Flash Metering Modes
TTL-OTF metering is a completely different approach to get correct flash exposure.
“TTL” means “Through The Lens”, ie. light is metered through the lens and all other optical elements that influence exposure. The meter will see the same light that the film sees. This means that filters, extension tubes etc. are automatically factored in the flash metering.
“OTF” means “Off The Film”. Light is metered while the shutter is open, and as it is reflected off the film during exposure.
Here's how the system works:
- The shutter is opened by the camera
- The flash is triggered and starts to emit a light pulse
- Light reflected from the film surface is metered. This includes both ambient light and light coming from the flash.
- When an amount of light is detected that results in correct exposure, the flash unit is turned off, ie. the light pulse is shortened.
- The shutter is closed.
The nice thing about TTL-OTF metering is that it gets things mostly right automatically. When flash output is reduced by a diffusor or by increased distance, or when a filter reduces light reaching the film, the camera simply lets the flash burn longer.
The downside is that the camera is fooled by subjects that are brighter or darker than mid-toned. The camera doesn't know what tonality an object is, it just sees the light reflected by it. When mid-toned exposure is reached, the flash is cut off. So if you photograph a white wall with this system, you will get an image of a grey wall instead of a white wall. Just as with ambient exposure you have to apply exposure compensation to correct exposure for subjects that are not mid-toned.
TTL-OTF metering is the default with most Minolta film cameras and flashes. It's also the fallback mode for more advanced metering modes.
Caution: A few newer and lower-level Minolta film cameras are not equipped with a OTF metering cell. These cameras use either Pre-Flash TTL metering or ADI for flash exposure. This means that older Minolta flash units that don't support either mode do not work on these cameras. Only D-capable flash units can be used in auto mode. The cameras in question are the Dynax 30/40/Maxxum 50 and the Dynax 3L/Maxxum 3/GT.
With this system, a metering flash is emitted when the mirror is still down and the shutter is closed. The metering flash has a known intensity, and the camera can see the effect of the flash on the scene. The camera uses its ambient metering system, which is usually more sophisticated than the TTL-OTF metering system. Metering also takes place through the lens, so again, filters etc. are automatically factored in. Using the knowledge gained from the pre-flash the camera then exposes the film with the mirror up and the shutter open.
This typically results in better flash metering than regular TTL-OTF metering, especially when the camera can use its multi-segment ambient meter.
With digital cameras TTL-OTF metering isn't even possible. So with these cameras, pre-flash metering is the default mode.
With Minolta film cameras, the built-in flash doesn't support pre-flash metering. Also, pre-flash metering is only available with HSS-capable (with HSS turned on) or D-capable flash units. See the Minolta flash table.
ADI was developed to overcome the limitations of TTL metering, specifically the problems with objects of very high or low reflectivity. The basic idea is to automate the processes of manual flash. The exact distance to the subject is provided by D lenses. A flash unit is required that allows adjusting the flash power in fine increments by the camera (D flashes). With such a system the distance, film speed and aperture is known to the camera, it calculates the required GN for correct exposure from these parameters, and sets up the flash unit to emit a flash of exactly that intensity. During the shot, no light is metered and flash output is not adjusted. This way bright objects again appear bright.
There are a few limitations for this system:
- The flash must be mounted on the camera. That's because the camera only knows the focus distance, not the flash distance. Only when the flash is mounted on the camera it can assume the two to be the same.
- The flash must point straight ahead. With bounce flash the distance the light travels is longer than the focus distance, and the reflecting surfaces absorb some light. The camera doesn't know these factors, so it can't include them in the calculations.
- Equipment that influences either flash output (diffusors, concentrators) or effective aperture (filters) can't be used. Again, the camera doesn't know these factors, so it can't include them in the calculations.
In a mixed light situation, the camera combines pre-flash metering and distance based metering. This is why ADI requires HSS and therefore pre-flash metering to be enabled on the flash unit. However, the Dynax/Maxxum 7 can also use pure ADI metering with the built-in flash, which does not support pre-flash metering.
Distance information is not only provided by D lenses. Also with non-D lenses, the camera can determine the focus distance. The camera initializes the lens by moving it to infinity focus, and then it uses parameters from the lens ROM to calculate the focus distance from the number of rotations of the AF drive. For this to work, a mechanical focus range limiter of the lens has to be disengaged when the camera is turned on so that inifinity focus can be reached at that time.
Furthermore, cameras that have an electro-mechanical clutch and rotation encoder in the AF drive (these are the cameras that support DMF) can track the movements of the focus ring even in manual focus mode. These cameras can use ADI even with non-D lenses in manual focus mode. Some of these cameras also have a Smooth MF feature, which mechanically de-couples the lens AF drive from the camera AF drive (including the rotation encoder). When Smooth MF is enabled, the camera reverts again to TTL metering.
© 2009 Michael Hohner; This page was last changed on 2009-06-10