Since our last post we have been exploring the vignette properties of the 20" x 24" ground glass, resolving the focusing issues of the Schneider Fine Art XXL 550mm wide angle lens and taking some more photographs.
In the first update we wish to expand on the vignette properties of the ground glass that we explored in the previous blog. In the last blog we posted some images that displayed the changes of the vignette when standing close and afar from the ground glass, in this blog the image below display the vignette changes when you view the ground glass to the right, centre, left, up and down.
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| Click on image to enlarge : Vignette properties of the ground glass when viewed from different angles |
The vignette is not an issue when focusing as you use a loupe just as you would on a 4x5 or 8x10 large format camera view glass and the vignette is not present. The vignette is of special interest when we invite students and the public see how the camera works and take photographs as it is a wonderful real time example of what happens to the reflected light rays from a subject as they pass through the lens by refraction and are defused on the ground glass resulting in an inverted image.
The second update concerns the issues we were having with the Schneider Fine Art XXL 550mm wide angle lens in that we could not close up the focusing distance on the camera due to the design of the front lens carriage. At first we thought we would have to redesign and rebuild the front carriage module but after a bit of head scratching we realised that all we had to do was to rotate the lens mount module around 180 degrees and that would shorten our focal length. By inverting the front lens mount module we have in effect created a recessed lens board which is commonly used on 4" x 5"cameras to accommodate wide angle lenses. To invert the front lens mount we had to remove the bellows from the lens holder but this should not be an issue in the future as we will use quick release slide clamps. It is a joy to use the Schneider Fine Art XXL 550mm wide angle lens as it opens up two more f stops than the 1100mm lens at f11 to f20 giving a brighter and shaper image rendition on the ground glass.
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| Front lens mount in the front postion which is perfect for the Schneider Fine Art XXL1100mm lens focal length but we need a shorter focal length for the 550mm lens |
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| Click to enlarge : Lens mount rotated 180 degrees |
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| Click on image to enlarge : Front lens module rotated 180 degrees resulting in a shorter focal length required for the 550mm wide angle lens |
In the third update we have taken some more photographs to check if there is a vignette and light leak issues across the full 20" x 24" frame of camera back, to do the test we used 6 sheets of 8" x 10" Foma 100ASA film to simulate a piece of 20" x 24" film. The film was tape roughly into place but in the future we will develop a holder for the option to load 2, 4 and 6 sheets of 8" x 10" film into the 20" x 24" film holder as it yields interesting and on Foma film inexpensive results for students. For exposure the film was shot at 50 iso as at 100 iso the Foma film struggles with capturing mid tones. The Schneider Fine Art XXL 550mm wide angle lens was used for the photograph now we have a shorter focal length on the bellow movements. The weather was very overcast so we gave it a long 9 seconds exposure and set the lens at f90 to increase the depth field to keep everything sharp and in focus to see if there were any issues with the image across the 20" x 24" capture plane.The film was scanned using an Epson V750 Pro scanner with all automated settings turned off. There are some issues with the scans in the sky areas though this maybe down to shoddy film developing of my behalf as I raced to see the images and the developer was too warm due to the weather turning warm yet our heating is still on at maximum.
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| Click on image to enlarge : Six 8" x 10" film negatives 'badly' tape down in the 20" x 24" film holder and exposed together. |
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| Click on image to enlarge : Detail form the six 8" x 10" negative photograph |
We are off to Manchester this weekend for a two day workshop with John Brewer to learn wetplate collodion
www.johnbrewerphotography.com, will report back on the workshop and process.
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