Background exposure control in flash photography

[haris]

I am frustrated with a seemingly simple situation. I am shooting with

D70 and SB-600 speedlite and would like to have a proper main subject

flash exposure along with slightly underexposed bacground. That's all

I want: complete control over the background while the camera/flash

properly illuminate the main subject. Is this too much to ask?

 

I thought that setting the flash to slow sync mode and dialing in -1

exposure compensation would do the trick, but this combination results

in an underexposed main subject. At that point, I went to the manuals

only to find they are thoroughtly confusing. They talk about using S

and not A mode for background exposure control. They also talk about

using spot metering and not matrix or center-weighed. I started

experimenting but it's late, I am tired, and working in my room I

can't set up a situation with a good separation between the main

subject and the background. The camera is also doing weird things:

when I use A or S exposure modes and rotate the main control dial,

exposure compensation sign lights up. I did not even know that the

main dial was active in A mode at all. Apparently, with the flash up

it is! Exposure indicator, which is supposed to move in 1/3 EV

increments, all of a sudden jumps by a full stop when I cross some

kind of boundary.

 

It would be nice if someone could kindly explain in English how this

whole system works, please?!

 

At a minimum, all I want is a simple thing (or so it seemed): how can

I get the background underexposed by 1 stop and the main subject

properly exposed by the flash? I would like to avoid using fully

manual exposure because the lighting of the background may be

changing, but I am not even sure that setting a manual -1 exposure for

the scene would give me the poper main subject flash exposure. Would

it? I am pretty sure that D70 metering system can isolate the main

subject in the center of a frame for proper flash exposre

independently of the background.

[erik_loza]

Do a few tests in manual mode, get the exposure nailed, then start working in whatever

other mode you prefer. IME, direct-on-the-subject flash, especially when bounced off a

caucasian person's fair skin, really can play tricks on a DSLR's metering system. You might

consider moving your speedlight off the shoe and using some diffusion to even the light

out a bit. Good luck with your project.

[chris_leck]

Here's a superficial, simple answer: Set the camera to underexpose the background by the desired amount. Try shutter priority with exposure compensation. Then set the flash to over-expose by the same amount. Make sure it's not set to balanced fill flash or it probably will underexpose and become more unpredictable. Try matrix or center-weighted metering. Spot metering with flash is problematic.

 

Experiment with lighting ratios from there. Depending on the composition, reflectivity of the subject, etc., you may have to play a bit with the exposure compensation on the flash to get what you want.

 

Lots of things are interrelated and the D70 may be trying to do too much thinking for you. When you become more familiar with your camera, you'll be able to exert more control over it.

 

WRT the exposure compensation, it's all configurable and depends on how your camera is set up. On my D100, the main command dial is set to aperture, the sub-command dial to shutter. When in aperture priority mode, the sub-command dial becomes exposure compensation. The reverse is true for shutter priority. Works for me, but these may not be default settings.

 

Believe it or not, for a static situation, manual is quick and easy -- and likely to give more consistent, repeatable results. Set the background exposure manually, then dial down the flash in manual mode to get what you want. It should take about thirty seconds to get it in the ballpark.

[david_h._hartman]

<em>Is this too much to ask? --Haris Subacius<br>

</em><br>

No!<br>

<br>

The camera bodys exposure compensation affects both the

ambient and flash exposure. Try setting the camera bodys

exposure compensation at -1 and the flash compensation at +1. The

flash compensation does not affect the ambient exposure. What you

should get with this setup is -1 for the ambient exposure and (-1)+(1)=0

or normal exposure for the flash exposure.<br>

<br>

If this does not work then try doing it the hard way. You are

working on a tripod with slow sync right? Set the camera to

manual exposure and meter the background. Thats your

ambient exposure. Dont use exposure compensation but rather

use the graphic meter display to set the exposure to 1 stop under.

At this point you can try TTL flash or set the flash to manual

and use the variable power option 1/1 to 1/64 to get the proper

exposure for the subject. Be warned that the guide number and

calculator may be optimistic. Its not uncommon for a

speedlight to test with a quality flash meter at half what the

published guide number would indicate.<br>

<br>

Manual flash with a flash meter is more accurate than TTL flash.

If you are using TTL flash and your subject is off center try

using the FV function or Flash Value Lock. Sometimes layering

automation on automation makes it hard to figure what went wrong,

too many variables. You want to change one at a time to figure

out what up. This is why I prefer to take control of the ambient

exposure with manual mode. If Im afoot Ill use TTL

where the flash is the main light and TTL BL where the sun is the

main light.<br>

<br>

Good Luck,<br>

<br>

Dave Hartman.<br>

<br>

---<br>

<br>

<em>Believe it or not, for a static situation, manual is

quick and easy -- and likely to give more consistent, repeatable

results. --Chris Leck<br>

</em><br>

With some practice this is true. The most extreme example Ive

read about was a major NY wedding photographer who had all his

Hasselblad lenses set to 7 feet (2.1m) with set screws. He set

the aperture, focused with his feet and had perfect exposure. <br>

<br>

I didnt go quite this extreme but I used to shoot daylight

fill with Nikon FM2 and Vivitar 285. I calibrated the 285 but

using 0.5x ISO, e.g. EI 100 for a film I normally shot at EI 200

(Thats EI 100 on the flash calculator and EI 200 on the

camera and spotmeter). I also used to sync stained glass in the

background with flash using a Rollei 80/2.8 Xenotar, Pentax

Digital Spotmeter and Vivitar 285. Once I calibrated the 285

using the calculator dial wasnt so difficult.<br>

<br>

I also had a system for my Hasselblad where I put labels with f/stops

on the quick focus handle. Id focus, note the f/stop at the

top, set the aperture and shoot. This was more than likely

to be consistent and repeatable. It was dead on and it was quick

and easy. The subject did not need to be entirely static.<br>

<br>

Best,<br>

<br>

Dave Hartman.

