Intrepid 8x10 - 4x10 Mask

4x10 Options

There are several ways to obtain a 4x10 image from an 8x10 camera. You can cut down a spare 8x10 darkslide to a sort of L shape, and mask half the film during exposure, you can find a 4x10 back, you can use a camera with a 4x10 sliding back, or you can insert a mask into the back of the camera in front of the removable back. Or just expose the full 8x10 frame and crop the image!

In 2023 Intrepid announced half darkslides for the three formats they produce - 4x5, 5x7, and 8x10. At this time it is too early to comment on them in use.

The problem with the darkslide method is that you have to mark the ground glass with a framing mark. The other options provide a 4x10 image on the ground glass as well as the film. 4x10 sliding backs are uncommon on newer cameras, though some manufacturers have produced 4x10 cameras. There are also two ‘standards’ for 4x10 film holder dimensions which makes the actual format harder to use. Any of the methods that expose two images on one 8x10 sheet assume that one can make notes about which half has been used, and that the development of both exposures is the same. All the pros and cons I can think of are listed at the end of this article.

Creating an Insert Mask for the Intrepid

The inside of the Intrepid camera back has a 255mm square opening, which is 6mm deep (this is the first model Intrepid 8x10 - there may be design changes in later models, so investigate first). On the inner edge are the bellows, and on the outer edge the frame for the camera back. This makes it possible to insert a masking board 255mm x 130mm (slightly more than half of 255mm) to get an offset 4x10 opening. It has to go on the bottom if one relies on gravity to hold it in place. This means that the back has to be reversed (insertion of the holder on the other side) for the second exposure on the piece of film.

Diagram showing a 4x10 mask covering the bottom of the field of view on the ground glass.
Diagram showing a 4x10 mask covering the bottom of the field of view on the ground glass

For verticals, the mask has to be inserted on one side or the other. In this case it makes sense to open the camera back and move the mask rather than try to operate the camera with the film holder inserted upwards. There is no reason that a vertical and a horizontal 4x10 exposure cannot be mixed on the same sheet, though good notes will be needed.

If the mask is inserted vertically, and the film holder goes in horizontally, the result is approximately 5x8 (8 being the vertical). Going the other way, with the 8 inch dimension being horizontal, is harder because the mask has to be located at the top of the back. Unless some form of retainer is devised, this is not going to work. Tilting the camera on its side would mitigate this, but adds extra work.

Lenses for 4x10

Obviously, anything that works for 8x10 will work for 4x10, and if the lens is centered there will be a little more room for movements. Among smaller format lenses, anything that has better than 273mm of coverage is a contender. Most 210mm 5.6 lenses intended for 5x4 will cover that with limited movements. Wide angle designs in the 120mm to 180mm range are also candidates. Centering the lens helps, especially where the lens is limited in coverage. With 4x10 you really need a 50mm (2 inch) rise/fall or shift either side of the 8x10 center for a neutral alignment.

Making the Mask

I chose to laminate two pieces of 3mm birch plywood to make a 6mm thick panel. This was partly because I had 3mm on hand, and partly because laminating two sheets should reduce any tendency to warp. The measurements were verified one last time, and then the two panels were cut. The two panels were sandwiched with a thin layer of wood glue and clamped down to a flat surface. Once dry, the panel was painted flat black to reduce the chance of light reflection.

The Mask in Use

To use the mask, the camera back is removed and the mask inserted in the space behind the bellows. The orientation of the mask determines the orientation of the format. The camera back is then replaced. The orientation of the back compared to the mask determines the format (4x10 or 5x8). The back is set with the film holder insertion direction parallel to the mask long axis to obtain 4x10. The other way gives 5x8. To zero the camera, apply shift or rise as necessary for the chosen orientation.

Intrepid with 4x10 mask inserted and the back off.
The mask in place before remounting the back. The front standard has been raised to center the lens.

The Intrepid provides (according to published specification)

Rise 70mm

Fall 60mm

Shift 60mm

The 4x10 mask requires:

Horizontal 50mm Rise to center the lens, leaving 20mm.

Vertical 50mm shift to center the lens, leaving 10mm in the shifted direction.

Intrepid with rise set for horizontal 4x10 mask.
The camera with the front standard raised to align the lens axis.

