Wednesday, October 21, 2009

16. Does moving the focus points affect flash power?

Lately there have been several comments made by experienced photographers stating that selecting different focus areas in the camera will change the place in the image that the flash meters from. I knew that this wasn't true based on my knowledge gained by designing flash and camera integrated circuits. I knew that the flash meters the entire frame based on center weighted brightness. However, I needed some proof, so I took the following series of images.

Firts, I arranged a white panel and a black panel so that they split the frame exactly in two. These images are unedited, directly out of my D3, low ambient conditions, f/5.6, 1/200th, ISO 400, regular TTL.
Image 1) In this image, the focus point was moved to the far left in the center of the white panel (the panel appears gray, because the flash system has averaged the center weighted frame and chosen a power that balances the entire image to 18% gray.


 
Image 2) In this image, I moved the focus spot all the way to the right side in the middle of the black panel. Notice that this image is exactly the same exposure as Image 1) showing that the position of the focus point is not used for the regular TTL metering system.


In the next few images, I investigate the effects of the focus system on TTL compared to TTL-BL.

Image 3) In this image the flash was in TTL and the subject was placed in the left part of the frame and the leftmost focus point selected. You can see that the statue is slightly overexposed from being off-center, but that is not what I am studying this time.



Image 4) In the next image, (still in TTL mode) I moved the focus point to the far right on the rope extending below the hats, and allowed the camera to focus there. You can see that moving the focus point did not affect the flash metering. The subject (although now out of focus) is exactly the same brightness as the previous image, as is the background.

 
 
Image 5) In the next image I switched to TTL-BL mode and placed the focus point back on the statue. You can see that the statue is no longer overexposed, as the balancing equations try to balance it with the darker background.



Image 6) In the next image I remained in TTL-BL mode and moved the focus point to the far right so the rope below the hats would be in focus. Immediately you see a change in the flash power. The statue is now overexposed and the background is much brighter. This is what has been causing even experienced photographers to think that the focus selection has an effect on flash power. However, the change is not being caused by moving the focus points. It is being caused by the change in focus distance. In this image the hats are about 15 feet behind the statue, and in TTL-BL mode, the distance is the prime factor that is used to set flash power. So, the farther distance caused the power to increase.


Image 7) In the next image I left the flash in TTL-BL mode, and switched off the auto focus and moved the focus point back to the statue. the important thing to notice is that the exposure of this image is identical to the previous image. This proves that moving the focus point does not affect flash power.



Image 8) In this image I moved the focus point back to the hats. Again the exposure did not change.


Image 9) In this image, I moved the statue into the center of the frame and selected regular TTL. Now the exposure of the statue is correct (the color is now accurate as well). This shows that even with all of the advancements of how TTL-BL deals with a dark background, regular TTL is still better as long as the subject is centered properly in the frame and occupies about 20% of the frame so the monitor preflashes can meter it accurately.



CONCLUSION:
These images prove that moving the focus point does not affect flash metering in either TTL or TTL-BL flash modes. The change in exposure that has been observed in TTL-BL mode when moving focus points is due entirely to the change in focus distance as reported by the D lens.

41 comments:

alm said...

I've seen Thom Hogan claim that the TTL-BL flash exposure is largely based on the focus points that camera considers in focus, your results are also consistent with that theory.

The only easy way to test this is using a non-D lens I think, although it is possible that the camera uses a slightly different algorithm when a non-D lens is detected. With a D-lens, you could compare the exposure of a white versus black subject in focus, and a white vs. black subject of the same size as seen from the camera in the background (out of focus), in the same place in the frame (left vs. right probably doesn't matter).

Another way might be to tilt the head up or use remote flashes to prevent the camera from using distance information.

Herman Au said...

I forgot how I stumbled across but I must say I'm pleasently surprised to find the very practical information that actually make sense on your blog, which is indeed a rarity on the Internet these days. Keep it coming! =)

-Herman Au

Russ MacDonald said...

Hi Alm,

Unfortunately, I sold all my non-D lenses a long time ago.

