Flash Compatibility
Issues in Digital Stills Photography
Digital Compact
Cameras
In recent times, there has been a tremendous upsurge in the use
of compact digital stills cameras, of the type with built-in
flash and no official provision for the use of external flash.
These cameras can be adapted for use underwater relatively easily,
and with great success, but some attention to the details of
the flash and auto-exposure systems used is required in order
to obtain the best results.
The flash built-in to the camera is likely to give poor results
when used underwater. Results may be acceptable in conditions
of good underwater visibility, but even then, the lighting direction
is likely to produce strong reflections, which may overload the
auto-exposure system and produce bleached highlights in the picture.
This, of course, can be a problem with all types of camera, but
digital cameras tend to have less exposure latitude than film
cameras and the problem can be exacerbated. Some housings have
a built-in flash diffuser to help reduce direct (specular) reflections,
but a diffuser reduces the intensity of the flash and the distance
at which effective lighting can be achieved will be limited.
Other housings block the forward direction of the built-in flash
altogether, and render it unusable except for triggering an external
slave-flash system. For best results, the use of an external
light-source is always advised.
Using an aimable external light source improves underwater image
quality for all types of camera by reducing backscatter (light
reflected from suspended particles between the subject and the
lens). Digital cameras can be used either with flash or with
video lights, but flash usually gives the best results provided
that the triggering and exposure control systems operate correctly.
In the majority of instances, a slave-triggering system must
be used, and due to the use of pre-flashes, specially designed
(digital compatible) flash equipment may be required. The issues
are as follows:
Pre-flashes:
Most digital cameras issue a pre-flash in order to evaluate
the reflectivity of the scene and thereby determine the required
exposure*. The pre-flash will mis-trigger a conventional slave
strobe (i.e., cause it to fire at full power during the pre-flash,
and not be ready to fire during the main flash which occurs a
fraction of a second later). This has lead various manufacturers,
notably Ikelite, Epoque, Sea & Sea, and Inon to produce digital
compatible slave equipment. A basic 'digital' slave expects to
be triggered twice, and fires on the second triggering. This
functionality can be switched off if normal slave triggering
is required, giving compatibility with non pre-flash and film
cameras. More sophisticated digital slaves (Ikelite) repeat both
the pre-flash and the main flash and so work with most cameras
without the need for mode selection. If your digital camera definitely
does not use pre-flash, or has a special slave mode which does
not use pre-flash, you may be able to use conventional (pre-digital)
slave-flash equipment, but a TTL slave system or some means of
varying the flash output manually will be required (see 'flash
metering' below). New purchasers should consider only pre-flash
compatible strobes.
* There may be more than one preflash if the
camera manufacturer decides to include flashes for red-eye reduction,
crowd control, stroboscopic disco lighting, etc, and some systems
use more than one pre-flash for the exposure evaluation.
If you already own a Nikonos Compatible (N-type) or Motormarine
(S-type) strobe designed for use with film cameras, you may be
able to use it with one of the digital slave adapters made by
Heinrichs-Weikamp.
Flash Metering:
Full-flash Cameras: Some cameras fire the internal
flash at full output and control exposure by adjusting the sensitivity
of the imaging device (CCD). In order to obtain the correct exposure
with an external flash, such cameras should be used with a variable-output
manual slave-flash such as the Epoque ES-150DS, an Ikelite DS-series
strobe with an EV controller, Sea & Sea YS90DX or YS25DX.
Metered-flash cameras: More-sophisticated digital cameras
use a built-in light-metering sensor or pre-flash data to control
the output of the internal flash. Control is achieved by interrupting
(quenching) the flash burst when sufficient exposure has been
achieved. In this case, a TTL (cut-off detecting) 'digital' slave
flash system (Ikelite DS-50 or DS-125 with 4100.5 sensor) will
provide best results by making full use of the camera exposure
control system.
