Flash metering question

[screeny]

I'm planning of buying a couple of radio triggers and a flashmeter, all in order to be able to shoot slides (No I

don't have the $$$ for a decent D-SLR so that's not the solution) with 2 or 3 of camera flashes in manual mode

and hopefully the auto meter will get me to some decent exposed slides. I'm reading my self into the matte ron

how actually use the flash meter, and I start understanding how it all works.

However, one thing just hit me which I don't understand: In general you dial your required shutterspeed in the

flash meter, flash away while keeping the meter near your subject (most of the time plant the meter on the nose

so to speak) , and the flashmeter gives you the correct aperture F8 or such. Hence you transfer the shutterspeed

you dialed in the flashmeter and the aperure the meter advices to your camera and all should go well...

 

However...I alsways understood light looses power the further you move from your subjet. In the old days with now

adjustable flashes, the power output was controlled by moving the flashunit further or closer away. So how does

the flashmeter know the distance to your camera? Its fine with it reads F* for correct exposure with a certain

setting but how the hell it knows whether my camera is 2 mtrs or 20 mtrs from the subject?

 

Is the concept that the aperture setting the flashmeter advices correct within a certain range in which the

camera must be, think of studio work? The reason I come up with this is because what if I want to use flash with

ambient outside? useing a 300mm lens so I would be quite far away from the subject. Would the resuklts from the

flashmeter still be correct when metered at the subject?

 

gr

 

Marc

[mark u]

The camera to subject distance is irrelevant. All that matters is the light that falls on the subject. So long as you meter the incident light (including the flash) at the subject, you should get the correct exposure. The flash power may be adjusted either by changing the power setting on the flash (the degree of flexibility offered varies according to the model) and/or by changing the flash to subject distance and/or by using modifiers (umbrellas, soft boxes, reflection surfaces etc.). With X sync flash, the aperture/ISO combination determines how bright a given flash power/flash to subject distance/modifier setup is, while the shutter speed determines the degree of influence of ambient light on the overall exposure - it has no influence on the brightness of the flash itself at all.

[screeny]

"The camera to subject distance is irrelevant"

now that I don't understand. The whole guide number calculation is based on when a flash is on-camera. Taking it of camera in your hand (flash with 1 mtere cord) will merely change the distance between the camera/flash and object. However if I go wireless and place the flash 2 meter from the subject but use a 500mm lens and stand on 14 meter distance between me and subject my intuition says the flash will need to put out much more power than when standing with camera only 2 meters from the subject.

 

Is my sense of logic playing tricks on me?

[gt1]

The only thing that matters is that the light falling on the subject stays consistent. If the flash is off camera and 2 meters from the subject then you can shoot from wherever, the aperture (and shutter speed for ambient light) is still going to be the same.

[Matt Laur]

I'll confess that this is contrary to seeming common sense. The camera DOES record the light that is reflecting off of

the subject. The strength of any light falls off at the square of the distance. So the light hitting your camera (having

bounced off your subject's face) is, by definition, stronger one meter away than it is ten meters away. Anyone have a

link to a clear explanation for the apparent laws-of-physics contradiction in the distance not mattering to the quality

of the exposure?

[richard_cochran]

Imagine a very dark field on a moonless night, with a lone cabin at the edge of the field. One wall of the cabin

is painted perfect 18% grey and is uniformly illuminated for an exposure of 1/60 @ <i>f</i>/4, ISO 800. Opposite

this wall is a huge picture window, facing the empty dark field, and the curtains are open.

<p>

If you stand inside the cabin and take a picture of the grey wall, you'll use an exposure of 1/60 @ <i>f</i>/4,

regardless of whether you're a foot away or across the room.

<p>

If you step outside the cabin and photograph the wall through the window, you'll still use an exposure of 1/60 @

<i>f</i>/4. Run across the field and use a telephoto lens, and it'll still look like an exposure of 1/60 @

<i>f</i>/4.

<p>

But if you and a model both step out and stand inches away from the big picture window, your exposure will be

close, but maybe you'll have to open up a stop from that 1/60@<i>f</i>/4. But as the two of you run across the

dark field together, with the picture window being your only source of illumination, you'll quickly run out of

light. Yet, aim that telephoto over toward the cabin interior so that the interior fills the field of view, and

you'll still be able to take a picture of the interior of the cabin at that same old exposure.

