GB2460654A

GB2460654A - Autofocus system test chart

Info

Publication number: GB2460654A
Application number: GB0810105A
Authority: GB
Inventors: Ian David Taylor
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-06-03
Filing date: 2008-06-03
Publication date: 2009-12-09
Anticipated expiration: 2028-06-03
Also published as: GB2460654B; GB0810105D0

Abstract

The invention provides a three-dimensional chart for assessing the accuracy of the autofocus system of a camera. The chart comprises a target element 7 and at least one further element 4. The face of the target element and the face of the further element each comprise a flat surface having markings for recognition by the autofocus system of a camera and assessment of focusing accuracy. The face of the further element is parallel to the face of the target element and is at a different depth on the chart from the target element. The target element may have a pinhole 6 for transmitting a collimated beam of light from the chart at right angles to the face of the target element. The markings may be a pattern of lines of differing thickness and spacing. The chart may fold flat about a central axis (3, figures 2A to 2C).

Description

AUTOFOCUS SYSTEM TEST CHART

The present invention relates to a system for testing the accuracy of autofocusing (AF) in a single lens reflex camera (SLR).

Single lens reflex cameras have the ability to focus the lens on the subject automatically. However, SLRs can be fitted with a range of interchangeable lenses. Some lenses reveal variations in the accuracy of the camera's AF system. There can also he discrepancies produced by different lighting conditions; artificial lamps vary in colour and are very significantly different from daylight. SLR manufacturers have, until recently, been reluctant to admit that their AF systems are less than perfect.

AF systems can be adjusted by a technician using an optical bench and specialist equipment, but this is time consuming and expensive. Recently, some SLR makers have installed software in their cameras to enable the AF system to be adjusted by users. However, at present this is a trial and error' process and users are warned that caution must be used and that incorrect adjustment could make the focusing errors worse. The AF checking system described here is designed to simplify the correct assessment of focusing errors and to provide data to make the adjustment process easy for the user.

The present invention provides a three dimensional test chart that enables the accuracy of the AF system of a camera to be determined quickly and easily. The invention also provides the use of the test chart to assess the accuracy of the AF system of a camera and a method for checking the accuracy of the AF system of the camera by taking a photograph of the test chart.

Where the AF system of the camera is not optimally adjusted, the test chart provides an indication of the size of the focusing error. This indication can be used to determine the adjustment that is needed to correct the AF system.

The accuracy of an AF system may depend on the lens attached to the camera body. The invention therefore allows adjustments to be made to adapt a camera to a particular lens.

The test chart provided by the invention contains a number of smaller flat (two dimensional) test charts, herein referred to as elements'. The chart is three-dimensional because sharpness of focus of the chart at varying distances (depths') from the camera needs to be assessed.

The three-dimensional chart for assessing the accuracy of the autofocus system of a camera comprises a target element and at least one further element, wherein: the face of the target element and the face of the further element each comprise a flat surface comprising markings for recognition by the autofocus system of a camera and assessment of focusing accuracy; and the face of the further element is parallel to the face of the target element and is at a different depth on the chart from the target element.

Where the test chart comprises multiple further elements, the elements are set at various depths on the chart. The depths of the elements are precisely determined. The target element is typically at an intermediate depth on the chart, i.e. the chart comprises one or more elements that are deeper than the target element and one or more elements that are shallower than the target element. This may be achieved by ananging the elements in a stepped pattern. The steps are typically of equal sizes.

When the test chart is used to test the accuracy of the AF system of a camera, the distance of each element from the camera can be precisely determined. Each element of the chart is parallel to eveiy other element and, in use, is accurately at right-angles to the principal axis of the camera lens.

Each element is flat so that the camera's autofocus system can lock onto a target at a precise distance. This has at least one major advantage over a two-dimensional chart which achieves depth by being inclined to the principal axis of the lens. That is, the whole of an element is at the same distance from the camera because it is on the same step; so there is no ambiguity about the distance of the target on which the AF system has to lock.

In addition to being flat and at right angles to the axis of the camera lens, all the elements may comprise the same markings. This has the further advantage that any differences between the elements in an image taken by the camera are caused by focusing inaccuracies only, and not by perspective changes in the appearance of markings on the different elements.

