A few facts about TTL BL flash, D lens protection, and FV Lock exposures

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WayneF

Senior Member
If interested, I added a very simple demonstration of TTL BL direct flash, showing how it is protected by D lens distance in a good case (meaning, which is sometimes accurate, and sadly sometimes not, sometimes it is worse. Primes are better than zooms).

Flash Indoors

And also showing how FV Lock can affect the metered value, apparently when TTL BL direct flash no longer pays attention to the D lens distance protection.

It will be trivial to reproduce this to see it for yourself, which is the purpose. It is certainly something we ought to know, but one point is, if your TTL BL direct flash is coming out underexposed, FV Lock may override it. FV Lock is not a feature on entry cameras without a commander however.

It is not a beginners subject, more for those that have explored their flash system, and who have questions. And not sure how organized this is... it may relay on information at other links, but the links are provided.

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daveg

Senior Member
Wayne,

I have already posted my example of how it can be used elsewhere.

Will you be posting any "real world" examples now?

Another interesting factoid about FV Lock:

You can take an FV Lock reading in a CLS setup and then change your camera to subject distance and it will give the same result
DG
 
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WayneF

Senior Member
daveg said:
Will you be posting any "real world" examples now?

Another interesting factoid about FV Lock:

You can take an FV Lock reading and then change your camera to subject distance and it will take the new distance into account when calculating the flash output - as long as you don't excede the flashes maximum output power.

DG
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I agree my pictures of this may not win many art awards. :) But they were real world photos with their situation selected to clearly demonstrate the details how how the D lens distance works with TTL BL mode direct flash, and how FV Lock ignores that D lens distance for TTL BL mode. Agreed, some users don't care to know, and may not need to know.

I have to disagree with your statement that FV Lock takes the new distance into account. It does refocus of course, but the idea is that "FV" is "Locked", and if you move signifiicantly closer and take another, the second will overexpose (inverse square law). Part of my point is that FV Lock does NOT take D lens distance into account (like TTL BL direct flash tries to do when not FV Lock).

Maybe try more extreme cases of movement, and you will see it. Two pictures at distant and close distances, with FV Lock with TTL BL, will show same effect as two pictures with Manaul flash or GN flash (all unvarying flash level).
 
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daveg

Senior Member
Hi Wayne,

I just tried it with my new model with the following settings:

Main Flash - SB800 on camera set to Master (Flash Off) Commander On.
Group A Flash - Camera Left approx 3-5ft from model - Zero EC.
Indoors so FP not active.

Take FV reading from Models face. Take picture.

Before the meter times out, move camera back to three times the original distance. Take Picture and compare in CNX2.

The Group A Flash gives exactly the same output when the camera is in both the original position (3-5ft) and when it is three times the distance away (10 - 20ft). This is repeatable.

DG

P.S. If you try to do this with the on-camera flash the (no CLS) then yes, I agree with you entirely, the flash output is locked in and changing distance will not produce the same result. This time MY statement was misleading and I have amended it.

P.S.S. I will introduce you to my new model soon. She is terrific and doesn't cost me anything - very low maintenance!!
 
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WayneF

Senior Member
daveg said:
Take FV reading from Models face. Take picture.

Before the meter times out, move camera back to three times the original distance. Take Picture and compare in CNX2.

The Group A Flash gives exactly the same output when the camera is in both the origanal position (3-5ft) and when it is three times the distance away (10 - 20ft). This is repeatable.

Sorry Wayne, I can't tell it any other way.:)
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There is absolutely no need to try that one Dave. :) I believe you, but I cannot say it any other way either - Flash exposure depends on the distance from flash to subject. NOT camera to subject. You can repeat that result with ANY fixed flash, in any mode.

I'm using hot shoe iTTL instead of Commander, but it is NEVER about camera distance. The D lens distance does assume hot shoe flash (at same distance as lens), otherwise it is not applicable and ignored.

So my link to Flash Indoors is another subject, about some Factors of TTL Metering Exposure. D lens and FV Lock are BIG factors of TTL BL exposure, stuff that is good to know what to expect.

Specifically, TTL BL simply does not act like TTL BL with FV Lock.

Direct flash TTL BL being protected from overexposure by the D lens distance is bypassed if using FV Lock (and also, the D lens distance is poor at best in many zoom lenses, often counter productive).

Balanced TTL BL flash in bright sun is bypassed if using FV Lock.

It then acts much more like TTL mode, except Exif still reports TTL BL mode (which it ain't). This seems sort of a biggie.
 
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daveg

Senior Member
Wayne we were writing at the same time and I have edited my post.

I was refering to a CLS setup and got a little "flash of inspiration" which turned into confusion!!

DG
 
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WayneF

Senior Member
daveg said:
P.S. If you try to do this with the on-camera flash the (no CLS) then yes, I agree with you entirely, the flash output is locked in and changing distance will not produce the same result. This time MY statement was misleading and I have amended it.
Click to expand...

That is because then you are moving the internal flash. Moving the flash does affect the exposure of the subject. However, if you are using TTL, then TTL will try to compensate it, if allowed to remeter with the flash at the new distance.
 
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