Help Please: TTL Flash- How does it work?

Pegasus_Thrust

Pegasus_Thrust

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I am an advocate of not using flash and as and where possible but I do understand that it is a must in certain conditions and at certain times of the day.

What I am trying to get my head around is how actually the TTL element works, I had until recently thought that the TTL part of the flash calculates the amount an image is to be under exposed based on information from the camera and powers accordingly to make up that short fall.
However upon doing a bit more reading I am now understanding the flash itself meters the subject/composition from within the flash unit and uses power appropriate to produce correctly exposed images.

Firstly I take it my more recent understanding is the correct one?

if this is the case:

What other info does the flash take from the camera other that focal length?

If the flash powers to create a correct exposure on it's own and in conditions where the camera can correctly too would I expect to get an over exposed image?...... if this is the case do I just dial down the flash compensation or the exposure compensation.


Don't get me wrong, when I have to use flash I do eventually get a correctly exposed image but it is hit and miss and with a bit of tinkering, I just want to understand in my own brain so as to dial the settings in correctly and rely on it without chimping.

I shoot nikon and with a flash on the settings I usually use are aperture, usually shooting at f2.8 at about 1/200th. I only really use flash to help illuminate indoors, as a bit of fill in bright conditions and on the odd occasion shooting outside at night.


Lots of questions I know, if anyone can shed some light (hu-huh!) on it, or at least put it into layman's terms for me that'd be great.
 
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TTL flash means metering through the camera lens. The camera decides the exposure. It does this by asking the flash to fire a short low power pte-flash, which the camera meters. It then divides how much power would be required to get a proper exposure, and tells the flash to fire that amount. If you're using some kind of auto exposure settings along with the TTL flash it may also adjust other things to bring the flash power into a good range, such as ISO or aperture.

Note that there are other ways of controlling flash power to get the right exposure without using the camera, using a meeting system built into the flash, such as the auto-thyristor flash popular in the era of film photography. But if your camera and flash say TTL then it's the camera deciding the flash exposure.
 
Well this is in line with my first thoughts and has thrown me, but I had read elsewhere that the flash assesses the lighting also- is that nonsense then?

Also if I had a TTL flash off camera via some TTL sender/receivers how does this work then?

Incidentally I use auto ISO in aperture, should I disable this?
 
Just to add to what Chris has said.
You need to think of the flash exposure and ambient exposure as 2 seperate things (because your camera does).

Also, you should wise up on how dumb a light meter is. Your camera doesn't see something dark and see underexposed, it sees dark. And if the average* of the objects its looking at are dark, then it'll assume they're mid-tones and overexpose the image. The opposite is true for bright things where it'll underexpose.

So once your meter has decided on the measurement of the ambient light, and then you decide to add flash, the flash prefire metering is as likely to give an incorrect or correct result as the ambient meter is. My rule of thumb is this:
If the flash is used as 'fill' light, I'll use auto modes and dial down the flash exposure compensation (the camera likes it a bit hot for me).
If the flash is the key light, I'll switch to Manual to fix the ambient exposure, that might be fixed at just under (effectively becoming a fill light) or way under (to provide a hint of atmosphere to a flash lit image).

Whether the flash is on camera or off, the TTL measurement works the same, but. If you're using off camera flash in a static (studio type) situation, the TTL metering is much less reliable than Manually setting the flash.

Consider this: You've set up your studio, with a grey background and you're shooting headshots. Your TTL flash will output different amounts of light for a blonde Caucasian in a cream top than it will for an Asian in a navy blue top. Do you want to fix those in post, or switch to Manual and get consistent results.


*Matrix metering tries to work out what objects might be in front of the camera, and it might be pretty good generally, but you have to remember it's still guessing.
 
Phil V said:
Consider this: You've set up your studio, with a grey background and you're shooting headshots. Your TTL flash will output different amounts of light for a blonde Caucasian in a cream top than it will for an Asian in a navy blue top. Do you want to fix those in post, or switch to Manual and get consistent results.
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This to me was one of the critical things. Shooting an event, every shot differing distances, exposures and all the other variables, shoot TTL, there will be variances but you'll get more right than shooting manual.

