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TTL Metering


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#1 carl_goodier

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Posted 20 January 2003 - 10:27 PM

(sorry to put a film question in the Digital part )

Question, which alot of UW photographers cannot answer me so far:-

Situation : If you are shooting on land without a flash ( i.e. Island / beach scene ), you may choose a particular hue in the sea for metering, lock it with AE lock, re-frame you picture and shoot.......... knmowing that the light / colour will be metered according to your selection.

Configuration : - Under water, I use Nikon F80 in a Sea& Sea NX-80 housing with 2 YS -90 Strobes ( one slaved )

Question:- If I were to aim the spot meter square in the view fnder on a, lets say yellow subject, and then press AE lock, and then move the view finder to get the right frame , where by now the spot meter is in a black portion of the frame............... when firing the shot, would the TTL strobes remeber teh AE-locked value from the yellow subject, or would it now control the strobes based on the new ( black ) value which is currently in the spot ?

Thanks a million for your help / advice.

p.s. I usually shoot in Manual, but is it better to shoot in S or A in TTL ?carl_goo@telecomasia.co.th

#2 Cybergoldfish

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Posted 20 January 2003 - 10:36 PM

Check out your post in film cameras

#3 carl_goodier

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Posted 20 January 2003 - 10:45 PM

ooopsssssss

#4 carl_goodier

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Posted 20 January 2003 - 10:47 PM

Appologies posted !............. Ooopsssss

I mis-interpreted your statement ( have seen them before in digital forums ). Please accept my appologies and thanks for your help )

#5 carl_goodier

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Posted 23 January 2003 - 02:45 AM

Thanks for your kind help........ but slightly in a different direction than expected.............

Lets assume that the object of desire is well within the strobe distance ( i.e. with YS-90 and Velvia film, I can in theory shoot a object using F8 at one meter distance, and the strobes will light it up nicely. So lets assume that I am using F8 at a distance of 0.7 meter ( well within range ) ( so there is enough power !).

Secondly, I am aware that the TTL sensor will shut down the strobe when it senses that enough light has been received by the film plane......... So here is the question one more..........

Question

If I set the spot metering square in the view finder on a lighter subject, then hit AE lock, and then re-frame the picture ( for composition ) , where by the Spot meter no falls on a darker colour.......... when the strobe fires, I assume now the TTL exposure will be based on the Spot meter square in
the view finder which is now on the darker subject, even though I really wanted to meter it on the lighter part.

Example, and very light skined diver in a dark wet suit. If I try to make the TTL exposure based on thier skin tone ( and the AE lock ) and the re-frame to get the diver in the frame whereby no the spot meter is pointing on thier chect area ( black wet suit ), [COLOR=red]will the TTL flash less power based on the original face ( which I tried to AE-lock ) or will it now use the current place of the spot meter on the black wet suit. If so, then the result as we all know would be an over exposed picture as the TTL will over compensate for black objects. [COLOR=blue].

Thanks for your views !

#6 bobjarman

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Posted 23 January 2003 - 04:44 AM

If I understand what you are asking I think the answer to your question is no. When you hit your AE lock button your telling your camera only the shutter speed or aperture or both depending on what mode your shooting.

Flash is an entirely different beast. You are correct that the flash system will read the amount of light that hits the film and shut down. But this amount is a fraction of time versus the exposure time you locked into your camera. The flash system has no way to know what to do other than to read the light on the film surface.

I think what you are wanting to do is called FEL or "Flash Exposure Lock". In this method your would do exactly what you are asking. find a flash metering point, press your FEL lock button, recompose and shoot. On Canons a brief flash is omitted to allow the camera to read the light of the area your want to base the flash output on. Than, the camera stores this information while you remeter the scene and shoot. FEL lock is available on most high end and some midrange cameras.

Look at it this way, the exposure lock is controlling the ambient light, the FEL lock is controlling the flash lighting. Its actually two separate meterings.

Cheers!

#7 james

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Posted 23 January 2003 - 06:42 AM

Think of it this way:

The camera has "two" light meters. One is used to set fstop and shutterspeed (what we call exposure) and the other meter is used to control the flash during TTL.

The first meter does its work before the shutter is open, setting the aperture or shutterspeed or just one of those two (if you shoot in A or S mode).

The second meter (the TTL one) ONLY works WHILE the shutter is open. It doesn't care what the aperture or shutterspeed is, all it cares about is how much light hits it WHILE the shutter is open.

I shoot a Fuji S2pro - which is basically a Nikon F80 and that's how my camera works - and most Nikon film cameras. So, I hope that I've helped you. The Nikon F80 actually has 3 TTL flash modes depeneding on which flash is connected. For underwater use though, the "regular old TTL" is what the F80 uses.

Preflash cameras may be different (as Bob stated above RE flash exposure lock) as the Canon E-TTL and Nikon's D-TTL actually use the flash light meters before the shot.


HTH
James Wiseman
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Photo site - www.reefpix.org

#8 carl_goodier

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Posted 23 January 2003 - 10:02 PM

Thanks guys !. I suspected so, but could not find any documentation to support the gut feeling.

Thanks for your help

Carl