Thank you to all of you!
This post can be used as a guide to "how to use the flash light"!
Once again thank you, Marry Christmas to those who share it and Happy New Year.

I believe you are correct, Panamon_Creel, insofar as I failed to stress that last point you made: "as long as the flash has enough power". Play with the setting - it is as I described, the camera is limited from choosing the exposure it wants because of the minimum flash speed setting.

EDIT:

Let me clarify - for I think I see where I was less than crystal:

The default behavior is to attempt to /balance/ light. It shows underexposure because it believes, regardless of flash power available, it can not allow enough ambient light to properly expose the background with the flash providing the foreground light. It is a warning (no preflashes needed) that you run the risk of out-of-balance light.
IF, and only if, your subject to background distance is short then the camera is wrong and you can achieve balanced light by going ahead and taking the photo (as the flash will illuminate more of the background than the meter is projecting) but if the subject:background distance is greater (which the matrix meter is assuming) you will have broken the "balance" attempt and underexposed the background while correctly exposing the subject.

EDIT 2:
The answer is, in the end, to gut-shot know what shutter speed you need in the situation to freeze motion, and allow the camera to go as slow as possible (in order to achieve balanced light) while still at the "freeze motion" point. 1/60th is a safe default setting, but there are times when you are happy letting it go slower.
(This assumes you _want_ balanced light - something which you likely don't when shooting a concert, for example, as waiting for the background illumination to catchup to the foreground illumination will likely equal massively slow shutter speeds, best to either go manual or small-center-weighted and meter for the subject as matrix will get confused.)
(IMHO matrix metering fails in most super-high-contrast scenes (concert/bar/snow/bright sand) and you should meter for subject.)

soap said:
"When the flash is open it tells that the picture will be undercompensated if the flash will not fire."

As far as I see that statement is not wrong, the light meter just shows the ambient light without flash, that's all. Flash output is only calculated with the pre-flashes prior to shutter release and is not taken into account prior to that. So flash exposure will still be correct even if the meter shows underexposure as long as the flash has enough power to illuminate the scene sufficiently.

D90 setting e1 is set to 1/60th by default in an attempt to prevent motion blur on walking-speed people when using the flash in balanced fill mode.
If you change it to a slower speed you should be cognizant of the fact, and monitor your shutter speeds at all time. 1/15th of a second = blur in many human situations.

Your are right Soap, you can actual get closer to 1/30,000th of a second on lower power, that is how pictures can be taken of bullets, water droplets etc.

But to the metering balance, If I am shooting a event in a Church, I do keep an eye on that reading, I do want the Church to well exposed but at the same time my shutter speed have to be in control for when people are moving

Anyway that readout is just one of the many tools that you camera gives you to help taking better pictures

Pete

PS - D90 setting e1 (Bracketing and Flash) defaults to 1/60th. Therefore 1/60th is the slowest the camera will go when the flash is up. This, I'm 99.9% sure, is why you're seeing the underexpose indicator. Camera is telling you that at 1/60th (the slowest it is allowed to go) and flash at full power will possibly equal underexposure. I mentioned this before, and I don't understand why it got ignored.

"When the flash is open it tells that the picture will be undercompensated if the flash will not fire." is simply wrong. Shoot the salesman. The exposure indicator shows exposure with everything calced, it does not tell you "what might happen if part of the system doesn't perform."

Change setting e1 to 1/15th and you will see the bars (in this situation) go to zero, as you have uncrippled the settings and allowed auto exposure the latitude it needs.

Mind that the flash's duration is bloody short, regardless of shutter speed. If your primary lighting source on subject is strobe and not ambient you can effectively freeze motion with 1/60th of a second shutter speeds (or lower) and flash.

This is very easy to demonstrate, and can happen all the time in a manual exposure situation with multiple strobes where you are effectively "killing" ambient through choice of ISO/aperture/ND filters. In such a situation (where ambient is "dead" and primary source of light is strobe) you're effectively setting exposure through strobe output level, and since the strobe duration is sooo short, shutter speed has little bearing on exposure level.

You _often_ don't see this when in any sort of auto mode on Nikon, because the default behavior of their flash system is to "balance" ambient light. Meaning that the brighter the scene the more strobe duration your camera calls for. This is a beautiful thing as it creates a very "natural" photo w/o glaringly obvious flash-bleached subjects. This is also why when shooting portraits, say, in front of a bar at night, the camera will default to a slow shutter speed (even though you have enough flash power on board to illuminate the whole scene @ 1/250th.) By going to a slow speed your smart body is trying to allow more ambient light from the background so that the flash is only ~50% of the exposure.

