How do you use fill-flash?

cumi · Apr 3, 2006

Hi!

How do you use fill flash technically (metering, mode, etc)?

I want to have a same photo with fill-flash as without, except of fill-flash's effect? Is it a good option to meter light (for example with shutter priority at 1/500 at dayligth) without the flash, then simply use the same settings, but with flash switched on (assuming you can sync 1/500 with flash)?

Any other practice, methods, experiences?

Thanks

ksmattfish · Apr 3, 2006

You use the aperture to control the lighting on your flashed subject (based on power of flash and distance to subject: GN divided by distance = f/stop), and then adjust shutter to the background exposure. In general it's often recommended to underexpose the flash 1 stop compared to the backgroundto make the use of artificial light less obvious.

If I was using a flash with a GN (ISO 100/feet) of 80, and my subject was 10' away, then f/8 would be a decent exposure if I was using ISO 100. If I want to underexpose the flash a stop I'd go to f/11. Then I'd meter the background for whatever aperture I've decided on. If I don't want to use such small apertures then I'll need a flash with variable power settings, a camera with a high flash sync speed, and/or slower ISO.

cumi · Apr 4, 2006

Thanks ksmattfish!

So, if I understand you correctle, you would do following:

1. Meter the background: F/11 & 1/250
2. Meter the subject with 1 stop "higher" aperture. Save the GN (with my D70s it's possible). eg. F/8 & 1/100
3 Take the photo (with exposure settings for backround) at F/11 & 1/250 & GN saved at step 2.

Is this, what you ment?

ksmattfish · Apr 4, 2006

I determine the subject lighting first. Since the subject is lit by flash, shutter isn't really all that important; it's effectively 1/10000th, or however long the flash lasts. If you get correct flash exposure at f/8, you would go to f/11 to underexpose by 1 stop (if you want to do the 1 stop underexpose thing).

When I know what aperture I'll be using for the subject, I use that to determine the shutter speed for the background.

1) I figure out my aperture based on flash power and distance to subject.
2) Close the aperture down 1 stop (go from f/8 to f/11) to underexpose the flashed subject 1 stop. This would be optional depending on the look you want.
3) Use the aperture that I've decided on to meter the background, and determine the shutter speed.

Here's where the problem comes in. Yesterday I was photographing some friends' kids in the park. I was using ISO 100, and had the flash dialed down from full power so that I was getting a normal exposure with f/4 at 10' (distance times f/# = GN). When I metered the background for f/4 I got 1/1000th. I needed to get it down to 1/250th or less for the flash sync. I put on a polarizing filter to block a couple of stops, and got the background exposure down to 1/250th @ f/4. But now the pol is also blocking the flash 2 stops, so I dialed the flash power up 1 stop, and that gave me my normally exposed background, and -1 exposed subject. Here's one of the pics taken at about 1:00 in the afternoon (harsh sunlight) in dappled shade.

DocFrankenstein · Apr 4, 2006

1) I determine the normal exposure for the subject. (The one I'd used without a flash)
2) I decrease it by 1/3 or 2/3 of a stop, because the flash will put some additional light on the subject.
3) Make the flash output one or two stops less than the normal exposure.

If I have time, I make the model hold the lightmeter.

You set the Tv=200 (sync speed), pop the flash and the lightmeter tells me the percentage of flash in exposure, the F stop that achieves correct exposure. And with rotation of the wheel it juggles the numbers around so that I can adjust % of flash by varying the shutter speed.

saulmr · Apr 4, 2006

So, basically I underexpose the subject by 1/3 or 2/3 stops to allow the flash to complete the subject's exposure and help even out shadows and features with a uniform light source?

DocFrankenstein · Apr 4, 2006

saulmr said:
So, basically I underexpose the subject by 1/3 or 2/3 stops to allow the flash to complete the subject's exposure and help even out shadows and features with a uniform light source?

You might... if that's what you're going for. Most of the time - yes, when you're using fill flash on people in "normal" situations.

I wrote that as a reminder to compensate for the flash. If your subject is exposed normally AND you add the fill flash without compensating, you're going to overexpose.

There's no rules, because sometimes you're trying to do different things.

IE if you have a model on a beach with the sun behind her, then there's no point compensating, since the flash is the major source of light.

ksmattfish · Apr 4, 2006

saulmr said:
So, basically I underexpose the subject by 1/3 or 2/3 stops to allow the flash to complete the subject's exposure and help even out shadows and features with a uniform light source?

You use the sun as your key light. If you set the flash to expose normally you would be getting a 1:2 lighting ratio. 1 part light where only the flash lights, and 2 parts where the flash and the sun light.

Underexposing the flash by 1 stop gives you a 1:3 ratio. 1 part where the flash hits, 3 parts where the sun and flash hit (1 part for the flash, and 2 parts for the sun). 1:3 is a nice starting place for color.