But you are right; the TTL-BL coefficients do change when using a non-D lens, shifting all of the flash power decision making to the monitor preflash.

I think I understand your experiment, and it should prove the point. However, I know that moving the focus points affects ambient metering, so in this test, you would have to be very careful to keep all ambient out of the exposure in order not to be mislead. You have to look only at the flash contribution.

I may run this test as you suggested and add the results to this blog (time permitting - I still have several more weddings scheduled this year).

Also, FYI, distance information is always included in TTL-BL calculations, whether the flash head is tilted up or not. The reason you normally do not see as much effect from distance when the head is tilted up is that you tend to run out of flash power very quickly if you focus on a farther subject. Also, when the head is tilted, the coefficients in the BL equations are adjusted to give more weight to the monitor preflash.

Thanks for your very inciteful inputs!

Russ

Russ MacDonald said...

Hi Herman,

Thanks for the nice feedback!

I looked at your wedding site, and think you do very nice work! I like the effect of the Remote flash firing back at you in the reception scenes. If you hit it just right, it adds some great halos effects. I use that technique myself once in a while. One problem is that kids tend to knock down the stand with the flash on it! I lost an SB-600 that way! Now I tie the stand to something so it can't be knocked over.

Russ

Anonymous said...

I know it sounds ridiculous but does focus distance and focus point equate to the same thing? By changing my focus point to an object that may be closer or further away it thus changes the focus distance with it, so I get a change in flash power.

If I focus on something in the same distance my focus wouldn't need to have changed.

Russ MacDonald said...

No, focus distance and focus point are two different things.

Focus distance is the distance from the lens to the subject. This focus distance is reported to the flash computer to be used in the BL equations.

Focus point is the little square lighted symbol you see in the viewfinder that you can move around. The position of this focus point is never reported to the flash computer.

As I showed in this blog, you can move the focus point all over the viewfinder and it does not make any change to the flash power unless you allow the lens to focus on what is behind the focus point.

For example, think about the procedure known as 'focus and recompose'. When you half-press the shutter button, the camera locks focus on whatever is behind the focus point you have selected.

Then, holding the focus locked, you recompose by aiming the camera at a different place. This means that the focus point is no longer placed on the point that is in focus. However, the TTL-BL mode doesn't care. It uses the focus distance that was locked in. It doesn't know or care where the focus point is placed when the picture is taken.

Russ

Anonymous said...

Hi Russ, I just recently switched from film to a D200, and your blog articles are a great help. With film, I just learned the minimal to use my SB24 for fill in broad daylight and keep my fingers crossed when shooting indoors. Now the D200 is an entirely new beast. Since I will be using the built-in flash for now, would it be possible for you to dedicate one article on how to get the most out of it? Or, can you identify the current articles that are applicable to it? TIA.

p.s. I will contibute one hint: take off the lens hood when using the built-in flash! That should give you a hint what a newbie I am :-)

Russ MacDonald said...

Hi Anonymous,

That's a good idea for another blog entry. Let me offer the following as a start to that new blog.

To get good results from the built-in flash, you have to understand which mode it is in (TTL or TTL_BL) and how to select between those modes. There is no choice in any menu to select TTL or TTL-BL.

The built-in flash can also work in Manual Mode, and as a Commander, but those are mre self-evident, so will not be part of this discussion.

The mode the built-in flash works in depends on which metering mode you select on the camera.

If you select Matrix Metering mode on the camera, you force the built-in flash to work in TTL-BL mode. Its TTL-BL mode works exactly like the SB-800 in TTL-BL mode, except the built-in has much lower power. Refer to my articles about TTL-BL, to understand how and when to use TTL-BL mode.

If you select Spot Metering mode on the camera, you will force the built-in flash into regular TTL mode. This works exactly like the regular TTL mode on the SB-800, except, again, it has much less power. Again, read my articles about the TTL mode to understand when and how to use regular TTL mode.

Hope this helps,

Russ

Anonymous said...

Russ, thanks for the response. I look forward to a built-in flash blog entry. Including some of the following will be very helpful.

