Focus and Focus Spread

You might have noticed that the camera doesn't have any measurement scales. That is because it doesn't need any.

The rear carriage can be unlocked from the focusing mechanism and is free to travel in its bearings; it's a very direct way to set up the rear of the camera for coarse focus and swing. Fine focusing the camera is by means of a screw drive, actually a trapezoidal spindle, which moves the camera back via a handle. Drive to the shafts is engaged and disengaged very simply and directly, with a flip lock to each side.. 

You might have noticed that the camera doesn't have any measurement scales. That is because it doesn't need any. There are two basic distance measurements needed on a camera; overall extension, for calculating the bellows factor, and a focusing scale, to give you the distance between near and far focus, which is used to determine the aperture needed to control depth of field.

Setting up a camera for optimum depth of field usually requires you to focus for the near point, emerge from under the darkcloth, take a note of a position on a scale on the camera bed, then focus on a far point, emerge, make a note of the new position on the scale, subtract the numbers to calculate the spread, adjust movements and focus to minimise spread, then repeat the whole process until satisfied, then set the focus on the halfway point. Or stop down and check the screen, if a wide aperture makes that possible. Or whatever other method you feel comfortable with.
Using a screw focus, controlled by a handle and not a wheel, you can simply count the revolutions- one revolution gives 2mm of travel. By the time you've set movements, focused for near, focused for far, then reset for half the distance, you already know the aperture you need before you emerge from the darkcloth and without having to continually re-emerge to check and calculate the numbers on the scales.

This does require learning some aperture values, related to the focus spread, which can be found towards the end of a rather excellent article written by QT Luong, under the heading Table of optimal f stops (2). Or they can be printed out for reference...

Focusing the rear of the camera has some advantages over focusing by moving the lens, though those advantages can be marginal for subjects at low magnification. The main advantage in not moving the lens to focus is that the position of the lens controls perspective, and moving it will alter that. The second advantage becomes apparent when working at high magnification; it can be difficult to find focus by moving the lens alone. Of course, if you're shooting landscape, then these effects are less important.

The screw focus on the prototype is quite long, and allows around 200mm of travel, which is a lot more spread than you'd ever need. The screw drive on the next camera will offer a much more modest focusing range, which will lead to some weight savings. As mentioned earlier, in operation, rough focus is achieved by manually pushing and pulling the camera back with the flip locks disengaged, the focus handle is only used to calculate fine focus. If you find yourself needing more than about 12mm, then you'll be way down at the tiny end of your aperture scale anyway...

As for the bellows factor, a small 1m measuring tape can be used, whenever it's necessary, though if you're confident at estimating the distance between things, then that will work too.

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