Yet another question about ambient light ... sorry

[northlondon43]

Colin and William thank you so much for taking the time to go into so much detail. I'm at work right now but will take a good long look at what you've both said later tonight - particularly pleased that William knows so much about the Samsung NX system! Oh and yes William it is an external Samsung flash - think its the SEF42a.

Cheers

Adrian

[William W]

Ok . . .

[William W]

it is an external Samsung flash - think its the SEF42a.

That's the one I used with one of my Students . . . I gotta get back to the beef.

Maybe I will post some photos of dinner.

There does not seem to be much info on the Samsung Flash Units readily available: maybe I could arrange to get a copy of the User Manual from you? - it would assist me help my Student, maybe.


WW

[Colin Southern]

Haha!

This is my latest find we had it with Pasta, last night – very nice:


But I am cooking Beef tonight so you just talked me into trying this >


. . . got to go and stoke the fire, CU later.

Sigh, so many vineyards around here ... just another boring day in paradise

[William W]

Maybe I will post some photos of dinner.

WW

[northlondon43]

William, given all your help the least I can do is try and provide you with the info ref the Samsung flash. However I only have a hard copy of the user manual somewhere, I think, I'm not sure how to get any info to you. To be honest the manual isn't very detailed as I recall.
Adrian

[William W]

Don't sweat it - I expected the User Manual to be 'not very detailed'.

Let us all know how you go with the testing with the Flash in Manual Mode.

WW

[pnodrog]

Sigh, so many vineyards around here ... just another boring day in paradise

Collin they are both North Island wines - Bill must be a good swimmer. If you want some you only have to swim the Cook Strait not the Tasman Sea as Bill did.

[William W]

I didn't know that there were TWO pieces of New Zealand

[Colin Southern]

Collin they are both North Island wines - Bill must be a good swimmer. If you want some you only have to swim the Cook Strait not the Tasman Sea as Bill did.

Wouldn't need to swim anywhere for a good wine ... more vineyards than I can shake a stick at in my parts

[northlondon43]

http://a248.e.akamai.net/pix.crutchf.../305SEF42A.PDF

Hope this helps William :-)

Adrian

[revi]

Wouldn't need to swim anywhere for a good wine ... more vineyards than I can shake a stick at in my parts

Same here, though I'm not sure they all produce drinkable stuff (well, someone must find it drinkable, I guess...)

[William W]

Hope this helps William :-)

Adrian

Yes thank you it does: and it also confuses me a little bit too.

I have just had a quick read. My Student has the same unit but has no user manual, I believe she bought her camera and Flash second hand.
She does have a double page one A4 sheet - an information brochure on the Flash.
On that document the 'A-TTL' is referred to as "Automatic Through The Lens" . . .

The plot thickens!

If you keep on with the testing I shall appreciate very much any results you share.

Thanks again,

WW

[northlondon43]

HI everyone...I started this thread some time ago to try and understand how the Samsung ATTL system worked and indeed differed from the Nikon/Canon ETTL system.

I posted last night on another forum (see below) and whilst intrigued by the answer I must admit it seems way too technical for me. Can anyone put it in slightly easier to understand terms for me please?

My question

'Hi, I've a Samsung nx11 and sometimes use the sef42a flash with it on attl. The thing is I can't find anywhere on the web explaining how the system works. The manual is next to useless and I've tried a number of forums to no avail. When I contacted Samsung I waited several weeks and got a very poorly translated explanation from a south Korean engineer.

All I've found out so far is that it doesn't work the same way as on Nikon and , which I think has ETTL.

Why am I interested? Well I sometimes get quite strange results and would like to know what the Attl is doing.'

THe answer I received

A-TTL (Advanced TTL) uses an infra-red preflash for ranging the subject. It then sets the aperture to balance between depth of field requirements at that distance and the background exposure required; then fires the main flash burst and controls the flash exposure (duration) by measuring through the lens OF THE FILM PLANE.
E-TTL (Evaluative TTL) uses TTL metering for ambient and flash exposure, but NOT off the film plane. A preflash is fired for ranging, subject location and brightness, and its results are compared with the evaluative metering for the background. The main flash burst is then fired at a predetermined (calculated) level/duration, based on the preflash and evaluative ambient measurements.

[Inkanyezi]

The verbosity of the answer is fairly easy to parse. However the actual problem is not solved only by knowing how the system is supposed to work.

Automatic flash has been around since the sixties. The first auto flash systems expected flash direct on, and worked primarily as the guide numbers do, by adjusting the aperture to the distance. It worked, it worked well, but was very limited, as it was used only with steady output flash and with direct flash.

Came the event of thyristor control of flashes, the first ones dumping excess energy into a built-in load, losing as much of the battery power for a small flash as for a big one. Now flash could be controlled by a photo diode that cut the current when sufficient light had been returned from the scene. Upgrading the technique it could also save battery power by not dumping excess energy, but saving it in the capacitor, which also decreased recharge time and enabled the flash to fire repeatedly as long as there was sufficient energy stored.

The drawback with an automatic electric eye flash was that it would not know which your main subject was, the one to expose correctly, so engineers had to make a qualified guess to just how much to over- or under-expose the general scene. A misplaced finger would be a fried out blob, making the scene behind pitch dark, due to the amount of light that reached the sensor.