There is no choice in any menu to select TTL or TTL-BL.

Thanks for confirming. I must have gone through the manual a thousand times not being able to find a menu for this. Nor is there any indicator in the displays to tell you which mode you are in.

With my SB24 on a film camera, I have learned to dial down TTL flash by -1.7 ev to add a small amount of fill in daylight. I suppose I can do the same with the D200's built-in flash for the same results.

I have also learned to set Rear Sync on the SB24 to create a trail when shooting moving subjects. But my attempt to do the same with the D200 built-in flash was a disaster, ruining a very important shoot. Yes, I should have tried this out first.

The problem was that with Aperture priority, setting the Rear Sync means SLOW Rear Sync, which resulted in huge over exposure! Now I know better.

As an aside, how can I use my SB24 on the D200?

The D200's Flash Sync Speed setting is very confusing to me, and I don't know how it relates to the shutter speed. An explanation would be greatly appreciated.

In one of your entries, you referred to a flash duration of 1/1000 sec to eliminate motion blur. Since there is no indicator for the duration, how can I tell what it is?

Sorry for all the questions. But this is a complicated beast, at least to this flash newbie.

Russ MacDonald said...

Hi again anonymous (please sign your name so I know who I am talking with),

For daytime fill flash, the D200 built-in flash works extremely well as long as you use Matrix Metering, so the flash will be in TTL-BL mode. Then, it adds just enough flash to make the subject brightness equal the background. Then, if you don't want it that bright (I normally don't) you turn it down the power with the flash compensation button. I normally find about -0.3 ev to be about right. Don't use the camera compensation button or you will darken both the ambient and the flash.

You are right that Rear Sync means Slow Rear Sync and if you are using camera A mode, this will create a very slow shutter speed.

The best way to use Rear Sync with the D200 camera is with the camera in Manual mode. Then, the shutter and aperture stay where you put them, and all you get is Rear Sync. Incidentally, this is the same whether using the built-in or an external CLS flash in the hot shoe.

The term 'Flash Sync Speed' is defined the same as it always has been on all previous cameras. It is the highest shutter speed that will sync with the flash, and on the D200 this is 1/250th sec. The camera will not let you select a higher speed than this.

The term 'Flash Shutter Speed' is a relatively new concept. It is only used when in camera A and P modes. In those modes, it will not allow the camera to automatically select a shutter speed slower than the Flash Shutter Speed. The default for this is 1/60th, and that was chosen because it is widely believed that 1/60th is the slowest shutter speed that most people can handhold a camera without allowing motion blur in the background. However, if you have very steady hands, you can go into the menu and reduce this speed to a slower shutter. You can't increase it above 1/60th. If you want a higher shutter speed, just switch the camera to manual.

The flash duration is set by the flash power that is calculated. There is no way to know what this power is or the duration of it. (I wish there were).

Please email me directly at rkmacdonald@gmail.com for more questions, since you have drifted way off-topic for this particular blog.

Regards,

Russ

Kevin S. said...

Russ
I have been going crazy looking for a TTL-BL setting on my D-80. Went through the manual and all menu's on the camera. I thought maybe it was just not on my model.
I see from your post to anonymous that it is a function of matrix or spot metering. Is that for the D-80 also?
I found your blog yesterday and being new to flash (just bought a nissin Di866 flash) and never realized there was all this "stuff" to concider.
Your blog has enlightened me as to the complexities of flash photography but has also been making it seem, at least to me,somewhat easy to understand. Thank you and keep up the good work.

Kevin

Russ MacDonald said...

Hi Kevin,

TTL-BL is a setting on the flash. It may be called 'Balanced Fill' or just 'Fill'.

The TTL-BL mode is often not available on third party flashes. It is one reason the Nikon SB flashes cost so much.

I don't know whether the Di866 has the TTL-BL mode or not.

Russ

Redwine said...

Hi Russ,

Really appreciate your very informative site.

You mention that the Flash (build-in or external) use TTL for Spot metering and TTL-BL for Matrix Metering.
How about when you use Center-Weighted Metering ?

