Mosaic Engineering, Inc.

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VAF-Series Optical Anti-Aliasing Filters: FAQ

Frequently Asked Questions
and Important Information
about Mosaic's Video Aliasing Filters

Our VAF-series optical anti-aliasing filters are an easy-to-use and effective solution to most of the aliasing and moiré problems typically encountered in DSLR HD video.  Nevertheless, their unconventional placement behind the camera's lens produces some peculiarities and limitations, of which you should be aware before deciding to purchase or use them:

Remove VAF filters for still photography.  VAF-series filters are specifically designed for HD video - and should be removed to shoot high-quality still photography at your DSLR's full, native sensor resolution.

Doesn’t my DSLR already have an anti-aliasing filter?  Yes... but your camera's internal filter is designed for shooting full-resolution still images, and has little useful effect when shooting in reduced-resolution HD video modes.

Optical focus and indicated focus differ (but infinity focus is always insured).  The VAF-series filters work well with most commonly-encountered DSLR lenses.  However, their placement behind the lens alters the camera's effective optical flange focus distance, and this has certain effects on the way lenses will focus with the camera.

One result of this is that when a VAF filter is being used, the lens' actual optical focus distance will differ from the focus distance indicated on the lens barrel.  This effect is most pronounced on wide angle lenses.  Nevertheless, the ability to achieve focus at infinity is always assured with any lens.  (Note that with the DSLR's critical focus and very shallow depth-of-field, when shooting video, filmmakers typically do not rely on the lens barrel focus indications, instead focusing manually with the internal or an external video monitor.)

Zoom lenses can be used, but do not track focus well while zooming.  Another result is that, while zoom lenses are easy to use with the VAF filters, they must be considered “variable prime lenses”, since accurate focus tracking while zooming is not possible when a VAF filter is installed.

When shooting high-quality HD video with a zoom lens, it is always the best practice to first zoom to compose a scene, and then lastly to adjust for critical focus by using the camera's built-in live-view digital magnification function.  Really this principle applies just as well without a VAF filter as with one, as even the most expensive zoom lenses do not track focus perfectly while zooming, even without a VAF filter.

Wide-angle lenses.  All of our current VAF-series filters work very well with most typically-encountered wide-angle lenses, without significant softening or resolution loss.  Full-frame cameras fitted with VAF filters are often used with lens focal lengths down to 13mm, and crop-sensor cameras with focal lengths down to 11mm; nor do these figures represent the limits of useful operation.  Please contact us directly if you have specific questions about a particular lens or configuration.

Note that the original version of the VAF-5D2 had a limitation with wide-angle lenses, showing in some cases significant off-axis resolution loss with lenses wider than about 24mm.  This limitation has been corrected by optical enhancements incorporated in the current VAF-5D2b, so that all current VAF-series filters now give very good wide-angle performance.

Aliasing in the LCD viewfinder does not indicate aliasing in the recorded HD video.  When using a VAF filter, the live video image on the camera's small, built-in LCD screen may appear to be the same as without the filter, and may show moiré and other aliasing artifacts.  These artifacts (with or without the VAF filter) are caused by the camera's internal downsampling algorithms that generate the viewfinder display, and as such are unrelated to aliasing artifacts (if any) in the full-resolution HD video being simultaneously recorded.  The low-resolution electronic viewfinder display is really only intended and adequate for framing and focusing.  (The exception to this principle, is of course the Nikon D800, which is capable of driving an external monitor at full 1080p resolution.)

Circular polarizers.  As is the case with most digital cameras, if a polarizer or polarizing filter is used with the VAF-5D2, it needs to be a circular polarizer, or have a circularly polarized output.

What happens when the camera tries to lower its reflex mirror?  What about when the camera leaves "live view" mode, or if I accidentally try to take a still photograph while a VAF filter is installed?  Can the camera be damaged?  No - in none of these cases, is there a danger of damage to your camera.  The reflex mirror in a DSLR is lowered by the pressure of an internal spring - it isn't forced or driven downard by a motor or other mechanism.  The presence of the VAF filter within the camera merely retains the mirror gently in its raised position, so that it doesn't fall downwards under spring pressure.

Further to this topic, since a VAF filter retains a DSLR's reflex mirror in the raised position, it prevents the use of the optical viewfinder while it is installed.  (Of course during video photography, the optical viewfinder is always disabled anyway, regardless of whether or not a VAF is being used.)

Will it hurt my camera to leave the VAF filter installed?  We don't believe that it will.  Many users (ourselves included) leave the filter installed indefinitely, with no negative effects.  (Of course as noted above, the VAF filters should be removed for high-resolution still photography.)

Why doesn't Mosaic make an anti-aliasing / anti-moiré filter for mounting in front of the camera's lens?  To work well, this requires a different filter for each lens focal length - whereas a single VAF filter can work with any focal length lens.

Cosmetic Imperfections  VAF-series filters are complex multi-layered optical devices, and may occasionally exhibit very small, pinpoint-like imperfections of internal AR coatings.  Such small imperfections may be detected in any optical lens, but are more easily seen in the VAF filters due to their open construction; they are an unavoidable result of the manufacturing process, and do not affect either image quality or resolution.

Is there light loss through the filter?  The light loss is minimal - less than 1/8 F-stop, and normally not perceptible.

Using the Canon 5D Mark II and VAF-5D2b with older Nikon lenses.  (This applies only to the Canon 5D Mark II with the VAF-5D2b - It does not apply to any of our other VAF filters for other cameras.)  Many Nikon lenses, particularly older mechanical lenses, have a "Maximum Aperture Indicating Post" - a metal post or stub that protrudes from the flange face of the lens in the direction of the focal plane.  Some early users of the original VAF-5D2 had reported that this lever could impact the filter when installed in the 5D Mark II, resulting in possible damage to the filter, the lens, and/or the camera.  Design changes in the current VAF-5D2b housing have significantly reduced the likelihood of this interference.  Nevertheless, we recommend using extreme care and close awareness with this configuration - please contact us with any concerns if you intend to use such lenses with the VAF-5D2b.