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Archive 2013 · Boy!, I Really Do NOT Understand This Flash Result!!
  
 
SweetMk
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p.1 #1 · p.1 #1 · Boy!, I Really Do NOT Understand This Flash Result!!


My brother mentioned getting pleasing results by setting a minus 2 offset for the flash (My camera is a Nikon D7000).
I had never had a reason to do this or attempt it before.

So I took a couple pics.
This pic was taken with no offset, the SB-700 flash (in a softbox, camera left) illuminated the table surface as well as the Manfrotto.







Next I changed the camera to a minus 2 offset and retook the pic. The reduced flash light power, allowing the window light to illuminate the table.







I have no understanding of what the camera is doing to cause this.

The camera was set to aperture priority, and f8.

Any help would be welcomed. Thank you



Jan 23, 2013 at 10:57 PM
BrianO
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p.1 #2 · p.1 #2 · Boy!, I Really Do NOT Understand This Flash Result!!


SweetMk wrote:
I have no understanding of what the camera is doing to cause this.


What exactly is your question? I'm not following what the "unexpected result" is.



Jan 23, 2013 at 11:04 PM
SweetMk
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p.1 #3 · p.1 #3 · Boy!, I Really Do NOT Understand This Flash Result!!


BrianO wrote:
What exactly is your question? I'm not following what the "unexpected result" is.


The Manfrotto looks similarly exposed in both pics, but, the amount of light from the window is a much greater portion of the lighting in the second.

What is the camera doing?

I would expect the overall picture to get darker, but, that is not what it appears to be. As I do not understand what is happening, I guess I can not predict results!



Jan 23, 2013 at 11:16 PM
ChiShutter
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p.1 #4 · p.1 #4 · Boy!, I Really Do NOT Understand This Flash Result!!


It depends whether you adjusted flash compensation, or exposure compensation. I'm guessing you adjusted flash compensation, which dialed the flash down. But in Aperture priority the camera just adjusted by lengthening the shutter speed to expose properly.

This is my guess.



Jan 23, 2013 at 11:23 PM
joove
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p.1 #5 · p.1 #5 · Boy!, I Really Do NOT Understand This Flash Result!!


I am not sure what the Nikon flash algorithm is (quite similar I would guess), but when I was with canon and researching this, this is what I learnt regarding TTL flash (auto exposure).

Loosely: when the camera computes exposure during the pre-flash, with the flash down by -2, you'll get a longer calculated exposure time to compensate for the smaller flash output. So when the shot is taken, you will expose the background longer which is what you see here.

Learning and controlling this will also allow you to do the opposite: completely wipe out the ambient light by overpowering the flash.



Jan 23, 2013 at 11:26 PM
SweetMk
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p.1 #6 · p.1 #6 · Boy!, I Really Do NOT Understand This Flash Result!!


The adjustment was made on the camera (hold down the flash button, spin the main wheel)
I just checked, both were 1/60th sec., f8, ISO 400 in the EXIF data.

The ambient light just seems so much more a part of the pic in the second pic, but, how?
Same f
Same time
Same ISO




Jan 23, 2013 at 11:39 PM
BrianO
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p.1 #7 · p.1 #7 · Boy!, I Really Do NOT Understand This Flash Result!!


SweetMk wrote:
...The ambient light just seems so much more a part of the pic in the second pic, but, how?


Angle of incidence = angle of reflection.

Your second shot was from a lower angle, so more of the backlight was reflected off the table and into the lens.

To really test what difference changes in flash output make, you need to keep all other variables constant. A tripod would have been indicated for these two shots.


Edited on Jan 23, 2013 at 11:57 PM · View previous versions



Jan 23, 2013 at 11:51 PM
cgardner
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p.1 #8 · p.1 #8 · Boy!, I Really Do NOT Understand This Flash Result!!


Look at the shutter speed information in the two files. As Joove says you'll see the shutter time is longer in the second shot. That's what is allowing the ambient from the window to dominate and why your brother suggested the -2 offset from the default flash calculation: so the flash wouldn't overpower the ambient modeling. In the first shot the flash appears overpower the ambient completely.

Conceptually the technical reason to use flash at all is when exposing for highlight detail does not record detail in the shadows: scene range in f/stops exceed sensor range in f/stops. In situations when exposure for highlight detail causes a loss of shadow detail you'd want to put the ambient light behind the subject as you have here as "rim" light so the flash will not overlap, then add the flash to the front side the camera sees from ABOVE the lens (not off to the side). You then adjust the flash offset until the highlights on the flash lit side are exposed correctly.

That strategy is like sun lighting the back and the sky lighting the front outdoors.

Why not put the flash off the side as you did here in the second shot? Because a flash off to the side will create shadows the camera sees and those shadows will not have detail UNLESS flash footprint is spilling off the opposite wall and ceiling creating "spill fill".

Your flash placement here from the left and behind only worked as well as it did because from the look of the shadows in both shots you appear to have lots of "spill fill" or fill from some other source putting detail in the black objects it the flash on the left isn't hitting directly.



Edited on Jan 24, 2013 at 12:02 AM · View previous versions



Jan 23, 2013 at 11:56 PM
SweetMk
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p.1 #9 · p.1 #9 · Boy!, I Really Do NOT Understand This Flash Result!!