[darrengold]

Haris, I'm a little confused, but your thread suggests you are working indoors, which may be your problem. Outdoors you would achieve your goal by all the methods so far mentioned, but I would switch off Multi balanced TTL and use ordinary TTL.

 

Indoors, you are using the TTL meter only. The ambient light isn't relevant as its far too low anyway. That's why AE -1 underexposes everything. To really achieve what you want you have to narrow the flash range. Try bouncing off the ceiling at differing angles, or use remote triggering and set the flash behind the camera so the exposure is correct for the subject but falls off quickly to the backround.

 

Happy New Year.

[haris]

Thanks for your replies. The general gist I get from all the posts is that too much automation can unautomate itself.

 

I like the suggestion of using manual exposure and underexposing the ambient light with the exposure meter itself rather than exposure compensation. It does not sound too difficult as long as light remains constant and compensation is within one stop--the meter does not show more than that. I assume that TTL flash remains fully automatic with manual exposure and give me a proper exposure for the main subject as long as it is in the center of the frame or FV is used, right?

 

+X flash compensation along with -X exposure compensation also sounds good. Will need to experiment.

 

I remain bafled about the reasons why exposure compensation with SLOW SYNC has anything to do with flash exposure. I don't even want to start thinking about throwing TTL-BL in the mixture. At that point the camera-flash combination are way beyond my limited powers of comprehension and they are doing their own thing. Can anyone suggest a good read on this flash mess? Or is this really a feature overload situation? More specifically, how does the BL option change exposure calculations in theory? Anyone?

 

Thanks to all.

[photo5]

How are you using your flash? Straight-on at your subject? Bounced from the ceiling? Diffusion?

 

I usually shoot indoors with the shutter set to 1/30th and the flash on matrix Program mode, with no flash compensation. In most rooms, depending on the amount of ambient light, 1/30th will render the background visible, but not overly so. In brighter rooms, 1/60th a sec will work well too.

 

Remember, the shutter speed does not affect your flash exposure, just the ambient light. The f-stop affects your flash exposure.

 

Back in the pre-AF Matrix 6-speed automatic days of photography, I shot with an FM2n loaded with Fuji NPH exposed at ISO 250, and a Vivitar 285 flash on a bracket. For indoor shots, the camera was set to 1/60th sec and f5.6-5 (halfway between 5.6 and 8). Worked every time.

 

Good luck!

 

Dave

[david_h._hartman]

<em>I remain baffled about the reasons why exposure

compensation with SLOW SYNC has anything to do with flash

exposure. --Haris Subacius<br>

</em><br>

Traditionally exposure compensation on Nikon SLR(s) is tied

directly to the ASA/ISO film speed control. Its no

coincident that the exposure compensation control on the Nikon F3,

F3HP, FE2 and F4s/F4 is concentric with the ASA/ISO control as

they both use the same FRE (Functional Resistance Element). Meter

coupling with AI and AIS lenses is also connected to the FRE by a

thread in these cameras. <br>

<br>

Many of the features of Nikon speedlights are built into the

camera body rather than the speedlight. Besides tradition there

are probably technical reasons why the camera body exposure

compensation affects both the ambient and flash exposures with

more modern Nikon SLR(s). If Nikon changed it would confuse the

hell out of long-time Nikon photographers.<br>

<br>

Slow sync is a feature of Aperture Preferred automation. In

manual mode its up to the photographer to drag the shutter

by selection. I didnt mean to imply or state that exposure

compensation on the body affects the flash only with slow sync if

that is what I wrote. Camera exposure compensation affects the

flash in any mode that a Nikon TTL flash SLR or DSLR offers, M,

A, S or P, any mode.<br>

<br>

So is it technical, traditional or both? Ill guess both.<br>

<br>

Best,<br>

<br>

Dave Hartman.

[w. shinn ii]

I recently purchased a Nikon D50 body and a few minutes ago was researching the use of the built in flash. Interestingly enough, the D50 has an option called FV Lock (Flash Value Lock) which if activated using the "D" series Nikkor lenses will allow you to meter a flash value on the subject and lock it. I have not tried this, but here's a thought...

 

I suggest, if this feature is available for the D70, you could try setting the D70 to "Manual Mode" meter the background using with an 18% reflectance gray card. After adjusting to get the right aperture and shutter speed for a "correct" exposure you would set your camera to the next highest shutter speed or the next smallest aperture. As an example, if the D70 built in meter indicates an exposure of 1/30 at f:8 for the background, manually set the camera to 1/60 and f:8 (this would give you a 1 stop underexposure for the background).

 

After adjusting the camera settings for the background exposure, position the subject and your self in the shooting position, activate the D70 flash (with the FV Lock activated), focus on the subject and press the shutter release halfway down (this tells the flash what power to fire), hold the shutter at the halfway position, compose and take the photograph.

 

The flash should output light enough to make your subject the right exposure at f:8 and the background should not be significantly illuminated by the flash to affect the underexposure you adjusted for.

 

Of course this assumes that the background is at least 1.4x farther from you than the subject; i.e., if the subject is at 8 feet, the background should be at least 11 feet or farther away. Hope this helps.