One of the problems encountered when duplicating slides to positive or negative materials is the build-up of excessive contrast in the image. Professional slide duplicators avoid this by pre-flashing the entire film with a small amount of light before making the copy or fogging the film during the copy.
This article describes a modified traditional method of making film to film copies. If you have access to a digital filmwriter, scanning, retouching and writing to film is the best option.
This method is available to anyone who has a camera with multiple exposure(ME) facility and if they have some lens arrangement capable of giving 1:1 reproduction. After talking with Dave Bassett from the Canberra Photographic Society about problems he has encountered using a zoom copier & solutions he has employed, I came up with this method for my equipment. Dave's main problem using a zoom copier is that it is a fixed F22. He finds checking the focussing very difficult. At 1:1 f22 the depth of field is very small. Diffraction reduces images quality. I decided to try using a close-up filter and telephoto lens which I already had rather than buying yet another "gadget". To my surprise, the first results were more than acceptable. This was especially surprising given that I began with a slightly overexposed slide which I was trying to correct. There is a slight loss in image quality on close inspection however the results exceeded my expectations.
A camera without multiple exposure(ME) facility can also be used. When you load the film into the camera, mark the part of the film leader that aligns with some part of the film transport mechanism very carefully. Close the camera back and wind on the film. Point the camera at a grey card lit by the appropriate colour temperature lighting and shoot the entire roll 2-3 stops under exposed or as many frames as the number of dupes you want to make. Unwind the film and carefully reload so that the mark on the film aligns with the selected part of the film transport. The pre-flashed frames can now be re-exposed with the duplicates.
To explain, the method involves pre-flashing the entire slide emulsion with a plain grey tone of 2.5% - 4% grey (almost black). This pre-flash stops the build-up of excessive contrast normally associated with slide - slide copies or cheap prints made via an internegative. To date, I have only used slides but in principle, negatives should work in exactly the same way.
Though you would probably get better results with specialist duping film, this method can be used with normal E6 daylight film. I'd advise against using high contrast films like Velvia or it's Kodak equivalent. Provia 100 works well as does Sensia 100. Kodachrome 64 with it's legendary accurate colour reproduction should work well though I have not tried it to date. The nice thing about this technique is that you don't have to use an entire 36 exposure duping film but can use a general purpose film which is in your camera and then use it for other things. I seldom want to dupe 36 slides in one go! Usually one or two.
I have been using a flash as a light source and a telephoto lens with a good quality two element close-up filter as my slide copier. The slide sits at the end of a short cardboard tunnel. Using flash ensures no problems with vibration or colour balance. The method works with exactly the same principles for other types of lighting. I have used tungsten film (Fujichrome 64T) with a tungsten light source. You should be able to use a white or grey card in daylight, or a tungsten light source with an 80A filter & daylight film or use tungsten film with tungsten light source.
The Method using flash
A) Set the multi exposure facility for two exposures.
B) First expose an out of focus image of a white or grey card with no slide at 2 -> 3 stops under the meter reading or use -2 -> -3 stops compensation on a TTL flash.
C) Then make an exposure of the slide at 1:1 or greater if you want to crop the slide. You can use the meter reading but as usual for continuous light sources or TTL for flash, watch the subject material & adjust accordingly. Overexpose for snow & sand under expose for dark subjects just as you would have for the original subject.
Using a Flash
On my camera I set 2 exp ME & I bounce the flash (Guide No=28) off a grey card and dial in -2 -> -3 eV compensation(full manual mode). The grey card is photographed well out of focus to ensure no surface texture shows up.
The amount of pre-flash is estimated depending how contrasty the original slide is. A flat slide gets 2% or if it's really flat - nothing. A contrasty slide gets 4% (-2 eV compensation)
TABLE 1
Pre-flash settings
18% grey - 2 stops = 4.5% grey use for high contrast original
18% grey - 3 stops = 2.2% grey use for medium contrast original
no pre-flash use for low contrast original
My grey card is a sheet of 18% grey matt board($9) If you don't have a grey card, use a white card. The metering system will still render it 18% grey and a -2 or -3 stops will render exactly the same tone. The important thing is to have it a neutral colour. If using flash, you may need to check that the flash to card distance is not too small otherwise the TTL system will not reduce the light intensity sufficiently to produce the 2-4 % grey.
Set the multiexp to 2. Pre-flash the film with the first exposure, then copy the slide with a 1:1 lens looking down a black cardboard tunnel at the slide with a white piece of cardboard for the flash to bounce off. I use a two element close-up filter on a telephoto lens. They produce excellent images for their cost and there is none of the light loss associated with extension enlargement so focussing is easy.
If using the TTL flash take care not to bounce at less that the minimum flash distance or you will overexpose. I use 1 metre with a GN28 flash to the white card to copy the slide and two metres to the grey card to pre-flash. The black tunnel stops the scattered light of the on camera flash hitting the slide.
In my opinion, this works much better than a cheap zoom slide copier because you can focus at full aperture and expose at f11-f16 for sharp imaging. The cheap zoom copiers are always at f22 and are difficult to focus.
The method requires careful technique & application for good results
Using this technique, you can darken a slightly over exposed slide or brighten a slightly underexposed one.
Easy traps to fall into (believe me!):
1. Forget to set multi exposure and put the pre-flash and the slide copy on two different frames of film.
2. Forget to change the exposure compensation, from -3eV for the pre-flash, to 0 eV for the slide copy and under expose the slide by 3 stops.
3. Don't forget that the finder is 92% of the image when framing with many cameras so if you carefully crop the image, it may not come out that way on the final.
4. Make sure that the slide surface is perpendicular to the optical axis of the lens or you will distort the image and you may lose focus across the image. This is a problem for me because my camera is on tripod and the slide is attached to my rectangular cardboard tunnel which sits on a table.
If you employ this method, do it without interruptions or distractions.
Write out every step in big writing on a card in front of you
or use a checklist.
It is complicated but the quality of the reproductions I have achieved using this method have been excellent and available to anybody with a TTL flash and a camera body capable of exposure compensation and Multi exp or is possible with any equipment and a little thought. If the camera doesn't have compensation you can possibly change the ASA settings to fool the camera. Using a set of Cibachrome enlarger filters, it is possible to do colour corrections. I use this method to correct small exposure errors (<1/2 stop) and only on otherwise very good slides. Colour correction without an analyser is tricky.
This method can be performed using a white card in sunlight as the light source rather than flash. Pre-flashing ratio etc is the same. I find it is more convenient to be able to work in the evening hence the use of flash. This method can, with careful execution, deliver better results than the lowest quality dupes available from a prolab at 1/5 of the cost. If you are after reproduction quality dupes, the $5 dupes from the prolabs are probably the way to go.
This diagram is not to scale. In my apparatus, the cardboard
tube is about 200mm long and the card is 1 - 2 tube lengths from
the end of the tube. If the white card is too far, the TTL flash
has insufficient grunt, if it is too close, the tube casts a shadow
across the white card.