Russ MacDonald said...

Hi Redwine,

TTL-BL will also work with Center Weighted metering. However, it doesn't work very well, because the amount of background that is metered is often minimal. TTL-BL depends on a close proxy to the background ambient in order to balance the flash power to it accurately.

The only time I ever use CW with TTL-BL is when I want to exclude something in the background from the metering (like the sun or any really bright light that I will allow to blow out).

Russ

Anonymous said...

Dear Russ

First of all, thanks a lot for this awesome blog. I'm recently introduced to the amazing world of speedlights (I own a SB600), and your blog is of great help !

My question is, I read and re-read again but I couldn't get why there's a difference in your image 3 and 5.
Image 3: Focus point is on statue: background is dark in TTL-BL mode.
Image 4: Focus point is on hats, brighter background again in TTL-BL.
Image 5: Focus point back on statue (like in image 3), but background is still bright. Why the background is brighter than the image 3rd, although you focused back on statue, and always with TTL-BL mode ???

Thanks in advance for your answer,
Fatih

Russ MacDonald said...

Hi Fatih,

This subject is very difficult to explain clearly.

Let me explain again in a slightly different way:

The reason that the background in image 5 is brighter than the background in image 3 is that, even though I moved the focus point back to the statue in image 5, I did not refocus the lens on the statue. The focus remained on the hats. I did this by turning off the autofocus in image 5 before moving the focus point back to the statue.

This shows that moving the focus point alone does not affect flash power. If you compare image 4 and image 5 you can see that they are identical, even though I have moved the focus point from the hats to the statue.

This also shows that, in iTTL-BL mode, Flash power is primarilly determined by the focus distance that the lens reports and not the focus point selected.

Hope that helps,

Russ

Anonymous said...

Dear Russ

Thanks for immediate reply,
If I understood you well:
In TTL-BL mode,in your image 3; the flash power is a function of the distance of your camera to focused area (hats in image 3) + the exposure value of the library (as the preflash will reflected on your library which is in the center of the image).

Thanks,
Fatih

Russ MacDonald said...

Hi again Fatih,

No. You are are thinking about the monitor preflash. However, when using TTL-BL mode, the monitor preflash has a minor secondary effect. When in TTL-BL mode the variable that has the most effect on flash power is FOCUS DISTANCE.

When the focus point is shifted to the statue, it does nothing to the flash power unless the focus distance changes.

In regular TTL, distance is not used, so the monitor preflash is then the only variable that adjusts the flash power.

Russ

Anonymous said...

Hi Russ

Thanks again for your prompt reply.
A final 'naive' question to better understand the focus distance:
In your FV lock image example of bride and groom (blog no:4); would you have a different result if you take the photo in TTL-BL; instead of using FV lock in TTL mode and recompose&shoot?
=> In other words; focusing on bride&groom by moving the focus point on them (and focus distance would change in that case) without having to put them in the center as in your example.
Considering your previous answer, TTL-BL would adjust the flash power based on focus distance in that case & you may not need FV lock?

and TTL-BL should work decently as your subjects are darker than the ambient light, am I right?

Hope I'm clear, thanks for enlightening me !
BTW, I agree with others, you should definitely write a book, these info don't exist in any Nikon sites / tutorials !

Fatih

Russ MacDonald said...

Hi again Fatih,

Yes, if I had used TTL-BL for the shot of the B&G in blog 4, it would have worked as you said.

However, it would have also made the subject dark.

I don't recommend TTL-BL when the ambient background is dark. In the past TTL-BL would not have worked right at all. In the newest cameras, the TTL-BL system has been improved so that it works OK but still not as well as regular TTL.

You see, in TTL-BL, the system tries to make the subject the same brightness as the ambient background. This means that the brightness of the subject is usually decreased until the image is underexposed. In the most recent versions of TTL-BL, the system recognizes this situation and switches to a 'pseudo' Regular-TTL and tries to keep the subject from being darkened too much.

