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Beach shots... SUnny day with flash
Old 09-26-2006, 09:25 PM   #1 (permalink)
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Somewhere here on GG/G1, I had read a tip about underexposing the background one or two stops to make the sky dark blue, then lighting the subject with your flash to make her pop instead of being underexposed as well.

Anybody know were that post is or can you offer advise on the lighting?

I imagine in camera metering for the BG, but when using a speedlight, is there a rule of thumb to ball park the flash setting or is it just trial and error? I'd like to limit my trial and error time once I'm on the beach
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 10:35 PM   #2 (permalink)
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Essentially what you're doing while doing this is metering your flash and allowing that to control the exposure on your subject and then you are using your shutter speed to reduce the amount of ambient light. So...

Let's say that to get a perfect exposure of the background you're shooting F8 at 125. But you want to make the background darker. So you would increase your shutterspeed maybe 250 gives you the look you want. Fine. Your flash is set to F8 and you will get the image you want. You do need to experiment with it though.

This shot was shot that way using two flash units:



This one was done using the same technique but with an on-camera flash (SB-600):

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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:05 PM   #3 (permalink)
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A flash meter can really help you here, as you can take both an incident reading to get the available light exposure down, and then take a flash reading to set your subject exposure. It's not trial and error, but rather a matter of physics.

That said, the way to approach this is as follows: establish your available light exposure first. I would take an incident meter reading of your background scene and then set the camera to underexpose by 1-3 stops so that you achieve the background look you want. This is easy with digital, since you can see what you've just shot on the LCD. Choose a combination of shutter speed and aperture that allows for a shutter speed below your maximum sync speed. That is, if your camera's maximum flash sync speed is 1/250th. of a second, choose a combination that lets you use that (or a slower) shutter speed. For the sake of this discussion, lets assume you settle on 1/250th. of a second at f/22, and that these values represent 2 stops of underexposure (that is, for "proper" exposure you should use 1/250th. at f/11).

Once you've established the look for your background, you need to decide how you want your subject to appear. You're going to use your flash for this, so the only thing that matters is the lens aperture. This is really simple to do if you have a flash meter and the ability to set the power level of your flash. Assuming you want your subject exposed "normally", you take flash meter readings from the subject position and adjust the output of your flash until the meter reads f/22. Now, when you make your shots, your environment will be underexposed by two stops and your subject will receive an amount of light necessary to achieve normal exposure.

If you don't have a meter, you can still easily set the flash if you know it's guide number. (This is where the physics part enters our discussion.) The guide number represents the amount of light emitted at full power. To figure your exposure, divide the guide number by the distance from the camera to your subject. The result is the f-stop you need to use. (Example: guide number of 160, 10 feet from camera to subject. 160/10 = 16 or f/16). You can multiply the guide number by the power setting to change the needed f-stop. (1/2 power * 160 = 80. 80/10 = 8 or f/8).

Ok, so you're set at 1/250th. at f/22. But your flash has a guide number of 160 and you are 15 feet from your subject. To properly light your subject the aperture needs to be 160/15 which is 10.667 or ~f/11. Now what? You need to open up the aperture by two stops and raise the shutter speed by an equal amount to keep the background the way you want it. But your camera won't sync with the flash at 1/1000th. of a second. You might be able to lower your ISO by two stops, but if you were shooting at ISO 100 to start with you probably don't have that much room.

A real good solution to this problem is to use a neutral density filter. A two stop ND filter would allow you to open up your aperture two stops while keeping the shutter speed at 1/250th. ND filters give you more control over your apertures in this situation too. You may want to shoot at f/5.6 in order to control depth of field. Given the above situation, a four-stop ND filter would let you do that.

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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:16 PM   #4 (permalink)
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Quote:
Originally Posted by mcherry
Essentially what you're doing while doing this is metering your flash and allowing that to control the exposure on your subject and then you are using your shutter speed to reduce the amount of ambient light. So...

