A neutral density (ND) filter is a “grey” filter, a filter that transmits equal fractions of the incident radiation at all wavelengths. A perfectly neutral filter over a broad range of wavelengths is an idealized concept, and one very difficult to implement in practice. There are different approaches to making filters approximating colour neutrality. We here compare the spectral transmittance of of ND filters of three different types available for use on camera lenses and explain why the use of some of them can introduce strong colour casts in the photographs we take with them.
A comparison between the Baader U filter and the StraightedgeU filter, both with sun and a modified flash as light sources. Examples of flowers from two species, which display different false-colours with the two filters. Continue reading Filters for UV photography
Using the E-M1 converted to full spectrum with the Pinhole Pro objectives is possible. Using a 58 mm NIR filter (Hoya R72) attached to the front of the 11 mm Pinhole Pro S11 worked fine, with no increase in vignetting. Using the StraightEdgeU 52 mm or Baader U-filter 2″ with a step-down ring blocked the corners of the image completely. The original 26 mm Pinhole Pro suffers a lot less from vignetting and can be used with these filters of smaller diameter than the front thread of the lens without problem.
For those interested in photography “beyond the visible”, some of the filters available from Midwest Optical Technologies Inc. under the MIDOPT brand name should be very interesting. They are distributed in European countries by Stemmer AG. Both companies are specialised in the supply of machine vision equipment. What adds additional interest is that filters are supplied in very many different sizes (from M13.25 all the way to M105, mounted and unmounted, and even with mounts suitable for installation at the back of objectives with C-mount).
Until very recently I was not aware that optical glass filters can fluoresce when exposed to ultraviolet radiation. In this short post I use two filters from Heliopan that I own as examples of this. However, many other filters, and even the glass elements in some camera lenses can also fluoresce. Continue reading Fluorescence of glass filters