spraynpray said:
Perhaps a automatically generated test flash could take care of the reflectivity and flash position info.

That is already what it does, but just before the capture. Remember this is with Manual exposure settings.

Here's the issue with the idea - everything can change up to the capture. It would be a great idea if everything in the scene was static, but outside of studio work with models, items etc. something can always move and change the scene.

For instance, side light from a lamp or window can be blocked. So let's take the idea of showing correct exposure with flash. As you take the shoot, someone walks into the pathway of a window. You metered the scene (with the "compensation" for flash) correctly but when you fired the flash, it would be dark due to the ambient light being blocked. As it works now, the flash automatically adjusts power to fill the scene to the "correct" exposure. There is no way for a camera to anticipate the future.

The only real way to do this is to set the flash to manual and use an exposure meter or use studio lights. Realistically most just take the photo and adjust compensation as needed. Images are free and are immediate - no longer have to worry about wasting money or waiting for film to develop.

Yes with one or more test flashes the "proper" exposure can be determined (that's essentially how D-TTL and newer systems work).

My comments were about the camera's inability to calculate the final exposure without firing a test flash first. Sorry if I wasn't being clear.

@Ade: I see what you mean regarding reflectivity, but I think the CLS could cope with number of flashes being used. Perhaps a automatically generated test flash could take care of the reflectivity and flash position info. I may be wrong now, but who knows in three years time?

@Goddesss: The D7000 is a great camera, enjoy it!

@spraynpray: while the camera can work out "tones" and has a rough idea of distances (aside from the focus point), it does not have any information about the "reflectivity" of various objects / surfaces, nor their true 3-dimensional shapes and surface angles.

Hence the earlier comment about the mirror (a high reflectivity surface). From a pure exposure-contribution perspective (neglecting any "artistic" arguments for a moment) the presence of a mirror does not have to be treated differently in D-TTL or TTL systems. A flash in the so called "SU-4" slave mode will behave similarly to a mirror, and is not treated separately either.

However without the above information, a camera's meter will not be able to accurately compute the effect of a flash.

Hi All
Thank you for the answers.
Ade: It totally makes sense what you are saying. I just was not sure if there is something in the camera that would calculate the exposure as spraynpray suggests.

TaoTeJared: I am using a Nikon D7000.

Thank you again for your responses.

Simone

Interesting question Goddesss15. Welcome to the forum - come and learn with the rest of us!

Ignoring the mirror point that Ade bought up (because a mirror would usually have an effect that has to be considered separately anyway), I can't see why what you ask would not be possible at some point in the future. OK, we may not be there in terms of processing power etc yet, but the idea in principle is valid. If the camera can work out the tone and distance of everything in the viewfinder and can calculate the effect of flash on that situation then why can't it give you advance notice of the expected outcome - even showing highlights etc. in the viewfinder?

Maybe flash still has a long way to go in the future.

I'm not sure if you are using a DSLR or an advanced compact but they work about the same.

One thing to add is that your flash actually sends out a "pre-flash" that the camera then meters off of. It is so quick that you can't even see it but it is enough time for the camera to recalculate the exposure. The camera will adjust the flash output for the settings you choose unless you go manual flash as well.

@goddesss15 You understand TTL means "through the lens". As Ada states so clearly, the camera has to see the flash "TTL" to know what is happening.

It is not possible for the camera to calculate how much additional light the flash will actually contribute to the scene, without actually firing the flash.

For example, if the scene contains a mirror, pointing back at the lens, then the flash might contribute a ton of new light from the reflection. But the camera doesn't "know" there's a mirror in the scene, so it cannot take that into account.

Conversely, if the flash is pointed up, outside, into a pitch-black night sky, then its actual contribution might be zero. Yet again the camera has no way to know this.

There are two common ways to meter the flash:

1. Experiment. Test fire the flash, measure what happened, adjust the flash strength, then fire the flash again. Repeat until the flash output is correct. You can do this manually (with a "flash meter" or by looking at the histogram) or automatically using one of the "Digital" TTL modes. In the automatic mode, the camera will fire one or more "pre-flashes" to measure the scene before finally taking the picture with the desired flash output.

2. Go ahead and fire the flash, but quickly shut it off when enough light is emitted. Essentially, this is the old film-based TTL mode. As the flash is firing, a special circuit monitors the exposure (as reflected from the film), and can "cut off" the flash when the scene is properly exposed.

In neither case however can the camera "know" how the flash will actually affect the scene before the flash is fired. Therefore the in-camera meter does not take flash use into account.

It gets even more complex when there are multiple flashes present. I.e., the main flash could trigger other flashes to fire, and there is no way for the camera to pre-calculate all the different flash contributions for all the possible scenarios.

I have a question regarding the exposure meter and Flash. How come the camera does not adjust the exposure meter when flash is selected. Example: I am in manual mode and want to take a picture of something on my desk. The flash is not activated. I notice that my Aperture and Speed combination will give me a picture that is underexposed. So now I decide to use my flash and activate it. However, the exposure meter still shows that my picture is underexposed because nothing increased.

1. Am I expecting to much from the camera to know that now I am using flash?
2. Is there a way to calculate the increase in stops that using the flash would provide?

By the way, great Forum. I am still a novice with all of that, so please bare with me if this question seems weird.

Simone