Modern Classic SLRs Series :
Canon T-90 - Its Metering Capabilities

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Double area SPC for center weighted average metering and also handles the partial metering function.

SPC (Silicon Photo Cell)
for spot and A-TTL/TTL metering.

Two views of the SPC that handle the TTL flash at the mirror box.

- Tom Scott -

Optical Path.jpg (15k)

One of the key selling points for any camera is its metering capability. Whether we like it ornot, everyone uses the camera's built-in metering as a reference point prior to clicking the required scene. And metering technologies have advanced to such a state that Evaluative and Multi-pattern metering can even take into considerations the subject's distance and color temperatures of the scenes (Nikon triumphed in this area, with its
F5 model). In 1983, Olympus launched its twin flagship model OM3 & OM4 that incorporated multi-spot metering to replace its OM1 & OM2n cameras.

Canon was ambitious to make use of the T90 as the ultimate metering machine, packing all known metering patterns that were available by its competitions - EXCEPT Nikon's multi-segment metering that was first seen on the Nikon FA of 1983. To begin with, the Canon New F-1 has three metering options which were upgraded for the T90 by making it unnecessary to physically change the focusing screens like the New F-1 to achieve the desired pattern of Center-weighted, Partial and Spot. These metering can be changed with a couple of pushbutton controls plus a twist of the Electronic Input Dial. The Olympus's way of Multi Spots Metering and Highlight/Shadow Control was "borrowed" but enhanced further and all these various functions can be use in combination with the AE (Auto Exposure) lock.

A more detailed descriptions on these are available next page.

Partial Metering works best in some selective situation since it measures 13% within the picture area for more precise control of un favorable lighting condition.

The conventional center weighted average metering is provided for general photography. Giving preferences more to the center area and this can work with AE lock, exposure compensation.

Spot metering: concentrate at the 2.7% of the picture area. Giving extreme precise light measurement. Can work in conjunction with another feature, the H/L Control.

On top of these three metering patterns, the T90 provided the users with an even more precise metering method, the Multi-spot metering capability. See next section.
Note: PLEASE USE the MAIN MAP for the viewfinder and LCD display for cross reference in this site. Another page is also equally helpful for its layout of controls and buttons.

These three major features in metering can work along with supplementary features like: its Exposure Compensation, AE Lock and H/L Control.

The T90 features two sensor elements, one of which is a double area SPC like the one used in the Canon T70. This element is set at the upper part of the eyepiece. The other element is located at the bottom of the mirror box. The double area SPC is a sensor element used for center-weighted average metering and partial metering. Spot metering is handled by the second sensor element. Both sensor elements are controlled by the micro-circuitry which makes up the brains of the metering system. For example, a
single IC is used for spot metering and another for TTL automatic control.

Spot Metering Button
Spot metering can be used in three ways: One point spot metering, multi-spot metering, and H/S Control. When the Spot Metering Button is pressed, exposure values are stored in the camera.
The other metering sensor located at the base of the mirror box has 3 functions: Providing ordinary spot metering, pre-exposure spot metering at FEL when using the 300TL dedicated flash, and real-time metering with the A-TTL/TTL automatic dimmer modes. This innovative sensor is directed by the sub CPU through an I/O device.

* The A-TTL metering is a clear departure from the New F-1n way of handling its flash metering - the
Canon F-1 has a automatic CAT system while the Canon F-1 developed from there and among the top professional models available during the early and mid-eighties, Canon is the only camera manufacturer that is still resisting of incorporating TTL flash meter under the main reflex mirror. (We will discuss that part of WHY Canon opposing to main stream technology when the New Canon F-1n site is up).

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Next section will detail all the above metering capabilities and various control on the T90.

Its Metering, Exposure control, Flash photography, Viewfinder display, Film Backs, Built-in Motor Drive, the brains, Focusing Screens Other capabilities and the eyes of Canon; Full specifications with details illustrations of its various controls, available in HTML / PDF (184K) format.

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Instruction manuals for
300TL Flash & Canon T90
Canon A & T series SLR Models

Lenses: FL | Canon FDn lenses. | Early versions of FD lenses

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Maintenance Team: Credit: Pawel Nabe for his image on the Data Memory Back. EEwyn Foo, my nephew, who has spent quite a number of nights converting the original manual in HTML format. Mr. Richard Yeow, General Manager -Optical Division of Canon Marketing for granting special permission to reproduce this manual into HTML format in his site as a form of obligation to all the T90 users worldwide. Maintainders of the T90 Message Board: Kaipin, Terry Carraway & Dr Strangelove; Tom Scott, for his images of the SPD cell, Chris Tutti for his initial effort to scan and prepared the T90 manual in PDF format. My staffs Miss Wati and Mirza for helping the basic setup work. * Canon, T90, FD Lenses, Canon Marketing are registered trade names or trademarks of Canon Inc. Japan.