Measuring The Entrance Pupil

Introduction

In The Entrance Pupil And Its Importance,  it was demonstrated that a requirement for high-quality image stitching was knowing the position of the entrance pupil. How do you find the entrance pupil?

  1. An educated guess will sometimes suffice, as the entrance pupil is the apparent position of the lens’ physical aperture as seen from the front of the lens. I have used this method many times when working with an unfamiliar lens.
  2. Look up the manufacturer’s data, not all, but some do include the entrance pupil position. For example, Zeiss Loxia 2.4/85 data sheet gives the position of the entrance pupil at 58.7 mm in front of the image plane.
  3. Look up the data on the Panotools Entrance Pupil Database.
  4. Never rely on the methods discussed in 2, 3, and 4- for all critical work, the entrance pupil must be measured.

It is so important,  I will repeat it again: for critical work, the entrance pupil must always be measured, as the entrance pupil varies from lens to lens even of the same type.

Measuring The Entrance Pupil

There are several methods for determining the entrance pupil, and the method laid out here I first saw at Panohelp.com and take no credit for it. However, I have made one slight change to the procedure, and this change improves its utility. This method makes use of the observation that the entrance pupil of a lens is the apparent point at which the image inverts.

Entrance Pupil Measurement Setup

Figure 1 Entrance Pupil Measurement Setup

With reference to Figure 1:

  1. I recommend working in millimeters for all length measurements.
  2. Hang a metric tape measure against a wall, post, window frame, door frame, etc. I recommend the use of a flat back tape measure such as Fastcap PMMR-FLAT16. Also, I use blue painter’s tape to hold the tape measure against the wall such as Scotchblue Painter’s Tape.
  3. The camera and lens to be measured are mounted in the portrait position on a tripod. The tripod height is adjusted so that the camera points approximately to the center of the tape measure. A bubble level (spirit level) is used to level the camera- it is essential that the camera’s line of sight and the tape measure form a right-angle. If your tripod or tripod head does not have a bubble level then use a hot-shoe bubble level such as this 3 Axis hot shoe spirit level.
  4. The distance from the lens to the tape measure, X, has to be measured. This can be done with another tape measure; however, more accurate results can be obtained by using a laser measure such as the Bosch GLM 20.
  5. The length of the lens must be measured from the lens front cap to the rear flange, and for this, I use calipers. I recommend the iGaging Absolute Origin 0-12″ Digital Electronic Calipers– the length of the calipers depends on the largest lens you are planning to measure.
  6. Use a notebook to record all measurements.

Figure 2 Tape Measure Attached to Post

Figure 3 Sony f4 70-200 mm Lens to Be Measured

Figure 4 Complete Setup

Entrance Pupil Formula

From the setup in Figure 1, the entrance pupil, K, can be determined by taking two sets of measurements of T, B, and X- see Figure 5.

Figure 5 Entrance Pupil Formula

Entrance Pupil Measurement

Once setup the measurements can begin:

  1. The camera and tripod are placed close to the tape measure. Stop the lens down to its lowest value, focus on the tape, and take an image. The exposure should be set as to make the tape measure easily readable.
  2. Using the display on the back of the camera or its EVF (electronic viewfinder)  select the image just taken, zoom in, and examine the top and bottom of the tape measure. Record the first top measure, T1, and record the first bottom measure, B1.
  3. Carefully place the lens cap on the lens, and record the measurement from the tape measure to the lens cap, X1.
  4. Move the camera and tripod back, and repeat procedures outlined in 2 and 3. Record the values T2, B2, and X2.
  5. For a zoom lens, the procedure is slightly different. For example, if measuring a 70-200 mm lens you must decide upon the focal lengths of interest, say, 70, 100, 135, and 200 mm. In each position, there will be four sets of measurements- one for each focal position. If the length of the lens changes with focal length there will be four X measurements. In this case, a zoom lens with four focal lengths of interest is the equivalent of measuring four prime lenses.

Does The Method Work?

Yes, the method does work. But firstly, continue on to the Entrance Pupil Calculator.

If you keen to see some measurements made with this method please see Measuring the Entrance Pupil Of the Zeiss Loxia Lens Series.

 

 

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