Modern Classic SLR Series :
Canon EOS-1N Series AF SLR camera
Flash Photography - Part II

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Another point you must know is, Flash exposure is automatic even if any of the capable EOS camera is set to manual exposure mode. As mentioned earlier, TTL automatic flash exposure control is carried out based on the actual aperture setting. In other words, even if the aperture is set by the user in manual exposure mode, the TTL program still controls the exposure automatically. When using manual exposure mode, both the aperture and shutter speed can be set freely by the user in the same manner as during normal AE shooting.

Perhaps, some of the new Canon user also may ask: The meaning of "A" in A-TTL. The "A-TTL automatic flash exposure control" name originated with the development of the T90 and 30OTL. At that time, while other companies were still using very basic TTL flash control, Canon saw a need to provide functionality to solve the following problems: i.e. The photographer had to determine the proper aperture value taking into consideration both depth of field and the flash coupling range. There was no way to judge if the flash photograph would be exposed properly until after the photograph was taken. Proper settings for daylight fill-in flash and slow-synchro operation were difficult to determine. The technology that solved all three of these problems at once was Canon's A-TTL automatic flash control system utilizing information obtained from a near-infrared pre-flash (which detects distance information from the level of infrared light reflected from the subject). With the development of this system, flash exposure at the proper aperture value finally became as simple as pressing the shutter button. Since at that time this was the most highly-evolved TTL automatic flash exposure control system on the market, we attached "Advanced" to the beginning of the name to distinguish it from other companies TTL systems, and abbreviated the name to A-TTL. However, though the "Advanced" concept actually embodied all five of the above points, the overall complexity of the system resulted in "A-TTL" being used simply to refer to systems with pre-flash, and "TTL"used to refer to those without. In A-TTL, Canon uses a preflash to actively judge the effective flash-to-subject distance rather than passively. When used indoors in program AE mode, this preflash information assists in the automatic determination of the proper flash aperture in close-range or medium distance situations (explained in detail later), and in the triggering of the out-of-coupling-range warning in long-range shooting situations. The preflash is also used to trigger the out-of-coupling-range warning in shutter-priority AE and aperture-priority AE modes.

Warning: A-TTL (when used with Canon 540 EZ or 430EZ etc.) works only in programmed AE mode ONLY.

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Mixing available light with flash can be a complicated issue those days (especially in multiple flash setup). One way of another, modern SLRs with their advance TTL flash exposure control simplifies the complication and be able to attain more assuring results with little presence of artificial illumination.

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US based commercial photographer Charlie Borland was on assignment photographing the annual report for a worldwide manufacturing client when he when he captured this industrial image. These environments are often dirty and Mr. Borland likes to enhance the photo with vivid color. Using a Canon EOS-1N and 20mm lens, the client later cropped the image to fit the page layout.

Credit: Image courtesy of Charlie Borland who owns and runs borlandphoto.com. Selective of his older works can also be found at Aspenphotoworkshops.com and represents by fogstock.com stock agency. Image copyright © 1996-2005. All rights reserved

This preflash signal is used for automatically determining the proper flash aperture value in Full Auto and P modes in all EOS camera models, but due to various considerations is no longer used for triggering the out-of-coupling-range warning in EOS models following (and including) the EOS 10. Prior to the arrival of the improved EOS-1N/540EZ flash system, the previous EOS-1 + 430EZ combination presents some differences between the two old and update of EOS-1N/540EZ system. The EOS-1N + 540EZ system is based on the EOS-1 + 430EZ system but somehow the A-TTL is confined to Program AE and other shooting modes like Shutter Priority, Aperture Priority AE works in normal TTL flash modes (Some users complaint the A-TTL is not particularly effective in some earlier models.). However, we have reevaluated the system in consideration of (1) changes in the way flash is used,by today's photographers, and (2) the balance required for the multi-points AF system, and have consequently made the following functional improvements and partial modifications to the control system. Also, though the following descriptions assume combination of the flash unit with EOS-1N, the unit functions in the same way with the EOS-1N RS. With the EOS1, automatic flash exposure compensation can only be performed on the 430EZ flash unit. With the EOS-1N, however, flash exposure compensation can also be set on the camera body using the procedure described below (a few paragraphs down).

Flow chart and theory of A-TTL flash with EOS-1N
Basically, as far as EOS-1N flash photography is concerned, the camera body section provides various flash modes, features and also the maximum permissible sync speed. On the other hand, what it has also requires a dedicated Speedlite to maximize its potential. For an instance, not all Canon Speedlite can perform A-TTL or slow sync flash, other features such as repeating flash, diffusers, bounce/tilt capability etc. but nonetheless, Canon EOS-1N can deliver fully-automatic flash photography when used with dedicated Speedlite(s), one such example is the high-performance Speedlite 540EZ which links with various EOS-11N features for truly sophisticated flash photography.

By using an EOS-dedicated Speedlite, flash photography, including fill-in flash and slow-sync flash exposure, can be automatic and made easy. In the case like the top flash model, 540EZ is used, focal length information of zoom lens can be transmitted from the lens to the Speedlite. The head then zooms to ensure proper flash coverage. If the subject is too dark or the contrast too low for autofocusing, the Speedlite emits AF auxiliary light automatically. The AE mode activates the A-TTL program flash AE for fully-automatic flash photography. When the camera is set to aperture-priority AE, shutter speed-priority AE, or manual exposure, the Speedlite automatically sets to TTL flash exposure to suit the camera's exposure mode.