I think Nikon has the goal of making TTL-BL work correctly in all lighting conditions, and each version is better in that regard than the previous, but it will be a few more generations until we can simply use TTL-BL all the time.

And thanks for the compliments! But I'm not planning to write a book, because that would be way too much like work! I do this for fun and relaxation.

Russ

Anonymous said...

Thanks Russ

Now it's crystal clear,

Best,
Fatih

Raymond said...

Hi Russ,

Let me tell you my understanding of your article and please correct me if I'm wrong!

So in TTL flash mode, the distance info. from the focus point was not used in flash power calculation (only the center focus point which pointed to the bookcase in the middle) and therefore the flash output of image 1 & 2 were the same; however, when in TTL-BL mode the subject distance was used in flash power calculation so the outputs were not the same due to the change of subject distance (hence image 3 & 4), right?

Russ MacDonald said...

Hi Raymond,

Yes! That's exactly right.

In TTL mode the distance is not used, but that was not my main point in this article.

My point was to show that in TTL-BL mode the flash power is determined by the distance, and not the focus point selection.

Russ

Anonymous said...

Hi Russ,
pls clarify further:
focus point selection in your words means only selecting a focuspoint in the viewfinder, not letting the camera focus by pressing the release button and getting a new focus distance. Right?
(If i am thinking of "select a focus point" i think of focusing the lens using this point.)
Regards, Wolfgang

Russ MacDonald said...

Hi Wolfgang,

Yes, your first description is what I mean. In my blog article I have separated the selection of the focus points from the actual focusing.

Regards,

Russ

mark rychel said...

The focus ponts come into play when you are using spot metering along with ittl flash.

Russ MacDonald said...

Hi Mark,

I have clearly shown in this blog that moving the focus points does NOT affect the flash power when in TTL flash mode.

And, Spot Metering is a camera mode. It has nothing to do with flash power. The flash has its own metering system, and that never changes when in TTL mode. You can switch between Spot and Matrix mode when in TTL and the flash power will never change.

However, when you select TTL-BL mode, THEN the flash uses information from the camera metering, so the camera has to be in either Matrix or Center Weighted mode before you can even select BL.

But even in BL mode, moving the the focus points does not DIRECTLY affect flash power as I showed in this blog.

Regards,

Russ

mark rychel said...

Hi Russ , with all due respect, I must disagree based on experience. If you are in the spot meter mode using lash ans the bride and groom are next to each other, if I place the focus spot on the brides dress, I am under, on the grooms coat I am over if I place it either face. the exposure is accurate. What am I missing in your explanation.

Russ MacDonald said...

Hi Mark,

If moving the spot affects the exposure, and the distance to the subject remains constant, then the change in exposure is being caused by ambient light. The ambient exposure is set by the aperture and shutter, and those will definitely change if you are using camera A mode, and you move the spot from the groom's dark suit to the bride's white dress.

The flash power will remain the same, because the flash has its own metering system that has no connection whatever to the camera metering system or the focusing spot. It will see both the white dress and the dark suit in a center weighted way and will set a power that is based on the average brightness of the two.

Regards,

Russ

Marty said...

Excellent factual article, which proves the point unambiguously and visually. Your blog articles are extremely useful to other photographers who often struggle to understand the deeper techincal aspects and 'hidden depths' of today's complex technology. Thank you for this fabulous resource.

Russ MacDonald said...

Hi Marty,

Thanks for the great feedback!

Russ

Ulf Thausing said...

Hi Russ!Thanks again for your article.
I don't understand why TTL-BL leads to these great results?
Because it is not the standard situation you posted in your earlier Blocks.
(Backlight is brighter than subject)

So why the TTL BL works that fine?
Even better when standard TTL in this case.
thanks a lot

Russ MacDonald said...

Hi Ulf,

Of course, in this article I am studying the flash power and its relationship with the focus points, so exposure was not my concern.

There are two reasons that TTL-BL was acceptable in this article:

1) I brightened the TTL-BL images with LightRoom.

2) The newer cameras, like my D3, employ a pseudo-TTL system to avoid the drastic underexposure that earlier cameras made when using TTL-BL incorrectly. If I had used my D200, the TTL-BL images would have been so dark I might not have been able to brighten them acceptably.

