Shutter Priority Aperture and ISO

Shutter priority: aperture and ISO trade-off

Shutter priority mode (Tv on Canon/Pentax, S on Nikon/Sony) pins the shutter speed for you while the camera works out the aperture, and optionally the ISO, to hold exposure steady. On paper, keeping exposure constant means N²/t · 100/ISO = constant; halve t and you have to either double N² (open up one stop, e.g. f/5.6 → f/4) or double ISO. A few numbers worth memorizing. Sports and birds-in-flight want 1/2000 s or faster, a handheld portrait sits at 1/125–1/250 s, and everyday handheld follows the 1/focal-length rule (50 mm → ≥1/50 s, for instance). For light trails and that smooth waterfall look you're down at 1/8 s to 30 s on a tripod. Once you climb past ISO 6400 most APS-C sensors start showing noise, so open the aperture wider before you reach for ISO when you can.

Applications

Fast action like soccer or motorsport (Tv 1/2000 s), light painting and night-sky long exposures (30 s on a tripod with an intervalometer), silky waterfalls and sea shots (1/8 to 1/2 s with an ND filter), panning a moving subject while following its motion (1/30 to 1/60 s), and street work where you want pedestrians frozen mid-step (≥1/250 s).

FAQ

What if the aperture runs out of range? When the lens is already wide open and exposure keeps falling, Auto-ISO steps in to fill the gap. Lock the ISO instead and the shot comes out underexposed, with the meter blinking at you.

When prefer Tv over manual mode? Reach for it when the action matters more than depth of field and the light keeps shifting, like tracking a runner as clouds come and go. If the light is stable and you care about depth of field, manual or Av is the better call.

Does shutter priority work with flash? It does, but the sync speed caps you (usually 1/200–1/250 s). Go faster than that and you need HSS (High-Speed Sync), which eats into your flash power.