The flat surface of each element comprises markings for recognition by the autofocus system of a camera and assessment of focusing accuracy. The markings typically comprise black lines on a white background. The lines vary in both width and spacing. The ratio of width to spacing varies within the pattern so that there are recognisable contrast changes in the chart even when not focused; this helps the AF system to pull the element in to focus. Appropriate line widths are from 0.05 mm to 1 mm, with spacings over a similar range. The lines may be printed on the faces of the elements, for example using black ink on a white background. The lines are typically straight, but curved lines, such as concentric circles, may be used. The lines may be parallel. A mixture of straight and curved lines may be used. Preferably the lines are ananged in a grid pattern.

The markings on the flat surface allow the user to see, on a photograph, the camera's LCD screen or any other image produced by the camera, whether or not the element is correctly focused. The use of black lines on a white background achieves the highest possible contrast when the chart is photographed under a variety of lighting conditions. This facilitates the analysis of the resulting image.

Coloured markings may be used but the use of a black and white pattern with no colour is preferred because it enables focusing errors caused by varying colours of light in different lighting conditions to be determined.

The element may carry a clearly printed number, or other symbol, which identifies the element.

In use, the camera will be aimed to focus on the target element. A positive number on an element may be used to show that it is in front of the target element and a negative number to show that it is behind the target element. The number will be easier to read if it is a single digit and meaningful if it shows the depth of the element in the chart, for example in centimetres. In principle, any system of numbers, ktters or symbo's could be used to identify each e'ement.

The number, or symbol, on the most sharply defined element allows the user to determine the adjustment needed to correct the autofocus of the camera, for example by looking up that element in tables which indicate the AF correction needed by the lens and camera combination for each most sharply defined element.

Each element is typically large enough to cover the area of one of the AF points' in the camera.

A convenient size is about 5 cm x 5 cm for each element and this is close to the minimum suitable size. For example, rectangular or square elements with sides between about 5 cm and about 10 cm, or circular elements with a diameter of between about 5 cm and about 10 cm, could be used.

Elements may be of the same size or may be different in size; the target element, for example, could be larger than other elements.

The test chart typically comprises a plurality of elements. The chart may comprise any number of elements. For example, in addition to the target element, the chart may comprise from 2 to 34 further elements, such as 14 elements or 20 elements. The number of elements is preferably large enough to cover the likely error range of the lenses to be tested. The elements are arranged at various depths on the chart and there may be one element at each depth, or more than one element, such as two or three elements at the same depth.

The elements can be arranged in any order. If the target element is placed at the true centre of the chart, there are preferably an odd number of rows and an odd number of columns. For example, 5 elements in a single row would be easy to construct but would be less accurate than, for example, 35 elements in 7 columns of 5 rows, but the latter arrangement would be more complex to construct. 15 elements in 5 columns of 3 rows with 5 cmx 5 cm elements is a good compromise.

This arrangement also has the advantage that it will fold to the size of an A4 book.

The elements are best arranged in a pattern which makes it convenient for the user to compare the sharpness of elements at similar depths in the chart. Whenever possible, each element should be placed close to another element which is just one or two steps behind it or in front of it. The target element is typically at an intermediate height, above at least one other element and below at least one other element.

The elements are preferably arranged in a pattern which makes the manufacture relatively simple and which, in use, allows light to reach all the elements in the chart without casting significant shadows. The target element is preferably at the centre of the chart.

The test chart may comprise an alignment feature. When the three-dimensional chart is photographed, the principal axis of the lens has to be at right angles to all the elements and has to pass through the centra' target e'ement. To achieve this quicUy and easily, a collimated beam of light may be projected from the target element towards the camera. The target element of the chart may have a small circular aperture (pinhole) cut in its centre.

A lamp or light emitting diode (LED) may be positioned directly behind the pinhole so that the light emitted from the diode directs a collimated beam of light that is perpendicular to the face of the target element. This makes optical alignment very easy because the user simply points the camera towards the target area and then moves the camera up, down or sideways until the bright light at the centre of the target element is seen. The LED may be activated by an enclosed battery and switch, which may be set into the side or back of the chart.

