INON D-2000 S-TTL: Official Explanation

by INON Japan Friday, March 11th, 2005

Note from Editor: This article is an official reply from INON Japan that explains how the INON D-2000 strobe's S-TTL technology works. You can also download this article as a pdf.

Dear Underwater Photographers;

Our latest D-2000 strobe employs dramatically advanced technology comparing to existing underwater strobes. Part of the technology would be hard to understand and have had to be speculated by individual test like Mr. Scheunemann has executed. We would be appreciated those individual tests but some of them seem to lack accuracy. Please let us take this opportunity to show a part of our technology used to the D-2000 for deeper understanding of the product for D-2000 owner and better understanding for future D-2000 owner.

Explanation of D-2000 'S-TTL'

'The flash duration of the D-2000 is equivalent to camera’s internal strobe. This characteristic enables the D-2000 to be controlled by camera’s internal strobe from small flash to full dump and does not need complicated amplify/delay function used for external slave TTL circuit which is easily to have exposure variation.'

Below waveform images show that the flash duration and flash waveform of the D-2000 is very much similar to those of built-in strobe of Canon S-60 or Olympus C5060. (The flash of the D-2000 is much more powerful than built-in strobe and the waveform is higher than internal strobe’s waveforms. But in the waveform images, the height is intentionally adjusted to be same as built-in strobe’s for comparison.) This means that the D-2000 can exactly duplicate controlling signal of a camera when the signal reflects to built-in strobe. D-2000 is designed to have optimum flash characteristic (fundamental factor of a strobe) for slave TTL. There is an adapter for existing strobe to work the strobe as slave TTL. It should work in certain shooting distance, but it is hard to work in wide range of shooting distance especially underwater for following reason.

Flash duration of Z-220 or YS-90DX (and other YS-90 series) is approximately more than five times as long as D-2000 or camera’s built-in strobe. Therefore it is hard to control those strobe by internal strobe signal to work as slave TTL. Moreover as in waveform images of Z-220 or YS-90DX, existing strobe has moderate flash output decay with different decay curve which cause exposure variation in middle/long distant subject. There is a slave TTL adapter with amplify/delay function to minimize the variation. However it would be bit hard in its setting like gathering data underwater and it can work at certain distant subject but it is hard to completely adjust variation of amplification/delay at middle/long distant subject. Since existing strobe has longer flash duration, it is hard to completely synchronize in fast shutter speed.

The D-2000 has EV Compensator ('EV Controller' in user manual) to adjust exposure depending on camera characteristic or personal preference. The EV Compensator enables handy and easy exposure adjustment underwater. The EV Compensator is newly developed circuit to fine-tune the flash duration of the D-2000 within the flash duration of internal strobe at full power.

'D-2000 employs fiber optics system which prevents the D-2000 flash light from affecting to its sensor and acquires reliable performance.'

Existing slave TTL system has a sensor located away from the external strobe itself and easily affected by flash light from the external strobe itself especially when shooting in macro, and has possibility to fail to control if the reflection of flash light to the sensor prevents quench signal of internal strobe from detecting by the sensor resulting in overexposure.

D-2000 can not have such a problem for several countermeasures. The slave sensor of the D-2000 is designed to obscure from light and INON Optical Cable Holder (component part of Optical D Cable/Cap W* Set) fix a fiber optics so that the cable surely face to internal strobe. So the D-2000 does not need other apparatus like Recorded TTL to accommodate such a problem. TTL system of the D-2000 is not recorded but 'real-time' TTL. Since S-TTL is real-time, there is no restriction for shutter speed. (Recorded TTL normally requires shutter speed not faster than 1/250). Also the D-2000 is compatible INON unique Clear Photo System which shuts visible light of internal strobe to take clearer image underwater.

'Underexposing in macro shot (within approximately 0.3m) when shooting in S-TTL with its Focus Light being "ON', is normal characteristic of a camera.

The Focus Light of the D-2000 illuminates a subject much brighter. When pressing shutter release button half-way with the subject being illuminated by the Focus Light, Auto Focus (AF) is locked and then Auto Exposure (AE) is locked before the shutter is fully depressed.

