Technical Information

Mirror Centering Aid

The Prêt À Tourner mirror centering aid (available in our shop) helps you to center the camera with the chart. This smart tool is very useful and provides you with the most precise results.

  • Place the magnetic mirror on the center of the chart by using its 4 discreet magnets.
  • Place the camera approximately at the required distance from the chart.
  • Frame the center of the lens reflection in the mirror.
  • In doing so, the optical axis grows perfectly perpendicular to the chart. Because you frame the center of the chart, the center is equidistant to its 4 corners.
  • Last but not least, adjust precisely the distance with the sliding plate.

Your test can begin!

Sharpness calibration test

The P.A.T. charts give you the opportunity to check mathematically the resolution of your sensor or your lens. In order to do so, use the numbers of each circular and linear target.

Numéros-mires-1

The numbers indicated on concentric and linear targets stand for, in twentieth of a millimeter, the number of pairs of lines printed on 1 mm.

Adjust the position of the camera using the mirror centering aid.

The table above gives you the correlation between the numbers in the concentric and linear targets, the number of pairs of lines printed on the chart on 1 millimeter, and the number of pairs of lines printed on the film or sensor on 1 millimeter.

To use this correlation table, place the chart at a distance from the image plane of 50 times the focal length of the lens in test. Here is an example for a 50 mm lens : 50 x 50 = 2500 mm = 2,5 meters = 8 feet

Analyse your lenses and print them with stop wide open.

The degree of sharpness depends on the clarity between the lines. The resolving power depends on the number of lines seen.

Optical Wedge Scale

The optical wedge scale enables you to optimally define the results of the sharpness test.

You will find below how to use this tool, which we have tried to make as easy as possible to access. If you have any questions, we invite you to call us or send us an e-mail; we will gladly help you.

WEDGE-1

Write the information about the tested lens in the corresponding areas: brand, serial number, focal, stop.

WEDGE-2

Place the camera at the distance advocated by the optical wedge sheet from the P.A.T. chart (approximately 50 times the focal length of the lens).

PAPER-MM

Stick the center of a P.A.T. self-adhesive graph paper on the engraved reference mark of the lens.

WEDGE-3

Write in the « ratio » area the ratio corresponding to the lens. The ratio depends on the gap between focus engravings. It has to be multiplied by the measurement unit of the scale (mm).

Find the ratio corresponding to your lens on our page optical wedge sheet.

WEDGE-4

The measurement unit of the scale is based on millimeters. It goes from -5 mm to + 5 mm and corresponds to the variation of the lens ring in the minus or in the plus direction around the correct testing distance of the lens.

For exemple, if the ratio is 3, then -5 mm becomes -15 mm, -4 becomes -12 mm, and so on...

WEDGE-5

Write in the « eye » area the best focus reading (useful when it comes to calibrating the viewfinder of film cameras).

WEDGE-6

Using your erasable marker, circle the number corresponding to the variation distance of the optical lens adjusting ring. Start with -5. Record or print a few frames (2 or 3 seconds). Then wipe off the first marking and circle -4, move the lens ring to -4 mm, and record a few seconds. Keep going until you reach +5. You will get an approximately 30-second movie.

Digital Cameras

Play your recording and easily spot the best result on the monitor.

Film Cameras

Analyze the frames once developed (negative) with a microscope to find the best result.

Correlation table

Prêt À Tourner provides you with a correlation table, allowing you to interpret the numbers of the targets.

When filming the target at a distance of 50 times the focal lenght.

Chart numberPairs of lines / mm on the chartPairs of lines / mm on the sensor
45250
54200
63,333166,667
72,857142,857
82,5125
92,222111,111
102100
111,81890,909
121,66683,333
131,53876,923
141,42971,429
151,33366,666
161,2562,5
171,17658,824
181,11155,555
191,05352,632
20150
210,95247,619
220,90945,455
230,8743,478
240,83341,666
250,840
260,76938,462
270,74137,037
280,71435,714
290,6934,483
300,66633,333
310,64532,258
320,62531,25
330,60630,303
340,58929,412
350,57128,571
360,55527,777
370,54127,027
380,52626,316
390,51325,641
400,525
410,48824,39
420,47623,81
430,46523,256
440,45522,727
450,44422,222
460,43521,739
470,42621,277
480,41720,833
490,40820,408
500,420
550,36418,182

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