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How to test if eTTL is working?


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#21 james

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Posted 09 June 2004 - 06:51 AM

Hi Rob:
When you want to test something, it is a good idea to isolate that variable from all of the others. That's why it's best to test a flash metering system in a dark room.

You may well be correct that eTTL is a balanced fill-flash system that uses information from the camera's light meter and the flash meters. While that's relevant for real-world shooting (where ambient lighting plays a part) it's not as relevant for the ICU test.

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#22 whitey

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Posted 09 June 2004 - 06:55 AM

Whitey: Ever heard of balanced fill flash?  It's what your camera and the Canon EX speedlite are trying to do.

To do a proper test of TTL you should NOT let ambient light factor into the test.

Cheers
James


James, I have no knowledge of Nikon TTL systems. I do understand Canon eTTL and its practical application to nature photography. I have attempted to provide an example of what the ICU should do if it is functioning in eTTL mode (as it is supposed to do).

The primary illumination in the photos of the subject (the cup) is flash, not ambient (look at the f/22 shot!). This is not fill flash. You could call it balanced flash - which is exactly what the eTTL system does in M mode.

Using eTTL in nature photography (including underwater) is all about balancing flash vs ambient exposure. This is why it is a powerful tool. The way it functions is not well documented in the user manuals, but this information is available and it's important!

I'm happy to discuss this further if you folk are still a bit hazy about what I'm going on about

Best wishes,

Rob

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#23 james

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Posted 09 June 2004 - 07:06 AM

Rob and others:

Just to clarify - it doesn't matter what camera brand you are using:

A properly functioning TTL flash system should yield the same foreground exposure regardless of what F-stop is used.

Neither of the examples posted (either Curtis's or Rob's) seem to be working particularly well, although Rob's Canon flash seems to be working a bit better.

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James
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#24 whitey

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Posted 09 June 2004 - 08:48 AM

Rob and others:

Just to clarify - it doesn't matter what camera brand you are using:

A properly functioning TTL flash system should yield the same foreground exposure regardless of what F-stop is used.  

Neither of the examples posted (either Curtis's or Rob's) seem to be working particularly well, although Rob's Canon flash seems to be working a bit better.

Cheers
James


Yes, completely, totally agree with your first statement.

The point of my experiment is to show you the fact that the foreground flash exposure stays constant despite the fact that ambient light exposure changes with aperture.

Curtis's ICU clearly doesn't work for flash exposure. If it was reshot using my method above, it wouldn't work either, but it would provide me with a bit more information as to what is going wrong.

My example shows eTTL working correctly. I'm not sure why you think it doesn't. Foreground (flash) exposure is correct from f/2.8 all the way to f/22. If you shot at a higher ISO or with a more powerful strobe, f/32 would also be correctly exposed. f/22 looks underexposed at first glance, but this is only because of the low ambient light - subject is still exposed to the same degree as wider apertures - which is sort of the point of the excercise.

James wrote:
When you want to test something, it is a good idea to isolate that variable from all of the others.

In terms of your criticism of my study methodology, well I don't think my experiment will make it into Nature but I have only changed one variable - aperture. Remember, one of the most important criteria for a study is that it actually answers the question posed in the first place - in this case, how to best test eTTL. Testing foreground flash exposure only is almost, but not quite, the same.

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#25 scorpio_fish

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Posted 09 June 2004 - 10:15 AM

1. Sigh deleted

2. Alex's test is appropriate because one can have two results. If the exposure is the same, the answer is that TTL was working. If the results are not the same, TTL was not working

Your comments 1 and 2 suggest that you're not clear on what eTTL is and how it works.


3. I don't know how eTTL works, nor iTTL or dTTL or TTL. This is how I think it works.

Regardless of which type of TTL, your camera/flash looks at the scene. The flash determines how much light is needed to expose the scene "properly". The algorithm and technique varies from brand to brand and is proprietary, but the end result is that it controls the duration of flash to provide the overall "correct" exposure for the scene. With various speedlights this calculation may or may not include distance information. It is still looking at various points in the scene. In the case of Nikon, it is looking at 5 different points.

