lf-photo.org.uk

Hi

is there a way to work out image circle
I have just aquired a set of Docter optik len but can't find much infomation about them
I did read that the 750 mm has an image circle of 1300 mm but find this somewhat hard to belive

thanks

robin

There's an issue here.

First there's the actual physical image circle that a lens illuminates on an screen. But more important is the manufacturers recommended coverage/image circle at a given aperture. The first is where the image is unsharp as you go towards the edges, the second is where they suggest is still reasonably sharp (by their criteria).

So you might see a lens being sold as covering 7x5 possibly 10x8 while the manufacturer says it's only designed to cover 5x4, which for high quality images is the case.

The CZJ Tessar only has a Field of view of 43°at Infinity so the Doctor Optic 750mm (he bought the CZJ LF division) doesn't have a 1300mm image circle at Infinity, it's more like 590mm However these are process lenses so at 1:1 the image circle is far larger.

That raises a third issue as often lenses are shimmed to work at specific focus ranges, so for macro work the cells are shimmed to be further apart. Gets complicated as there's nothing definitive written

The factory made two different 750mm lenses the 750mm Apo Tessar, and two APO Germinars an f9 & an f14.5 which are slightly wider 46° - having an Image circle of 1230mm at 1:1, but half that at Infinity. The last version had a removable spacer between the front cells and shutter for normal (non repro use).

Some where I have a complete set of data on Docter Optic lenses in PDF format probably on a hard drive in the UK. Back to your original question you can't work out the image circle, that's measured on an optical bench or by physical photographing test charts.

Ian

I second Ian on this issue.
If the lens is an apo-repro, it was meant for use at short distances and 1390 mm of image circle does make sense for a 750 at 1:1 on a repro bench where you make offset plates for road maps, about A0-size... using the technology of the sixties.

A picture of a 750 mm Zeiss-Jena Apo-Germinar (the pre-Docter model) is shown here, at the 2006 galerie-photo conference in France, the proud owner is Herr Jörg Krusche.
Although the lens was designed for short distances, used at infinity and stopped down to f:22 - f/32, the results, as shown by Jörg at the conference, are outstanding... but even on a 10"x8" camera, you'll never be able to use all the available image circle.. even at infinity.