TTL flash Sensor
Under the Main Reflex mirror is one of the most important area for any modern SLR camera. This is where all the essential components for autofocusing, exposure metering locate. Canon has a safety measure deployed, even if you open the camera back, you will only see the shutter curtain, to view these component, just set your camera manually or using shutter priority to 30 sec. trip the shutter release and then open the camera back. The flash sensor is the only one that faces backward to the shutter curtain for measuring intensity of light during exposure while the rest are facing forward which takes light from the secondary mirror under the main reflex mirror (photographer location). Sometimes, "moderately priced" model may not has such a large TTL SPC cell, not just Canon - others as well, I guess it is purely economic sense.

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Mixing available light with flash is a technique.
Credit: Images courtesy of Joe & Mary Ann McDonald who own and maintains hoothollow.com. The link also provides some good field guides as wellas application of flash in wildlife/nature photography. Image copyright © 1996-2005. All rights reserved

Flash Modeling on A-TTL 3 segments illustration
Metering of light reflected off the film plane during actual exposure

TTL flash metering is carried out by a three-zone TTL metering sensor (the principle on the EOS-1N is the same as the eye-controlled AF capable Canon EOS 5) where it is located in the bottom of the mirror box. Weighted flash exposure control is carried out according to the selected focusing point, with the three metering zones corresponding to the center focusing point, the two left focusing points and the two right focusing points, respectively. In actual operation, flash light reflected off the film surface is detected by the three sensors, and flash control is carried out at normal sensitivity by the sensor corresponding to the selected focusing point, and at one-stop reduced sensitivity by the other two sensors. In other words, if light from the flash is uniformly reflected onto all three sensors, the sensor corresponding to the selected focusing point will reach the flash control level first and will terminate the flash output. (If one of the other sensors detects more than twice as much reflected light, that sensor will reach the flash control level first and will be used to control the flash output.) In manual focusing mode, however, all three sensors carry out flash control at the same sensitivity level, with no weighting assigned to a particular focusing point. The Canon EOS-1N flash control has other supplementary functions such as TTL Automatic Flash Exposure Compensation; as it is part of alternate exposure control added feature.

<<< --- Metering sensitivity distribution characteristics of the T`TL sensors are shown at the left hand side.

The main issue is: -Since this three-zone flash metering is controlled in the camera, it works with all EZ series flash units and with all direct-coupled multi-flash accessories.

Body metering during TTL and A-TTL automatic flash control

Previous EOS flash systems were designed based on the two premises that flash is used (
*1) as an artificial light source in low-light situations and (*2) as a supplementary (fill-in) light source in backlit situations. In other words, flash was assumed to be primarily used in conditions where existing light is insufficient for proper subject exposure at the desired camera settings. Looking at the recent practices of professional photographers, however, it is noticed that many photographers are also increasingly using flash in medium light level and front-lit situations to achieve new types of expression. Until now, body metering during EOS automatic flash shooting has beef weighted toward background metering.

Flash photography viewfinder display on various flash modes
However, even though the EOS-1N uses basically the same multi-points focusing and multi-zone metering system as the EOS 5, the EOS-1N is designed to carry out overall metering while the EOS 5 carries out rather extreme peripheral metering (peripheral frame metering). This change in body metering characteristics, which weights the main subject the same as the rest of the scene, was made in consideration of professional shooting practice during the mid '90.




| previous | NEXT | 2/4 A-TTL (Advance-TTL) Automatic Flash Exposure with various exposure modes

Flash Photography:- Part 1 | 2 | 3 | 4 - Canon 540EZ site

Selective Canon TTL Flash Models:- 160E | 200E | 300EZ | 300TL | 420EZ | 430 EZ | 480EG | 540EZ | Macrolite | Other non-TTL Canon flash models
Useful Resources at Julian Loke site: - Instruction Manual(s (PDF) for Canon 420EX | 580EX AF-TTL Speedlites

Suggestive weblink:- Canon Flash Resources at Photonotes.org or Bob Atkin Flash FAQ section

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Background and Various Issues | The Basic Features & various Setup
Manual & Auto Focusing | Metering Systems | Exposure Control | Viewfinder Optical
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Flash Photography (with Speedlite 540EZ extension & Selective info on Canon TTL Flash Models:-160E | 200E | 300EZ | 300TL | 420EZ | 430 EZ | 480EG | MacroLites | Other non-TTL Canon flash model

Reliability Issues:- Body Chassis | Shutter Unit | Electronic Circuitry | Film Transport & film handling Secondary Functions:- Custom Function Part One | Part Two | System Accessories:- Film Back Options - instruction for Command Back E1 | Macro/Close Up Part one | Part two and Part III Flash for Macro-Photography | Power Sources -BP-E1 | PDB-E1 | Focusing Screens | Remote Control with Wireless Remote Set LC-3 | System compatibility

Variants of Canon EOS-1N:- Canon EOS-1N RS | Canon/Kodak Digital DCS-1, 3 -5 & 520/560 Series | Full Technical Specification | Main Reference Map / Nomenclature | Resource Centre:- Comparative Charts between EOS-1 & EOS-1N / or with its active Competition(s) (Nikon); Quick Operational Reference Card (278k Gif File); Listings of 7-segment digital numbers/letters appeared on LCD display panel/viewfinders (HTML page); External Link:-Instruction Manual (3.3MB PDF file applicable for both Canon EOS-1N (RS). | Using EOS system for your photography | Bots & Nuts of EOS System - by Philip Chong |

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