PRIVATE AND CONFIDENTIAL

RESEARCH DEPARTMENT

Performance of 10 to 1 zoom lens

for image orthicon format:

(Type 10 x 358 No. 1095308)

RESEARCH REPORT No.T-144 1965/6

THE BRITISH BROADCASTING CORPORATION

ENGINEERING DIVISION

PRIVATE AND CONFIDENTIAL

RESEARCH DEPARTMENT

P.ERFORMANCE OF 10 TO 1 ZOOM LENS FOR IMAGE ORTHICON FORMAT: (Type 10 x 358 No. 1095308)

K.J. Wright

Research Report No. T-144

( 1965/6)

Head of Research Department

r-------------------------------~~·I

This Report Is the p~operty of the British Broadcasting Corporation and may not be reproduced or disclosed t~ a third party in any form without the written permission of the Corporation~

I

Research Report No. T-144

PERFORMANCE OF 10 TO 1 ZOOM LENS FOR IMAGE ORTHICON FORMAT (Type 10 x 358 No. 1095308)

Section

1.

2.

3.

4.

5.

Title

SUMMARY" " , • , . , , ........ .

IN1RODUCTION. . . . . . . . . . , , . . . . . . . . , . . . .

RESULTS . . . . , . . . . , , ,

2.1. Modulation Transfer Function 2.2. Vignetting Characteristics . 2.3. Transmission and Veiling Glare 2.4. Geometrical Distortion 2.5. Overall Assessment. . . ,

COMPARISON WITH OTHER LENSES, .

3.1. Comparison With Previous 10 x 35B Lenses 3.2. Comparison Wi th the Rill Lens, Type Varotal V

CONCLUSION .....

REFERENCES. . . . , , . . . , . , . .

Page

1

1

2

2 2 2 3 4

4

4 4

5

5

March 1965 Research Report No. T-144

(1965/6)

PRIVATE AND CONFIDENTIAL

P.ERFORMAN-CE OF 10 TO 1 ZOOM LENS FOR IMAGE ORTHTCON FORMAT:

(Type 10 x 358 No. 1095308)

SUMMARY

The characteristics of the redesigned zoom lens type 10 x 35B have been measured. These include modulation transfer function, vignetting, transmission and veiling glare and geometrical distortion. An overall assessment based on sharpness and vignetting is given, and comparisons are made with production lenses type 10 x 35B and with other zoom lenses.

1. INTRODUCTION

The zoom lens type 10 x 35B is a development of the lens type 10 x 35A described previously, 1 in which the geometrical aperture has been increased from f/4'5 to f/3 0 8. However, the type 10 x 35B lens reported upon here differs from previous production type 10 x 35B lenses, in that it is a completely new optical design; several production lenses type 10 x 35B have been tested in Research Department. Following criticism of the performance of the production lenses M. Masson, the chief lens designer of Messrs. P. Angenieux, visited Research Department. As a consequence of a discussion of the results of these tests, and some further measurements relating to the best type of correction for spheriGal aberration, Messrs. Angenieux have produced an improved design. The optical performance of this improved version is outlined in this report.

A brief specification of the lens is as follows:

Focal length range 35 mm to 350 mm

Maximum aperture f/308

Image format size 24 mm x 32 mm (image orthicon format)

Minimum focusing distance 00 96 m

The procedure used for the tests was similar to that used In measurIng preVIOUS 10 to 1 zoom lenses. 1,2