The speedlight is not necessarily evaluating "the cup". It is evaluating various point(s) (the size of the points is not known) in the scene. The results of changing the aperture can alter the overall exposure of the cup (a single reference point). This is why having the ambient background does not tell us if the eTTL was working properly. One of the possible results is that the exposure of the cup changes. But this does not mean that eTTL was not working or working properly. That is why I asked the questions. (Maybe I'm taking a photo of the lower left hand section of the tablecloth. If so, why is it different?). It just means that the brain that runs the eTTL decided to add or delete flash duration to provide a better result. Therefore, the result cannot be used to determine if TTL was functioning properly because there is no control results.

It's quite common for TTL flash to overexpose the foreground of scenes as it trys to increase the ambient exposure (which it cannot do). Current speedlights have added sophisticated algorithms to take this into account.
I'm not familiar with Canon's flashes. Perhaps the FE lock, i.e. spot/partial flash metering, allows only metering the cup (one would still have to define exactly what the metered area is). If so, the results could be measured to determine if eTTL was working properly, again only if you know the defined area that is being metered. Of course, this spot flash metering is a feature not available on underwater strobes.

I still stand by my original statement (sans sigh) that a fully lit scene provides the best test, because the results are easily interpreted as correct or incorrect. Nothing in the scene should be different (with modest exception).

#26 whitey

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Posted 09 June 2004 - 09:24 PM

3.  I don't know how eTTL works, nor iTTL or dTTL or TTL.  This is how I think it works.

Regardless of which type of TTL, your camera/flash looks at the scene.  The flash determines how much light is needed to expose the scene "properly".  The algorithm and technique varies from brand to brand and is proprietary, but the end result is that it controls the duration of flash to provide the overall "correct" exposure for the scene.  With various speedlights this calculation may or may not include distance information.  It is still looking at various points in the scene.  In the case of Nikon, it is looking at 5 different points.  

The speedlight is not necessarily evaluating "the cup".  It is evaluating various point(s) (the size of the points is not known) in the scene.  The results of changing the aperture can alter the overall exposure of the cup (a single reference point).  This is why having the ambient background does not tell us if the eTTL was working properly.  One of the possible results is that the exposure of the cup changes.  But this does not mean that eTTL was not working or working properly.  That is why I asked the questions.  (Maybe I'm taking a photo of the lower left hand section of the tablecloth.  If so, why is it different?).  It just means that the brain that runs the eTTL decided to add or delete flash duration to provide a better result.  Therefore, the result cannot be used to determine if TTL was functioning properly because there is no control results.

It's quite common for TTL flash to overexpose the foreground of scenes as it trys to increase the ambient exposure (which it cannot do).  Current speedlights have added sophisticated algorithms to take this into account.  
I'm not familiar with Canon's flashes.  Perhaps the FE lock, i.e. spot/partial flash metering, allows only metering the cup (one would still have to define exactly what the metered area is).  If so, the results could be measured to determine if eTTL was working properly, again only if you know the defined area that is being metered.  Of course, this spot flash metering is a feature not available on underwater strobes.

I still stand by my original statement (sans sigh) that a fully lit scene provides the best test, because the results are easily interpreted as correct or incorrect.  Nothing in the scene should be different (with modest exception).


The above asumptions about how E-TTL exposure works are mostly wrong. You mention you don't know how E-TTL works, and are not familiar with Canon flashes. FWIW, I do know how E-TTL works, at least more or less. I've got 30,000+ exposures on the 10D, and have been using E-TTL since Canon first bought it out in 1995 on the Eos 50. E-TTL is Canon's proprietary system, which was preceded by TTL and A-TTL. It functions differently from these, and you can't use the terms interchangeably. E-TTL is supported by all type A bodies, which include all Canon's recent digital cameras. It's this flash system that the Aquatica ICU is supposed to support.

"The ICU allows for full E-TTL capability with the Canon Rebel, enabling extremely accurate strobe exposure by basically mimicking the E-TTL of Canon land strobes.", to quote the March press release on Wetpixel. Aquatica have been very quiet on the ICU front of late, I just checked their website again and didn't see any mention of it. But they are selling housings with it, and it isn't cheap, so it's reasonable to expect this to work. To test if they really do have E-TTL support 'cracked', you need to know how E-TTL functions in the first place, which I've tried my best to explain.

