Bulb Exposure ISO Aperture

Bulb Mode Long Exposure: ISO & Aperture

In Bulb (B) mode the shutter stays open for as long as you hold the release down, which is how you get past the 30‑second ceiling most cameras enforce in Tv/Manual. How much time you need tracks the light. Drop a stop of ISO or close a stop of aperture and the shutter has to double, and t_new = t_ref · 2^(EV_ref − EV_new) handles any of those swaps. Trip the shutter with a remote or cable release so you don't shake the body, and if your camera offers mirror‑up and electronic front‑curtain shutter, turn both on to deal with leftover vibration.

Once you go past 30 s you're into astrophotography, fireworks, light‑painting and car‑trail work. With film, reciprocity failure (the Schwarzschild effect) kicks in: the emulsion starts losing sensitivity after a few seconds. Kodak Portra 400, for instance, wants roughly +1 stop at 10 s and +2 stops at 100 s. Digital sensors stay linear instead, but they pile up thermal noise. For long exposures, lean on in‑camera long‑exposure NR, or subtract dark frames in post.

Applications

Astrophotographers shoot the Milky Way at 20–30 s before the stars start to trail, or run for hours on a star tracker. For fireworks you set f/11, ISO 100 and hold bulb open through the burst. Urban shooters paint car light trails over 15–60 s, and architectural photographers stack exposures for HDR night scenes.

FAQ

Which remote triggers work in bulb? Wired releases such as the Nikon MC‑36 and Canon TC‑80N3 double as intervalometers and bulb timers. On the wireless side you have the Vello ShutterBoss, CamRanger 2, and Sony's own RMT‑P1BT for tethered control.

Do I need an intervalometer or just a cable release? For one‑off shots a plain release does the job. An intervalometer is what lets you dial in exact durations (4 min for star trails, say) and run timelapses without keeping a finger on the button.

Why does my long exposure look noisier than a stack of short ones? A single long frame gathers more thermal noise as the sensor warms up. Stacking shorter sub‑exposures (30 × 30 s, for example) averages the random noise away, and that's how most astro work is done now.