Many vintage cameras had some sort of simple rectangular frame to show the approximate size and location of the image captured by the camera’s lens. The frame finder often comprised nothing more than a bent piece of wire. The disadvantage of such a simple frame finder is that it can be very inaccurate: what you see varies according to the angle and distance at which you peer through it.
More sophisticated frame finders consisted of two frames, a smaller one nearer the eye and a larger one further away. The user has to sight the image to be photographed by centring the smaller frame within the larger one, and the larger rectangle gives an indication of what would be included in the shot. The sports finder is fast to use, with good visibility around the edge of the frame, and allows the image to be seen at its natural scale. The sports finder is good for press applications, and following fast action, but is only moderately accurate. The problem sports finders raise is that the rear frame (ocular) is out of focus when the eye is adjusted for distant subjects. However, by contrast optical viewfinders of the time produced a very small an image, which was difficult to work with when scenes contained movement. The sports finder was good enough to remain in production on some types of camera until recent times (e.g. modern underwater cameras).
The brilliant finder combined a mirror at a 45-degree angle between two lenses positioned at 90 degrees to each other. Brilliant finders are usually very small (about 1cm across), viewed from above, and provide as image that is reversed left to right, making them difficult to use. Nevertheless, they were very common.
A special version was of the Brilliant finder was the Sellar finder, which consisted just of a concave mirror with a targeting aid, to help the user position their eye to give the correct view.
A development of the frame/sports finder was the Newton finder. This has a single negative (plano-concaved) lens in the front frame, and a targeting aid near the user’s eye. The negative lens reduces the size of the scene viewed, allowing the front frame to be smaller (but it’s hard for people who are long-sighted to use it).
While still comprising a set of frames projecting from the camera, telescopic finders combined a negative lens at the front with a positive lens as the eyepiece. This arrangement is a reversal of Galileo’s telescope, and therefore sometimes called the reverse Galilean viewfinder. Like the Newton finder, they give an image of reduced size.
This development of the reverse Galilean finder has a half-silvered rear face to the front lens, which reflects an image of a set of frame-lines, painted around the surround of the eyepiece lens. The user sees the frame-lines superimposed upon the scene (creating an illusion that the frame-lines are further away). When camera designs began to enclose viewfinders within the body of the camera, this system didn’t work so well, leading to the development of the bright frame finder.
Bright frame finders
In this system, a bright frame is shown in a telescopic viewfinder by placing a half-silvered mirror in the finder, at an angle to reflect frame lines at the side, which is illuminated by light from a translucent panel on the camera front (often placed next to the viewfinder).
Keplerian viewfinders use a reversed Kepler telescope. To keep explanations simple, this is an (optical) improvement on the Galilean telescope, which produces an upside-down image. In camera viewfinders a prism is added to reverse the image so it is seen the right way up. This allows the path of the light to be folded (like in a set of binoculars) leading to a viewfinder that can fit the available space in the camera (as found in the tiny Canon Demi). The optical brightness and quality of a Keplerian finder is similar improved, in the same way that binoculars improve on a simple telescope.
Parallax errors and their correction
A parallax error results when an object is viewed along two different lines of sight, such as when the viewfinder is necessarily on a different axis to the camera lens (usually above and often to one side of the lens). The error varies with distance. It’s negligible for distant scenes, and very significant for close-up objects, leading to incorrect framing.
Correction of parallax error in telescopic viewfinders has been attempted in a number of ways. The simplest is a secondary set of frame-lines in a bright line finder, which show the edge of the region that will be included in the photograph at close focus. A more sophisticated solution is that the finder bright-lines mechanically adjust (reposition) according the camera lens focus adjustment. Another solution was to introduce a mechanism that adjusted the angle of a telescopic finder (this was not common).
Ground glass screens
This article would not be complete without a quick word about ground glass screens, which also function as viewfinders.
In reflex cameras, the subject is viewed via a mirror, which reflects the light from a lens onto a ground-glass focusing screen. The difference between single lens reflex (SLR) and a twin lens reflex (TLR) is that in the SLR camera, the mirror is moved immediately before exposure (to allow the light to be focussed onto the film), while in a TLR camera the mirror and focussing screen are permanently arranged as a viewfinder, and the photograph is made with a separate twin lens.
The simplest screen is viewed from above. This is called a waist-level finder. The view in these finders is reversed left-to-right. A fresnel lens (made of a number of concentric rings to diffuse light) is often placed on top of the focusing screen, to improve illumination of the corners of the screen.
This system – a ground glass viewfinder – was used in the earliest wet plate cameras, but without a mirror.
In modern SLR cameras, the light passes through a pentaprism to provide eye-level finders. The pentaprism not only alters the path of the light, but also reverses the image so it can be seen in the correct orientation. Prism finders do not suffer from parallax errors, since the scene is viewed through and photographed via the same lens.
Of course, advancements in electronics have presented new viewfinder options, but these are beyond the scope of this article.