I've talked at length above about balance between flash and ambient light. The reason that this is important is because if flash predominates too much, even if the exposure is correct, the result tends to look artificial. You see this a lot in macro photography, including underwater.

The E-TTL test I posted was the second one I shot, and is was done purposefully and after careful consideration. I initially shot with the exact same parameters as the initial poster had with his ICU (ie indoors with less ambient light, 1/200). I found that at the settings used the ambient light component was underexposed at all apertures, and therefore the test wasn't as useful for testing E-TTL as it is generally used 'in the wild' (this first test was similar to Dr Mustards'). It does test how you would use E-TTL if you're shooting in low ambient light conditions. Both tests are useful. My posted test models better how I've been using E-TTL both above and underwater.

Best wishes,

Rob

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#27 satura

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Posted 14 June 2004 - 09:33 AM

On http://www.muenster....litz/indexe.htm Matthias posted new infos about his ETTL converter. I'm in the process of buying a housing for my 300D and my Ikelite DS125 I think I wait until I see the results of this converter. I'm dreaming of a Hugyfot with manual and TTL support. Unfortunately there is no 300D housing from Hugyfot and no TTL support for my Ikelite 125......

#28 bvanant

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Posted 14 June 2004 - 10:01 AM

From seeing matthias's site, it appears that he needs to see the built in flash from the camera. This precludes any housing where the internal strobe can not be put up and those made of metal I think.

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#29 james

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Posted 14 June 2004 - 10:06 AM

I think his OEM converters are purely electrical and go in between the camera's hotshoe and the strobe bulkhead.

HTH
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#30 satura

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Posted 14 June 2004 - 12:28 PM

He has optical adapters but if you follow the link http://www.muenster....blitz/ettle.htm you'll find a fully hardwired converter.

#31 whitey

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Posted 14 June 2004 - 08:42 PM

- ETTL is converted to Nikon-TTL, not emulated. This means the exposures are similar to a standard TTL camera, like the Nikonos V

Whether this is a problem depends on whether 1/it actually works and 2/whether you wanted E-TTL functionality.

- Due to the complexity of the ETTL protocol currently only the Canon 300D/Digital Rebel is officially supported and the following strobes: Sea&Sea YS60/90/120

Mathias notes that other strobes 'might' be supported in the future. If he's engineering it with one particular strobe in mind, your odds of getting ds-125 support down the track are unkown.

mTTL ™ mode should still work fine! :lol:

Ike reckons he will have real E-TTL support by summer on his housings. Now he may be talking about the Australian summer, or he may be talking about summer in some far-off decade. But I'm still figuring his system is the most likely to work with a DS-50 or DS-125.

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#32 satura

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Posted 15 June 2004 - 12:23 AM

I already have a plastic camera - I don't want a plastic housing for it :-) It is unlikely that Ike will sell the conversion circuit without the housing....

#33 whitey

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Posted 15 June 2004 - 03:17 AM

Here's another Canon housing option we seem to have overlooked.

http://www.photo.net...hoto_id=1578143

Perhaps this is the one for your camera? I'm not sure about E-TTL, you may have to contact their R&D department. I'm proud to say this is Aussie engineered and manufactured. Cost is $AU1600. 1Ds owners sell your Seacam, this is the housing to shoot with!

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#34 satura

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Posted 15 June 2004 - 04:05 AM

Nice housing :-) :-)

I don't know enough about real ETTL to say if I need this or not. Standard-TTL sounds usable to me. I mailed Matthias for more informations about the differences between ETTL and normal TTL and why he decided to convert ETTL to normal TTL instead of emulating ETTL (I would say because the substrobes cannot fire the "special preflash"...).

What I really want is a 10 settings dial on my solid housing with 9 manual settings and a TTL setting. This would give the photographer a strong tool to work with.

#35 tshepherd

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Posted 15 June 2004 - 05:58 AM

What I really want is a 10 settings dial on my solid housing with 9 manual settings and a TTL setting. This would give the photographer a strong tool to work with.


Then buy a film camera or a Fuji S2. Sorry if this sounds sarcastic, but that's simply a fact of life at this point; no other verified, functioning options as of now.