The front standard has been set for a 270mm lens (it needs a little more racking out for infinity focus), and the bellows has enough flexibility.

Two 4x10 negatives

First 4x10 iamges.
HP5+, Thornton Two Bath with stronger Bath B (20g/l sodium metaborate) 6+6 minutes. Intrepid 8x10 with 4x10 internal mask, 270mm f9 G-Claron, 1/60 f32. Digital picture, so there are some reflections from the film. Only about 3 stops subject brightness overall.

The edge of the image was actually crisper than I expected, but it is not quite as sharp as the film rebates. That is to be expected, as the film holder slots are in contact with the film, and the mask is not.

Using the masks for 5x8

Vertical (mask is horizontal) 64mm Rise to center the lens, leaving 6mm.

Horizontal (mask is vertical) 64mm shift to center the lens, which is 4mm more than the maximum (probably not significant). If shift in the offset direction is required, switch the back 180 degrees, and move the mask to the other side to give up to 120mm of shift.

One way to get more shift or rise/fall is to use a lens offset in the lens board. This will give 10-20mm of built-in offset. With a Sinar type lens board, as used with this camera, the lens can be mounted in any orientation, though access to the lens controls may be awkward.

Lenses for 5x8

This format requires a minimum of 240mm of coverage with a little wriggle room. The 5x7 format is usually given as 208mm minimum, so some 4x5 and 5x7 lenses will do. Getting down to 90mm is going to be a challenge. A wide of 105mm to 120mm is more likely. Things get better around 210mm, where even the older lenses have some excess coverage.

With shorter lenses there is a risk of the bellows intruding with the amount of shift required. Careful use of a binder clip to alter which part of the bellows expands can help. In practice I have not been able to get my 90mm to center on 5x8 using shift without bellows intervention. Oddly, it is less of a problem with rise, probably due to the different behavior of the bellows with gravity involved.

Coping with the Offset Swing Axis with the Lens Shifted

The extreme amount of shift required to use a masked format vertically means that the swing pivot point is nowhere near the lens axis. One way to avoid this is to mount a slotted rail between the baseboard and the front standard. This rail is locked to the focus base using one of the existing threaded sockets. The front standard is mounted on the rail with the correct centering. If the front standard is aligned with the center notch on the new bolt, the pivot point is axial to the lens. This separates shift and swing.

Holding the mask in the upper position

After some experimentation, I have settled on a simple square ‘U’ shaped rod to hold the mask in the upper position. I chose to 3D print the support in one part - the vertical supports and a joining base - but a couple of balsa wood strips cut to fit and painted black will also work. The support needs to be sized to fit the space between the bellows and the back.

Mask installed in the upper position with the support below.
Mask installed in the upper position with the support below

As seen through the ground glass without a lens in place.
As seen through the ground glass without a lens in place

With the back on, but the ground glass removed, showing that the supports are not in the image area.
With the back on, but the ground glass removed, showing that the supports are not in the image area

The Pros and Cons of Doing 4x10 on an 8x10 Camera

Exactly what constitutes which will depend on personal motivation and the degree of complexity one can stand.

Using the masking panel give three options for format: 8x10, 5x8, or 4x10. Both the smaller formats are half the area of 8x10, but very different in aspect ratio.

The internal mask constrains the ground glass - no composing to marks, what you see is what you get, though the boundary line is not going to be sharp.

It is slower than a dedicated format.

The mask is smaller and more portable than a reducing back, though with some movement restriction. Not much different from a modified dark slide. 5x8 is easily masked to 5x7.

It is probably slower than the masking dark slide method. You would need two modified darkslides to get 5x7, and they would be flimsy. (Two dark slides, with a 5x7 aperture cut in them, one near the handle, the other near the end of the holder.)

Using the internal mask is about as complicated as using the modified darkslide - tracking exposed portions of film needs careful notes and thought setting up the camera.

The back has to come off the camera often, though if one does both horizontal and vertical 8x10 this may not be a big change.

Two exposures on a sheet of film have to be developed identically. This means using two holder sides if different development is required, which adds complexity. Cutting the sheet in two and processing separately is possible, but adds more complexity and extra handling.

The negative pairs will probably have to be separated for contact printing, but could stay together if enlarging.