A cheaper alternative is to allow for light from an external source positioned behind the chart to pass through the chart and out through the pinhole in the target element to produce the perpendicular collimated beam of light. Where the target element is hollow, this may be achieved by including a small circular aperture (a second pinhole) at the back of the chart directly behind the pinhole in the target element. A light source, such as a desk lamp, can then be placed behind the chart in such a way that it shines through the two pinholes.

The test chart may be constructed so that it can be folded. This makes the test chart easy to protect, transport and store. This simplifies testing the AF system in a variety of locations where lighting conditions may vary. Its portability and easy set-up makes it easy to check lens accuracy under lighting conditions of varying spectral distribution.

For example, the chart may be built on two hinged boards which open like a slim book with hard covers. When the boards are opened the elements of the chart are revealed. When in use, each element may be kept parallel to one of the boards by hinges and a supporting strut; so, when the boards are opened through a right-angle, all the elements are parallel to the one board and at right-angles to the other board. The elements may be kept parallel to one of the boards at all times, when closed, open or when partially open.

Any suitable material may be used to construct the test chart. For example, in the embodiment comprising hinged boards, the two boards, which need to be flat and warp-free, may be made from laminated card, composite plastic board, foam-cored paper board, or wood. The elements and supporting struts are preferably thin, flat and rigid. They may, for example, be made from card or plastic.

The markings for facilitating focusing and image analysis, for example, the grid pattern, may be printed directly on the card or plastic, or printed on thin paper and secured to the card by adhesive.

To provide hinges that are slim and in the plane of the elements and the struts, the simplest method is to score the card or p'astic so that it can be accurate'y fo'ded to form the hinges.

The invention provides the use of the test chart to test the AF system of a camera.

In use, the camera is aligned so that the principal axis of the lens is at right angles to the elements on the test chart. In a prefened embodiment, the camera is positioned at right angles to the target element by positioning the camera where the collimated beam of light from the centre of the target element can be seen through the viewfinder.

Once the camera has been aligned, the test chart is photographed from a measured distance. The distance is chosen to suit the focal length of the lens used.

If a digital camera is used, the image produced is examined closely. The image may, for example, be on the screen of the camera, on a computer screen or on a photograph produced from the digital image. If a film camera is used, the film is developed and the negative or photograph is examined.

The sharpest element on the image is chosen and the focusing error is then calculated. A number, or symbol, present on the element may be used to facilitate this. The user is typically provided with a table to enable the focusing error to be determined easily. The table correlates the number, or symbol, on the sharpest element to the adjustment needed to correct the AF. A sample table is provided herein as Table 1. Table 1 may be used in conjunction with the prototype, shown in Figure 1, which has steps of 1 cm numbered -6, -5, -4, -3, -2, -1,0, +1, +2, +3, +4, +5, and +6.

When the focusing error for a particular lens and camera combination is established it can be allowed for or corrected by a method appropriate to that model of camera.

Accordingly, the present invention provides a method for testing the accuracy of the autofocus system of a camera, comprising: (i) positioning the camera so that the lens is at right angles to the face of the target element of a chart according to the invention; (ii) focusing the camera on the target element; (iii) taking a photograph of the chart; and (iv) analysing the image produced to determine which element on the chart is most sharply focused.

The method optionally further comprises (v) determining the adjustment needed to correct the autofocus of the camera; and/or (vi) adjusting the autofocus of the camera.

Figure 1 shows a prototype test chart that has an overall size of 21 x 21 x 30 cm when open, and 1.5 x 21 x 30 cm when folded. The size of this prototype, when folded, is that of a slim A4 notebook The prototype test chart has 15 elements arranged in 3 rows and 5 columns. Three of the elements (the central target and the two marked 0') are at the same depth. The other 12 (numbered -6 to +6) are set at depth increments of 1 centimetre.

In the prototype shown in Figure 1, the chart is built on two hinged boards which open like a slim book with hard covers. This is further illustrated in Figure 2. When the chart is folded flat, the upper hard cover (1) (the back-board') and the lower hard cover (2) (the base-board'), which are joined by a hinge (3) are parallel (Figure 2A). When the back-board is opened it reveals the elements of the chart (4) (Figures 2B to 2D). Each element is kept parallel to the back-board at all times by hinges (3) and a supporting strut (5); so, when the back-board has been opened through a right-angle (Figures 2C and 2D), all the elements are parallel to the back-board and at right-angles to the base-board.

