Debate About TTL Theory

<p>I've been having a debate about why you can't focus and recompose when using TTL at a wedding. The reason this started was because I picked up a D700, which has all its focal points concentrated toward the center, more so than my old D300 (big flaw imo). So I can't thumb the focal point to the brides eyes in many portraits because they're too low in the frame.</p>

<p>Anyway, it was suggested that instead of thumbing my spots that I focus and recompose and lock in the flash exposure. But my understanding of TTL is that the pre-flash metering is done just a split second before the actual exposure. So wherever the center of the frame is when the shutter snaps, that's where the reading will be made. Therefore, you are metering the bride's dress or the groom's tux, regardless of flash locking.</p>

<p>So I guess the alternatives are using auto or manual. Manual isn't in the cards for me right now, with the myriad of other things I'm thinking about during a fast moving wedding.</p>

<p>Matt,<br>

You raise an interesting question, but I think that TTL judges the overall content of image. If you were spot metering, you'd definitely have a problem, but as a focus and recompose guy, I shoot TTL exclusively and have not had an issue. You might go to planetneil.com to see if Neil addresses this question specifically. I've done his workshop and this did not come up.</p>

<p>The consensus over here is knowing exactly how the camera is metering the flash requires reading the manual (dang it!). :D <br>

I keep my camera set for averaged metering, so I would assume that's how it's metering the flash. My personal question would be if your camera is set for spot metering, will it also spot meter the flash, or does it stay at averaging?</p>

<p>Assuming the camera is set for averaging and stays that way when using a flash - you're good to go for focusing & recomposing, which happens to be the method we use over here.<br>

It's a habit I picked up off Matt - Focus point of cameras is set to center of frame. Focus on the eyes, recompose, shoot. Never changed my habit or even thought about it w/ the flash, and I've been quite happy w/ the results. I'm certainly not exposing for the center of the frame, causing problems for other areas.</p>

<p>The TTL uses a kind of matrix metering to evaluate the (whole) scene for flash exposure based on the preflashes. Pre-flashes are metered before each exposure unless you have activated FV lock. See the manual on how to program FUNC to FV lock.</p>

<p>The spot/centerweighted/matrix decision is for ambient light, not flash. However, in some camera/flash combinations, it's possible that the ambient metering choice or the choice between manual/automatic modes affects also flash metering but it's not obvious how exactly this works. It also depends on the specific models you're using.</p>

<p>What Ilkka said but TTL flash uses something more aikin to center weighted than matrix, regardless of the selected exposure metering for ambient.</p>

<p>It's easy to test your combination of flash and body. (They don't all behave the same).<br /> Take your camera and go inside in a room with not too much ambient light. Set the camera to Manual and select ISO 100, 1/250s and f/11 or something that will give almost no ambient exposure but still possible for the flash to illuminate to a proper exposure.<br /> Pick something white next to something black (like two cardboards) and try out different exposure methods (spot/center/matrix) and frame differently and you'll see how the exposure will differ. It's easy if you have a f/2.8 zoom because you can make it all white or all black without even moving.<br>

PS. You should disable auto iso, auto contrast and everything else auto to make your test more valid.</p>

<p> </p>

<p>PS. If you can focus and recompose or not depends on your shooting - how you usually frame, if you shoot wide or tele, ambient to flash ratio, ambient light level, what camera body and flash combination you use and how picky your are about your exposures :-)</p>

<blockquote>

<p>I picked up a D700, which has all its focal points concentrated toward the center, more so than my old D300 (big flaw imo).</p>

</blockquote>

<p>I can't help with the flash question, but I can (I think) answer why the D700 has its af points closer to the centre of the frame than the smaller sensor D300. As I understand it, it's because the geometry of the phase-detection system used for AF means that the points can't be close to the edge of the mirror box and still give a sufficiently long baseline. If you make the sensor smaller then you can push the AF points further to the edge of the image while still keeping them close enough to the optic axis. </p>

<p>You see the same thing on Canon AF systems; the 1DIII has AF points closer to the edge of the frame than the 1DsIII, because the sensor is smaller. </p>

<p>On Canon bodies (at least 5D and earlier as that is all I own), FEL is biased towards the centre focus point. Consider this when using FEL. Can't speak for other brands....</p>

<p>Good info. When I have a moment, I'll have to run some tests and dig for my manuals.<br>

Thanks.</p>

<p>It seems that the Nikon flash metering system is not tied to the active focus point after all. But it should be using the distance info from the lens to get a better exposure.</p>

<p>TTL works with the selected focus points when used in spot, central points when using center weighted average, and all points when using matrix metering.</p>

<p>I use TTL in conjunction with center weighted and when i want to slightly overexpose, I half press the shutter buttton (not used for focusing) while pointing a something a little darker. If I want to under espose, I include some brighter part of the frame when making the exposure calc. It makes the flash do just what I want and is the same thing as focus recompose, only its more "meter recompose". If you do it a lot it becomes second nature.</p>

<p> </p>

<p>Heh. TTL Theory. I agree with one of the posters about reading the manual and the other poster with regards to flash value (Fv Lock) lock. On the D700 there seems to be about a half dozen ways to get the pre-flash to fire first. You can then recompose after that. Also, I would highly recommend you experience the joys of using your flash manually. Nothing gives you more precise control over light than the inverse square law and your feet.</p>