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D7000/SB700 in TTL and AF point selection

Arkayem

Richmond Hill, GA (Savannah), US
6134 posts

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"RE: D7000/SB700 in TTL and AF point selection"

Arkayem Moderator Awarded for his high level skills in flash photography Charter Member
Mon 30-Jan-12 12:33 AM | edited Mon 30-Jan-12 01:00 PM by Arkayem

>Just going through my stuff and testing everything for
>tomorrow’s Birthday party (kids, kids, everywhere kids).

I'm sorry that I didn't see this until Sunday, which I guess is the day of your birthday party shoot. Well, let me answer anyway.

>I’m
>doing this for my practice only and it will be OK if I’ll mess
>it up a bit, but I want it to be as good as possible for my
>first “party” with D7000…. And I suddenly realize I’m not
>fully understand how it will work “behind the scene”, I mean
>what I’m trying to achieve.
>I’ll be shooting in manual mode to get ambient light to where
>I want it to be and SB-700 with dome pointed up in TTL mode
>for main subject exposure (with color filter, if required). So
>far i understand what I'll be doing. I’ll have a stroboframe
>flash bracket with me, but most of the time I anticipate I’ll
>be using landscape mode for taking pictures of several kids at
>the same time. To get flash to TTL I’ll be switching metering
>to spot. I was going to use center AF point and recompose,
>but… here I realize it might be a wrong approach.

The SB-700 does not have a switch or any method on the flash to select between TTL and TTL-BL. It will always be in TTL mode when you select Spot metering on the camera, and it will always be in TTL-BL mode when you select either CW or Matrix metering.

>Usually, in matrix metering mode, I focus in AF-S on
>someone’s face/eyes,half press shutter release and then I
>recompose. But I’m not sure how D7000/SB-700 metering works in
>TTL.

Matrix, CW, and Spot metering are all Camera functions. The flash system has its own metering.

In Spot metering, the flash will be in regular TTL mode and does not receive any data from the camera metering system. The regular TTL flash sets its power entirely based on the reflected energy of the monitor preflash.

However, when in matrix or CW metering, the data that the camera collects is fed to the TTL-BL flash metering system so it can calculate the required power to balance with the ambient. You see, in CW and Matrix metering the camera meters the entire frame, and the flash uses that data to determine what the ambient brightness is, so it can balance with it.

The critical step that you must take before you shoot any flash pictures, is to determine the amount of ambient light you are dealing with.

If you are in low ambient (like indoors in artificial light), the flash will be primary, so you should use regular TTL with the camera set to underexpose the subject by about two to three stops. You do this by using camera Manual mode and setting the aperture and shutter to offset the built-in light meter by two to three stops on the dark side.

If you are in bright ambient (like outdoors in daylight) the flash should be used in TTL-BL mode to add fill. In TTL-BL mode it is critical that the camera meter be centered, or it will not work correctly. In this case, I normally use the camera in P mode so that it will automatically center the light meter.

>Will it get measurements from “matrix” metering or
>flash in TTL mode will use “spot” metering information? What
>happen if after recomposing “that” spot will be pointed to
>very dark or very reflective surface? The main subject will be
>over/underexposed?
>Can you please explain to me how does metering works in TTL
>(not TTL-BL) mode. And how it usually handled by others in
>such events as kid’s parties?

I think I have already answered most of this question, except one thing I did not mention was that the regular TTL metering is center weighted. This means that you must keep your subject close to the center of the frame to get proper flash power. If you want the subject to be off-center, then use FV Lock (which fires the monitor preflash) with the subject centered to get a properly TTL metering. Then, recompose to take the picture. The flash power will remain locked until you hit the FV Lock button a second time or the light meter times out.

>My visualization of the process is either to use FV function,
>which most likely will be impractical with kids or to switch
>to 39 points and first compose, and then choose the right
>focus point manually before taking a picture…

The focus points have no affect on the flash power. The regular TTL flash metering is very very simple. It sets its power only based on the reflected power of the monitor preflash from the center weighted frame. That's it.

Now, in TTL-BL, the system becomes much more complex. The algorythm that calculates the flash power uses the data from the camera metering system (which it uses as a proxy for the ambient even though the subject is also included), the monitor preflash, and the distance to the subject as reported by the D lens. In fact, the monitor preflash is used in only a minor way when using TTL-BL. The distance to the subject and the metering data are the two primary factors in TTL-BL that are used to set the power. The monitor preflash is still center wighted, but it is normally such a small effect that you can assume that the flash power will be almost the same no matter where the subject is placed in the frame.

I have only touched the highlights in this post. There is much much more to this.

I invite you to read much more complete explanations about this and lots of other things in my blog articles starting here:
http://nikonclspracticalguide.blogspot.com/2008/01/nikon-flash-two-separate-metering.html

Russ
Nikonian Team Member
Russell MacDonald Photography
Nikon CLS Practical Guide

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