Russ

eMac said...

Hola Russ,

"CONCLUSION:
These images prove that moving the focus point does not affect flash metering in either TTL or TTL-BL flash modes. The change in exposure that has been observed in TTL-BL mode when moving focus points is due entirely to the change in focus distance as reported by the D lens."

I'm not sure about your conclusions. Image 1 and Image 3 actually prove that TTL-BL uses the focus point to calculate flash exposure, whereas TTL only uses the center of the frame to calculate exposure... that's why you are getting an overexposed statue on Image 1 (TTL), and a correctly exposed statue on Image 3 (TTL-BL), even when focus distance is the same for both. As a matter of fact, when you move the statue to the center of the frame and use TTL, you get again a correctly exposed image (Image 7).

I found the above a while ago (with the SB-800/D200) but after reading your post yesterday, I thought I got it wrong. I did a bit of research and found this video http://www.youtube.com/watch?v=he35-IRSd2c where the gentleman proves what I knew. I did the testing again with the SB-900/D700 and works exactly as I knew and as seen on the video.

Maybe I'm just misreading your conclusion, and I'm saying the same as you are. If not, then section 8 needs to be reviewed as you also say there that flash always meters the center of the frame.

FV Lock meters a different area, depending on if one speedlight is used on the hot-shoe, or if more speedlights are used (CLS), and if using TTL or AA (Page 194 of the King James D700 manual version).

Last but not least, thanks a lot for this great blog.

I have a few other questions but need to reproduce your tests first, to be sure I'm getting it right.

Gracias,

Eduardo

Russ MacDonald said...

Hola Eduardo,

I watched the You Tube Video, and I'm sorry to tell you that this guy makes more incorrect statements about TTL-BL in this one video, than I think I have ever heard before. He is so far off base on all his comments that I wouldn't even know where to start correcting him. In fact, I have no idea where he got all these ideas! If you will send me your email address, I will send you a list all the arrors this guy has made in this video. You can get my email address from my profile.

Before I continue, let me explain that I worked on cameras and flash systems including the TTL-BL design as an engineer for about eight years up until 2001. Since then, I have remained in contact with that design team, and there have been no major changes in how the TTL-BL system works.

Also, let me be very clear to define the term 'focus point' the way I use it. I mean the little brackets in the viewfinder that you can move with the multifunction selector.

Please look carefully at images 4 and 5.

Notice that they are identical.

In image 4 I placed the focus selector bracket on the hats and I manually focused on the hats until the green 'in-focus' lamp illuminated in the viewfinder. You can see that the flash fired at the power necessary to light the hats.

Then, in image 5 I moved the focus point selector (the bracket in the viewfinder) over to the statue, but I did not refocus the lens. I left it focused at the same distance that the hats are at.

If the focus bracket in the viewfinder was what the flash was using for its power setting, then you would expect the statue to decrease in brightness.

However, the image remained exactly the same. Therefore, without any further examples, you can already see that moving the focus bracket in the viewfinder has no effect on the flash power.

As far as the flash metering in the center goes, I am refering to the monitor preflash system, and yes, it always meters the monitor preflash only in the center of the frame (center weighted, actually). This is the only metering that is used when using regular TTL.

However, in TTL-BL mode two sources of information are used to determine flash power. 1) Distance is used as the primary source of flash power data, and 2) the Monitor Preflash return is used as a secondary source of flash power data. Most of the time Distance is all that is used, but under certain lighting conditions, the monitor preflash data can become more important and have a bigger effect.

Hope that helps,

Russ

Russ MacDonald said...

I received an email that Jim Thomas had posted a question on this blog, but when I went to the blog, the comment was not here.

Anonymous said...

Hi Russ,

I have gone over this post and most of the comments above me.

I use my SB-900 in conjunction with my D3s and ALWAYS use my flash in TTL-BL mode. I find it to be very reliable most of the time. For when it is not reliable such as producing underexposure, I normally work my way around that by adding some positive flash compensation. I find this to be much more convenient than always switching between TTL and TTL-BL when there are already so many things to keep track of on camera.

