I thought I would share this thread, in case someone is looking for a centre filter for a 90mm lens or is thinking that centre filters are all made equal!
A while ago I purchased a Rodenstock Grandagon 90mm f5.6 lens. This was primarily for 5x4. I then sold all my 5x4 gear and moved up to 5x7 and 6x17. I though the 90mm would work fine, as there was plenty of coverage. The issue was with achieving an even exposure with the 6x17. This wasn't too much of an issue with b/w, as I could correct it in Photoshop with the local gradient tool. The problem arose with colour, as changes in density altered the colour. I looked around for a centre filter and was offered a Schneider Center Filter IV. This was for the Schneider 90mm f5.6. I thought it would work fine, as the focal length, aperture and coverage angle were the same for each lens. The first roll of colour, with the filter attached, showed that the exposure was still uneven and there was a colour shift towards the edge ( blue/cyan ). I figured then that there must be a difference between the centre filters for each manufacture, although the lenses are equal on paper. I now have the correct Center filter for the Rodenstock lens. There is a clear difference between the filters. The attached picture is a scan of the two filters together, to show the difference. I haven't used the Rodenstock filter on the camera yet but I hope to achieve even colour and exposure with the correct filter.
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Originally Posted by Paul Greeves
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