Modern Classic SLRs Series :
Canon New F-1 - Metering/Focusing Screens

 
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The focusing screen, widely regarded as just a piece of focusing aid. You have to give credit to Canon's attention to such detail and put on such a great deal of effort into enabling New F-1 enjoy a level of viewing comfort not seen or heard before it.

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The focusing screen has another important function in the New F-1 camera because it can be use to change the metering systems in the camera, you will have three choices of either conventional center weighted average, Partial (12%) to spot (3%). Making the New F-1 the only professional camera in the early eighties that provides, three options to choose from to handle various lightling condition in a more precise and personal way of metering.


Why Canon opposing to the mainstream of metering method adopted by many others such as putting a metering cell inside the mirror box instead? Canon claimed their tests performed in Canon's laboratories showed that the cell-in-the mirro-box method are more suited for aperture-priority AE mode, but does not function too smoothly when operating in shutter priority AE mode. (Because metering is performed after the lens has been stopped down, film plane metering systems often require shutter speed readjustments for maximum accuracy.) And since the New F-1 has shutter priority automation when it couples along with a motor drive or winder, this has become a technical requirement. Canon also claimed Film plane (TTL OTF) metering systems may pose some problems in using special films having a different reflective surface, including infrared film where they felt that TTL OTF system would limit the full performance of a professional-grade camera. The best course, according to Canon, would be to upgrade the focal plane metering system. Was it true or merely a marketing decision ? But it did made the Canon New F-1 has something that can be picked on by its rivals, whatever the desision made, Canon ate their own words by incorporating the TTL/OTF metering feature in another classic, the marvelous Canon T90 back in 1986 - the only thing significant feature in that camera is, the focusing method was replaced by dual SPCs, one under the mirror box and another at the eyepiece.

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Credit: Image courtesy of Mr. Claudio®. who is a collector for Canon photo gear, he also has an Ebay Section as well as maintaining a website on his own where occasionally trading some photo equipment. Image(s) copyright © 2003. All rights reserved. Please respect the visual property of the contributing photographer.


Focal plane metering in the F-1 The New F-1 employs an improved version of the focal plane metering system used in the original Canon F-1. The focusing screen is actually fore the core of improvements made to the New F-1's focal plane metering system. Each focusing screen unit includes a specially designed photometry element which incorporates a wedge-shaped horizontal element..

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This element, a micro-beam splitter, is the same distance above the mirror as the film plane is behind the mirror. When light is reflected up to the eyepiece, a portion of this light is deflected by the micro-beam splitter's grating to the silicon photocell (SPC) metering element behind the focusing screen.

SPC location.jpg
The SPC that did all the miracle/tricks of the thrice metering patterns on the New F-1.

The SPC is placed as close to the focal plane as possible, so that in effect, metering is performed at a point equivalent to the film plane. The SPC replaces the CdS photocell used in the former F-1.The focusing screen system comprises 13 different interchangeable screens which, besides helping you focus accurately, permit you to change the metering sensitivity pattern. All 13 screens are available for centerweighted average metering and selective area metering and 6 for spot metering, giving a total of 32 focusing screen/metering system combinations. The system includes two Bright Laser Mattes which are nearly 20% brighter than the other laser mattes in the series. The Cross Split facilitates focusing by dividing the subject in both horizontal and vertical directions and the standard New Split/Microprism solves the problem of prism darkening with slow lenses. Other screens are available for specialized uses. The A/B Size Laser Matte is useful for publishing and advertising, featuring engraved crosses in each corner for cropping. Changing screens is easy, requiring no special tool. For the first time in an SLR, one has a choice of several metering systems. The sensitivity pattern is changed by replacing the focusing screen. Three patterns are available.

Center-weighted average metering: For normal shooting in either AE or manual mode. The sensitivity de creases progressively from the center to the edges. The pattern remains unchanged regardless of the lens focal length, and center-weighting in the horizontal direction is greatly accentuated to minimise difference in exposure between horizontal and vertical positions.

Selective-area metering: A pattern that is particularly effective for backlit or contrasty subjects since it provides a precise reading of the subject or certain area of the frame. It is effective, also, for AE photography and stopped-down AE in close-up applications. Metering sensitivity is restricted to 12% of the image area.

