Static grain optical phenomenon at full wide aperture?

yzmotoxer807

yzmotoxer807

Well-known member
There is an optical phenominon where at wide open apeture on the 35mm lens, there is no Static grain. but when the lens' apeture is stopped down, magically the grain appears.
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Why is this? How does this phenomenon work and why? Also, if this happens, why can't you just leave the 35mm lens at full wide and stop down the lens on the camera when less light is needed?
 
Smaller apertures make a smaller (diameter) hole for light to go into the adapter internals (bigger diameter), and casts 'shadows' on the focussing screen grain revealing the contours. Could be completely wrong, just a thought.
 
My guess would be stopping down the lens=greater DOF which brings the grain into sharper focus.

You do want to stop down your 35mm lens in a few situations, such as increased resolution at its "sweet spot" and increase DOF. When you have an extremely fast or telephoto lens, DOF becomes so short that's it's unmanageable sometimes. Not fun to have to pull focus in those situations.
 
Okay now I understand why you would want to adjust the aperture on the 35mm lens but I dont think that a greater DOF would bring the grain into focus because the 35mm lens is being projected onto the GG so a larger DOF would only affect the projected image.
 
I've never heard a good explanation for this or come up with one myself. its a very good question.

lenny levy
 
I think its that when there is a lot of light it starts to bloom enough and overlap the little grain holes in effect hiding them in light. The Brevis can be shot static at wide open with no show of grain and when I stop down slowly it looks like its just the light spill or bloom that hides them. Did I say that clearly? Sounds good in my head but looks weird in text. I think i said the same thing twice. :P
 
It's got a lot to do with increased aperture angles and contrast as diffusion levels go up. That's why a static adapter with decent diffusion has very little usable aperture range...the lenses must be fast and used wide open in very good light to avoid grain masking over your images. With a moving GG, you get a lot more usable aperture/light range from your lens collection.

In our extensive testing here with lenses, most of them get about 100 extra lines at the corners when aperture is stopped down to the f4 or f5.6 range. That's why we've biased CF1Le in the Brevis to be able to dial this in and only see 1 stop or so of loss to the camera. This clip illustrates: http://www.cinevate.com/images/lightloss.mov

Cheers,
Dennis.
 
Denis that gave me an example of what to do but I am still curious about physically why this phenomenon happens. Can someone explain what actually causes this?
 
If you draw a ray diagram of what happens as the aperture blades are stopped down, you see a sharp increase in angle of incidence to the GG. This is likely why the grain shows up, even with spinners under high contrast situations. One might compare it to the longer shadows from mountain peaks as the sun sets...in this case the mountain peaks are GG structure.
 
Okay that makes sense. So the "ridges" in the gg cast small shadows when the iris is small and when the iris is open there is more light to eliminate the shadows. Right?
 
Dennis Wood said:
If you draw a ray diagram of what happens as the aperture blades are stopped down, you see a sharp increase in angle of incidence to the GG. This is likely why the grain shows up, even with spinners under high contrast situations. One might compare it to the longer shadows from mountain peaks as the sun sets...in this case the mountain peaks are GG structure.
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so my response was on the right track. yay :2vrolijk_08:
 
What I don't get is why the grain would be static if the GG is moving or in the case of higher f stops even spinning.
Riddle me that Dennis.

Lenny Levy
 
Lenilenapi I dont really understand what you mean. This thread is talking about a static 35mm adapter. I think that you are referring to is a vibrating or spinning 35mm adapter but a higher f stop doesn't have anything to do with that. A spinning adapter will work with higher shutter speeds better than a vibrating adapter because with high shutter speeds and a vibrating adapter the camera will sometimes pick up the grain when the gg gets to the top or bottom of its stroke.
 
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