The camera already calculates correct exposure at this stage. A camera with TTL strobe control in manual exposure mode, calculates correct exposure factoring in outside light if the outside light is bright. (Certain Canon camera does not have TTL Strobe control in manual exposure mode). When the shutter is fully depressed, strobe emits pre-flash and strobe exposure is fixed. Finally the strobe emits main-flash.

When the strobe emits pre-flash and main-flash, AE is locked and exposure level is calculated with Focus Light would be ON. Therefore the main-flash amount will be remainder deducting Focus Light amount from total amount of light for the calculated exposure. (This is only in macro shot where the Focus Light affects) A camera does not expect the Focus Light being OFF during main-flash and calculate exposure as if the Focus Light were ON during main-flash. This is normal characteristic and feature of a camera.

The Focus Light of the D-2000 automatically turns off during shutter is open to prevent exposing focus light on image. So in macro shot, a subject illuminated by the Focus Light at pre-flash gets dark at main-flash for the Focus Light being OFF resulting in underexposing for the reason mentioned above.

There might be considerable influence in test shot for close subject on land but the influence gets less underwater since light decays underwater. However, in macro shot with open aperture or slow shutter speed, the influence could be observed. To prevent this influence, we provide two countermeasures. One is red sticker on the Focus Light and the other is red filter on diffuser panel. The red filter is detachable so user can use it when necessary depending on personal shooting style. The red filter/sticker also helps to soften focus light not to stimulate shy subject. The blue filter is for non pre-flash type camera in slow shutter to increase color temperature of focus light and not to have a flat spot in case focus light exposing in an image.

The left image (in Mr. Julian’s test) by Z-220 shows that Focus Light and Laser have been exposed on the image. Focus Light exposed on the right side of the subject and the laser on the shoulder. In this case, the camera calculated exposure with Focus Light amount so the image can not be underexposed. If the Focus Light had been turned off, the image should have been underexposed as same as D-2000. You may also download our D-2000 test method and results (pdf).

