A Brief History of Focusing, The Begninning

What makes a camera, and by extension an image produced by one, different from how we perceive the world with our eyes is focus. Our eyes and brain work together to insure that the world is always in focus regardless of where and what we look at. However a camera and lens cannot simply reproduce the world as we perceive it, nor is that in general desirable. Focus and depth of field are inherent artifacts of lenses and using them is one of the ways a photograph can fame the world in a unique perspective when making a photograph.

Focusing, the actual process of adjusting the lens’s position, has changed little over the history of photography. Even modern autofocus lenses still shift the position of one or more lens groups.  However the way we measure and control those changes has. Hopefully this series of articles will shed some light on the evolution of focusing.

What’s perhaps not immediately obvious is that one doesn’t actually look though the lens when working with a camera. What, in fact, you’re looking at when looking though the viewfinder is an image projected on a focusing screen. This may seem a bit odd at first, but it’s necessary because the lens is actually focusing light over a large area at a fixed distance to form the image on the film.

The Ground Glass, How To See Though Film

Figure 1: Focusing on a ground glass.

Imagine for a moment the problem that’s posed when one uses a view camera.  A view camera provides no viewfinder at all, and even if it did it would be rather useless given the amount of adjustments that can be made. Instead the operator looks thought he same lens they’ll be imaging with and makes their adjustments to movements and focus. However the problem should be pretty obvious, if the film is in place, there’s no way to see though the lens. If the film is removed, the photographer’s eye would have to be in exactly the right spot to form a proper image.

The solution is a ground glass, the most basic focusing screen. A ground glass is exactly what it sounds like, a sheet of glass that has been ground on one side to a matte finish. The matte finish is key, that’s what allows the image to form on the surface of what would otherwise be a transparent piece of glass.

The ground glass does offer some advantages, even though it might not seem that way at first. First, there aren’t any special optics between the lens and the image used to focus. This means that there is nothing to mask the detail allowing the photographer to insure focus is placed exactly where they want it. However as much as that is an advantage for focusing, it doesn’t make for a bright surface to look at. In fact, the basic ground glass focusing screen is the darkest focusing screen.

The other advantage has more to do with the size of early cameras. Most early photographic work was done with what we’d now call large format cameras, with frames larger than 60mm on a side.  When the focusing area is that large, it’s practical to employ another aid such as a loupe to magnify the image yielding even more accuracy when focusing.

The Need for a Focusing Aid

As photographic formats became smaller the problems with focusing changed. No longer was there room for a large glass screen that could be used with a loupe against and inspected while hiding under a curtain to reduce glare. Compounding things, the smaller film size allowed for new compact camera designs with the objective of being easily portable. Even if it was desired to use a loupe it would defeat half the purpose of the smaller camera.

A view of the focusing screen from a pre-war Zeiss-Ikon Ikoflex II

To illustrate the problems with a simple ground glass, the image to the right shows the view through an pre-war Ziess-Ikon Ikoflex. The illustration is a bit deceptive, as I actually ended up use a flash to illuminate the scene after focusing under as much light as I could get and guessing some. With out the flash the metered exposure for the screen was about 2 EV. The actual light levels metered at about 7 EV.

The easiest solution, if it could be called that, to the focusing problem is to simply not focus. In fact there have been several cameras made that used hyper-focal focusing (most cell phone cameras do this today) in an attempt to get around the focusing problem. However hyper-focal focusing removed the ability to use focus and depth of field to creative ends. It also be necessity limits the maximum focal length that can be used and fixes the aperature, usually to a small one around f/11 or f/16.

The next easiest solution is to simply guess. That is, mark the lens at points that correspond to different distances and turn the lens to the distance that seems about right. While it might make distances scales sound completely useless, they aren’t. In fact, they persist on many lenses even today though often in a simplified for.

Guessing works okay in some situations, but proves to be almost useless out side of them. The two biggest problems are when used with fast lenses, lenses with long focal lengths or worse both. Fast lenses, necessary for selective focus and blurred backgrounds, have very shallow depths of field necessitating more accurate focusing. Long focal lengths too have very shallow depth of field, and require accuracy at quite great distances before the depth of field is large enough to matter. The combination of a fast long lens compounds the problems even more.

Clearly a better way is needed. Next time we will look at how to find the distance to the subject without using a ruler or guessing, or even leaving the camera.