Light &
Lens.

The setup calculator for this yard: one Sony a7S III at its 12,800 second base ISO, three lenses, two shooting positions, several feeders. Pick the rig; it answers the three questions that decide whether the footage is sharp: is the whole feeder inside the focus zone, how big is the bird in frame, and what will the light let you get away with.

v1.0.0 · Updated Sat / 2026-08-08 / evening CT · all glass math uses full-frame CoC 0.030mm

The grid

Every lens at both staged positions, computed from the same engine, using your current light anchor and bird choice. "Max usable f" is the highest aperture number the light affords at the shutter floor; "needs" is what full feeder coverage demands at that lens's longest reach.

The field method

No light meter needed; the camera is the meter, exactly as you guessed.

1. Frame up, dial the default: widest aperture, 1/1000, ISO 12,800.
2. Take the test shot. Read zebras or the histogram and count how many stops off you are (over reads as +, under as −).
3. Put that number in "stops off" above. Every readout and the grid re-solve instantly for the real light.
4. Set the aperture this page tells you the feeder needs, take the shutter it says you can afford, and roll.

Standing rules baked into the math

ISO lives at 12,800 because that is the a7S III's second base ISO: same color, controlled grain, and it buys the fast shutters. Shutter floors: 1/1000 for songbirds, 1/2000 minimum for hummingbirds (wingbeats near 53 per second; 1/4000 freezes wingtips). The f/9-to-f/11 instinct is validated here: the background trees sit far enough back that the blur disc stays creamy even stopped down, so when light allows, take the depth. Shooting through clean low-reflectivity glass costs roughly a third of a stop; the test-shot method absorbs it automatically.

Spec note: the long zoom entered conversation as "f/48": it is the Sigma 60-600mm f/4.5-6.3. Both Tamrons carried their real names already: 35-150mm f/2-2.8 Di III VXD and 50-400mm f/4.5-6.3 Di III VC VXD.