I1

2

2. RESULTS

2.1. Modulation Transfer Function

Measurements of the modulation transfer function were made at six focal length settings (namely 35, 60, 100, 150, 260 and 350 mm) with a test object effec­tively located at infinity. The setting-up procedure for zoom operation was the normal method of successive approximations in which the back-working distance was adjusted for optimum 9-cycles/mm performance at the short focal length position, and the focusing control on the lens itself was similarly adjusted at the long focal length posItIon. During this procedure it was found that the lens would not focus correctly for objects beyond about 450 metres, for this reason the light source was displayed to simulate an effective test object at 450 m. The modulation transfer functions at the maximum aperture of f/3'8 are shown in Figs. 1 to 6. These curves represent normal zoom operation, no allowance being made for any mechanical tracking error. The fact that the performance of the lens could be improved at some focal length settings by refocusing shows that the profile of the cam used to effect zooming was somewhat inaccurate. A summary of the performance, shown in Figs. 1 to 6, is given in Fig. 7, which shows the modulation transfer factor at a spatial frequency of 9-cycles/mm, corresponding to the cut-off frequency of the British 625-line system.* Fig. 7 shows that the performance at 150 mm focal length is poor in relation to the expected performance particularly on axis and a check was made for tracking error. Refocusing by a change of 240 ~ in the back-working distance gave the improved performance, at a focal length of 150 mm shown in Fig. 8, The 9-cycles/mm modulation transfer factor, as a function of position off axis, is shown in Fig. 9, the normal zoom performance is also plotted for comparison. The higher level of modulation transfer factor main­tained over most of the field is of particular interest,

The performance of the lens at a reduced aperture was not investigated fully, due to the limited availability of the lens. The performance at three {,cal lengths (35, 150 and 350 mm) with an aperture of f/5'4 was measured on axis and at 14 mm offaXls; the modulation transfer functions are shown in Figs. 10, lland 12, and these may be compared with the maximum aperture performance in Figs. 1, 4 and 6.

All results so far have related to a test obj ect effectively located at -li large distance. The axial zoom performance of the hms with the maximum aperture-of f/3'8 and a test object distance of 1'26 m is shown in Figs. 13 and 14. The'choi'ce of 1'26 m rather than the design minimum of 0'96 m was determined by experimental limitations, no significant difference in performance being expected at 0'96 m.

2.2. Vignetting Characteristics

The illumination at the periphery of the image field'as a function of focal length, for the two apertures of f/3-8 and f/5-4, is shown in Fig. 15. The, subjective assessment of vignetting in limens IS gIven in Section 2.5.

2.3. Transmission and Veiling Glare

The axial transmission of the lens is an improvement on the type 10 x 35A, but somewhat less than that of the Varotal V 10 to 1 zoom lens manufactured by Messrs. Rank, Taylor Hobson. 2

The British 625~line system with 5·5 Mc/s bandwidth corresponds for the image orthicon to a cut-off spatial fr'equency of 8· 8 cycles/mm.

3

The results are glven 1n Table 1.

TABLE 1

TRANSMISSION % COLOUR OF LI GHT

10 x 358 10 x 35A VAROTAL V

Tungsten white 70 57 81 Red 73 59 86 Green 70 57 80 Blue 52 41 69

The effect of low transmission is important in terms of the light flux available at the photocathode of the camera tube. For this lens with a geometric aperture of f/3'8 the photometric rating is T/4·5.

The veiling glare index in tungsten light is shown in Fig. 16, vary1ng from a minimum value of 1'2% to a maximum value of 2%. Although an improvement on the type 10 x 35A, the veiling glare indices obtained are approximately 0'5% higher than those measured on a Varotal V lens. An improvement in both transmission and veiling glare is anticipated as the result of an investigation by the manufacturers into the efficiency of lens coatings.