The grid pattern shown on each element (4) in Figure lB works well, but it is not the only possible one, and the line width and separation can be changed to suit specialist lenses.

A small circular aperture (6) is present in the centre of the target element (7) in the prototype of Figure 1. This is also illustrated in Figure 3A. A light emitting diode (LED)(8) is set into the back-board (1) directly in line with the aperture (6) in the target element. The LED may be activated by a battery and a switch which are set into the back-board or attached to it. This makes optical alignment very easy because the user simply points the camera towards the target area (7) and then moves the camera up, down or sideways until the bright light (9) is seen at the centre of the target (as shown in Figure 3B). As an alternative, a small circular aperture may be cut into the back-board directly behind the hole in the target element. The user can then place a light source such as a desk-lamp behind the back-board so that it shines through the two apertures.

Two non-folding prototypes are shown in Figures 4 and 5; these are less portable than the folding design but can have a wider range and variety of arrangements of the elements. Figure 4 shows 15 elements in an arrangement different from that in Figure 1. Figure 5 shows a prototype with a larger target element and 21 different levels.

An equation that can be used to construct a table of focusing errors is shown in Figure 6. This equation generates tables which can be used to determine the focusing adjustment needed. Symbols used in the equation are: = focus shift on the test chart. For a lens focusing in front of the target element (called front focus') u is positive.

= focusing error at the focal plane of the camera. If u is positive then iv will be negative.

x = focusing distance set on the camera lens. This is the distance from the focal plane of the lens to the target element of the chart.

f = focal length of the lens used.

Whilst an embodiment of the invention has been described above, it should be appreciated that this is illustrative and is not intended to limit the scope of the invention, as defined in the claims.

In particular, the dimensions given are intended as guides and are not intended to be prescriptive.

TABLE 1

Sample Chart for Calculation of AF errors --Camera to target distance in metres ________________ ____________ 1 ___________________ ______ sharpest focus--> 4 -4 4 0 1 2 3 4 5 6 focal_length_in_millimetres ________________ 16 AFerrorinpm--> 17 14 11 8 6 3 0 -3 -5 -8 -11 -13 -15 18 AF error in pm--> 22 18 15 11 7 4 0 -3 -7 -10 -13 -17 -20 24 AFerrorinpm--> 41 34 27 20 13 6 0 -6 -12 -19 -24 -30 -36 27 AF error in pm--> 52 43 34 25 17 8 0 -8 -16 -24 -31 -39 -46 AF error in pm--> 91 75 59 44 29 14 0 -14 -28 -41 -54 -67 -80 AFerrorinpm--> 122 100 79 59 39 19 0 -19 -37 -55 -73 -90 -107 43 AFerrorinpm--> 143 118 93 69 45 22 0 -22 -44 -65 -85 -105 -125 AF error in pm--> 158 130 103 76 50 25 0 -24 -48 -71 -94 -116 -138 AFerrorinpm--> 200 164 130 96 64 31 0 -31 -61 -90 -119 -147 -175 AFerrorinpm--> 302 249 197 146 96 47 0 -46 -92 -136 -180 -222 -263 AFerrorinpm--> 433 357 282 209 138 68 0 -66 -131 -195 -257 -317 -376 AFerrorinpm--> 597 491 388 288 189 94 0 -91 -180 -268 353 436 -517 Camera to target distance in metres ________________ 2 ________________ ______ sharpest focus--> -6 -4 4 0 1 2 3 4 5 6 focal_length_in_millimetres ________________ AFerrorinpm--> 43 36 28 21 14 7 0 -7 -14 -20 -27 -34 -40 AFerrorinpm--> 53 44 35 26 17 9 0 -8 -17 -25 -33 -41 -49 AF error in pm--> 63 52 42 31 21 10 0 -10 -20 -30 -40 -50 -59 AF error in pm--> 88 73 58 43 29 14 0 -14 -28 -42 -56 -69 -83 AF error in pm--> 118 98 78 58 38 19 0 -19 -38 -55 74 92 410 AFerrorinpm--> 193 160 127 95 63 31 0 -31 -61 -92 -122 -151 -180 AFerrorinpm--> 383 317 252 188 125 62 0 -61 -122 -182 -241 -300 -357 AFerrorinpm--> 492 408 324 242 160 80 0 -79 -156 -233 -309 -384 -458 AFerrorinpm--> 769 637 506 377 250 124 0 -123 -244 -364 -482 -599 -714 Camera to target distance in metres ________________ 3 __________________ ______ sharpest focus--> 4 -4 4 0 1 2 3 4 5 6 focal_length_in_millimetres ________________ AF error in pm--> 49 40 32 24 16 8 0 -8 -16 -24 -31 -39 -47 AF error in pm--> 78 65 52 39 26 13 0 -13 -25 -38 -50 -63 -75 AFerrorinpm--> 151 125 100 75 50 25 0 -24 -49 -73 -97 -121 -144 AFerrorinpm--> 191 158 126 94 63 31 0 -31 -62 -92 -122 -152 -182 AFerrorinpm--> 288 239 191 142 95 47 0 -47 -93 -139 -185 -230 -275 AFerrorinpm--> 368 305 243 182 121 60 0 -60 -119 -178 -236 -294 -351 250 AFerrorinpm--> 628 521 415 310 206 102 0 -102 -202 -302 -402 -500 -598 300 AFerrorinpm--> 993 824 656 490 325 162 0 -161 -320 -478 -635 -790 -944 Camera to target distance in metres ________________ 4 _______________ ______ sharpest focus--> -6 -4 4 0 1 2 3 4 5 6 focal_length_in_millimetres ________________ AFerrorinpm--> 101 84 67 50 33 17 0 -16 -33 -49 -65 -81 -97 AF error in pm--> 150 125 99 74 49 25 0 -25 -49 -73 -97 -121 -145 AFerrorinpm--> 190 157 126 94 62 31 0 -31 -62 -92 423 153 183 250 AFerrorinpm--> 315 261 209 156 104 52 0 -51 -103 -153 -204 254 304 300 AFerrorinpm--> 483 401 320 239 159 79 0 -79 -157 -235 -313 -390 -466 350 AFerrorinpm--> 703 584 466 348 232 115 0 -115 -229 -342 -455 -567 -678 400 AFerrorinpm--> 987 820 654 489 325 162 0 -161 -321 -479 -637 -794 -950

