Bronica Learning Center :: TTL-OTF Metering
When electronic flashes were first introduced, they were fully manual. In other words, they put out a consistent amount of light every time they were triggered. Proper exposure was achieved by consulting a table to calculate the appropriate aperture based on film sensitivity and the distance of the subject to the flash. The correct aperture was only correct if all the factors remained constant.
Automatic flashes included an external sensor which automatically adjusted the power of the flash units based on distance of the subject to the flash while maintaining a pre-selected power range and consistent aperture. This alleviated manual calculations, but required a consistent aperture choice for proper exposure.
With the advent of TTL-OTF flash metering, all of these calculations, settings and external operations have become an internal automatic and self-checking system.
TTL - OTF (through the lens - off the film plane) flash metering is a feature built into all current Bronica cameras with the "i" designation and the GS-1. It allows a cell in the camera to actually measure the light hitting the film and adjust the flash to achieve proper exposure. The GS-1 has always had TTL-OTF and is activated when used with the dedicated GS-1 Speedlight, available from Bronica. The ETR-Si and SQ-Ai also have TTL - OTF metering which works in conjunction with an adapter in the case of the Metz unit or with Sunpak and Quantum TTL-OTF compatible flash units equipped with their dedicated adapters. The adapter acts as the translator between the camera and the flash unit. To achieve proper exposure, the flash fires rapidly multiple times which builds up to the correct density on the film.
The
advantage of TTL-OTF is that light is actually being metered off the film plane
by a cell incorporated into the design of the camera body. When the proper
exposure is achieved (based on 18% gray), the power of the flash is
automatically cut off by the body. By having a camera and flash integrated in
this manner, a photographer can instantly choose any aperture without having to
adjust the power of the flash or consider distance calculations, giving the
photographer more creative control and far quicker handling. The TTL-OTF system
also provides an internal check and will let you know instantaneously if, for
any reason, the system failed to achieve proper exposure.
In order to achieve all potential
advantages of the TTL-OTF metering system, it is necessary to be able to
synchronize your flash unit at any shutter speed. Instead of 1/30th or 1/60th
of a second as your top synchronization shutter speed, all current Bronica
cameras can sync with the TTL-OTF flash system at all shutter speeds up to the
maximum shutter speed of 1/500th of a second. This high sync speed design is
inherent in the design of all Zenzanon leaf shutter lenses. This allows the
photographer the ability to control, on the fly, depth of field and selective
focus as well as allowing additional latitude for creative lighting ratios.
Though the camera and
flash can be used in conjunction with an auto exposure prism, this is not
always recommended because these two automatic exposure systems (camera and
flash) operate independently of each other. Just because each component is in
an automatic mode does not mean that all systems are working in concert. Though
metering of AE prisms is technically through the lens (TTL), its metering is
actually done off the ground glass. This is not the same as through the lens
and off of the film plane metering (TTL-OTF). The TTL-OTF cell in the camera
body measures light from the film plane and cuts the flash power upon proper
exposure, but does not control the shutter speed of the camera; the shutter
speed of the camera is controlled by the meter in the prism in this case. In
sunny outdoor situations this is generally not a problem. Indoors, however,
using an AE prism on auto with TTL-OTF flash metering will often result in long
exposure times which will cause ethereal or ghostly images followed by a
subject, frozen by the flash unit. This can be a desirable effect in some
cases, but can be frustrating if that is not the visual objective.
The metering pattern of the TTL-OTF cell measures light in the middle of the ground glass, an area approximately the size of a quarter. Since the subject is not situated within this center-weighted average-metering pattern, one will risk inaccurate exposure.
It should also be
noted that the TTL-OTF cell is measuring reflectivity off of the actual film
stock and in some cases the ISO on the film box will not be the effective ISO
to the TTL-OTF cell due to different reflective indexes of various films. Many
film manufacturers have compensation information available, but the best way to
determine how the OTF cell is reading light is to shoot a test roll with the
films that will be commonly used. While shooting, it is also a good idea to
experiment with ratios of fill flash to available light. This is done by
changing the ISO on the TTL-OTF adapter to create over or underexposure.
