Plate DDS comparison
A roll of 120 format film has been included for scale measurement & comparison.
On the left is a standard half-plate double dark slide [DDS].
Its smaller axis dimension (the most important) measures 150mm.
On the right, is a British standard whole plate double dark slide. This measures 195mm.
So what have we here?
This is a Double Dark Slide Film Holder [DDS for short]. It is a later development from the book form holder.
It's smaller axis dimensions here is identical to the British Gandolfi Bookform Plate Holder: 150mm.
This is very important, since with a little modification, these DDSs can fit into a half-plate camera like a Gandolfi, but NOT a Thornton Pickard half-plate camera.
These DDS are made by Kodak and labelled "For 4 3/4 x 6 1/2 inch plates".
There are similar 5"x7" DDS slides which have different axis measurements and are not half-plate compatible, particularly with modified half-plate cameras.
From the top left: a blank sheet representing the area of the half-plate film. On the top right is a closed DDS. On the bottom left is a withdrawn slide from a DDS and on the bottom right, is the exposed area of the DDS where the film will be positioned.
This is it.
From the top, the plate holder now has its sheet of glass inserted.
As film is expensive, I've not loaded a sheet since this would be wasted. Also, it would cover the glass.
From the viewer's plane towards the back of the image, here are the layers as a cross-section:
1. Plate-holder Sheath (removed for exposure)
2. Film sheet (emulsion facing viewer and/or the lens)
3. Glass sheet
4. Metal septum (with two prongs)
5. Glass sheet (rear)
6. Film sheet (rear)
7. Plate-holder sheath (closed in this image).
The sheath has been withdrawn and folded over and the tint of the glass can be seen against the tungsten (yellow/orange) light.
The quarter plate mask here has been taken out and placed on top of the half-plate box of film. The area of black of the mask represents the dimensions of the half-plate film and the aperture area represents quarter plate.
Bookform Holder Open
[In day light]
The middle plate swings and locks down with a tab, just visible at 3o'clock.
There are many methods for loading dry plates with film. This is the method I use.
1. Load the plate holders with the glass plates first. Have the holder flat as illustrated.
2. Starting with the right side, a glass sheet fits into the wooden grooves covering the black area.
3. Lock down the swinging black light baffle and the pressure prongs will apply against the glass plate, holding it in position.
4. Now load a glass plate in the left hand side: the left side of the holder should be flat as the glass plate is lowered in place. The rest of the right-handed assembly can be swung over and closed on top of the left hand side.
The book form holders are now ready to accept film.
Now things are coming together:
1. Book Form Plate/Film Holder
2. 2mm cut-glass (deburred edges)
3. Half-plate film [118mm x 163mm]
Efke PL25M film, also known as Efke 25 is also known as Adox 25 after a rebranding exercise. It's one of the late great vintage high silver single layer films manufactured from Croatia and is available in half-plate size [April 2007].
The plate holder has been unsheathed for demonstration purposes only. The exposed black area is where the film fits in. Behind the film fits the sheet of glass which can be seen overlaying the film box.
Bookform exposed II
Here we have the half-plate book form holder with a sheath withdrawn to its furthest reach. There is a self-limiting stop for the sheath and it can not be withdrawn any further, hence no force should be applied.
The holder on the top is fully closed. The holder on the bottom is unsheathed and unclasped.
The position of this holder is for demonstration only - the holder should never require the clasps and the sheaths to be removed simultaneously.
1. The clasps are unclasped for loading and unloading film/plates.
2. The sheaths are used sequentially (front side then rear side) for exposures.
3. Any time the clasps and the sheaths are raised, an error in operation will result.
Notice the smooth deburred edges.
The glass can be cut using plain 2mm picture glass using a glass-knife/scorer and ruler.
Deburring of the glass edges can be used with a limestone knife sharpener. The reflectance of the glass plates does not matter if these plates are to be used as sheet film pressure plates. However if these are to be used for liquid emulsion or for collodion type negatives, then it is imperative that the glass is cleaned down with isopropyl ethanol to remove grease stains from handling which might otherwise cause the emulsion to lift from the plate or fail to adhere completely.
It would be highly unusual to find any glass plate within a bookform holder. Most of the used book form holders a photographer ever comes across will be void of internal glass plate.
Why does this matter?
Inside the book form holder is a blackened divider (see image below) which holds two pressure prongs. These prongs apply pressure against the plate in order to attain a flat level surface. If film is placed directly onto these prongs, the film will be scratched and the film surface will become warped.
Therefore a card cut-out, plastic sheet or glass plate can be inserted so that the same functional effect of a glass plate can be obtained.
I choose to use glass plates for several reasons, including the reduction of dust and debris. One day I hope to coat my own glass plates with liquid emulsion.
These glass plates were cut down to size. For half-plate film, these plates must be: 118mm x 163mm. The margins must be burred otherwise the glass will present sharp edges and hazards to skin during loading in the dark.
On the top, the bookform plate holder is closed.
On the bottom, the bookform plate holder has had its brass side-clasps unfastened and has not been opened and rotated 90 degrees.
Observe the following:
1. On both sides of the middle dividing plate are black areas
2. These black areas are the reverse side of the removable holders
3. Therefore these black areas represent the actual area of exposure
4. The black areas are also hinged with black cloth tape
5. This enables the holder cover to be flexed when withdrawn
6. The middle dividing plate acts as an internal darkslide to prevent fogging of the complementary negative.
7. The dividing plate holds two prongs which exert pressure against the plate.
However this is not all. A half-plate holder holds more. The current open position is primed to accept glass plates only. Users of glass plates place the emulsion side towards the hinged cover; fold over the middle divider, and carefully lock this before proceeding to do likewise with the contralateral side. The technique for loading film requires very little modification using the same principles.