D2000 has shown to be extraordinarily fast and I certainly do agree that this is the best way to meet camera’s internal strobe characteristics. I think a 5 times faster discharge @full power would have been worth advertising from the beginning as I could not think of any reason to not mention this strength. On the other hand I can not confirm suspicion that exposure accuracy is decreased in case of DA2 in combination with a slower strobe (Z220, ys-60, Mini, etc.). The DA2 gain/amplify feature is user settable as well and set to no gain by default. The accurate topside results (exposure series) are not decreased underwater. The virtual gain numbers given in the article are simplified values basically demonstrating this feature. In reality light output is not proportional to flash duration. Most of the energy is dumped in the beginning because the discharge envelope fades out to the end. This characteristic differs from strobe to strobe. This means half power would require a lot less than half of the flash duration. Even with gain factor 1 (no gain) and a slower but more powerful amphibic strobe connected to the DA2, a great amount of energy is dumped in a relatively short duration. Depending on the discharge envelope and speed a quiet powerful strobe may not dump its complete full power in certain conditions when DA2 does not apply gain. Depending on which strobes is used, it probably could emit a bit less, the same or even a bit more amount of light in the same time compared to the D2000 max light output. In other words: Compared to the D2000 the DA2 is not an amphibic strobe but enables usage of a lot of existing strobes as slave TTL solution. In order to compare actual max power output it would be necessary to evaluate the actual connected strobe’s corresponding light output in the same amount of time instead of comparing full discharge speed only (speed only leaves out the intensity and overall guide number as it differs a lot between several strobes). This is hard to determine as many compatible strobes can be used in conjunction with the DA2. For example: ys-350 strobe might dump even more power than the D2000 within the camera internal strobe’s lightning duration (which differs a little bit between several camera models as well) as the big strobe’s intensity is higher. Of course this is not confirmed as several parameters have an influence on this: mother strobe’s lightning duration, external strobe’s intensity, discharge speed and envelope fade characteristic, etc. Beside the technical stuff which might be of limited real world usage the article is focused on the fundamental differences and circumstances between a slave adapter combined with a preferred strobe and an integrated solution. In my opinion this is what counts when it comes down to individual needs. For example: A friend of mine has spent a lot of money on dual ys-350 strobes in conjunction with his film SLR rig. Having the DSLR upgrade in mind he uses a compact digital camera for the time being. He was quiet happy by just adding a DA2 for 90 bucks to each strobe (he could have used just one DA2 but y-cable would have been more expensive than two DA2). Would be hard to convince him for two D2000 instead as he likes the powerful ys-350’s and is going to use them in the future with sync cord connections (DSLR). On the other hand a digital compact camera user gets the best available handling when choosing the D2000 (+/- compensation, unlimited manual settings with every camera, etc.). A C5060 owner might want to obtain full advantage of the camera’s hot shoe and the PT housing’s bulkhead and prefers a sync cord TTL solution (leaving out all of the mentioned optical slave TTL circumstances) and goes for a Subtronic Mini/Midi/Maxi with integrated Olympus TTL circuit and manual power settings available on the back as well. There are a lot of solutions out there and it depends purely on the individual requirements and eventually already existing gear. I hope the article has mentioned some useful points in this respect. Julian
Posted by Jolly on 2005-03-16 12:19:14 The article provides information on slave TTL solutions in general, describing technical details and ‘tricks” based on the DA2 features which are available to the public. Accuracy lacking is a bit controversial to mention because opposite to DA2’s available documentation the lack of provided D2000 information clearly could not and did not claim any accuracy in terms of D2000 / S-TTL internal operating method. Based on the provided description I kindly doubt (in an advantageously way) a pure and steady 1:1 internal strobe copy as the D2000 in my hands has proven to feature a reliable working exposure compensation dial. It may not be required to understand the method of the compensation dial for underwater usage but I still would be happy to see an explanation due to theoretical interest as this was the item my assumption was actually focused on. As a manufacture has taken the opportunity to share its point of view on a foreign product I would like to comment a few issues. Basically it is recommended to mount the DA2 completely free of ambient and foreign strobe light in order to avoid any influence - similar to the D2000 (supported mounting and optical cable is mandatory). Furthermore there is an additional feature called “recorded TTL”. This feature is user settable and allows save slave TTL operation even if the DA2 is not mounted properly (I was surprised how uninfluenced operation was possible even if the mounting is very poor). External strobe output is only delayed if Recorded TTL is switched on. Therefore this feature is no drawback but a handy additional tool for worst cases where other solutions suffer from ambient influence. This makes sense as DA2 is designed to work with many different cameras/housings and many different strobes. In several cases camera’s fastest sync speed is even untouched when Recorded TTL is active (S60: 1/60s fastest TTL sync speed). But you don’t have to use it as long as the mounting is properly done. Z220 + DA2 enable focus light suppression during flash output. In spite of this feature the Z220 user manual explains how focus light (and target laser) might be recorded under certain conditions. I guess this is because the strobe does not know when the camera opens the shutter and focus light can not quench before this happens. This limitation applies to Nikonos cameras in general. As the DA2 provides standard Nikonos connection this issue shows up in the same way it does on land with my Nikonos V (see exposure series). The Z220 can not distinguish between pre- and mainflash (basically a Nikonos strobe – no preflash) as the D2000 seems to do (and cuts of the light accordingly before or exactly when the shutter opens). However even small amount of light being recorded at the beginning of the exposure - the suppression applies to pre- and mainflash in the same manner and does not influence exposure. Moreover this issue is less effective underwater like the D2000 exposure deviation tendency due to the preflash focus light influence. In spite of the Z220 manual stating this issue I have never seen any focus light on my Nikonos V underwater pictures. Furthermore DA2 incorporates a special Z220 mode (user settable as well) to take care of focus and laser light. I have to admit I’ve never tried this mode and can not confirm the usefulness. The normal mode was fine underwater.
Posted by Jolly on 2005-03-16 12:18:40