How did photographers focus before autofocus, and how did fixed-focus cameras work?

Question

Before autofocus existed, how did photographers reliably achieve sharp focus? On a modern DSLR, manual focusing through the viewfinder can feel difficult because the image appears small. Were older cameras easier to focus, or was precise focus simply less critical in the film era? Also, how did simple fixed-focus cameras work without any focusing controls?

autofocus depth-of-field focusing-screen fixed-focus manual-focus

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

9y ago

user11924 · Accepted Answer

Through-the-lens focusing cameras had focusing screens — usually ground glass or fresnel lens (related: What is a focusing screen?). View cameras (the old-style large cameras with bellows) projected the image onto the focus screen. The photographer directly inspected the image on the focusing screen (perhaps using a loupe to magnify areas of the image), often under a blackout hood. When it came time to capture the image, the focusing screen was replaced by the film holder, at which time the film could be exposed.

^{Image on focus screen of Sinar F 4×5. Image © Guillaume Piolle, CC-BY-SA-3.0}

Split-prism focus screens were the most common on SLRs. The prism pattern in the center of the screen purposely "bent" the out-of-focus rays in opposite directions, magnifying the visibility of out-of-focus areas. Most split-prism screens also had a micro-prism (also called micro-raster) ring or collar surrounding the split-prism circle in the center of the screen. The micro-prisms especially helped for the fine focus adjustment near in-focus. This 5 minute YouTube video demonstrates the use and effect of both the split prism and the micro-prism ring.

Below is an example of the split-prism focusing. The ring of micro-prism "stippling" surrounding the split-prism is also visible in the unfocused image:

^{SLR split prism focusing screen images, unfocused (left) and focused (right). Images © Dave Fischer, CC-BY-SA-3.0.}

There used to be small but active community of people retrofitting their DSLR with split prism focusing screens. However, most of the companies that made split prisms for DSLRs have stopped making them. See also: Do focusing screens exist for modern DSLRs?

Here is an animation of a Nikon Df retrofitted with a split prism focusing screen in action (note, there doesn't appear to be a micro-prism ring in this split prism):

When the objects are no longer misaligned, then the lens is perfectly focused on the objects.

The corollary to being in focus is being able to determine the distance to the subject. This same technique was also used for optical range finding (not rangefinder cameras). Split-prism coincidence rangefinders were used in militaries to determine distance to target, in order to set artillery coordinates and propellant charges:

^{View from a coincidence rangefinder. Public domain, from Wikimedia Commons}

Rangefinder cameras used a separate optical path for focusing, the range-finding focusing mechanism. This showed two overlaid images. When the images were perfectly overlaid, the subject was in focus. This example from Wikipedia illustrates the concept:

How did photographers focus before autofocus, and how did fixed-focus cameras work?

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