Wratten Gel Filters
Posted 20 May 2007 - 01:51 AM
I'm shooting some test footage tomorrow and unfortunately, I wasn't able to aquire any filters. I'm going to be shooting 16mm, on an ARRI 16s, using the new FUJI 160T.
How are the Wratten Gel Filters when used in front of the lens? Do they stand up? I need an 85B...any suggestions or recommendations? And tweaking it in post is not an option.
Posted 20 May 2007 - 02:19 AM
Tweaking it in post is not an option - ok how bout tweaking it at transfer ?
Posted 20 May 2007 - 03:52 AM
Posted 20 May 2007 - 04:04 PM
I was getting a print made...no transfers and I was hoping to do it in camera
That can be adjusted when you make a print, provided you give the colorist a good greycard.
Posted 20 May 2007 - 04:15 PM
Firstly I discovered 'newtons rings' as I had sandwiched gels together and the gap between them was in the range of the wavelengths of visible light and was causing nice rainbow interference patterns like you see on oil spills, if you had enough control you could make a funky color filter that you could dial in the color you need just by changing the distance between two bits of ND - off topic tho ... basically make sure if you stack them there is a large enough gap between them to avoid this ...
If your lens is wide enough for the given format you are shooting you will actually see these rings and any other blemishes in the filter as distinct features of your neg... If you are shooting telephoto they will integrate into a general color cast and any scratches will just cause a slight loss in clarity and contrast - so like any filter the cleaner the better and also the longer your focal length the better ...
I also had trouble without a holder in attempting to keep them flat - if some sections are angled and other arent any light travelling through that section will travel through more gel on its path to the lens than other parts - just like a normal flat filter anyway ... but at least with a flat filter the effects are consistent throughout the image circle, if you have funky crumples and pinch points here and there they are noticeable with wider lenses...
All these factors lead me to a mucho betterer solution of using gel behind the lens - you use much less, it is protected and in a part of the light path that is still incoherent ...
This method may play hookey with your focus settings in terms of a general shift (you might lose infinity, or is it the other direction - cant think, too early !) - its near enough to a simple back focus maladjustment so if you zoom your focal plane will move ...
Most people dont even notice or care though - (like me)
Posted 20 May 2007 - 04:48 PM
You got it right about losing proper back focus. If you gel behind the lens you will optically increase the flange focale depth by appx. 1/3 the thickness of the gel. (.0015") Not to much to worry about on longer lenses but it wreaks havoc on wide angles. Also if you recollimate the lens for use with a gel behind it, you have to have one in there at all times even if it's just a clear one or you will not be able to achieve infinity focus.