Claims

CLAIMS1. A three-dimensional chart for assessing the accuracy of the autofocus system of a camera, which chart comprises a target element and at least one further element, wherein: (i) the face of the target element and the face of the further element each comprise a flat surface comprising markings for recognition by the autofocus system of a camera and assessment of focusing accuracy; and (ii) the face of the further element is parallel to the face of the target element and is at a different depth on the chart from the target element.
2. A chart according to claim 1, wherein the target element comprises a pinhole for transmitting a collimated beam of light from the chart at right angles to the face of the target element.
3. A chart according to claim 2, which further comprises a light source directly behind the pinhole for projecting the collimated beam of light from the chart.
4. A chart according to any one of the preceding claims, comprising from 2 to 34 further elements, wherein the faces of the further elements are at various depths on the chart.
5. A chart according to claim 4, wherein the target element is at an intermediate depth on the chart.
6. A chart according to claim 4 or 5, wherein the target element is at the centre of the chart.
7. A chart according to any one of the preceding claims wherein the markings comprise a pattern of lines of different thicknesses and spacing.
8. A chart according to any one of the preceding claims wherein the markings on the further element comprise a symbol to indicate the depth of the element within the chart.
9. A chart according to any one of the preceding claims wherein the chart folds flat.
10. Use of a chart according to any one of claims 1 to 9 to check the accuracy of the autofocus system of a camera.
11. A method for testing the accuracy of the autofocus system of a camera, comprising: (i) positioning the camera so that the lens is at right angles to the face of the target element of a chart according to any one of claims 1 to 9; (ii) focusing the camera on the target element; (iii) taking a photograph of the chart; (iv) analysing the image produced to determine which element on the chart is most sharply io focused.
12. A method according to claim 11, wherein the camera is positioned at right angles to the target element by positioning the camera where the collimated beam of light from the centre of the target element can be seen through the viewfinder.
13. A method according to claim 11 or 12, which further comprises: (v) determining the adjustment needed to conect the autofocus of the camera.
14. A method according to claim 13, which further comprises: (vi) adjusting the autofocus of the camera.