Underexpose the flash by 2 stops, and you get a 1:5 ratio (1 part flash in the shadows, 1 part flash + 4 parts sun on the sunlit side), which is popular for BW. Like Doc said, it just depends on what you want.

cumi · Apr 5, 2006

My main problem is, that I never no in which mode (aperture, shutter prio, full manual or program) to shoot with flash. If I shoot without a flash in AP mode and set F4, the shutter speed will be set automatically by the camera, depending on the ligth conditions. If I shoot with flash in AP mode and also set F4, the shutter will be (mostly) always the same (like 1/60), but the GN and flash power will be set by the camera. So for the the subject, there will be no problem, but the backround... OK, when it's too dark, there is nothing to do with the background. But when I want to use flash only as a fill flash I always have a problem. I know what do I have to do, but I can not implement it technically.

Let me go into technical details:

ksmattfish said:
1) I figure out my aperture based on flash power and distance to subject.

My camera has GN 15 (ISO 200), so if the subject distance is 3m (you are talking about feet and not meter???*) I set F5 ( 3 x 5 = 15). I don't use the camera to figure this out (?).

ksmattfish said:
2) Close the aperture down 1 stop (go from f/8 to f/11) to underexpose the flashed subject 1 stop. This would be optional depending on the look you want.
3) Use the aperture that I've decided on to meter the background, and determine the shutter speed.

OK, I set F7 (1 step lower than F5) and meter the background (in AP-mode). Let's say I get 1/100.

Now I go back to subject again, switch to manual mode (is the only way to this in maual mode???) and set F7 & 1/100, turn the flash on and shoot.

Is this correct? Can you comment the my questions in the 3 steps?

Thanks a lot, I think I'm starting to get it...

* I checked some photos in BreezeBrowser I took with my PowerShoot A520, and found these values with GN:

Distance: 1,65m
Aperture: 3,2
Focal length (35eq): 47mm
Flash EF guide number: 8,66 (1,65 x 3,2 would give 5,28)

Distance: 1,29m
Aperture: 4
Focal length (35eq): 47mm
Flash EF guide number: 8,34 (1,29 x 4 would give 5,16)

Distance: 1,58m
Aperture: 4
Focal length (35eq): 83mm
Flash EF guide number: 7,22 (1,58 x 4 would give 6,32)

I don't get it...

DocFrankenstein · Apr 5, 2006

You either set to M or Av, since you want to control the aperture.

Can you even adjust manually the flash output of the 520?

To make it easy:
1) dial FEC 2/3 of a stop in Av mode
2) use ISO 50
3) place subject 3 meters from the camera or further
4) fire the flash at full power

cumi · Apr 5, 2006

DocFrankenstein said:
Can you even adjust manually the flash output of the 520?

Yes, I can choose 2/3 or 1/3 power.

Can it be, that my problem in the calculation of guide number is, that I use TTL metering, so the camera is not taking the maximum power, but is reducing it when needed? That's why there are less than GN 9-s... But the equotion is still incorrect somehow...

ksmattfish · Apr 5, 2006

cumi said:
I never no in which mode (aperture, shutter prio, full manual or program) to shoot with flash.

(you are talking about feet and not meter???*)

I say use the camera and flash on manual until you get it figured out. After you have it figured out in full manual, you can determine whether or not you can trust your gear in auto-modes. I have yet to find a camera/flash combo that I trust on auto. Some of them do a pretty good job most of the time, but there will always be times when auto-mode fails you. Particularly when trying to carefully balance flash lighting and ambient lighting.

Guide Numbers are usually listed in feet and meters for ISO 100. Use whichever one you are more comfortable with.

ksmattfish · Apr 5, 2006

cumi said:
My camera has GN 15 (ISO 200), so if the subject distance is 3m. I set F5 ( 3 x 5 = 15). I don't use the camera to figure this out (?).

OK, I set F7 (1 step lower than F5) and meter the background (in AP-mode). Let's say I get 1/100.

Now I go back to subject again, switch to manual mode (is the only way to this in maual mode???) and set F7 & 1/100, turn the flash on and shoot.

Your steps are correct.

GN 15 (ISO 200/meters) divided by 3m = f/5. This would be "normal" exposure for ISO 200. If your camera or flash can do the math for you that's fine. Even before flashes became computerized they usually had some sort of slide rule calculator thingee on the back where you could set ISO and distance to subject, and get the correct f/#.

It doesn't matter what exposure mode you meter in. It may matter what meter mode you meter in. Depending on the background segmented metering might come up with something different than spot metering. I'd probably just go with whatever metering mode you normally use.

One thing to consider is that GN are determined by the manufacturer in optimum laboratory conditions, and may not perform quite as well in the real world. Often I find that it's not quite up to the levels advertised. I have flashes that perform pretty close to the GN, and some that require an extra 1/2 stop or so. With personal testing of your flash you'll figure out if you need to add a little bit.

cumi · Apr 5, 2006

Thanks ksmattfish a lot, I got it! I will experiment with my new d70s...

Any comments on these numbers:

cumi said:
Distance: 1,65m
Aperture: 3,2
Focal length (35eq): 47mm
Flash EF guide number: 8,66 (1,65 x 3,2 would give 5,28)

Distance: 1,29m
Aperture: 4
Focal length (35eq): 47mm
Flash EF guide number: 8,34 (1,29 x 4 would give 5,16)

Distance: 1,58m
Aperture: 4
Focal length (35eq): 83mm
Flash EF guide number: 7,22 (1,58 x 4 would give 6,32)