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R.F. Hunter, Purma Special 127 roll film camera, c1937

The English made Purma Special is a "Bakelite" camera introduced in 1937 (continued to 1951 apparently) and attracts interest due to it's ingenious shutter and claim to be the first camera to use plastic optics. In truth this was limited to the viewfinder optics, these being made by Combined Optical Industries Ltd,using "Perspex". This same company produced plastic taking optics much later in 1959 with the Kodak Brownie 44A. There are quite a few sites that deal with the Purma Special on the internet, so we will restrict our description to the shutter mechanism. Usually described as gravity controlled, whilst this is true - it in no way does justice to it. In use the camera would be used with fast speed for bright sunlight, medium for bright but not sunny and slow for overcast days. Aperture and focus are fixed.

Click on "this camera's pics" button, below, to see sample images.

R.F. Hunter, Purma Special 127 roll film camera, c1937
Body No.N/a
Shutter, Focal plane ty
pe, speeds 25, 150 & 450th
Lens, Beck Anastigmat 2.1/4" f/6.3
Condition, 5F

The gravity controlled shutter works in two complementary mechanisms;

Firstly, the focal plane shutter consists of two blinds that sweep the film gate. The gap between these two blades alters depending upon which way the camera is held, this gap being controlled by a weighted cam wheel. Holding the camera horizontally results in a medium sized gap (3mm), this still holds true even with the camera horizontal but inverted. With the camera held vertically with the advance knob uppermost results in the gap closing to the smallest size (2mm) and hence a faster speed and finally holding the camera vertically again but with the advance knob at the bottom results in the gap opening to it's widest (12.75mm). Changing the orientation of the camera has no effect on format as the negatives are square. The pictures below illustrate this, the shutter being stopped during its travel so that these pictures could be obtained. In the picture at right the weighted brass cam that controls the gap is just visible.

Medium Speed

Medium Speed

Fastest Speed

Fastest Speed

Slowest Speed

Slowest Speed


 This is only half the story however, the shutter is gravity assisted in the normal sense too. The combined weight of the shutter blinds and associated brass cam is swept across the gate by a small spring, when running horizontally it derives no benefit from gravity and moves at a speed determined by the spring. However when running at the fastest shutter speed the whole assembly is running downhill, and moves considerably quicker. Conversely the slowest speed setting requires the whole mechanism to run uphill which slows it significantly. Another lovely facet is the design of the cam which maintains a constant radius for a large part of it's arc, this is to offset it's displacement as the shutter accelerates from a standing start during medium speed exposures. Finally, in order to protect the film whilst the shutter is cocked, a secondary shutter is placed just behind the lens - this is retracted by the action of pressing the shutter release button. The secondary shutter is visible in the right hand picture, above, because this image was taken whilst the shutter was being cocked, had the image been taken during release - the lens would be visible. Whilst the design is nicely thought out, one design shortcoming became apparent whilst our example was stripped for examination, it is not possible to get the gap to set correctly if going from fast to medium speed. The design hopes that sufficient acceleration is experienced by the blind for it to be partly left behind, therefore setting the gap correctly - in practice this doesn't always happen. Best practice is to go from fast to slow and then to medium, this ensures medium will be set. For a humble little Bakelite camera, it certainly is interesting.

 

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