cast) Round automotive / tractor taillights are arounf 4-5 inches in diameter and fit very well over the reflector, and seem to do the best to remove the bluish cast from the strobes, but tend to be unpredictable.

do you know which specific Rosco gels you are using? Also, which specific lights are you using?

Perhaps I might be able to help. I think that what you may be observing and reporting as "fading out" of the color of your flash (too "blue"), even when using your Roscoe gels, is not the fault of the gel conversion, or their specifications, at all.

There is probably one of two issues - or both - that are defeating your attempts to balance your light color. One is that tungsten room lighting, to which you are trying to match your flash lighting, even with so-called halogen lamps, is never even close to 3200K in color; it's usually at about 2600 to 2900 K and sometimes even lower, especially if it's on a dimmer. Even if you replace the architectural lamps with photo lamps, many of them are often 3000K or lower, unless you specify certain standard 3200K lamps. Even those nominal 3200K photo lamps are sometimes warmer than specs should allow, and they all change to become warmer as they age. Therefore, if you gel your lights by calculating a conversion to 3200K, you're probably "under-gelling" your strobe light source. If this is the case, just gel your strobes even warmer and check your preliminary results on a laptop before you shoot for real.

The second possible reason for your "fade-out" problem is another common gremlin -inadequate covergae of the gels over the lightsource. As you indicate that you have a space problem precluding your use of large indirect diffused modifiers like lightbanks, which can be pretty well completely gelled on the outside of the front diffuser, and because you mention your lighting as using possibly true "bare-bulb" setups, you must know that it is extremely difficult to "cover" all the light emitted by a bare bulb light source using sheets of conversion gel, no matter how big the sheets are. Therefore, practically speaking, especially where space is at a premium, there will almost always be some part of the projected light from the flash lamp that will escape the gel and add some white ("blue") light to your mix, making the overall color cooler than you anticipated. This can sometimes be cured by setting up a thin guage wire cage, made of "chicken wire" around your bare bulb and taping your gels all around the bulb on the outside surface of the wire cage. You may have to juggle this around quite a bit to get it right and not cast a tell-tale wire shadow into the picture area. Heat hint: don't turn on the modeling lights for more than a few seconds at a time!

I hope this helps.

David

I'm going to go with the "under filter" theory. Try stacking two sheets of the gel. Seriously. There's an old way of looking at color temperature, called "MIRED" that really works well in these situations. MIRED means "millions of reciprocal degrees). Take a color temperature and divide a million by it. That's MIRED. Filters correct color temperature in MIRED. So, calling the flash 5500K, the gel is designed to turn 5500K into 3200K, and you've probably got 2600K lights in the building you're shooting. Now, the difference between 3200K and 2600K doesn't sound like much, it's just 600K, and that get is doing 2300K, converting 5500K into 3200K, right?

1,000,000/5500K = 182 MIRED

1,000,000/3200K = 312 MIRED

So, a gel that converts 5500K to 3200K is -130 MIRED

1,000,000/2600K = 385 MIRED

You need another -73 MIRED to get to where you want to be, the -203 MIRED difference between 5500K and 2600K. The gel didn't even get you 2/3 of the way there.

And, in the real world of gels, the gel makers cheat a little, so the gel you buy to go from 5500K to 3200K probably isn't the actual -130 MIRED you want, it's probably closer to -110 MIRED, and they thing you'll never notice that you got "shorted" 20 MIRED. So you can actually take two sheets of that -110 MIRED gel, stack them on top of each other, and you'll get -220 MIRED, which is only 17 MIRED more than what you needed.

You've already learned the first lesson of light: everyone notices when a fill is too blue. The second lesson is that they seldom notice when a fill is too orange. This is true of almost every area of photography, from portrait to fashion to product to architecture. Double gel the strobes, and I bet you like it, and it will be less "hit or miss" than a taillight.

Taillights don't have MIRED values, they're not "spectral tilt" filters, they're "cutoff filters", and the difference would take me 3 pages to explain...