OR, get a strobe with multiple manual settings and learn to shoot it in Manual mode.

Tom

#36 Jolly

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Posted 15 June 2004 - 12:24 PM

I mailed Matthias for more informations about the differences between ETTL and normal TTL and why he decided to convert ETTL to normal TTL instead of emulating ETTL (I would say because the substrobes cannot fire the "special preflash"...).


Hi,

yes you are right. It is just the inability of amphibic strobes to send this special "modulated linear preflash". Instead a common preflash is used and the converting system is calibrated for the differences.

The metering system will of course maintain E-TTL as the camera does it including flash output adjustment. It's "just" the preflash replacement with a common preflash.
And I understand it won't feature high speed sync and you will have problems when selecting an aperture value equal or greater ambient light.

But this should also apply to Aquaticas system, as no amphibic flash features the Canon preflash method. If there really is an Aquatica E-TTL setup. I have heard it is more mTTL :-)

Concearning the TTL and E-TTL tests:
Alex test is of course correct and applies to every standard TTL metering system. For E-TTL it tests the basic flash light metering and strobe adjustement. A perfect method to start if there is a problem with the system.
But E-TTL always takes ambient light into account with several metering zones. This can not be disabled to one simple metering zone/mode. To confirm the function of E-TTL Robs setup makes sense as a proper system has to handle this. If the system is correct it will work with both test setups.

@Rob
As you mentioned bad news on Heinrichs converter for our 10Ds for the time being.
He says it is quiet sure that the converter will not work with the 10D right now. The reason is this preflash item. Because of the simple preflash the converter has a programmed correction table. This correction will be different for the 10D because the camera meters with more zones and the results are different. however the 10D "upgrade" of the converter will be just a firmware update.

some minor good news: It is not confirmed by tests until now but Matthias mentioned he is absolutely sure the Inon Z220 will work. He is about to confirm this very soon.
The DS-125 is no choice for me. In Germany you get two Inon Z220 for the price of one DS-125. And I am quiet sure that the DS-125 won't beat two Inons.

Maybe we should get used to the idea of housed system guns for some time :-(
I guess Matthias is the only promising chance left right now. Aquatica and Ikelite don't seem to finalize their E-TTL solution to soon.

greetings,

Julian
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#37 Alex_Mustard

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Posted 15 June 2004 - 11:10 PM

Just to clarify. My test was done at the end of 2002/start of 2003 with a Nikon D100 and a housed SB80DX strobe using Nikon's dTTL protocol.

When I started with my D100 I used dTTL for all my macro and standard (28-70mm) lens images UW. It worked very well. But I now prefer manual strobes for everything and my last 9000+ digital images have been taken without TTL. I still use TTL on film (because I don't have a LCD and histo on my film camera!).

I am off to the Red Sea on Sunday and I am not even taking the housed SB80DX with me on this one week trip.

Alex

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#38 Jolly

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Posted 16 June 2004 - 01:43 AM

Hi Alex,

that's interesting to read as I have some kind of incurable target fixation that TTL is a must have for macro. :lol:
If the amphibic strobes could do TTL would you still do a lot of manual with macro shots?
Would be nice to see there is a chance to handle macro work manually as it would make my housing/flash decision more easy. With film I never tried closeups and macros manual, only WA.

thank you and regards,

Julian
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#39 Alex_Mustard

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Posted 16 June 2004 - 02:55 PM

It is a common issue with anyone coming from film to digital. Nobody (myself included) can understand how they don't need and won't want TTL on digital until they have bought the camera and spent loads of time and effort on getting TTL 4 digital!

Alex

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#40 curtisleo

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Posted 16 June 2004 - 04:08 PM

Thanks to all with all the responses. I'm quite happy that everyone here have been quite helpful. I would have to say thank you to Ryan Canon at Underwater Camera Pros for taking care of my setup.

Unfortunally I was unable to get a working eTTL ICU from Aquatica however I did receive a call from a engineer. They are working and testing it so very soon, this will be resolved.

Now I'm off to my dive trip to the Phillipines tomorrow and will post my pictures from there when I get back.


Curtis