2.4. Geometrical EUstortion

The geometrical distortion of most fixed focus lenses is ,not greater than about one tenth of that specified in the BBC lens specification TV/88/2. 3 The situation with zoom lenses is somewhat different and it was thought desirable to make measurements particularly at the two extremities of the range of focal lengths where the geometrical distortion is likely to be most severe. The proposed specification for zoom lenses TV/139* sets a limit of 0'02 for geometrical distortion. This limit 1S defined as follows:

'The magnification and alignment shall be so arranged that there is no geometrical error at the centre and at the periphery of the field. The distortion is expressed in terms of the gradient of the curve of radial displacement of an image point plotted against its true position. The gradient shall not exceed 0'02. '

Fig. 17 shows the geometrical distortion plotted in terms of the lateral displacement of an image point from its ideal position with the test object effectively located at infinity. The form of distortion changes in sign from 'pin-cushion' distortion at the short focal length to 'barrel' distortion at the long focal length end. The peak displacement is approximately the same magnitude for both the long and short focal lengths; however, the field position corresponding to the peak displacement is different giving a distortion gradient of + 0'04 at 35 mm and - 0'08 at 350 mm focal length. This lens does not satisfy the specification. According to the designer however, an improvement of 2 to 1 is expected in the final design of this lens.

* Not yet published.

4

The geometrical distortion with 35 mm focal length and the test object at a distance of 1 m is shown in Fig. 18. The distortion remains pin-cushion in form but has an increased displacement and a distortion gradient of + 0'05.

2.5. Overall Assessment

The integration of a modulation transfer function up to the cut-off spatial frequency gives a result related to slope of the equivalent time function (the response to unit step) at the 50% point and this has been found to correlate well with the subjective assessment of sharpness. 4 Further integration can take into account various parts of the field and in this way an index can be determined which 1S a measure of the overall impairment of the picture sharpness due to the lens. S The subjective assessment of vignetting has also been assessed. S

The subjective assessment of the lens at the full aperture of f/3'8 is g1ven in Fig. 19, the total impairment varying from - 1'3 to - 2'1 liminal units. The impairment due to lack of sharpness varies from - 0'6 liminal units at 60 mm focal length to - 1'2 liminal units at 350 mm focal length; the remaining impairment is due to vignetting. Fig. 19 also shows the subjective assessment of vignetting at f/5'4 and from this cause alone an improvement of 0'2 to 0'8 limens will be observed in the overall assessment.

3. COMPARISON WITH OTHER LENSES

3.1. Comparison With Previous 10 x 358 Lenses

Fig. 20 shows the zoom performance of the lens plotted in comparison with the zoom performance of a previous production type 10 x 358 (No. 1078029). Averaged over the field, the improvement in modulation transfer factor at 9-cycles/mm is better than 25%. This is a considerable improvement and may in part be attributed to the close co-operation between Research Department and the manufacturers.

3.2. Comparison With the RTH Lens, Type Varotal V

The RTHVarotal V 10 to 1 zoom lens 2 has a somewhat similar specification to the Angenieux 10 x 358. A numerical comparison of the differences is given in Table 2.

TABLE 2

FEAlURE VAROTAL V 10 x 3SB

Near focusing distance 1'83 m 0'96 m

Maximum geometrical f/4 £/3'8 aperture

Transmission to tungsten 81% 70% white light

Photometric aperture T/4'4 T/4'5

Veiling glare index 0'85 to 1'2% 1'4 to 2%

5

Table 2 shows that the two zoom lenses are comparable with the exception of the ability of the Angenieux lens to focus down to half the distance possible with the Varotal V and still maintain normal zoom operation. This facility of being able to focus to 0·96 m (3 feet) is considered to be important in studio production.

Dealing now with a comparison of subjective assessment at similar photo­metric apertures, Fig. 21 shows that the redesigned Angenieux lens is slightly superior in sharpness assessment to the Varotal V when averaged over the focal length range. The overall performance including vignetting has an average degradation no different from that of Varotal V. For further comparison, the subjective assessment of the lens type 10 x 35A is also shown in Fig. 21. An important further point of comparison is concerned with the method of focusing the lens. Varotal V is a constant volume device; operation of the zoom or focus control does not force air through the lens. Operation of the focus control on the Angenieux 10 x 35B causes the front element to move by about 44 mm, air being forced in or out of the lens. This could quickly lead to a considerable impairment of the veiling glare performance as a result of operation in dusty environments.

