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TTL Metering


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#1 carl_goodier

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Posted 20 January 2003 - 10:25 PM

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 - 10:33 PM

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

The metering system is measuring the ambient light reflected from the subject. Holding the AE lock, locks that particular reading and keeps it when the shutter is released.

Matrix metering may remember this value in P setting only.

Most UW strobes have VERY basic TTL systems and do not dedicate in P mode. This means you may over/under expose depending on the reflectivity of the metered subject.

#3 carl_goodier

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Posted 20 January 2003 - 10:51 PM

So using AE lock will lock the metered value and control the strobe output even if you re-frame the picture ? ( I am sure it works in P, S and A mode )

#4 wetpixel

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Posted 21 January 2003 - 02:07 AM

Right. That's if your camera supports flash exposure lock. :)
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#5 scorpio_fish

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Posted 22 January 2003 - 02:39 PM

The short answer is no, although the answer can be "depends".

Let's see if I can explain it decently. AE lock will not affect TTL strobe output. TTL strobe control via your camera; the camera will send a signal to the strobes to quench when the TTL sensor sees that the film has gotten the "correct" exposure. This "correct" exposure is programmed into your camera based upon some ANSI luminence value which equates roughly to the 18% gray we here so much about.

First, as soon as your strobes are turned on you will notice that your camera will immediately change shutter speeds to 1/60, regardless of what your exposure meter reads.

Secondly, in your example, the yellow object that you want to as your exposure point... Is it within your strobes range of light? If it is and you meter the yellow object and lock that exposure, then the strobe output must be zero to maintain that exposure. Any strobe output will overexpose it.

If the object is beyond the light output of your strobes and you meter the object and lock that exposure (easier just to do this in manual mode), the strobes will still want to light the entire frame to 18% reflectance. Your yellow object will be exposed properly, but your foreground will be potentially overexposed.

Assuming your object is beyond the strobes distance (i.e. background), the typical technique is to meter the spot you want to expose proplerly and set your strobes on manual power to light the foreground based on a guide number. This can be done most easily by setting your camera in manual exposure mode. Meter the spot, set your exposure and then recompose.

The above does not apply to land flashes, e.g. a Nikon speedlight.

How a camera is programmed to work with flash also varies from manufacturer to manufacturer. Canon uses a different technique for controlling its flash.

Lastly, grab your camera manual and find the subject "slow synch". Slow synch will allow you to do what you want to do. Slow synch capabilities vary from camera to camera. This allows you to do long exposures to expose backgrounds while still using flash.
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#6 carl_goodier

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Posted 23 January 2003 - 02:36 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 !

#7 Cybergoldfish

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Posted 04 February 2003 - 09:09 AM

AE lock only works with the natural light exposure.

Using spot metering and thinking this works with TTL flash is a nice idea but the flash uses different sensors that either take a generalised centre weighted area off the film e.g. Nikonos or using matrix sensors from the whole film area.

It would be wonderful if you could use spot in TTL flash, saving a lot of burnt out fish shots against
an infinite blue.

Holding down AE lock will only influence the natural light reading whether you are in manual, s.auto, a.auto or program.

#8 yahsemtough

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Posted 04 February 2003 - 09:53 AM

So, I think what was said by Scorpio is meter the background, ie the blue for proper exposure of it as the strobes do not really change that then, adjust your strobe power and distance for the main subject with the desired stop.
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