Friday, July 13, 2018

Digitising old slides using an Ohnar zoom slide duplicator

I have loads of old 35mm slides, mostly from my days of film photography using the Olympus OM2 system, but also some even older. There are some good wildlife photographs here as well as stuff of historical, family and personal significance. Over the years, I have used a number of means of capturing these including a CanoScan III film scanner, a "Compact Digital Film Scanner" from Jessop's and a 35mm holder for a flatbed scanner. None of these have been particularly satisfactory and none of them work with my current hardware.

Be that as it may, there are times when I want to recover images from my old slide collections. So I have recently tried yet another approach: I have an old device - an "Ohnar Zoom Slide Duplicator" by Bush & Meissner Ltd.

I don't remember when I bought this, but probably in the 1980s. It is designed to fit on a 35mm SLR camera using a T2 mount - and mine has an Olympus T2 mount, so clearly dates from my Olympus OM2 days. The 35mm negative or slide goes in the holder (at the right hand end in the photo above) and the whole device is pointed at some bright light source (e.g. an electronic flash). The holder has a translucent white diffuser built in. The holder allows the slide to be moved from side to side and can be moved up and down and rotated. The device also has a built in 1 - 2.5x zoom. So it is possible to photograph the whole slide or any portion of it.

A problem is that this device was designed for use on a 35mm SLR. I expect that it would deliver an image of the whole 35mm frame when on a full frame DSLR body but, mounted on a crop sensor DSLR such as my Canon 80D, the minimum magnification is effectively 1.6x (the body's crop factor) and it is not possible for me to capture the whole frame.

In practice, it mounted easily enough using a Canon T2 mount. I find that a good exposure is only achieved by trial and error, which when using a DSLR, is not a problem. I simply inserted a slide, pointed one of my Yongnuo 560 III flash guns at the diffuser, put everything on manual, set the ISO to 100, shutter speed to 1./250 (the flash sync speed of this body) and then took a series of trial shots, adjusting the flash power each time until I got an acceptable histogram. I find that the density of slides varies quite a bit, so the flash power needed for a good exposure varies by at least a stop either way between different slides, but typically around 1/16th power.

The main problem I have encountered with all of these devices (and this is no exception!) is that they tend to significantly increase the contrast of the image. Even using the best method and exposure, the resolution and dynamic range of 35mm colour slides are not actually very good compared to what we have become accustomed to with modern digital images, so the resulting images are rather disappointing whatever the capture device.