Back to my point. ;)
IF you have a situation where the flash is (nearly) the sole source of light on subject (imagine the subject is off-center in the shadows while the majority of the scene is brighter lit), there will be no ambient on the subject to cause blur during a 1/30th second exposure (1/30th picked by camera to bring up the rest of the scene's ambient) AND there will be perfect freezing of motion on the subject because the strobe (subject's only source of light) is actually only "lit" for 1/1000th (or so) of a second - regardless of shutter speed.

Mind, this is not the "normal" or "average" situation - but IF I was anywhere near clear enough in my description (rather doubt it), it gives you food for thought on one aspect of what is happening.

RossN said:
Hello Guys,
Thanks for bearing my armature questions.

Or summarizing my thoughts, what is the reason of having this indicator, when it gives information that simply said is obvious and not giving me let say "confirmation" that current adjustment are ok (speed, aperture, flash est.)

Regards,
Ross (Rossen)

The reason is so that you know that your flash is exposing the area it can reach with the correct exposure with a shutter speed that should prevent blur but that will also underexpose the area that is not exposed by the flash, with the build in flash properly anything 20 feet and further away, so if you have a larger room and you want full exposure of a that room you need to adjust your camera ISO etc to get that reading closer to 0

You know try it out and you will see the difference

Pete

Well, I've found the answer and it is definitely that I'm becoming an idiot.
Went to the store, played with similar units and discovered the same behavior. Spoke to the salesman, he told me that he had used 400D and the indicator had the same behavior. When the flash is open it tells that the picture will be undercompensated if the flash will not fire.

So thanks for trying to resolve my nonsense problems!

how about quick reset (press two buttons with green circle next to them)
that will clear all the random settings that you might of had and forgot about them

but what guys (above said) should work

did you change this one by any chance (under\over expose)
http://www.kenrockwell.com/nikon/d90/users-guide/images/D3S_2761-top-comp.jpg

Hello Guys,
Thanks for bearing my armature questions.

Soap, no, the camera is set to 1/60. I haven't seen setting's menu/option that allows camera to go lower or higher speed automatically (for flash and for manual modes (use to call them SPAM modes)). That drives to the next question, how can I set shutter speed higher than 1/60? It is mentioned that if VR is activated (page 265), then camera won’t go higher than 1/60? And still this is not valid for "S" mode?

jonnyapple, I tried going down to 1/30-1/2 and you were right, the indicator went to lower and even to zero. But FV lock doesn't change the indicator's value.

....
Or summarizing my thoughts, what is the reason of having this indicator, when it gives information that simply said is obvious and not giving me let say "confirmation" that current adjustment are ok (speed, aperture, flash est.)

Regards,
Ross (Rossen)

Is your camera set to go no slower than 1/60th with flash on?

The camera is metering without flash, and that's what the indicator is showing. They do it that way so that if you want to expose for the scene and still use flash you can. For your flash settings in this picture (TTL on the front shutter curtain) the flash will fire a metering preflash before opening the shutter so that it knows how high to fire the flash during the actual exposure. There's something I haven't used before called FV lock (flash value lock). That fires the metering flash and stores the proper value so that you can then recompose for the final shot.

I guess the short answer is the camera meter is telling you what it's seeing right now, not what it will see with the flash during the exposure.

edit: BTW, if you want to get rid of it, you have to (easy way) shoot in M or (hard way) change the flash shutter speed (custom setting e1 on the D90). But Pete's right. You need to know what you're doing and probably have a tripod to get that to work right.

Thanks Man!

Referring to my case, I tried Auto mode, still the same indication, somethimes the indicator even flashing (out of compensation). I'm completely confused. After the shoot the picture looks ok (normal histogram).

What's on the desktop screen in the background is a familiar sight to a lot of people around here. I'm laughing at myself for recognizing it. Welcome to the forum, Ross.

Appreciate!
Thanks!

What it mean is that your shooting area is underexposed (close to 2 stops) and the flash is now helping exposing your picture at a shutter speed that will prevent blur, but anything out of reach from the flash will be underexposed by app 2 stops.

For most pictures that is fine, however if you want that reading to get closer to 0 then you would either need lower shutter speed, larger aperture and/or higher ISO. You should also change to A setting otherwise in P the camera will actual adjust the aperture higher if you were to increase your ISO.

Note: that lower shutter speeds(less than 1/60) even with a flash can result in blur to anything that is not completely still,

If you want a large room to get exposed properly then getting close to 0 is needed

hope this will help

Pete

Hello All,
Can somebody tell what is the meaning of the underexposure indicator showing negative values when flash is active and the scene (to be taken) is dark?
Does it mean that at the present adjustment (shutter speed, aperture, ISO est.) the picture will be underexposed if flash will not fire or something is wrong with the camera?