Let's say that to get a perfect exposure of the background you're shooting F8 at 125. But you want to make the background darker. So you would increase your shutterspeed maybe 250 gives you the look you want. Fine. Your flash is set to F8 and you will get the image you want. You do need to experiment with it though.
That makes sense to me. So let me ask you this.... I'm using an SB-800, If I've got the Aperture and shutter set where I want, How do I confiugre the flash? It seems as though I would *have* to use a flash meter or am I missing something? Thanks for your help.
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:18 PM   #5 (permalink)
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Quote:
Originally Posted by ChipBulgin
A flash meter can really help you here, as you can take both an incident reading to get the available light exposure down, and then take a flash reading to set your subject exposure. It's not trial and error, but rather a matter of physics.
Hey Chip... Thanks for the response. I think I will break out the flash meter tomorrow or brush up on the GN calcs for my SB-800 (needed when using as a remote. On the camera I guess it has all that I need on the display). We will see.
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:39 PM   #6 (permalink)
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Quote:
Originally Posted by PhotoDave
That makes sense to me. So let me ask you this.... I'm using an SB-800, If I've got the Aperture and shutter set where I want, How do I confiugre the flash? It seems as though I would *have* to use a flash meter or am I missing something? Thanks for your help.
A flash meter is the best way. If you don't have one on had you can spot meter your subject using your camera's meter as you normally would or use the guide number as Chip said. Not sure what camera you're using but if you have the SB-800 you might have high-sync flash available to you (I can with my D2x) which will eliminate the necessity of an ND filter. To be honest, the only time I've needed and ND filter is when trying to overpower the sun around mid-day with a porty unit that I can only sync up to 250. For sunsets (those last few moments before the sun dips below the horizon) it's never been a problem. No if you use an external flash (such as I did in the first sunset pic I posted, I used a Ring Flash) then a meter is pretty essential.

[EDIT: I just realized I didn't answer your question, I set my SB-600 manually]
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:40 PM   #7 (permalink)
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Chip,

Thanks for taking over where my laziness left off... Great post.

Matt
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:40 PM   #8 (permalink)
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Hey Dave,

I just realized I made a mistake when talking about reducing the power of your flash. You don't multiply the power setting by the guide number, because the guide number is not a linear value. Every time you halve the power of your flash, you divide the guide number by the square root of 2. Setting your flash to 1/2 power would require opening up your aperture by one stop, not two. The progression of opening up the aperture is as follows:

1/2 power : 1 stop
1/4 power : 2 stops
1/8 power : 3 stops
1/16 power: 4 stops
etc.

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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:52 PM   #9 (permalink)
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Quote:
Originally Posted by ChipBulgin
Hey Dave,

I just realized I made a mistake when talking about reducing the power of your flash. You don't multiply the power setting by the guide number, because the guide number is not a linear value. Every time you halve the power of your flash, you divide the guide number by the square root of 2. Setting your flash to 1/2 power would require opening up your aperture by one stop, not two. The progression of opening up the aperture is as follows:

1/2 power : 1 stop
1/4 power : 2 stops
1/8 power : 3 stops
1/16 power: 4 stops
etc.

-Chip
All good info. I will try to apply some of this info in some tests tomorrow. No models unfortunately, but maybe a fence post or two
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Re: Beach shots... SUnny day with flash
Old 09-26-2006, 11:57 PM   #10 (permalink)
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Quote:
Originally Posted by mcherry
A flash meter is the best way. If you don't have one on had you can spot meter your subject using your camera's meter as you normally would or use the guide number as Chip said. Not sure what camera you're using but if you have the SB-800 you might have high-sync flash available to you (I can with my D2x) which will eliminate the necessity of an ND filter. To be honest, the only time I've needed and ND filter is when trying to overpower the sun around mid-day with a porty unit that I can only sync up to 250. For sunsets (those last few moments before the sun dips below the horizon) it's never been a problem. No if you use an external flash (such as I did in the first sunset pic I posted, I used a Ring Flash) then a meter is pretty essential.

[EDIT: I just realized I didn't answer your question, I set my SB-600 manually]
You bring another question to mind... I have a D70 and am using an SB800. When the camera is set to Manual and I have the shutter and aperature set as desired, how doe sthe flash work whe in iTTL mode? Manual is cut and dry - I got that, but what about the TTL or iTTL modes? WIll the output of the flash be dependant upon the shutter, aperature and distance reported by the camera or will the flash intensity be based on the spot metering (i.,e. the models face or torso, hence changing the distance calc fo the exposure).

I Hope that question made sense anyway
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