Some thoughts on the design of a lightweight 4x5

[Neil Barnes]

Currently busy with a lathe and a slab of one inch bar, making tripod mounting bushes. In spite of the fact that every camera ever made has one of the damn things, they are impossible to find, so I had to make them.

I've also got the bi-directional back finished, and the focusing screen almost done - it just needs the glass (polycarb to start with) grinding and fixing, and a couple of springs to hold it all together.

Neil

[Neil Barnes]

It's coming along... here's the tripod bush - I made a couple - and the focusing screen and dark slide holder. It still seems to work, in spite of me breaking a screw when mounting the springs. Aye well!

Neil

[banana_legs]

Neil,

Looking good; those tripod bushes are sweet. I welded a nut to a steel plate for my first 4x5 and it really was not pretty!

Best regards,

Evan

[Neil Barnes]

The one underneath is carefully posed to hide the fact that one of the screwholes is a little off-line... but to be fair, it's nearly forty years since I last used a lathe in anger. My godfather, who was a clock and watch maker, and left me the lathe, would have been disgusted. But the next ones will be better.

At present, I have the main body of the camera complete, with a polycarb focusing screen and the slide carrier which works in 4x5 or 5x4 orientations. Next step is the dual-extension - one with pins and one with a rack and pinion. That uses something else I made this week - PTFE slide bearings which have an internal diameter of 3mm and an outer diameter of 4mm. They weren't easy, either!

Neil

[dave_whatever]

Lookin good Neil. You should sell some of them tripod bushings, like you say there's clearly a gap in the market. I'd buy one.

[Neil Barnes]

I wish you'd told me last week! Now the lathe and the bit of brass is six hundred miles away. I'll have to see what my father can do...

3/8 thread, by the way, 25mm overall diameter (give or take a couple of thou to round the bar up) with the screws on a nominal 19mm circle.

What's a fair price for one? I have no idea, beyond the material cost.

Neil

[IanG]

There's a wide variety of different T-nuts - tripod bushes available from McMaster Carr http://www.mcmaster.com/ - Very inexpensive. I also need 3 or 4 along with rack & pinion parts for a geared trackbed and they have a good selection.

Ian

[Neil Barnes]

I'm investigating them, thanks, Ian. The weld nuts http://www.mcmaster.com/#weld-nuts/=d43pid in 3/8-16 are hard to argue with at six bucks for fifty. Not as pretty as brass, though!

http://www.hpcgears.com/ are where I got inexpensive plastic rack and pinion gears. What I *can't* find is camera-baseplate sized sliding parts - everything I come across is 'industrial' size and accordingly priced. A combined 45 degree interlocking slide, a couple of mm thick, with an included rack and pinion, would be lovely.

Neil

[dave_whatever]

There's a wide variety of different T-nuts - tripod bushes available from McMaster Carr http://www.mcmaster.com/ - Very inexpensive. I also need 3 or 4 along with rack & pinion parts for a geared trackbed and they have a good selection.

Ian

Unless I'm going blind it doesn't look like there are any BSW tnuts or weldnuts on there. Or are we saying that the american class 1B is close enough?

(P.s. shirley there must be a uk-based equivalent to that site? Ideally selling true oldschool whitworth stuff!)

[Neil Barnes]

3/8-16 UNC *nuts* are a loose fit on a 3/8-16 Whitworth bolt. The other way around and they'll jam.

Whitworth thread is better, though (like the tap I used )

Neil

[Neil Barnes]

Progressing nicely - the second-extension slider is in and moving, though I haven't thought in detail about a lock for it. Probably just a pin through a hole - i don't expect this to move much. It carries the rack and pinion for focusing and the front carrier support rails - and the carrier, of course.

Neil

[Neil Barnes]

I knew all that time spent playing with meccano wasn't wasted. Here's the front carrier and main focusing rack. The wooden dowel on the gear drive will be replaced by something stiffer, when I've worked out how to attach the gears and knobs.

Neil

[Neil Barnes]

It's coming along. I ran out of velcro for the back of the bellows.

The front isn't rigid enough. I may remove the sideways shift and restrict it to twist only; it's the slot that's causing most of the twisting in the base; the verticals are fine.

Neil

[Robert J Fallis]

That is very nice neil, shows a lot of skill and hard work

bob

[Neil Barnes]

Thanks, Bob.

This is of course just a pre-prototype to get the feel of how things might fit together, but I'm learning stuff already...

- weight at present (still missing a few knurled knobs) is a touch under a kilo. Around a quarter of that is in the lens and bellows, and the commercial bellows weigh about the same as my homebrew. The carbon fibre I intend to use is stiffer than the ply at one third the thickness, but has a touch under twice the density, so that should give a weight reduction in the sheet materials of around 30-35%. Use of rectangular section CF tube instead of the beech blocks will save weight, too. So it's possible that the weight could still be close to the half kilo, sans lens.

- the focus traveller works really well with the CF bar runners and the rack and pinion, though I may need to find different adhesives to to fasten the rack. (There are very few screws or bolts on this proto - just where I need to go through metal. There are a couple of of concealed dowels locating wood-wood joints.)

- the focus screen is simple and easy, and works the same as my MPP - just slide the dark slide in between it and the rear reference plate. The thickness of the offset to the screen is critical and can't be guaranteed in CF sheet; this may need shims measured in manufacture. At present though, I got lucky laminating two different thicknesses of ply together and a light sanding to get the 4.8mm required. Lightly sanded polycarb appears to work as well as glass, though I haven't tried it in anger yet.

- the sideways shift and rotate mechanism isn't strong enough. This is down to the the bottom bar flexing around the slot, and not having sufficient fore-aft contact area to prevent the whole thing rocking. I have an idea or two as to how to fix that, but I need to make some more bits and play with them. I don't want to lose all movement there, but I may restrict it to rotate only.

- equally, I'm not sure, now it's there, about the lens tilt. With a pivot point in a slot, it's too easy to add tilt when you want shift and vice versa.

- I'm wondering if I really need the double extension. Removing it makes things lighter and a little simpler, and improves the baseplate stiffness. It also means I can use a shorter bellows (= cheaper - they're not cheap at that length!).

These latter three points are raising the issue of intended use. This is meant to be a light view camera; something one can carry up a mountain without requiring a Sherpa or drag all day through a city in search of interesting architecture. When you *need* the movements, that's when you use your Sinar or Ebony or Horseman - at four or five times the weight. In most of my work, the only movement used is vertical shift - converging verticals do my head in!
I think also that for both landscape and architectural work, it's not often that long prime lenses are required; when I do it I find myself crying out for wider angle (and I can't afford a Super-Angulon!). At present, the bed, without extensions, is 200mm long. Focal plane to foremost lens position is about 200mm, and there's another 150mm available on the extension. But my lenses are 110mm and 135mm... a back of the envelope calculation tells me that with a 90mm lens on infinity, the last couple of centimetres of the board will be in shot.

So, I think on that basis, I shall simplify the design a little. I can always unsimplify in a later version... but I'd like to hear people's views on this.

I will bang a few sheets through this one, once I've done something about the front carrier (and got the knurled knobs I'm waiting for!) and see how it feels.

Neil