The SLR Camera

What You See, What You Get

On a rangefinder or viewfinder camera, the finder window sits beside the lens. What the window shows is not quite what the lens records, and the nearer the subject, the bigger the difference. This is parallax.

The window can’t show where the lens is focused either, and it doesn’t change when a longer or wider lens goes on. The SLR’s answer is to look through the taking lens itself.

Red frame: what the lens records · dashed frame: what a window beside it sees (at 0.45 m)

A Mirror at 45°

Behind the lens a mirror stands at 45° and reflects the light up to the focusing screen. The screen is as far from the mirror as the film is, so the image on the screen is as sharp as the one on the film.

The lens forms an inverted image: upside down and reversed left to right. The mirror turns it the right way up on the screen, but it is still reversed left to right.

Lens → 45° mirror → focusing screen

The Roof Pentaprism

Above the screen sits a roof pentaprism. The light first meets the “roof” on top and reflects once off each of its two faces, then once more off the silvered front face, and leaves through the back towards the eyepiece.

Three reflections in the prism and one at the mirror make four. An even number of reflections never turns the image into a mirror image, and the roof swaps left and right back. Through the eyepiece the image is upright and the right way round.

Roof twice + front face once · upright through the eyepiece

Pressing the Release

While you frame, the aperture stays wide open and the screen is bright. Press the release: the aperture closes down to its setting, the mirror swings up and the finder goes dark, the shutter opens and closes, the mirror drops back and the aperture opens again.

The mirror returns by itself; this is the instant-return mirror. The OM-1’s mirror has an air damper to soften the shock as it moves. The whole sequence takes less than a tenth of a second; here it is slowed down.

Aperture closes → mirror up → shutter → mirror down → aperture opens

Two Curtains

The OM-1 has a cloth focal-plane shutter just in front of the film. Two opaque cloth curtains cross the 24 × 36 mm frame sideways: the first uncovers it, the second follows and covers it again.

At slow speeds the second curtain sets off only after the first has arrived, so the whole frame stands open for a while. At fast speeds it sets off early, and only a slit between the two crosses the film. The narrower the slit, the faster the speed, but every part of the film is lit for the same time.

Shutter speeds 1 s to 1/1000 s · slowed down

Flash Sync

A flash lasts a few thousandths of a second. It fires when the first curtain arrives, and lights only what is open at that moment.

At 1/60 s or slower, the whole frame is open then, and all of it is lit. At faster speeds there is only a slit, and the picture shows one lit band. That is why the OM-1 syncs with flash at 1/60 s.

1/60 s: all of the frame lit · 1/250 s: one band

Winding On

After each picture, the advance lever. The OM-1’s lever swings 150° and does several jobs in one stroke: it winds the film on one frame, moves the frame counter, cocks the shutter, and sets the mirror and automatic aperture for the next shot.

135 film has 8 perforations per frame, 4.75 mm apart, so each frame is 38 mm: 36 mm of picture and 2 mm between frames. While cocking, the two curtains go back overlapped, so no light reaches the film.

One frame = 8 perforations × 4.75 mm = 38 mm

Taken Apart

An SLR comes apart into a few assemblies: the lens, the mirror box, the finder (screen, pentaprism, eyepiece), the focal-plane shutter, the film transport, and the body around them. The OM-1 body is 136 mm wide, 83 mm high and 50 mm deep and weighs 490 g; in 1972 it was the smallest and lightest 35 mm SLR.

The assemblies separate; whichever one the pointer rests on is lifted out and named.

Cut Open: One Light Path

Cut along the lens axis, the light’s whole route shows: through the lens, folded up by the mirror, imaged on the focusing screen, three reflections in the pentaprism, and out of the eyepiece.

The OM mount is 46 mm from the film. The mirror lives in that space, and swings up flat beneath the screen.

Section: lens · mirror box · pentaprism · shutter · film

What It Changed

What the eyepiece shows is what the lens records. Telephoto, wide-angle or macro, any lens frames itself, with no separate finder to fit. Interchangeable lenses, finders and focusing screens, and motor drives grew into whole systems.

The Nikon F of 1959 was the model of the system SLR. At the 1964 Tokyo Olympics it replaced the large-format press camera as the reporters’ main tool, and it was everywhere in the coverage of the Vietnam War. For decades after, the SLR was the standard camera of professional photography.

1936 Kine Exakta · 1949 Contax S · 1959 Nikon F · 1972 OM-1