BrianO wrote:
Angle of incidence = angle of reflection.

Your second shot was from a lower angle, so more of the backlight was reflected off the table and into the lens.

To really test what difference changes in flash output make, you need to keep all other variables constant. A tripod would have been indicated for these two shots.


I m more intrigues now, and will reshoot on a tripod tomorrow when I get light from the window.




Jan 24, 2013 at 12:01 AM
 

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BrianO
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p.1 #10 · p.1 #10 · Boy!, I Really Do NOT Understand This Flash Result!!


cgardner wrote:
Look at the shutter speed information in the two files. As Joove says you'll see the shutter time is longer in the second shot.


No it isn't. As the OP stated above -- and I double checked with Elements -- they're both at 1/60, f/8, ISO 400.



Jan 24, 2013 at 12:04 AM
BrianO
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p.1 #11 · p.1 #11 · Boy!, I Really Do NOT Understand This Flash Result!!


SweetMk wrote:
I m more intrigues now, and will reshoot on a tripod tomorrow when I get light from the window.


I'd also suggest trying the experiment at the lowest possible ISO setting. In this case, the flash may have been firing at such low power that the difference between no FEC and -2 FEC was not readily seen. Most of the light in both shots may have been from direct and bounced ambient.



Jan 24, 2013 at 12:08 AM
SweetMk
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p.1 #12 · p.1 #12 · Boy!, I Really Do NOT Understand This Flash Result!!


OK, I reshot on a tripod, using a 85 watt CFL as the "window" light.
Starting at +1, and progressively going to -3, the flash gives less and less light. In the raw pic, the Manfrotto keeps getting darker.

I guess it is surprising what the slight difference is when hand holding.

The tripod pics show different surfaces changing source of lighting as the offset is changed, interesting.

Not minimally, BAM , the surface has a different light source. I was surprised.



Jan 24, 2013 at 12:36 AM
BrianO
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p.1 #13 · p.1 #13 · Boy!, I Really Do NOT Understand This Flash Result!!


SweetMk wrote:
...I guess it is surprising what the slight difference is when hand holding.


Yeah, it's pretty amazing sometimes. We tend to focus so much on the subject that we sometimes fail to see what's happening in the background/surround, and the changes in the ambient light can have significant impacts on the scenes.



Jan 24, 2013 at 01:12 AM
cgardner
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p.1 #14 · p.1 #14 · Boy!, I Really Do NOT Understand This Flash Result!!


BrianO wrote:
No it isn't. As the OP stated above -- and I double checked with Elements -- they're both at 1/60, f/8, ISO 400.


I sit corrected. Missed that in the narrative and couldn't check the data on the files. Also didn't notice the change in POV and your observation about the angle of reflectance off the table makes sense.

For the OP: If you put the flash and camera in manual mode and eliminate variables like the camera position you'll better understand the cause and effect of mixing ambient and flash.

With the camera on the tripod (to eliminate that variable) start by setting shutter at 1/125th and adjusting aperture to correctly expose the highlights on the object (not the table) per the clipping warning with just the window behind the subject and your starting evaluation baseline. You'll see how well your sensor range handles the contrast of the ambient lighting. If you see detail on the front side in the black parts you really don't need to add any flash.

Next put the flash over the camera at a 45 degree downward angle to the table. Adjust flash power manually until the front side looks normal. Have the ambient lit highlights you exposed with detail in step 1 changed because the flash added light? If so increase shutter speed until the clipping on the subject (not the glare in the table) disappears

Tie a string to the stand just under the flash so you can keep the distance (and manual flash exposure) the same then move the flash around to the side in an arc in 15 degree increments.

Compare the results you get by moving the frontal light around. Where does it create shadows? Does does the detail on those areas change?

You'll learn more about cause and effect of mixing flash and ambient in less time with that one variable (fill position) systematic approach.




Jan 24, 2013 at 01:19 AM
Allynb
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p.1 #15 · p.1 #15 · Boy!, I Really Do NOT Understand This Flash Result!!


By dialing down the flash, the camera compensated by adjusting the aperture and slowing the shutter speed. This slower shutter gives more ambient light to your background. Remember, the flash controls the aperture and the shutter speed controls the ambient light.
Allan



Feb 03, 2013 at 01:29 AM
BrianO
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p.1 #16 · p.1 #16 · Boy!, I Really Do NOT Understand This Flash Result!!


Allynb wrote:
By dialing down the flash, the camera compensated by adjusting the aperture and slowing the shutter speed. This slower shutter gives more ambient light to your background.


Nope. As already stated by the OP in post #6 and repeated by me in post #10, both shots are at 1/60, f/8, ISO 400. The only difference -- as explained by me in post #7 -- is in the camera-to-subject angle, which lets more of the ambient light from the rear bounce off the table into the lens.



Feb 03, 2013 at 11:41 PM
swoop
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p.1 #17 · p.1 #17 · Boy!, I Really Do NOT Understand This Flash Result!!


The camera is exposing using the ambient light and the flash is simply filling in the shadows. It's called fill light.


Feb 12, 2013 at 04:07 AM





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