Spot metering: Functioning as a 3% spot meter, this pattern is advantageous in tricky exposure situations where extreme contrast exists between subject and background. Typical examples are a singer in the spotlight on stage and subjects with the sun or bright snow behind. It can be used to great effect with telephoto and super-telephoto lenses. Spot metering is also appropriate for close-up work and photomacrography when the subject is small in relation to the image area and/or when metering against the light. With a comparatively large subject, use of a selective-area screen is recommended.


A little additional info on the Silicon photocell (SPC) SPC provides a wider metering range, fairly fast response, and superior linearity make this photocell the ideal choice for camera photometry systems


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Enlarged View of the SPC from the above illustration.You may Click here to find out more in the glossary section on SPC, SPD, Cds).

Combined, these traits give the New F-1 an impressive coupling range of EV-1 to EV20 in 22 steps (a candle's flame) (ASA 100, 50mm f/1.4). This gives six more steps than the former F-1, which had a coupling range of EV3 to EV18. If you can see an image in the viewfinder, the New F-1 can meter it. How does it fared with its competitions ? The Nikon F3 has a rather conservative standard of EV1- EV18, while the Pentax LX posed the most impressive figure of EV -6.5 to EV20 in auto mode and EV1 to EV20 in manual mode. Similarily, the top of the line automatic
Olympus OM models (OM2n, OM4, OM4Ti) are another few impressive model due to its real time metering capability off the film plane. During an exposure process, upon striking the SPC, the light is then immediately converted into an equivalent electrical signal. Since the strength of this signal, however, is only one-millionth that of the CdS photocell, it naturally requires amplification. The amplifier used in the metering system of the New F-1 and the paired SPC are fabricated together on a single silicon chip IC. This IC is then housed in a hermetically sealed package to protect it from the elements. Consequently, this metering system provides the reliability required in a professionalgrade camera.

Metering sensitivity pattern Metering patterns control the functions of the photocell. The same lighting conditions measured with the same metering system will result in different readings (and consequently different exposure values), depending on how the photocell "senses" the light. The ability of the photocell to determine actual lighting conditions is controlled in large part by the metering sensitivity pattern. This pattern tells the photocell just how much light is falling on which areas of the image. Generally, the applications for which the camera was intended determine which metering sensitivity pattern is to be used. The New F-1 has been designed primarily for the professional user, making the consideration of metering sensitivity patterns an important issue. In the former F-1, a portion of the condenser lens was coated with a beamsplitting reflective surface to obtain a selective-area metering pattern (approximately 12% of the frame). The New F-1 developed a completely new optical technique which minimizes the disadvantages and emphasizes the advantages of metering sensitivity patterns.

Micro-beam splitter (MBS) provides multiple metering patterns
The focusing screen units of the New F-1 system are quite complex. Each consists of a focusing screen, a light-gathering plate, and a condenser lens. The principal element of the light-gathering plate is the new microbeam splitter.

Main Blow up SPC.jpg (19k).
The development of this special grating and beam splitter coating gives the New F-1 the unique ability to provide multiple metering sensitivity patterns through interchangeable focusing screens. And personally, I admire Canon's effort for trying out in a general and relatively disgarded area in the focusing screen and it is amazing to see so much technologies packed inside these to make it works.

The micro-beam splitter (MBS) consists of two thin, transparent plastic sheets bonded together. A wedge-shaped grating is formed at the bonded surface. A beam-splitter coating covers the grating and directs most of the light toward the eyepiece. However, some light is reflected back to the photocell (see above). When this light strikes the flat surfaces of the light-gathering plate, it is reflected repeatedly until it reaches the SPC photocell. This system is known as the focal plane metering system, due to the position of the SPC photocell. Only the portion of the focusing screen covered by the MBS and MBS coating is used to calculate exposure. It follows that by expanding or contracting this portion, the metering area can be changed. It is this principle upon which all the New F-1's interchangeable focusing screens are constructed. The screens are available with three different metering sensitivity patterns, namely center-weighted average metering, selective-area metering, and spot metering. ALL these basic info will lead you to another page, contain and outlining the detail descriptions of how these special focusing screen works, technically.

| Next | Full Description of the New Split Screen Or | Next | The Focusing screens that did the tricks Or | Next | All the available 32 screens for various applications

Instruction Manual: Canon New F-1 Camera | Motor Drive FN | Canon High Speed Motor Drive Camera |
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