35mm Film SLR Camera Product

Overview

A 35mm SLR puts one lens to two jobs: composing and exposing. Between shots, the Reflex Mirror inside the Mirror Box sits at 45° and diverts the lens image upward onto the Focusing Screen; through the Pentaprism the photographer sees exactly what the film will record. At the moment of exposure the mirror swings out of the way and the Focal-Plane Shutter uncovers the film for the dialed time. The whole sequence — mirror up, diaphragm closed, shutter run, mirror down — completes in well under a tenth of a second.

Everything mounts to the Chassis Casting, an aluminium die-casting machined in one setup so the lens seat, mirror hinge, and film rails hold their relative positions to roughly 0.02 mm. The Bayonet Ring is shimmed at assembly to the system's 46.5 mm flange focal distance; the Pressure Plate in the Film Back Door holds the emulsion at the other end of that tolerance chain.

The exposure sequence

Pressing the Shutter Release Button trips a chain of spring-loaded levers. The Mirror Lift Lever releases first, flipping the mirror frame up against its damping bumpers in about 30 ms while the Aperture Stop-Down Lever closes the lens diaphragm from its wide-open viewing aperture to the set taking aperture. Only then does the shutter fire.

The shutter is a vertical-travel focal-plane design built on the Shutter Base Plate. The First Curtain Blade Set — a stack of overlapping duralumin blades — accelerates downward under its drive Coil Spring and uncovers the gate; the Second Curtain Blade Set follows after the timed delay and closes it. At 1/1000 s the second curtain starts before the first has finished, so a narrow slit sweeps the frame and no point on the film sees light for more than the set time. Slow speeds from 1/30 s down to 1 s are held by the Timing Escapement, a clockwork pallet retard selected by the Shutter Speed Dial cam. Flash must wait for the moment both curtains are fully open, which is why X-sync through the Flash Hot Shoe is limited to 1/60 s.

When the second curtain closes, the mirror returns instantly — the photographer loses the image only for the duration of the exposure.

Film path

Loading drops a 135 cassette into the left chamber; the leader threads across the gate onto the Take-Up Spool. Each stroke of the Film Advance Lever does three things through the transport Helical Gear Pair: it tensions both shutter curtain springs, steps the Frame Counter, and pulls film exactly one frame. Frame pitch is metered not by the spool — whose effective diameter grows as film winds on — but by the Sprocket Shaft, whose teeth engage the perforations; eight perforations equals one 38 mm frame advance, so spacing stays even from frame 1 to frame 36. A slip clutch in the take-up spool absorbs the difference.

During exposure the Pressure Plate, loaded by leaf springs, presses the film against the outer rails of the gate; film flatness is what makes the difference between centre and corner sharpness at wide apertures. At the end of the roll, the photographer disengages the sprocket clutch and winds the film back into its cassette with the Rewind Crank; pulling the crank upward also pops the Door Latch. Aged Light Seal Foam foam in the door channels is the most common source of leak-fogged frames in old bodies.

Viewing and metering

The Focusing Screen sits at the same optical distance from the lens as the film, so an image sharp on the screen is sharp on film. Its central split-image wedge displaces out-of-focus detail laterally, giving an unambiguous manual-focus reference. The Condenser Lens evens illumination across the screen, the pentaprism erects the image, and the Eyepiece Lens presents it at about 0.86× magnification.

Metering is through the lens: two Silicon Photodiode Cell photodiodes beside the eyepiece read the screen with a centre-weighted pattern. Their photocurrent feeds a bridge on the meter Bare PCB, biased by the ISO Resistor Ring (set by the Film Speed Dial) and by the Aperture Coupling Ring ring that follows the lens aperture setting. The output drives the Meter Galvanometer at the finder edge; centring the needle by adjusting aperture or shutter speed sets correct exposure. Two Silver-Oxide Button Cell silver-oxide cells power the meter for a year or more — and because the shutter and transport are purely mechanical, a dead battery costs only the meter, never the picture.