The TTL-BL, as you mentioned, calculates the distance between the camera/flash and the subject, so it renders FV lock useless as long as I have my picture focused and framed properly. Is that correct? I seem to be getting very consistent results by just focusing-lock focus-recompose-shoot WITHOUT the FV lock. (I am using my AF-On button with the camera switch set to AF-S)

This said, may I still be possibly losing out on the benefits of the regular TTL that the TTL-BL cannot?

By the way, keep up with the great work!!! :)

Best,
Charles

Russ MacDonald said...

Hi Charles,

This is quite a bit off-subject for this blog, but I will go ahead with my comment.

TTL-BL is really designed for adding fill flash in bright ambient conditions.

It is true that TTL-BL uses distance primarilly for the flash power, but it also uses the average ambient brightness. In low ambient conditions TTL-BL will cause the flash to increase when there is a bright light in the background compared toi when the background is dark. Sometimes this works and sometimes it doesn't.

Regular TTL was designed for shooting in low ambient conditions and providing all the exposure on the subject. This usually makes the subject much brighter than TTL-BL and much better pictures overall.

In general, regular TTL will give you more consistant results when in low ambient conditions. Just set your camera in Manual mode, ISO 400, 1/80th shutter and f/4 and shoot. You will get remarkably consistant results.

Have you read my cookbook blog? http://nikonclspracticalguide.blogspot.com/2009/12/camera-flash-cookbook-for-any-lighting.html

Russ

Russ MacDonald said...

Hi Charles,

Something happened and your post and my reply have both disappeared.

Here it is again:

"Thanks Russ for the comment. I found your post informative as usual, but it didn't really address the questions I brought up. That is:

1.) If I were to ALWAYS use TTL-BL and adjust its inconsistencies (ie: underexposure) by adding flash compensation and vice versa, am I losing out on the benefits of the regular TTL?

2.) If the above is true and I use TTL-BL all the time, then there is no significant reason to use the FV-lock in combination with TTL-BL.

Thanks in advance, Russ!

Best,
Charles"

Here are the answers to your questions:

1) No. If you were to always use TTL-BL and adjust FC to compensate for flash inconsistancies, you may or may not be successful. Unless you are on a tripod, you will get differences in flash power with even slight differences in framing. This means you may find your shot too dark, increase the FC, shoot again, and it may be too bright.

2) answered in 1) above.

Russ

Eugene Bogorad said...

Hey, Russ! As far as I understand you have to use the following settings:

- focusing: af-s (single)
- metering: spot

this way the flash metering uses the space around the single focus point.

Tridib Biswas said...

Hi,
I use a Nikon D7000 with 18-105mm Kit lens
I find the "Flash warning on" most of the time on in the viewfinder even when I am shoot a fairly bright lit out door scene - I normally use Matrix metering - Aperture priority or Program Mode . The eventual photograph comes out ok even if I ignore the Flash warning .
•Is it that I have made some wrong settings in my camera or I need to take it the service station.

I was trying to figure the issue and am still not very clear for the cause of the same. However in between I conducted some tests of my own as follows:

I took some shots of a white CFL light against a off white wall in my bedroom in manual mode single point focus and spot metering with out flash . I then repeated the same with single point focus and evaluative metering. The results in both the cases where same wherein the flash warning was blinking in both the cases and the CFL lamp was bright and rest of frame was dark.

Next I took shots outside in bright daylight with a tall building and a tree in front of the same. I found when I focused on the leaves of tree which well lit , the flash blinker was on, but in the same frame if I focused on the well lit reflective wall of the building behind , the Flash Blinkers were off. In both the cases the shot come out well exposed with a flash on. I was in manual, Single point , Spot Metering and as well as evaluative mode.

The above exercise might sound very trivial , but for noob like it is bothering as I feel have not been able to grasp something fundamental.

It would be nice if it could be explained. I hope it is nothing to do with my camera's metering mode.