I dont think this will work if you are using glass filters. If you are using acetate filters then it could be a good way to check filter factor.
Hey, Don, if it's not going off on a tangent too much, mind if I ask why you find yourself doing more incident than spotmetering these days?
This isn't a trick question and I don't have an axe to grind. I'm just often curious about the metering techniques we use.
It *does* make sense - but ...Quote:
Originally Posted by dnmilikan
"Spectral response" (energy transmission at a given frequency) is usually determined with the use of a "Spectrophotometer". This is a device consisting (simplified to beat @#$@#$) where there is a controlled w-i-d-e spectrum light source, a *very* sensitive device for reading light energy, a controllable "slit" and a and a rotatable (and controllable) prism. In use, the prism and slit are set to a given angle to allow a narrow beam of light at a given frequency, the energy output is measured without the medium in question; then with the medium. The two values are then compared and a transmission attenuation factor is calculated for that one specific wavelength.
The prism is then rotated to supply a different wavelength of light; before and after measurements are again taken ... etc.
After the required range has been investigated, the spectral transmission is then plotted.
At least that *was* the way it was done, back in the neolithic ("neo" indicates "new" - what indicates "old"?) age, before fancy computers and lazy optical technicians. Now, I imagine, one turns the wildly expensive machine ON, stuffs the "filter" into it and pushes a button.
You speak of "channels" with the densitometer. I would guess that there are a few at discrete wavelengths of light. Probably the values at those particular wavelengths would be valid; I can't see why they would not. .
However, all films - black and white as well as color, are sensitive to energy at a wide range of wavelengths - so I'm not sure how useful the (accurate) information would be.
Comment: Ho boy!! Memories of an Optical Bench .... and a sore back...
On the alleged superiority of acrylic vs. glass, I have a lot of 3x3" acrylic filters, mostly made by Ambico when they were trying to compete with Cokin. These come up fairly cheaply on eBay, so it's not too hard to accumulate lots of them, and there are many holders that will handle 3x3" filters, if that size is large enough for you. Voss, for instance, made a clip on 3x3" filter holder, which I use for all sorts of odd series sized lenses. I happened to acquire enough of the Ambico adapter rings (these are not so easy to find) when it was in regular production to make it worth while, though now I have several lenses that need a larger filter, so I'll have to find another system eventually.
In any case, I tested all my filters of various types once for flare, and while I found that even a cheap glass multicoated filter was better than a fancier glass single-coated or uncoated filter in supressing flare, the acrylic filters fell somewhere in between.
Acrylic filters can have other problems, such as ND and ND grads not being really neutral, and I'd be concerned that acrylic sheets not made for a specifically photographic purpose may not be optically flat.
Originally Posted by David A. Goldfarb
Thanks for sharing your experience and information. I think that I have a compendium bought (will know tomorrow) that requires 4 inch filters. The reason that I am shying away from gel filters is that they have always struck me as susceptible to damage in the field. Do you know of an alternative for 4 inch filters?
It has to do with the thickness of the material and being able to read it with the densitometer probe. If the material is too thick you will have extraneous light going into the probe, plus the aligment of the sensor will not be the same as if it was a thin material, for which these instruments are designed.
Suppose you turn off all the lights to read the glass or acrylic filters, then at the same time you have light from the densitometer bulb escaping due to thickness.
4" square filters come as gelatin filters, polyester, and acrylic, as far as I know. I know B+W and Heliopan make glass filters in frames to fit 3x3" (really expensive last I checked), but I'm not sure they make them in 4x4".
I don't necessarily think that acrylic is a bad choice, but I'd be a little concerned about flatness if they aren't manufactured for optical use. The Lee and Hi-Tech filters are supposed to be good, as far as new acrylic filters go. I know that my Ambico ND filters and Cokin ND filters and ND grads are slightly warmer than neutral, but the Lee filters are supposed to be much better in this regard, if you're shooting color.