Freezing the Finish Under Fluorescent Flicker
An APS-C body (1.5× crop) with a 70–200 mm f/2.8 is the only long lens available at a high-school swim meet. The deck is lit by 50 Hz overhead fluorescent tubes. A meter reading on a swimmer's face gives ISO 3200, f/2.8, 1/500 s. You must freeze the 100 m freestyle finish, keep the face above the water sharp, and include the lane rope about 0.8 m in front of the swimmer. The camera's files are clean to ISO 6400 and become mushy at 12800. Flash and strobes are prohibited. You are shooting through a glass barrier that shows a faint reflection of the stands, from a deck seat about 10 m from the finish, level with the water. The client wants a horizontal frame for a 3 m banner and needs ten selects within three hours.
1. Set the exposure for the finish and decide whether to stop down so the lane rope is sharp. Show your arithmetic and state what you are willing to sacrifice.
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Freezing hands and splash needs roughly 1/1000 s. From the baseline ISO 3200, f/2.8, 1/500 s, that is one stop: ISO 6400, f/2.8, 1/1000 s, which sits exactly at the stated clean-ISO ceiling.
The alternatives fail. f/4 at ISO 6400 returns you to 1/500 s, which risks motion blur; f/4 at ISO 12800 reaches 1/1000 s but destroys skin and water texture. Motion blur cannot be recovered in post; a soft foreground rope can.
Depth of field at 200 mm and 10 m is only about 0.3–0.4 m (m ≈ 0.02, so DOF ≈ 2Nc(1+m)/m²), and the rope at 0.8 m forward is well outside it. The rope is context, not subject, so let it go soft. If the frame feels too tight, shoot at 135 mm, which widens the field to about 1.8 m at that distance and roughly doubles DOF, then crop - but keep 1/1000 s. Protect the face highlights; expose so the water sheen is not clipped, and check the first frames at 100%.
2. The venue's fluorescent lighting is causing frame-to-frame brightness swings in a burst. Explain the cause and what you will change on the camera and in your shooting pattern.
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Mains at 50 Hz makes fluorescent tubes ripple at 100 Hz, a 10 ms cycle. A 1/1000 s exposure samples only about 1 ms of that cycle, so each frame catches a different point in the rise and fall - typically a third of a stop to a full stop of variation, and horizontal banding if the shutter curtains cross the ripple.
Actions:
- Enable the camera's anti-flicker (flicker reduction) function. It delays the shutter to the bright peak of the cycle and evens out exposure at the cost of slightly lower burst rate and shutter lag - acceptable at a finish.
- Use the mechanical shutter; fully electronic/silent shutter with a slow row readout is more prone to banding under ripple.
- Shoot short bursts of three rather than long machine-gun runs, and glance at the histogram between heats so a drifting exposure is caught early.
- Do not chase the swing with exposure compensation frame by frame; you will bake in inconsistency across the set. Lock one exposure that protects highlights and normalise the small residual differences in post.
Test on an empty lane before the race: three frames, compare histograms, then commit.
3. How do you deal with the reflection in the glass barrier while retaining a usable autofocus strategy for a swimmer closing at speed?
Read the worked answer
The reflection is a dielectric surface problem, so geometry beats filtration. Press a rubber lens hood flat against the glass, or drape a dark cloth around the lens and glass, to exclude the stands from the reflection path; shoot perpendicular to the glass, because reflections strengthen at grazing angles. A circular polariser would suppress the glass reflection, but it costs about 1.5–2 stops - at f/2.8 that forces ISO 12800, which you have already ruled out. Skip it.
For focus: use continuous AF with a single point placed on the swimmer's shoulder or cap, not a wide zone that will grab the rope or splash. Back-button focus separates focus from the shutter so you can pre-focus on the lane marker at the finish and fire as the swimmer arrives. Expect the face to be at the waterline, so place the point slightly high and accept a little water in front. Shoot a short burst through the touch, then refocus - do not let the camera hunt on a flat, splashing surface.