Slide digitizer

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This simple gadget makes it easy to DIY digitize old slides & negatives.

Slide digitizer 1909-8.jpg


Quality

These work ok, but dedicated slide scanners are better optimised. Alignment depends on the accuracy of the wire work, and contrast & saturation tend to be lower with a diffused light source than with condenser light sources.

Making it

Camera platform

No camera platform is needed, a mounting bolt (as used on tripods) will ensure its always in the right place. The platform is offset to one side so that when the camera sits in the middle of the platform its correctly positioned. It was cut to fit the camera's feet, and positioned so the lens would be in the centre of the board.

Check the screws are properly countersunk to avoid any bumps under the camera. Since we're only screwing into thin board, a countersink must be cut before inserting the screws.


Slide holder

Slide holder above 2065-3.jpg
Slide holder front 2062-4.jpg
Slide holder side 2059-3.jpg

The wire slide holder was made from 2mm iron wire. 1.6mm is rigid enough, but 2mm was chosen to ensure it survives a degree of abuse without damage. Thicker wire would make working it hard going.

Power up the camera, place it on the platform, and holding the slide in one hand, move the slide until its position suits the camera. It needs to almost fill the camera frame, and of course the camera must be able to focus on it ok. When a satisfactory position is found, mark where the slide needs to be on the baseboard, and drill a pilot hole there in the centre for the screw.

Before making the slide holder, we need to determine at what height it needs to hold the slide. Measure the height from baseboard to the centre of the camera lens. This equals the height we want from baseboard to the centre of the slide. Subtract half the slide height and you've got the distance from baseboard to the bottom edge of the slide.

Make the wire slide holder, starting at the bottom and working up. The wires that support the bottom of the slide should be horizontal so that any slight sideways movement of the slide doesn't make it rotate a bit. Put the slide into place after every bend you make, so you can correct any misalignment before continuing. The main thing is to avoid bending the wire that's already been bent, so a pair of long thin nose pliers was used to hold the already bent wire still, while the new bends were made just using hands. Finally a few scrapes of a file smoothed off the cut wire. The wire work took about 5 minutes.

3D views of the slide holder to show how it works. in use and empty.

Negative holder

You may want to add a separate negative holder placed as close as possible to the slide holder, right behind it. Almost any computer image manipulation program can turn negatives into positives. Check the wire of the negative holder doesn't intrude on the slide's picture area. Negatives aren't rigid, so ideally use more wire framing to support the neg as fas as possible.

Another option is to use an empty slide frame as a negative carrier. Minor modification of a plastic slide carrier allows negatives to be slid through. This option was chosen for the model pictured.

Camera mounting bolt

- as used by tripods etc. I didn't fit one initially, and its not essential. Adding one makes the digitizer usable while hand held. It makes it a bit easier to use too.

A bolt fits through a hole drilled in the baseboard, and screws into the base of the camera to hold it still. Fit a wingnut onto the bolt and tighten it hard aginst the bolt head; now you can operate the mounting bolt by hand. (There are various other ways to improvise this of course.) The standard camera mounting bolt thread is 1/4" whitworth.

With a bolt head sticking out the bottom the board will need feet adding. These might be a strip of the baseboard cut off and glued on the underside at each end.

Cover

A black card cover is fitted to cover as much as practical of the area between slide and camera. This prevents room light bouncing off the front side of the negative and reducing contrast & saturation.

Light

Any light source used is very out of focus to the camera, and this fact is used to eliminate marks and pixellation, common issues with ad-hoc light sources. The baseboard was left long to ensure light sources are always strongly defocussed.

This defocussing doesn't mean you can use an uneven light source, any source used needs to present an even light field.

sky

The sky is the best ad-hoc light source. It provides plenty of light, and plenty of blue content. The digitizer can sit flat on a table outdoors looking at white card at a 45-ish degree angle. Direct sun isn't needed.

monitor

A computer monitor displaying blank white (eg windows notepad) on max brightness doesn't provide anywhere near enough light for a digital camera to achieve decent shutter speed or ISO. Because the slide area is so small, it must be strongly lit to make the camera happy.

lamp

A 100w lamp could be used to light white paper just inches from the bulb. Ensure you don't end up with the paper on the bulb though. This will never give as good quality as skylight, as the blue output of domestic lighting is low.


Macro lens

Some sort of macro lens is required to enable focussing on the small slide just inches away. There are 3 possible ways to do this:

macro lens

These are available for SLR style cameras. Changing the lens is not practical for other camera types.

extension tube

This is a tube that goes between a standard lens and the camera body. This changes the optical characteristics of the whole lens assembly, enabling close up work. Again these can only be used with SLR cameras. These are much cheaper than a macro lens, but less flexible.

external lens element

This option can be used with all types of camera. A single additional lens element is placed as below, resting gently against the built in camera lens. This enables the camera to focus on very close objects, which is needed for a small slide to fill the frame.

 ___
|   |
|   |_
|    _| |)
|   |
|___|

 cam     lens

Choosing a lens

A lens of +6D works with zoom equipped digital cameras. Cameras with no zoom are more fussy about exact lens strength, and generally need a stronger lens. For these the procedure below is advisable.

To determine the exact lens strength you need, power up the camera on the baseboard, setting it to minimum aperture if possible. Move the slide by hand until its image nearly fills the camera's frame - it will be heavily out of focus, don't worry about that for now. Mark the slide's position. Now, the lens you need will have a focal length of approximately the distance from the slide to the camera. To convert this to diopters:

  • D = 1 / focal length in metres

When this doesn't precisely equal an available lens strength, pick the next lower diopter value.


Exposure

The camera adjusts its exposure according to the amount of light and dark in the slide picture. With automatic exposure adjustment this often results in overexposure, losing picture detail. Part of the cause is that with slides the ratio of average light level to peak white in the picture is generally significantly lower than paper photos, and digital cameras tend not to respond correctly to this. A simple solution is to set the cam to use exposure bracketing. This way each shutter press produces 3 pictures, with one lighter and one darker than it judges best.


See Also