Nikon Coolpix Cameras with flash
connector:
Nikon Coolpix 950, 990, 995, 4500, 5000, and 5700
cameras are provided with a sync connector and adhere reasonably
closely* to the Nikon film SLR (and
hence Nikonos) TTL flash interface specification. Ikelite housings
for these cameras bring the sync connector out to an Ikelite
TTL bulkhead socket, and may be used, via a normal Ikelite TTL
sync cord, with any Ikelite TTL substrobe (i.e. DS-series and
conventional Ikelite substrobes). Ikelite also make a cable for
Nikon SB-series Nikonos strobes. Aquatica and other manufacturers
bring the flash connector out to a Nikonos bulkhead, and their
housings may be used with most current or recent Nikonos compatible
strobes (Nikon, Sea & Sea, Ikelite with Nikonos cable, etc.).
* Possible issues:
Ikelite recommend a modification to their substrobes manufactured
before June 1st 2001, the issue appearing to be that of slightly
excessive X-terminal output current causing the thyristor in
the camera sometimes to refuse to de-latch after triggering.
To reset the system you have to switch the strobe off and on.
If Ikelite had a problem, then older strobes from other manufacturers
(particularly Morris Aquaflash F3) may be even more prone to
it.
The following Coolpix models do not provide TTL auto-exposure
via the hot-shoe connection with Nikonos compatible strobes:
Coolpix-5400
Olympus Camedia TTL Flash:
The PT-020 housing for the C-5060 has a connector for external
TTL flash, as do some of the later members of the PT-series;
and there are housings for the Camedia FL-20 and FL36 flash units.
The connector is not a Nikonos
connector, and the electrical interface is specific for Olympus
Camedia cameras, i.e., it will not work directly with
Nikonos compatible strobes. |
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Ikelite has developed a conversion circuit for the Olympus
Camedia protocol. It is designed to work with Ikelite DS-series
substrobes. This conversion module, using a microcontroller and
powered by the substrobe, was introduced with the Ikelite housing
for the C-5060, and is included in subsequent manufacture of
housings for the C-5050, C-8080, C-750, SP-350 and so on. A 5-core
Ikelite-to-Ikelite sync cord (#4103.51, marked with a blue band)
is required for the power take-off from the strobe. |
 |
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Matthias Heinrichs (HeinrichsWeikamp) has developed a range of
adapters for the Camedia TTL system. These are designed for the
C4xxx-series cameras, the C-5050, 5060 and 8080, and enable the
cameras to be used with a variety of pre-digital N-type and S-type
strobes. |
Digital
SLR Cameras
The problem for underwater photographers is that most camera
manufacturers do not produce flash units designed for underwater
use; and putting normal flash units into housings does not solve
this problem completely because such units lack the output and
coverage required for good wide-angle results. Consequently,
the cameras favoured by the underwater industry are generally
those for which the TTL flash interface protocols are known.
Unfortunately however, camera manufacturers generally do not
issue technical details of their flash interfaces; and so most
conversion modules, i.e., electronic adapter circuits which mediate
between the camera and a Nikonos-compatible or Ikelite strobe,
are developed by a process of reverse-engineering. This process
is compounded by the ever-increasing complexity of TTL interface
protocols, particularly in relation to digital cameras, and by
the fact that underwater flash-equipment manufacturers do not
get access to cameras until they are released on to the market.
Consequently, there will always be a delay between the introduction
of a camera which uses a new flash protocol and the resolution
of the issues so raised for underwater photographers. In some
cases therefore, early adopters of new cameras may find that
the available underwater housings only permit operation of external
flash units in manual mode. In these days of automated-everything,
this may sound like a serious limitation; but in fact, some photographers
prefer to use manual flash, and doing so with digital cameras
is straightforward if the following points are observed:
 Use a variable-output
flash unit, or use a TTL flash unit in conjunction with a Guide-number
(EV) controller.
Set the camera
LCD display to show the image histogram (if available), and use
this to verify that the light-levels captured are well distributed
over the available contrast range (i.e., not crowded at on end
or the other).
Use a 48-bit capture
mode (RAW or TIFF), so that there is plenty of redundancy in
the data to allow post-capture contrast adjustment without risk
of posterisation.