Static subjects, shoot manual, you'll pop a few frames getting the exposure so, but you'll learn how the lighting is working, rather than just thinking "well that worked" and scratching your head afterwards on the voodoo of TTL, and then you'll get consistent results for all the subsequent shots, whereas TTL may well vary the exposure because your subject has moved a step to the right and so sitting somewhere else in the metering. Strobist 101 or "Light, Science & Magic" are great reads.
 
As for OCF using receivers etc the flash is on a hot shoe on a stand the transmitter on the hot shoe on the camera and they talk to each other just the same as if the flash was on the camera
 
Thanks Lartamax, but what if I am using 2 OCF hitting the subject at different angles with camera in the centre (so 3 angles and 3 potentially different amounts of light)- will it calculate seperate amounts of power to be applied based on the 2 seperate pre-flashes?...... and would those amounts be reliable amounts with camera being in a 3rd position?
 
Sounds like a static situation, where manual would be better.
 
Yes when I have been faced with the situation I have switched to manual tbh, but based on the info above I was just trying to get my head around how the voodoo might work.
 
Pegasus_Thrust said:
Yes when I have been faced with the situation I have switched to manual tbh, but based on the info above I was just trying to get my head around how the voodoo might work.
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Re-read my post.
Of course it'll expose it 'correctly' but how would it know how you want it lighting? You're using 3 flashguns, how would the camera know which was the key light, what the ratio for the fill light should be and who knows what the 3rd light is (background light at a stop under, hair light at 1:1, high key white b/g at +1/2 stop)?

Lighting is craft, you can't expect your camera to do the craft for you.

In essence the 'voodoo' won't work. Your camera hasn't got a clue what you want to achieve until you tell it. Now you can still use 'the voodoo' by setting FEC individually on the flashes, but as I said earlier reflective meters are easily fooled and it takes the same amount of effort to set the power manually.
 
Pegasus_Thrust said:
Thanks Lartamax, but what if I am using 2 OCF hitting the subject at different angles with camera in the centre (so 3 angles and 3 potentially different amounts of light)- will it calculate seperate amounts of power to be applied based on the 2 seperate pre-flashes?...... and would those amounts be reliable amounts with camera being in a 3rd position?
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How does the camera know how many flashes you're using? It doesn't, so it uses one pre-flash whether you're using one OCF or ten.
 
Jonnybrad2001 said:
Static subjects, shoot manual, you'll pop a few frames getting the exposure so,
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Or use a flash meter and get it right first time.. like we did with film when you couldn't see the image :)
 
Pookeyhead said:
Or use a flash meter and get it right first time.. like we did with film when you couldn't see the image :)
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Probably right, however I learned to meter in the old days which makes it straightforward to me. But have you seen the experts debate which direction to point the meter?
 
Phil V said:
Probably right, however I learned to meter in the old days which makes it straightforward to me. But have you seen the experts debate which direction to point the meter?
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That's true.

Lost skills.. lost skills...
 
I blame the teachers!!! ;)
 
“Befowa the wowa” we used to scratch a formula in the sand with a pointed stick, it was something like … f = flash guide number / distance. I fink :thinking: :).

Rhodese.
 
Phil V said:
Probably right, however I learned to meter in the old days which makes it straightforward to me. But have you seen the experts debate which direction to point the meter?
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You forgot the quotation marks around experts!

Rhodese said:
“Befowa the wowa” we used to scratch a formula in the sand with a pointed stick, it was something like … f = flash guide number / distance. I fink
Click to expand...

Take the camera to subject distance in feet, divide it by 3.291 and multiply the result by the aperture. Subtract 7 if it's a Thursday or a bank holiday (except Christmas day) and add 3.6 if it's your birthday and the final number is the amount of flash powder you need in ounces.


Steve.
 
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