Film Camera Shutter and Exposure

What is modelled

A focal-plane shutter has two curtains. The first uncovers the 24 mm tall frame from top to bottom in the travel time t_c; the second follows after the exposure time t and covers it again. Every row is uncovered for exactly t, but at a slightly different moment, so a fast sideways subject is recorded with a skew. When t is longer than t_c the whole frame is open together for t - t_c, which is the window for flash.

Equations

• Exposure value EV = log2(N²/t); film exposure H = q L t / N² with q = 0.65.
• Meter: t = K N² / (L S) with K = 12.5, which gives H = q K / S for any aperture and speed.
• Row y opens at tc y/24 mm and stays open for t; slit width w = (24 mm / tc) t.
• Skew on film = v m tc for a subject of speed v at magnification m.

Checks run in the page

• Reciprocity of aperture and time.
• Uniform open time of 40 frame rows.
• Flash-sync window against max(0, t - t_c).
• Metering formula over 90 settings.

Honest simplifications

A linear curtain travel, a fixed lens transmittance, a simple S-shaped negative curve, no reciprocity failure at long times and no lens vignetting or diffraction. A teaching model; the 3D page shows the real mechanism only schematically.

• Press Fast action and look at the narrow slit and the skewed bar in the negative.
• Press Flash sync and compare a short time with a long one: the green band appears only when t exceeds the curtain travel time.
• Open the aperture one stop and shorten the time one stop: the exposure error stays the same.