<p>

What's going on here? When you use the picture window as a light source, its light falls off as you get away

from it. But when you use it as a photographic subject, the exposure stays constant. But it's the exact same

light! It doesn't know whether it's being used as a light source or a photographic subject! How can it be

falling off for one purpose, but staying constant for another?

<p>

It's because the apparent size of the picture window falls off with the inverse square of distance. Put a 50mm

lens on your camera, and stand at the point where the window just exactly fills the frame of your viewfinder.

Now double your distance from the window, and you'll see that you can fit the window in the top left 1/4 of the

frame. That's why it's not as bright a light source when you want to use it to illuminate your model's face.

When you use it as a photographic subject, the amount of light hitting each pixel of your CCD is the same as it

was when you were close, it's just that it's only illuminating 1/4 as many pixels. When you look at your model's

face illuminated by this light source, it's 1/4 as strong when you're far away than it was when you were close.

<p>

This is a long way of explaining that camera to subject distance doesn't matter for exposure settings, but light

source to subject distance DOES matter.

[screeny]

I think I understand...but probably have to read it again tomorrow :)

Thanks for the explanation!

[terry_spade]

You have already have some good answers above. Here are some links that may help.

 

http://www.lighting-essentials.com/small-strobes-knowing-your-flash-unit/

 

http://strobist.blogspot.com/2006/03/lighting-101.html

[mark u]

To complete Richard's picture: when you fill the frame from across the field using a 500mm telephoto at f/4 you are benefiting from the larger physical aperture of the lens - 10 times the diameter of the 50mm, and so with 100 times the area. This exactly offsets the inverse square falloff from being 10 times further away than filling the frame with the 50mm lens. Shooting from the same position as the telephoto with the 50mm lens, you will only fill an area with 1/10th the linear dimensions (and therefore just 1% of the area) as filling the frame. You are only capturing 1% of the light at the further distance compared with the close one due to inverse square falloff, but as it occupies 1% of the area, its intensity on the sensor/film is the same.

[Matt Laur]

So, I guess that's why they're the <i>laws</i> of physics, and not the <i>guidelines</i> of physics!

[wick beavers photographer]

Man: set the aperture to your figured flash exposure. Set your shutter speed to your ambient or how much relatively you want of it.

If you need to control your depth of field (how much is in focus), you need to be able to change settings on your flash(es). The

more depth of field, the more power from the flash(es). If the flash is limited in power cuz you cannot afford powerful flash(es), then

you're gonna have to be careful with your focus.

But get this!

If you want more light than you can afford, you can set your aperture high and shutter speed slow enough to pop your flashes

several times to get more light than possible in one pop. Problem becomes a photoshop one if you need to have your subject

stand still long enough to remain sharp.

Also, help out your low powered flashes by setting your camera's ISO at some ridiculously high asa, scuse me, ISO. Then you

play with grain/noise. Soooooooo, you can see there are all these trade offs for being broke. If you can, go rob a liquor store and

get some high powered lights.

Good luck!

Wick

[dave_holland]

You gotta think that those guys who invented the f-stop principle were pretty dang smart. Of course on the surface it

is confusing, with square root 2 each stop, strange numbers. But those principles make translations to exposure

really simple. Of course you don't need to know camera distance, or even lens focal length. They planned it that way.

 

In the days of separation of meter and camera, with a really slow feedback curve, it had to be simple. So they had to

make it the way they did. By using f-stop, rather than some other arbitrary measurement of light power, they took the

variables of camera distance and focal length right out of the equation. And those principles became commonly used

because they made life so simple for journeymen of the day. Of course they were based on the mathematical

principles of light dispersion. But fortunately we tradespeople don't need to understand all of that, as simple as it

may be.

 

What you end up with is long lenses that have really big objective diameters, with equivalent exponential price tags.

And tiny lenses that can brag about incredibly fast glass. By defining f-stop as focal length divided by diameter of the

lens (diaphragm) opening, they cancelled out messy details. Finally there was a way to compare lenses based on

ability to capture light, independent of subject distance. Genius.

 

Dave