Help with flash exposures

[astroNikon]

f-number = Distance / distance

wouldn't that always be "1"

typo ...

 

[Braineack]

f-number = Distance / distance

wouldn't that always be "1"

typo ...

stop trying to confuse.

 

[KmH]

My bad.
Fixed it.

 

[Derrel]

I'm struggling with determining correct exposures when working with manual flash off camera, especially A---when my flash represents the primary source of light in the scene.

Could someone please work me through it please?

A common problem is if I meter for ambient only my camera will need to be setup way beyond where I am happy, in terms of ISO and speed. So rather than setting a perfect ambient exposure, I get as close as I can without pushing my ISO and shutter speed too much and then I end up chimping with my flash firing to get enough light.

The downsides are in processing I realize my exposure on my subject can be up or down some ways; the LCD lies and it takes time and looks unprofessional.

I have a Portaflash meter for flash, but with no correct ambient exposure to work from don't understand how I can use it to resolve the above issues.

I would really appreciate some help, thank you.

Heres' some help in three different types of flash situations.

A: if the flash is actually the PRIMARY light source, which is called flash as main light photography, then there is no need to worry about ambient light, and the exposure is determined by using the flash meter's readings. In this situation, using flash as main light, you use flash power level/flash-to-subject distance/ISO setting/lens aperture setting as the four main variables in controlling exposures. The SHUTTER speed has basically ZERO effect on exposure of the things lighted up by the flash pop. The shutter speed controls ONLY the areas that are NOT lighted by the flash burst, like the distant background, or light sources in the picture, like windows with sunlight coming in; desk or table lamps;ceiling lamps;television set images; computer screen images.

B. If you want to use off-camera flash lighting to fill-in dark shadows, or to add eye-sparkle to peoples' eyes, then you do want to set the flash about 2.5 to 2.7 EV BELOW (dimmer; less-than;darker-than) the ambient exposure. This is of course using flash as a fill-in light. Much of the time, this can be done with the flash right on the camera, right in the hotshoe, and pointed straight ahead, which is a perfectly acceptable, well-known use of what is called on-axis fill lighting. The fill light is aimed straight ahead, from as close to the lens position as possible.

C: you might want to use flash as an accent, rim, or separation light source, in which case you'd usually place the flash off-camera, off to the side or slightly to the side and behind, or slightly to the side and behind and above the subject, and the flash would be set to deliver an exposure that is generally 3/4 to 1 to 1.5 to maybe even 2 "f/stops" higher than the ambient lighting's f/stop in use. In this case, again, the shutter speed has no effect on how bright the flash-illuminated parts of the scene are recorded, but the f/stop and the shutter speed DO control the exposure for any on-scene ambient-lighted stuff. You can use flash power level or flash-to-subject distance as two easy ways to control how bright the rim-lighting effect is.

Keep this in mind, if your flash has fractional manual power settings, that Full, Half, Quarter, and Eighth power settings mean it has four different "f/stop power values", which is often described as a four f/stop range.

 

[TCampbell]

Using the Guide Number (GN) of the flash unit :
GN = distance × f-number

and using some basic algebra:

Distance = f-number / GN
f-number = Distance / GN

Keith's info above helps establish the correct flash exposure. BTW the "distance" is the distance from the flash -- not distance from the camera. "Guide number" is based on the flash being used at ISO 100 and with no flash modifiers (e.g. if you put the flash in a modifier... a shoot-through umbrella, a soft-box, etc. then the fabric is going to "eat" some of your light.)

If you use modifiers (suppose you use a shoot-through umbrella) then you'd want to find out how much the modifier alters the exposure. e.g. test the modifier by comparing shots taken both without and then with the modifier (using otherwise identical exposure settings) to determine how much light you lose with the modifier. This is important because as soon as you start working heavily with flash, you'll quickly realize you probably prefer the look of softened light (hence, you'll be using modifiers of some type.) I use a hand-held "incident" light meter (Sekonic) to work this out with precision and it really helps to have a hand-held meter that handles flash exposures -- it eliminates the guesswork. (A Sekonic L-308s would be probably the most basic model that can meter for flash.) But you can work it out even without a meter.

Also you can vary the power output of most flash units (e.g. full, 1/2, 1/4, etc. power levels) and the guide number assumes full power. If you cut the power level by 1/2 then you'd need to compensate for that (e.g. open the aperture or increase the ISO).

Next...

Notice that Keith's equations don't mention shutter speed? That's because as long as the camera uses a shutter speed which does not exceed the camera's flash sync speed, the shutter speed won't affect the exposure (assuming no strong continuous source of light on the subject -- but that's usually true.) The flash is only bright for a split second... regardless of how long the shutter remains open it won't provide any more light than you got out of that initial burst.

So once you've worked out a correct "flash" exposure for your subject... you can collect light for the background (ambient lighting) by extended the amount of time that your shutter is open. This assumes the background is DIMMER than the lighting on your main subject (provided by your flash). In other words... if you needed a 1/200th sec exposure at f/8 for your main subject (I'm just making those values up) but the background is dimmer and you want to get some background in the shot, you can leave the shutter open longer (e.g. say... 1/60th sec -- again, I'm just making this up for the example) and that would let the camera collect about three times more light from the background than it would otherwise get if you just used 1/200th. It would have no noticeable effect on your main subject (lit by your flash) UNLESS there are other "strong" continuous sources of light on that main subject.

This technique is often referred to as "dragging the shutter" (leaving it open longer than necessary for just the exposure of the main subject... so that you can also expose for the background which was not illuminated by the flash.)

Most of the time (with indoor shooting) this is the problem... if there were plenty of available light, you probably wouldn't bother with the flash. So you probably need to increase the amount of time that the shutter is open to get a more pleasing background exposure.

But there is a possibility of the opposite situation happening too ... so this general process of "dragging the shutter" won't work when the reverse is true. What if the background is well lit... but the subject is not well lit? Suppose you are shooting a subject outdoors... and your subject is in the shade of a tree, but the background is in full sun. Now the situation is reversed... an exposure for your subject will over-expose the background. And the exposure set correctly for the background will result with an underexposed foreground subject. In this situation you set the camera to expose for the background (keeping the shutter speed at or below the max flash sync speed) and then supplement with flash to boost light on your foreground subject.

All of this is really just a "taste" of what you'll want to learn. There are more scenarios... suppose the background (back indoors again) is lit with incandescent lighting. That light puts out a rather yellow/orange color cast. But the flash puts out a "daylight" color (closer to a true "white"). You can end up with a situation where the subject looks right and the background looks orange because the color of the light sources (flash vs. ambient) are not the same "color". So you fix that by putting a "gel" on the flash (the gel has a bit of an orange tint.) This matches the color of the flash to the background and now you can use white balance controls (either on camera or on the computer) to correct and the color of the room will match throughout.

All of these are issues you run into when you deal with "mixed lighting".

We can't summarize everything in one post. strobist.blogspot.com is a good website (lots of articles). There are some great books on the topic -- specifically books on the use of "flash" photography.