Came TTL, and Olympus was first, with the OM-2. That camera measures light off the FILM PLANE, actually from the film itself. Other cameras later adopted the same system, taking the light reflected off the film for evaluating exposure. The engineer told you that the Samsung NX series measures light during exposure off the sensor, much akin to what the off the film measurement did.

The problem faced by flash manufacturers, whether using an external sensor or one in the camera, is that the flash duration to be controlled is so infinitesimally short, that delays within the electronics themselves are significant, effectively impeding precise control. Even the very short distance that light has to travel from the flash and back to the camera may influence the precision of the system. These problems have not been solved yet, but camera manufacturers are trying to make the firmware aware of corrections that have to be done and automagically apply a "corrected" flash duration.

Where we stand now, TTL flash still is largely moot. Measurement is made through the lens, yes, but not with the actual flash, but with a pre-flash, which tells the computer within the camera how much of the light will be reflected back, so that it may set parameters accordingly. Your A-TTL uses another approach, trying to measure the actual flash during exposure. It fails, as such systems always did. Your A-TTL uses the invisible pre-flash to measure distance only and to set aperture accordingly, then after the aperture is set, the flash itself is measured from the sensor, according to the information you got. You have confirmed that this system does not work as purported. However, it does work as designed, i.e. not very well.

Your engineer also used the term "evaluative". That term is about the most mumbo-jumbo term you may come across presently regarding cameras. The NX camera indeed may have an evaluative feature, as its measuring area, the sensor itself, may distinguish just any part of the image to use for evaluating against other parts. However, one of the largest and most hailed camera manufacturers, completely lacks any hardware to make such an evaluation, Canon. The only distinct points in any Canon camera than may be measured for evaluation are the AF spots. There is no information about how evaluative metering works, not from any manufacturer. "Evaluative" is marketing blurb, marketroid dribble that tells us nothing. If we cannot understand WHAT it does, how could it be used creatively?

So in essence, you have partially analysed the problem with flash metering and its inconsistency with your camera. You may hope that direct on camera flash will function well, when using it in its fully automatic mode, giving you no control whatsoever over the effect. Fill flash probably will not work with any consistency, and that leaves you with the option of manual control, if you would not want to be limited to what the incomprehensible automatic system might do.

What you really should do is try other ways of using the flash, whether on or off camera. The flash has a tilt head, and it can be pointed at other directions than straight on. Remember that you should not tilt partially, as this will supply your scene with some direct light only in the upper part of the image, leaving the bottom dark. Tilt must be done to the maximum angle, always.

When tilting the flash, you might experience dark eye-holes on your subjects, which is not very pleasing. To get rid of that problem, you may easily supply some direct light by using a rubber band and a piece of paper, that you place on the back side of the upwards tilted flash head, to reflect a portion of the light directly toward your subject, as per this image: http://uploads.ifokus.se/uploads/031...gummisnodd.jpg. The size of the paper will govern the amount of fill light, and it should be flush with the backside of the flash head, not tilted or bent forward. The main light source for the image should still be the ceiling.

To further enhance your flash experience, you may get an extension cable, so that you can hold the flash off the camera, and direct its head in other directions than straight up, just remembering that its IR window must always be pointed toward the subject. When using reflected flash, lighting usually is more consistent, as the flash will not go into those extremely short durations where results are unpredictable.

[northlondon43]

Your A-TTL uses the invisible pre-flash to measure distance only and to set aperture accordingly, then after the aperture is set, the flash itself is measured from the sensor, according to the information you got.

Firstly thanks for your comprehensive answer and background to the development of flash. I must admit to still being somewhat confused by the above. I use my camera in manual mode with ATTL so I don't understand how my aperture can be automatically set by the flash talking to the camera. Secondly does your answer imply that the level of ambient light is not taken into account?

[Inkanyezi]

Firstly thanks for your comprehensive answer and background to the development of flash. I must admit to still being somewhat confused by the above. I use my camera in manual mode with ATTL so I don't understand how my aperture can be automatically set by the flash talking to the camera. Secondly does your answer imply that the level of ambient light is not taken into account?

The answer you got from the technician implies that A-TTL works in Auto mode, and sets the aperture according to measured distance. I cannot check the validity, but as I understand it, if you set it yourself in manual mode, the flash still would be measured off the sensor, although depending on your choice of aperture and the distance, It might be less predictable, as the flash might work in a mode where its duration is outside some limit.

The background lighting will always influence measurement as well, as it is measured off the sensor, and all light that is received will be integrated into the measurement. So with very strong ambient light, the flash should automatically be weaker. However your trial showed some inconsistency, so I wouldn't bet on it.

But you should really try the suggestion, point the flash straight up, fix a piece of paper upon it with tape or a rubber band, and see how the pictures come out. If you do this manually, I'd suggest f/8 at ISO 800 as a starting point, then you can adjust either ISO or f-stop to make images brighter or darker. Compared to using flash with ambient light, this method has a great advantage, as the flash will be the main light source, but coming also from the ceiling. Otherwise, there is a stark difference in colour temperature between different light sources, which usually does not look very nice.

[northlondon43]

Quick reply... I think you've misunderstood the quote I started with.... It's not from a Samsung technician, must someone on another camera forum