Saturday, January 26, 2013

Digitising old negatives - Part 2 - exposure

In my last post I described how I obtained and assembled a negative or slide copier that will work on a crop-sensor (APS-C) DSLR.  In this part I'll explain how to I got consistent results.  I was aiming for three things

  1. The slide had (obviously!) to be in focus
  2. Exposure had to be more or less correct
  3. Colour negatives have a strong orange tint.  I wanted to remove this as much as possible (I could have left it all to post-processing on the computer, but figured that the less drastic the adjustments I made in software, the better the overall quality would be).
Focusing was easy, because the M42 to Sony lens mount adapter I have has a "focus confirmation" chip, meaning that when you turn the focus ring on the lens a green dot will show up in the viewfinder when the image is sharply focussed.  This isn't usually perfect, but in this case it's good enough.  I set the aperture on my lens to its widest (lowest number - f/1.7), loaded a negative into the holder, pointed the camera at the light on the ceiling and adjusted the focus until the green dot appeared.  If your adapter or camera doesn't have the benefit of focus confirmation, or "live view" or equivalent, then you may have to find the best focus point by trial and error. Adding one or more extension tubes to a lens reduces the depth of field (range of distance for which an image is in focus) drastically so be prepared for some fine tuning.

To get a consistent exposure, I decided to use my flashgun.  This also meant I could compensate for the orange tint in the negatives by putting flash gels (tinted pieces of plastic) in front of the the flash.  I bought a few sets very cheaply from and rather than buy a clip for them found it easiest simply to use the wide angle diffuser that came with my flash to hold them in place.  My flash (a Sony F42AM) can be triggered wirelessly by the camera but a cable-attached one would have worked too.

In order to get as sharp a picture as possible, I set the aperture on the lens down to f/8 (most lenses are at maximum sharpness around this point; the increased depth of field from a smaller aperture also meant it would matter less if the focus was slightly off).

I then experimented with various flash power settings and distances between the flash and the negative to get a reasonable exposure (one where the histogram displayed on the camera when reviewing pictures bulges more or less in the middle).  I found that setting my flash (which has a Guide Number of 42) on half power and placing it the length of a standard bic biro (14.5 cm) from the negative gave a good result.  The inverse square law applies here (light intensity falls off by a factor of four as the distance between flash and subject is doubled), so full power at about 20cm distance should give an equivalent exposure.

Because I'm using a fixed aperture and setting the flash power myself, shutter speed is more or less irrelevant but I put the camera into Manual mode and chose a shutter speed of 1/160.  A couple of minor camera settings which helped (again to ensure consistency between shots) - I set white balance to "flash" and the "DRO" optimiser off.  Finally I set the camera to save both RAW and JPG files; Part 3 explains why.

To work out which combination of gels to use, I put some an unexposed but developed part of a negative (from the start or end of the film) into the slide holder and took some shots, swapping various gels in and out, until the colour most closely resembled light grey; this took three Half-CTB (Lee filter 202) gels and one Quarter plus green (Lee 246) one.

My first few shots were spoiled by dust and dirt on the inside and outside of the diffuser glass at the end of the Accura; it's important to make sure that this - as well as the negative of course - is as clean as possible before shooting.  My negatives were in strips of four, and to start with I was lining them up through the camera viewfinder which was tedious as I had to open up the aperture so I could see enough to adjust the position of the negative, then close it down again (whilst remembering not to touch the focus ring) and of course ensure the camera was positioned correctly relative to the flash.  After a while I got a feel for where the negative should be (going by the positioning of the sprockets on either side) so was able to shoot, move to the next negative in the strip, shoot again, etc. reasonably quickly.  The results were decently sharp and exposed - how to process them into good positive pictures again?  That's the subject of Part 3.

One final point on the negatives - it's best to have the side with the film emulsion facing the camera which means the image will have to be "flipped" on its vertical axis later.  This means putting the negative into the holder (a) with the picture upside-down, and (b) with the lettering / numbers at the top of each frame showing back-to-front when looked at from behind the camera.

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