A friend called me up yesterday and asked me to post this question. What happens exactly if you were to put a matt over a lens that was smaller than the front element ? Not small enough to vingnette but large enough to obscure part of the front element from recieving any light ?
I've also heard that all glass has a certain amount of flare, thus reducing contrast depending on how much light hits it's surface. Using hard mattes will reduce the light hitting the front element, affecting contrast differently depending on lens / matte combo.
In my assisting days, I spoke with several well established firsts (read: 'ol timers) who said that they never used these mattes because it would suck light away from the lens itself. I myself wouldn't use anything tighter than a 75mm matte for this reason. Was I too paranoid, maybe. But one of these guys told me he did tests that confirmed this light loss. Of course, we're talking a third of a stop at most so...
Artificial Diaphragm. Like Matting out the front of a 200mm Nikkor, it would act as an additional Iris, and underexpose your image, also out of focus highlights would change shape to match that of the matte, at least that's what I understand to happen.
Steven is correct in saying a small matt will act as a reducing diaphragm, reducing the exposure reaching the film. This is assuming that the matt "smaller than the lens diameter" is virtually against the lens' front element. If it is far enough forward to begin taking shape it is simply a small matt. Wedding photographers often have used the effect Steven mentioned, that out of focus bright objects tend to take the shape of the diaphragm. They will make a heart-shaped or cross-shaped diaphragm, place it against the front of the lens, then shoot a closeup of the couple with out of focus candles burning in the background. The candle flames will take the shape of a heart or a cross. A related curiosity occurs during an eclipse of the sun. As the sun's disk is partially obscured, becoming a crescent, foliage on the trees become diaphragms and the spots of sunlight reaching the sidewalk are crescent shaped. --
Anything that reduces the front aperture reduces the stop. Focal length divided by aperture gives you F stop. Reduce aperture.....reduce F stop. Period.
Depends on the lens of course, but if you put a hard matte in front of a fast long lens you will often cause a 'waterhouse stop' effect. I mean like an 85 or 135 hard matte in front of a 200 or 300 Nikkor. You can tell this very well by 'a-b'ing it...the image is noticeably brighter without the matte. I first noticed this while doing a pickup shot for a MOW and saw that the image brightened suddenly when the AC swung the mattebox out of the way prior to checking the gate. It looked like an ND had been pulled--at least a full stop difference. Those big front elements are there for a reason, and they need to be exposed in order to gather in the light! I think this is not understood by many ACs, who are understandably anxious to keep out flares. In many movies with long-lens work you can see circles of confusion from distant highlights which have been turned into rectangles by the hard mattes. And the DP was probably wondering why those shots printed 6 points lower than everything else!
The matte will act as a Waterhouse stop, reducing the amount of light reaching the film. And on a long lens, you'll get those nasty square shaped edges to any out of focus highlights near to the edge of frame. I hate that.
There is an additional effect, and that's veiling glare (sometimes called flare). If you're using a zoom, eg 5:1 Cooke and you have a set with a lot of light coming in from the side and top (ie overall flat lighting) then masking down will dramatically reduce veiling glare, thus increasing overall contrast and reducing the stray light in the shadow areas. This can seem like reduced exposure. In isolation of course, you _want_ to reduce veiling glare, but if you're cutting together shots, then you should try to keep in constant. Of course, with video, you can adjust the black level and gamma to compensate, but we'll keep away from that particular discussion!
Then what is the best procedure when setting up a mattbox with an adjustable internal bellows on a zoom? For example, if you're putting using an 8-64 only at the long end, should you set the bellows for the extreme wide end only? And then use the eyebrow and side wings to cover the tight end? And aren't your eyebrows and sidewings doing the same as a hard matte, effecting your true aperture?
Don't worry about it, this problem only occurs with long fast lenses when they are being used at wide apertures. Think of it this way: a Nikkor 300mm T2, wide open, has an aperture that is 150mm in diameter, right? That's over seven inches. Obviously a hard matte for an 85 is smaller than that, therefore the hard matte becomes the iris, since it's the narrowest opening the light has go through to get to the film. That's why it's like a 'Waterhouse stop,' a sheet of metal with a hole punched in it--the most primitive kind of iris. On an 8-64 at the long end, wide open, your maximum aperture is only about 27m in diameter, about an inch. So you've got lots of room to bring in your mattebox, or shade, or hard matte, or whatever.
Also out of focus lights(with a long lens) turn into the form of the iris. Hexanganle or circular so you probably tell what lenses were used without seeing the credits thats if you know what your all irises look like.
I couldn't keep away: of course you know that you can adjust black level and gamma in telecine, if you're going to video only (no print).