Lens problems with The New Countess
Today was time to take the New Countess full plate camera to Clevedon and try some images of the wonderful Victorian Pier. Shame it was so breezy – not ideal for longish exposures with a huge camera like this one!
With two slides I had 4 exposures to make. It was a good opportunity to try the (supposedly) F384 lens too as it was a good focus length (12 inches – 300mm ) for the job.
Bringing the exposed paper negatives home for developing, I found that the lens I was using – Rapid Aplanat No.3 Ser.D.F.8 (R.O.J.A. vorm. Emil Busch, Rathenow) – was not operating at the indicated F stops – see the title image!
I had tested it in the garden on the pear tree using the widest aperture with no problem – but at the indicated f90 it way over-exposed the image (Paper negative). I tried a couple of other lenses at the same time successfully so thought I’d make some measurements of the diaphragm aperture with digital callipers and calculate the theoretical f-stop (dividing the focal length by the aperture diameter)
As the F stop numbers increase the variance between the indicated F stop and the calculated one increases greatly. Column headings are Focul length, Diameter (of aperture) in 100ths of an inch, The F stop indicated and the F stop calculated.
Using a more familiar lens at F45 for a 2 second exposure I achieved a reasonable if (wind) shaken exposure.
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