Optical Low-Pass Filters in front of the lens ?

[Christopher Lowden]

Hello
I acquired some Red Optical Low-Pass Filters (OLPF) filters hoping that I could mount them on my Epic MX ... and was disappointed to discover that they don't fit.
My particular interest is in the Low Light Filter. When I search the web, I get tonnes of information about OLPF lowering moiré effect but nothing about the red style, aesthetic filters.
Is there a particular reason why these filters are placed in front of the capture ?
Are there 4x4 alternatives?
Many thanks for any thoughts.

 

[Phil Holland]

Low Pass Filters are Anti-Aliasing Filters, usually a 3 stack of birefringent and a wave plate. The additional thing is what we often call a "Color Science Filter", which cuts/absorbs Infrared and Ultraviolet Light and may have some other color characteristics as well as Anti-Refective Coatings.

In terms of the optical path, the Low Pass Filter makes the most sense in front of the sensor. Doing this in front of the lens would lead to per lens and focal length variables.

In terms of the Color Science Filter, yes you can do that in front of a lens. Pros and Cons to that as well.

You can remove all of this and ideally you have an optical slug in there to make up for the back focus change. And there are Full Spectrum cameras out there with the color filters removes to allow for IR or UV imaging. Removing the Low Pass Filter though does greatly increase the potential for aliasing and moire, which is a headache that is difficult to handle when it shows up in frame. i.e. Hard to fix in post in most cases.

RED's initial DSMC and DSMC2 bodies allowed for different "OLPFs" to allow mainly for different color filtration and anti-reflective coatings, which opened up the IR and UV imaging potential. You need to find the ones for your specific camera really. There are a few cameras out there without Low Pass filters as well for those who need that, but that is a custom order essentially.

Low Light Optimized OLPF will indeed provide the most light transmission at the cost of being rather prone to internal reflections. The Standard OLPF became a very good balance between color and internal reflection control. The Skin Tone Highlight OLPF is the most light hungry, but provides a slightly better roll-off, good color, and the most internal reflection control.

DSMC3 is a fixed filter system, but also has the "best CSF/OLPF" to date in terms of overall image quality.

 

[Christopher Lowden]

Hello Phillip, Thank you so much for this explanation. It is much clearer as to the pros and cons of these filters in a Red context.

For those reading this, the OLPF filters seem to have been designed for Dragon and above so MX cameras cannot take them.

 

[Patrick Tresch]

I think it's since DSMC1. MX was also a DSMC1 sensor (before Dragon beast). I probably still have a OLPF for DSMC1 and it could be LLO.