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from Menlo Park CA on 4/6/2008
These comments relate to Cokin filters black and white filters 001, 002, 003 in size X and 002 in size P.
Positive points:
* Very convenient way to have one set of filters serve lenses of different sizes.
* Neatly fit into the Cokin 350 bellows/compendium unit.
* Wide selection of filters of various types.
* Inexpensive compared to glass filters.
Negative points:
* The optical quality of the filters is far below that of high quality glass filters such as B+W. They can introduce a focus shift or on-axis astigmatism. Whether the Cokin filters are "good enough" depends on the lens with which they are used. The longer the focal length, the more stringent the demands are for strict optical "neutrality" (i.e. no waviness or shift of focus). I bought the X filters to use with a Schneider 300 mm Symmar S, and the results are generally unsatisfactory on a 4x5" negative. With a 280 mm Leitz Telyt on 35 mm film having much higher demands for on-film resolution than 4x5", the result can be a disaster. On the other hand, used with an 80 mm Planar or 150 mm Sonnar on a Rolleiflex SL66, the filters are generally good enough to be useful.
* There is much variability in the optical quality from sample to sample, and in the case of the X filters that are much longer than the part directly in the optical path, there is variation along the length of the filter. That turns out to be good: the X002 filter was terrible at one end and relatively good at the other end. Test what you will use!
* It is essential to focus through the filter, because there can be a serious focus shift introduced by the filter. The effect of a focus shift increases rapidly with the focal length of the lens. The X003 sample shifted the focal plane by about 1 mm with the 300 mm Symmar and a similar amount for a 280 mm Leitz Telyt. That is about 5 times the depth of field at f/5.6 for the Telyt! However, once the focus shift was corrected, the filter performed relatively well, but still not great, with such long focal lengths. Of the four filters tested, only the X003 suffered from such a large focus shift. (No, it is not a chromatic aberration.)
The bottom line: I ordered three 105 mm diameter B+W glass filters for the 4x5 camera. Can use the Cokin filters with the Rollei in some cases.