This document is for a Saturn 1B but I think it still applies.  See below for the Saturn V info.

The vertical bar of black rectangles are called "vertical motion targets".

[![enter image description here][1]][1]

[Source][2] *APOLLO/SATURN CONSOLIDATED INSTRUMENTATION PLAN FOR AS-204/LM-1*  pp 3-5 & 3-6

Special cameras were pointed at the targets.

> 4.4.2 FIRST MOTION AND VERTICAL MOTION OPTICAL COVERAGE. This information is
> provided by 16mm Milliken and 35mm Mitchell cameras. The two Milliken (DBM-4) cameras are located on holddown arms, 180 degrees apart,
> looking at first-motion targets near the base of the launch vehicle.
> They will operate at a frame rate of 400 frames  per second (fps). **A
> 35mm Mitchell camera will be sited at the 110-foot level of the
> umbilical tower, centered on the motion targets (Figure 3-3) located
> on the Position II (Fin 3) centerline of the S-IB/S-IVB interstage.**
> The cameras will provide pictures showing first motion and the first 3
> to 5 meters of the vertical movement of the space vehicle.

(emphasis mine)

Once you know what the document is called...here is the same info for a Saturn V (Apollo 15).

[![enter image description here][3]][3]

> One camera at the 160-foot level of the umbilical tower, positioned to
> include a fixed reference target in the field of view will provide
> coverage during the first 5 to 7 meters of vehicle ascent. The frame
> rate is 96 frames per second.

[Page 4-4][4] of *APOLLO/SATURN V CONSOLIDATED INSTRUMENTATION PLAN FOR AS-510 (APOLLO 15)* 


  [1]: https://i.sstatic.net/hJ0nB.png
  [2]: https://www.ibiblio.org/apollo/Documents/UAH-19671016-ApolloSaturenConsolidatedInstrumentationPlanForAS204LM1.pdf
  [3]: https://i.sstatic.net/CLLyL.png
  [4]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710018696.pdf