4. CONCLUSION

The new design Angenieux type 10 x 35B zoom lens is a considerable improve­ment on previous production 10 x 35B lenses and, with the exception of the veiling glare index and its geometrical distortion at the longest focal length, it is in no way inferior to Varotal V. The image quality is good (impairment of just greater than - 2 limen) at the maximum aperture and when used one stop down, i.e. at f/5·4 the impairment in image quality is expected to be negligible.

5. REFERENCES

1. 'The Angenieux 10 to 1 zoom lens for the image orthicon format, Type 10 x 35A (No. 1016554)', Research Department Technical Memorandum No. T-1054.

2. 'The RTH 10 to 1 zoom lens for the image orthicon format, Type Varotal V (No. 617566)', Research Department Report No. T-112.

3. BBCSpecification of Lenses for Television, TV 88/2.

4. SPROSm, W.N.: 'The subjective sharpness of television pictures', Electron. Radio Engr., 1958, 35, 4, pp. 124 - 132.

5. SPROSON, W.N.: 'New equipment and methods for the evaluation of the performance of lenses for television' , BBC Engineering Division Monograph, No. 15, December 1957.

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Fig. 13 - Axial modulation transfer curves

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0 35 45 60 75 100 120 150 200 260 350

focal langth, mm

Fig. 15 - Peripheral illumination of image field as a function of focal length for two apertures

2~----~ __ ~--'------r-------'---'------i ~ r- .--~ ~.~------~--~-----+--~----+-----~-----+----~

~ ~---t--------+----t---------+----+-~-+--~-+-----=-' ~I ~ ------~----~------~------~­~:~--+------~----+------+---+----.-+------+----~ _~_~_~_-l

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1 I 1 I 60 75 100 120 150 200 260 350

I !

~~5----+.1~---~~--~----~~~~--~----~~--~~--~· focal Iczngth, mm

Fig. 16 - Veiling glare index for tungsten illumination

o 41,---" --.----.-,----,----,----,-----,--r----;-.j..----'----'--'--I ---r--r-T ~-'-i -..,-, -~! ---I I i ,..+-- - - ~ ..... I 11 i i ' r-~--+--+--4-~--~--~~~+_~--~--~~~~·-+--~I---~·L-~I--~-+I--~ I I .,/ 1 ' .... i, I I I i I

:>31'--+---+_-+---+--___!---+-~+_____+--_+_____!--_+_--+____+_--~~--_l__--+__--+---_i_____.;

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position off axis, mm

Fig_ 17 - Ge(>metnr;c'I! Jistortion for 35 and 350 mm focal length T.!'!SE o:·:ect at infir'!.,y

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6 8 10 12 position off axis, mm

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35mm !

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I \ 14 16 18 20

Fig. 18 - Geometrical distortion at 35 mm focal length test object at 1·0 m

o

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. 1. / 154 I -~ ~ vlgnattmg f .

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total impairment f/3'8 --. 1

45 60 75 100 150 200 260 350 focal langth, mm

Fig. 19 - Overall assessments of impairments

1·

E E "Uo 0>

.... o

0

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7mm ....... .... \.-.'-t-..2mm

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I 10 x 358 No. 1095306

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1

o

1

~m

45 60 75 100 150 200 260 350 focal length, mm

Fig. 20 - Comparison of two designs of type 10 x 35B lens

New design No. 1095308 Old design No. 1078029

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sharpness impairment -

J----- I 10 x 358 --- / ~ i --- -...t_ varo\al 'if

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............... 10 x 358 ./

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,

45 60 75 100 126 150 200 260 350 400 focal lel19th, mm

Fig. 21 - Comparison of a modified Angenieux 10 x 35B, RIff Varotal V and a Angenieux 10 x 35A. Each lens at full aperture

Printed by BBC Research Department, Kingswood Warren, Tadworth, Surrey

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