We will start by adjusting the lens distance and the tube length to obtain a view less than 100% wide, say 90%, to allow cropping, alignment correction and / or a slight inclination: useful for compensate for example a leaning horizon on the original. I printed a grid on tracing paper, in 38 × 35 format — that of the slide with its margins — placed in an empty slide frame (see photo on page 2). It allows me to check whether the slide holder is horizontal and to focus at least temporarily. It also allows to do a manual white balance with a flash stroke.
The camera is mounted on a tripod (not mandatory but more practical), as well as the flash, which can also be placed on its base or on any support. The white polystyrene screen is placed between the slide holder and the flash. The camera and flash are connected. The latter is set to manual mode, to adjust the power of the flash at will. Focusing is done by directing the camera towards a sufficiently strong continuous light source such as daylight or a fluorescent lamp. Then we direct the camera to the flash, adjust the lens aperture around f/8, the flash power to 1/4 and take a picture. Using the histogram, over / under-exposure is checked, and corrected by changing the flash power or moving it away from the screen. Naturally, when the actual slides are taken, a different correction will probably be necessary, but this first adjustment gives a good starting point.
Note: there is no need for a cable release or remote control, since the camera, the stand and the slide form a rigid set and there cannot be a camera shake!
Here is a sketch showing very schematically the indicative distances between each element of the system. It must obviously be adapted to each particular case. This is an APS-C sensor camera. One sees the interest of choosing a focal length short enough to limit the overall length. Normally, an actual macro lens will not need elongated tubes or bellows, since it is supposed to reach the 1:1 magnification without any accessories. I gave the values for a 28 mm lens as an indication, because I don’t have a 35 mm on hand.
Focal length ▼
If the preparation has been well done, there will be no major changes to be made. Be careful, however, to check the focus from time to time, even if, at f/8, the depth of field should tolerate a slight shift of the slide. But it is better to re-focus if you change batch and especially frame thickness. For each slide, pass a blower on each face before placing it in the slide holder. Dust and dirt are indeed the worst enemies of the slides, and you will see that there is enough dirt glued on the emulsion not to add some more! And of course, check the histogram after each shoot to avoid overexposure at all costs.