Isostigmar Lens

<p>I pick up a unusual lens since I have not seen much about it here, <br />Its a R&J Beck Ltd Isostigmar lens7.2 inch f-5.8<br />also what is stranger is it has three set of Iris scales on the alum barrel<br />the main scale is 5.8 to 32 but can go down 45.<br />the other scales are F 16 to45 but will go one more stop on the scale <br />the last set are b f 16 to 45 with one more on the scale<br />I have a funny feeling this is a convertible lens F= front element, B=back element<br />and then combined.<br /><br />The black and white shot at F-32+1 <br /><br />enjoy comments welcome</p><div></div>

<p>now photo taken with the lens :</p><div></div>

<p>From " A Lens Collectors Vade Mecum" :</p>

<p align="left"><em><strong>Isostigmar</strong></em> <em>These were made from from 1907 and seem all to have been introduced fairly quickly. The design was an unusual one at the time and still is today so they are a much more interesting lens than many others of the time. They were still made after WW1 into the 1920's, being last noted in 1922. This design was a Gauss type lens with an extra negative glass in the centre to improve the corrections.</em></p>

<p align="left"><em> </em></p>

<p align="left">It is a Series II and 7.25in (focal length) for 6.5x4.75in plate size. </p>

<p align="left"><em>It was a three focus lens, as either the front or the rear could be used alone, and for this the cells were used in front of the barrel. The typical example was the Series 11, which the 7.25in lens converted to an 11in front and a 13in rear cell. Some care was required that the rear cell did not touch the iris or shutter blades in use.</em></p>

<p align="left"> </p>

<p align="left">Being the f stop is determined by aperture opening as viewed through the front element and this is a triple convertible use only the calibrated scale for the configuration you are using even though the mechanical lever will go further. You can experiment with exposures and determine where the next f stop is for the combination in use but you may encounter diffraction distortion in the resulting image(s). </p>

<p>Mr Monday. Thank you very much for the information I will make sure not to go pass <br>

the last mark on the Iris scale :</p>

<p>I'm impressed with the contrast you have in your example photo. I had an Isostigmar but it usually gave low contrast and was prone to flare. If you take the lens to bits you will find a lot (8? 10? can't remember) of uncoated air / glass surfaces which I assumed led to the results I obtained. So - well done.</p>

<p>Colin I plan to use it for some more photo shots its Interesting.<br>

will try it at wide open and f-11 and f-32 again</p>