C100: Craziest question of the week: custom internal filters?

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drummondb

Well-known member
Hullo, friends! What do you think about the possibility of putting some type of filter in the space between the sensor and the rear of the lens in the EOS cinema cameras?

I've been experimenting with netting lenses (pantyhose basically). One common way to do that is to use what's called snot tape and stick the panyhose to the rear of the lens. It's a bit tricky with EF lenses because of the electronic contacts there, but it's not impossible. Some EF lenses are easier to do than others. Here's a cinematographer showing how he does it: https://vimeo.com/38670058

I was just looking in my C100, and I realized there's quite a lot of empty space in front of the sensor. I guess that's where the mirror would go in an actual DSLR?

What if I used the same netting method, but taped it inside the C100 in front of the sensor? There's even a convenient little raised lip in front of the sensor. Then that got me thinking, couldn't I also put a little piece of a glass filter there if I wanted to? If I used the snot tape like above, or maybe even if I cut it just right, it would probably just sit there on that lip all snug.

I know all common warnings about not messing with the sensor area. But theoretically, if we assume I never damaged the sensor by screwing around in there, is there any reason why internal filters wouldn't work?
 
The problem is that you throw off the back focus because of refraction. The system is collimated to a specific optical depth and anything in the path will shift that depth. That's why the internal filter wheel has a piece of clear glass for when you do not wish to employ ND. It still needs that refraction adjustment as part of the calibration.

That said, people have been doing this sort of thing for years. If you focus solely by eye or auto focus, then it will likely not be an issue. But do not rely on the focus reference marks on the lens barrel at all.
 
Very kinky stuff Mitch, I loved using the nets on the rear of the B4 mount on my old Sony D600, as well as my Panasonic HDX900, I used the I Rings, http://lightbreak.com/iring_article.php
I liked the look the Christian Dior Black 10 denier, I hear the Fogal 10 was a sought after model as well, I think that was Janusz Kaminkie's favorite net? I know a lot of the DP's use snoot tape to mount to the rear of PL lenses. I like the idea of the IRing sitting snug inside the camera lens mount just in front of the sensor :) I hate the thought of getting close to any camera sensor. I used museum wax to secure the Iring net , on a DSLR Novoflex adapter secured right in the middle of the Novoflex, looked great!IMG_2834.jpgIMG_2835.jpgIMG_2836.jpgIMG_2837.jpgIMG_2838.jpg

Broatch Berry
Venice CA

Screaming Dime

screamingdime.com
 
I used the Christian Dior and Fogals as well. Fun fact, there was one woman at Fogal's corporate office in NYC who understood that photographers liked to use their product for something other than dressing women's legs. She had a wonderfully thick Queens accent, and it was always a perverse joy to call her up asking for the nude stockings in extra-large size. Kinky indeed.

BTW, a stocking behind the lens does not change the back focus, as most of the light passes right through untouched. But a gel or glass filter will alter depth.
 
it was pretty well understood by the sales staff at the focal store in NYC, that if you were male and came through the door looking for stockings, they would peg you as a camera type. They always sold me the large size without even having to ask. The stocking were all the same price so get the large, not the petite...
 
Hey Mitch, let's say i put a square glass filter in the cavity, maybe 2 to 4mm thick. But I leaned it at an angle so it would catch on the bottom lip and not rattle around, would the angle of the glass make the focus noticeably non-uniform from the top to the bottom of the image? Since the glass would be a few millimeters farther away on the bottom than on the top of the sensor? Thanks!
 
No, but the lack of planarity to the image may cause other aberrations to the image. Plus the thickness of the glass relative to the projected image would be greater due to light passing through on a diagonal with the filter tilted. This increases the refraction and further offsets the back focus. The thicker the glass the greater the offset to the back focus.
 
So I did some preliminary testing. It SEEMS when a diffusion filter is placed inside the C100 behind the lens, the effect it has is much more mild than the same filter placed in front of the lens. Is that to be expected?

I had a particularly nasty filter laying around that I wouldn't mind cutting up--a Cokin P084 Diffusion 2. It is resin, not glass, so I was able to cut up a smaller piece to put inside the camera. So I shot my buddy Dexter clean, and then with the filter inside the camera, and then I removed the internal filter and put the bigger, remaining piece of the same filter in front of the lens.

No the filter was not dirty, and yes I checked focus on the external test. The filter just looks that bad.


 
I think I would stick to a matte box and some grades 1/8, 1/4, 1/2, Soft EFX glass filters, or some Black Promist. Or go for some lens netting.


Broatch Berry
Venice CA

Screaming Dime
 
drummondb said:
So I did some preliminary testing. It SEEMS when a diffusion filter is placed inside the C100 behind the lens, the effect it has is much more mild than the same filter placed in front of the lens. Is that to be expected?
Click to expand...

Yes of course the closer you get the filter to the sensor the milder the effect will be. Just like dust on the back element of a lens is absolutely fine while on the front element it's an issue. I think it's a depth of field charcter,. If you shoot at f/16 or so the effect of the back-positioned filter will become just as strong. But who shoots at f16 anyway.
 
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