An attempt to summarise the ever-changing situation with regard
to TTL protocol converters is given below:
Nikon D-TTL, i-TTL:
Nikon D100. D1, D1X, D1H,
The first proper generation of Nikon digital SLRs (i.e., true
digital cameras rather than modified film cameras) used the D-TTL
system. This feature enables the camera to perform balanced fill-in
flash using a multi-zone TTL sensor and distance information
from the lens (depending on the lens). The camera can also be
switched to "normal TTL", which sounds like the ideal
system compatibility option for the underwater photographer,
except that there's a catch. This "normal" TTL only
works with Nikon DX-series speedlights, so it is not quite as
"normal" as it could be. D-TTL cameras refuse to output
a TTL quench signal in the absence of the correct 'handshake'
signals from the strobe. In the absence of a protocol conversion
module in the housing, underwater users of D-TTL cameras must
use manual flash, or a Nikon DX-series speedlight in a custom-built
housing (if available).
D2x, D50, D70/s, D200
A extension to the Nikon D-TTL protocol, called i-TTL, was introduced
with the D70. The D70 requires an i-TTL capable speedlight because
it does not have a conventional TTL sensor (i.e., the pre-flash
method of exposure evaluation is the only method available),
but an i-TTL speedlight (SB600, SB800) will work with earlier
D-TTL and TTL cameras; i.e., the speedlights are backwards compatible,
but the new cameras are not. The i-TTL protocol also includes
facilities for controlling slave units by modulating the master
flash to send data, in a manner somewhat similar to that used
by Canon.
Ikelite has developed iTTL conversion circuitry and includes
it in its D50, D70 and D200 housings to give iTTL compatibility
with DS-series substrobes.
HeinrichsWeikamp
can supply OEM converters for D2x, D50, D70/s and D200 enabling
the cameras to be used with pre-flash compatible strobes.
Fuji:
The Fujifilm S2 is based on the Nikon F80 / N80 camera body,
and so uses the plain old Nikon SLR TTL flash protocol which
works with Nikonos compatible equipment. This camera is now sadly
discontinued. The S3 uses the D-TTL protocol.
Canon e-TTL:
Canon's extension to its e-TTL protocol for digital cameras includes
the ability to use a relatively long (500ms)
low-intensity pre-flash. This feature avoids overloading of the
image sensor (recipricity failure) during exposure evaluation,
but creates a problem for underwater photographers because standard
pre-flash compatible strobes cannot produce this special pre-flash.
Several research departments have come up with resolutions to
this issue, and two possible approaches emerge:
1) The full solution is to manufacture strobes which can produce
the extended pre-flash.
2) The e-TTL protocol permits the flash unit (i.e., the hot-shoe
conversion module), to instruct the camera to use a normal pre-flash.
This allows the production of conversion modules which will work
with a normal 'digital' strobe, but may require some exposure
compensation in conditions of high ambient light.
A standard pre-flash converter for the various eTTL cameras is
made by HeinrichsWeikamp.
Ikelite's e-TTL system consists of an in-housing conversion
module which produces special control signals for its current
production DS-series substrobes. It is available for EOS 300D,
350D, 5D, 10D,20D and PowerShot G6.
see www.ikelite.com/web_pages/2canonTTL.html
Sea and Sea has designed a housing for the Cannon 200EX
speedlight and has produced an eTTL strobe: the YS55TTL/E. These
are for use with Sea & Sea Canon housings with a 6pin TTL
bulkhead, such as the DX 300D.
One obvious solution to the eTTL compatibility problem is to
use a Canon EX-series (or Metz, ProMaster, Sigma) eTTL-compatible
speedlight in a custom-made housing (if available). Some developers
have experienced problems in trying to extend the Canon flash
unit away from the hot-shoe, probably due to the inter-wire capacitance
of the cable, and custom builders may discover that they either
need to incorporate some co-axial (screened) cables into the
bundle, or keep the cable short.
Note: for cameras which use the extended Canon pre-flash,
slave triggering from an EX-series speedlight will be manual
only, and will require a slave system which can suppress (i.e.,
ignore) pre-flashes.
Acknowledgement:
Thanks to Matthias Heinrichs for detailed discussions
of the E-TTL system and other flash-protocol conversion issues.
DWK 2004-2006 |
© D. W. Knight, Cameras
Underwater 2002 - 2006. |
