Using ND filter to trick a flash

[weasel_bar]

I have a thyristor flash linked off-cam with simple sync cord--no TTL flash

metering. I need to trick flash into throwing out more light, as I am shooting

at f/32 (for macro). Someone suggested using a piece of unexposed, developed

film over the flash sensor to do this. I tested it with my camera's meter and

the flash thyristor, and it acts almost exactly as a 1-stop ND filter.

 

Now, question is this: since the film is orange-ish and flash light is white,

will its filter factor change significantly depending on the color of the subject

I am shooting? That is, will an orange subject be very differently exposed than

a green or blue one?

 

I would like to use the film because it is cheap and convenient--can cut up a

strip and just tape it over the front of the flash sensor. It seems cutting up

a hard ND filter and finding a way to mount it temporarily on the flash is a

pain. I would not want to glue it on.

 

Thanks for any help with this question.

[twmeyer]

"<i>since the film is orange-ish and flash light is white, will its filter factor change significantly depending on the color of the subject I am shooting?</i>"<p>It might, but I imagine it would be minimal. ND filters are pretty cheap and one 3in sq filter would go a long way... t

[weasel_bar]

I just tested this by metering a reddish object and a blue object, with and without a piece of film in front of the lens. In both cases, the film appears to block just about 1 stop of light. The reason I thought it might be important is that I've heard of people using orange filters in b&w to darken the sky.

 

If I bought a ND filter for this purpose, how would I mount it to the front of the flash? Film is easy to tape on.

[garry edwards]

Metering systems ARE sensitive to light of different wavelengths, so the colour of the light WILL affect the measurement of exposure - but in your case this isn't relevant, because your filter isn't placed over the flash reflector, it's placed over the sensor, which is only 'interested' in how long the light from the flash takes to reach the subject and bounce back to the sensor.

[mark_sirota1]

Weasel, you can get ND filters in gel form that you'd cut to shape with scissors and tape onto your flash. <a href="http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=102706&is=REG&addedTroughType=categoryNavigation">Here's one from B&H.</a> They have several other strengths as well.

<p>

This is the expensive way. You can buy a whole sheet for not much more, which might be a good idea since eventually the heat from your flash head will warp the filter.

[ellis_vener_photography]

Rosco and lee Filters ware both have to send you filter swatch books if you request one.

These will have different strengths of ND filters as wel las different color balancing filters and

the swatches are just large enough to fit over the flash head of a Vivitar 283, Nikon

SB-800, Canon EX 580, etc. , size flash.

[bill_b2]

Other possibiolities:

 

Lower the ISO on the flash by one stop to get more exposure.

 

Tape some Kleenex on the sensor. One or two layers.

 

Do the flash manually. For the sensor to accurately read the subject, the flash would have to be too close to the subject to light it. If far enough away to light it, the subject is too small for a good reading.

 

If digital camera, no need to mess around with the lighting. Just use manual flash and check the test shots.

[helenbach]

Ellis' suggestion is an excellent one, especially as you are only covering the sensor. You can pick up a small swatchbook for free at theatrical lighting dealers and many pro photo dealers, as well as requesting them from Lee and Rosco. If you want larger bits email me and I'll post them to you gratis.

 

"...your filter isn't placed over the flash reflector, it's placed over the sensor, which is only 'interested' in how long the light from the flash takes to reach the subject and bounce back to the sensor."

 

Garry, are you sure about that? Isn't the sensor on most flashguns acting as a flashmeter rather than a distance meter? If what you said was true, the ND filter would have no effect.

 

Best, Helen

[picturesque]

I use pieces of unexposed, processed black and white film, so no orangy color (the film leader)--even CN film (chromogenic film) works. However, it is very convenient to have the swatchbook from Rosco. They are available for one cent from B&H.

[timsumma]

Ellis, a Neutral Density �filter� for your purposes is fine. The use of the Rosco, Lee or GAM dye based filters will do the proposed effect that you seek. A heads up is that when you are out and about photographing that ambient Infrared energy can create an error in the sensor that is the thyristor that turns off your flash when the �correct� amount of light has reached the receptor. The use of internal flash sensors inside the optical path are generally better choices. If all of a sudden you find your images underexposed this is a real possibility for the cause. As reference please note that the Neutral Density �filters� made as gelatin units are in deed FILTERS. A Kodak Wratten Neutral Density unit IS NOT A FILTER it is an attenuator. Know also that Lee sells repackaged Kodak Wratten Neutral Density Attenuators that are for use over the camera lens. I have no information on the dyes used by gel manufacturers as to what are the transmission ranges of their product. If you want specific information on the Wratten products from Kodak you will find it in the Handbook of Chemistry and Physics (53 Edition has it in around page E-222). Old copies are easily gotten in used book stores. This is a thick reference manual for science. Kodak pays to publish the Wratten information here rather than deal with supplying the information else were. What is quite possibly interesting to you is that there is quite a lot of infrared generated by a flash and in close this could have some effect if it is sent back to a flash units sensor.

 

Now to the point of this pile of information. Most digital camera sensors are sensitive to infrared energy. The manufactures go to great lengths to put a rather expensive piece of glass in front of the sensor to cut off the infrared energy. Weird thing is that the Wratten number 96 ND freely transmit infrared energy. So the Wratten 96 will stop visible light in measured amounts but transmit the infrared you are creating with the flash. More tech info: your flash, be it a studio unit or a hand flash with a power ratio control (the other control used instead of the thyristor control) should be set for half power to create the most output of infrared emission from the flash (this is a substantial amount of output by the way). If you were to drop some three to five stops of neutral density over your camera lens (.10 = 1 third of a stop, .3 = 1 full stop; so 3 stops = .3X3=.9 for 3 stops, and 1.5 ND = 5 stops.), and set your flash to half power you could be making images for your macro images with a combination of both visible light as well as infrared. To change the ratio of the infrared to visible light you would simply change the Wratten 96 ND in front of the lens up or down; each .3 unit would change the exposure by 1 f stop so you would open or close down by the unit removed or added. Exposure would remain correct but the volume of infrared energy recorded would rise or fall while the visible light would stay constant. And that is what I call taking technology, applying a bit of creative thinking, and getting a whole new way of seeing. And no, I don�t think anyone has thus far done this except of course that idiot Tim! By the way, I have done this with film and traditional cameras. It works best with the slow IR films, the Kodak HSI film is not suited to this purpose.

[helenbach]

Tim,

 

The difference between the ND filters that pass IR radiation and those that attenuate light and IR almost equally is that the former use dyes and the latter use something like carbon or silver particles, or a semi-silvered mirror (Schneider make ND filters, or attenuators if you wish, from semi-silvered mirrors). Absorbtive or reflective ND. This ability to control the relative amount of light and IR is very useful, as you say.

 

Best, Helen