I constantly see references to video being the application where an Optical Low Pass Filter is most useful, but it seems that still images do best without one. Would it be possible to then have a camera with a sensor sans the OLPF (such as the D7100) apply the benefits of an OLPF through a special filter on the lens? Do any manufacturers make such a filter?
In cameras without an Optical Low Pass Filter (OLPF) that are becoming more common, the high resolution of the sensor often approaches or even exceeds the resolving power of many of the lenses that will potentially be used with that camera. This means the limits of the lens itself provide the benefit of reducing moire. Reduction of moire, also known as aliasing, is the whole point of putting an OLPF, also sometimes called an anti-aliasing (AA) filter, in front of the sensor.
An OLPF works by refracting the light striking it into four paths. Since theses are very tightly spaced, this has the effect of slightly blurring the image focused on the sensor just behind the OLPF. You can't detect the four different images because the size of the spread produced by the OLPF is about the same size as a single pixel well, which is the smallest unit of detection the sensor is capable of.
There are at least these problems with trying to do this from the lens.
- The light at the front and even the back of the lens is not focused. Current OLPFs are designed to refract light that is focused.
- Changes within the lens, such as moving focus elements or zoom elements that change the focal length, would require different spreads of the lens mounted OLPF's effect to get the desired blur of the sensor's Nyquist Frequency.
- Interchangeable lenses must be compatible with a variety of camera models, many of which have different sensor designs from the other models that are also compatible with the same lens mount design. A filter optimized for a sensor with 4µm pixel pitch would not be useable in a camera with a 6µm pixel pitch sensor. This issue is also compounded by the way that different cameras deal with using high resolution sensors that often exceed 20MP to produce HD video, which is only about 2MP in terms of resolution. Some use down sampling, others skip pixels. Each method produces it's own flavor of aliasing.
- Even with a fixed-focal-length fixed-focus lens, designing a filter that could achieve the desired effect for a single specific sensor design would be difficult. If such a thing could even be achieved accurately enough from the front of the lens or even attached behind the lens' rear element within the limits of today's design and manufacturing technology it would be expensive and so sensitive to changes in temperature and other environmental factors that it could only be used in a strictly controlled environment.
Most of these issues are less severe if the camera in question skips pixels to produce video at HD resolution. Since the pixel size is the same but only about every third or fourth pixel is contributing to each frame of video, the margin for error in terms of the precise amount of blur induced would be much greater. There are a few such products on the market aimed at improving the anti-aliasing performance of cameras that already have an OLPF, but they are camera (not lens) specific and tend to install in the camera's mirror box rather than attach to the lens. They usually hold the reflex mirror in the up position the entire time they are installed.
The problem with aliasing in video is to do with the method of downsampling the high resolution sensor image to produce a 1080p video frame (roughly 2 megapixels, a fraction of the resolution of modern image sensor), which uses line skipping (only reading every three or four lines from the sensor).
This is akin to scaling images using "nearest neighbour" resampling in Photoshop (or other editing program). The gaps between the pixels that are read out is much greater than the 1 pixel blur radius of the camera's AA filter.
However there are after market solutions for video aliasing, including lens mounted solutions as you suggest, though I imagine the eventual blur radius will be somewhat dependent on focal length.
A better solution is to get a filter from Mosaic Engineering that mounts behind the lens, which are also specially designed for DSLR video (to provide sufficient blur for sensors that use line skipping).