ISO Shutter Aperture by EV

The exposure triangle: ISO, shutter and aperture

Three controls decide how bright a photograph is, and each one adds or removes light in stops. ISO is the amplification gain on the sensor. Shutter is how long the sensor sits there collecting light. Aperture is the area of the opening in the lens. Double any of the three and you gain a full stop, so ISO 100→200 is +1 stop, 1/125 s→1/60 s is +1 stop, and f/4→f/2.8 is +1 stop. The connection to exposure value is EV = log₂(N²/t) − log₂(ISO/100).

Raising ISO amplifies the signal, but it amplifies the noise right along with it, so the signal-to-noise ratio (SNR) falls off, and the shadows show it first. Take the Sony A7 IV: base ISO of 100, native range 100–51200, extended range 50–204800. Photographers who want the cleanest files lean on ETTR (Expose To The Right). You push the histogram as far right as it will go without clipping the highlights, then pull the exposure back down in post.

Applications

Freezing motion means raising the shutter speed and making up for it with a wider aperture or higher ISO. Handheld in low light, open the aperture first, then bump ISO, and keep the shutter above what you can hold steady. For stars and the Milky Way you want a wide aperture, high ISO, and a 15–25 s shutter. With studio strobes you lock the shutter at sync speed and do the rest with aperture and flash power.

FAQ

Which control should I change first? Start with whatever drives the look you're after. Aperture if depth of field matters, shutter if motion does. Then juggle the other two until the exposure lands.

Is high ISO always bad? No. Modern sensors stay clean up to ISO 6400–12800, and a sharp frame with a bit of grain beats a clean one that's blurry. Noise reduction software does a good job pulling the detail back.

What is dual-gain ISO? A lot of sensors carry a second analog gain stage (ISO 640 on the A7S III, for instance) where read noise suddenly drops. Files from that point come out cleaner than the math alone would have you expect.