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Old Aug 20, 2003, 8:35 PM   #1
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Default Light metering overules aperture & shutterspeed?

Maybe I'm overlooking something, but I thought that light in photos is a result of mechanical facts:
-Sensitivity, iso (asa)
-Aperture
-Exposure time

If all three would be set at a fixed value it should not make a difference were one spot metered light intensity, as the end result would be restrained to the previous summed factors.
Wrong. Look at the example crop of two images shot only a a second after each other in artifical light (don't look if you have sensitive artistic eyes). Both images have identical Exif information;

Image shot with Fuji F601.

Could somebody please explain how this can happen in a technical fashion? I start to think that whatever the user tells the camera, there is internal some silly digital image enhancer having last say, or something else is wrong.
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Old Aug 20, 2003, 8:44 PM   #2
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... Try fixed white balance, ie not on Auto!
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Old Aug 20, 2003, 8:50 PM   #3
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Blazzing fast answer:-))

But alas I forgot to mention that whitebalance was with both photos on 'Fluorescent Lamp type2'...

Other suggestions??
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Old Aug 20, 2003, 9:08 PM   #4
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Default Re: Light metering overules aperture & shutterspeed?

Quote:
Originally Posted by Mathilde uP
Wrong. Look at the example crop of two images shot only a a second after each other in artifical light (don't look if you have sensitive artistic eyes). Both images have identical Exif information;
Since I do not see the scene and the brightness of the blue and gold colors, my answer may not provide you with a complete answer. However, there is nothing wrong in the image nor with your camera. Keep in mind that all built-in camera meters are of the reflective type. This means these meters measure the light reflected from the subject you point your camera at. However, the meter does not know what the real intensity is. As a result, all meters are calibrated at the mid-gray or an subject that reflects 15% of the incoming light. When you point your spot meter at the blue area, the meter believe that blue area is 50% gray and guarantees that the blue area will be exposed as 50% gray. Since blue is darker than 50% gray, the blue area in the image will be over-exposed, and this is the reason the right image is over-exposed. On the other hand, when you point the spot meter to the gold area, the meter will assume gold will be 50% gray. Since goal is lighter than 50% gray, in the image the gold color is no more gold but a mid gray. Of course, the blue color will be even darker and the image is under-exposed.

So, there is no mystery. It is simply the law of physics behind building reflective meters. To yield a good exposer, you might want to buy a gray card, place the card in front of the subject, take a reading, and use the indicated aperture and shutter speed.

CK
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Old Aug 20, 2003, 9:08 PM   #5
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You hint that fluorescent lights were used, so I would guess the shot on the left caught the exact moment the light was flickering (they do flicker at 50-60Hz don't they?). That's the only explanation I could come up with. Try that same shot again using a regular light bulb for lighting. There really shouldn't be any such variation if everything is manually set to specific settings (only the available light or flash light output may vary in their intensities, then you have another problem, not camera related).
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Old Aug 20, 2003, 9:14 PM   #6
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I think marokero is on to something, either that or try to decrease the shutter speed to 1/30 or less so you can get at least two flashes (or frames as on a TV screen) with the same fluorescent light! 8)

The metering is irrelevant, after all the exposures haven't changed between the two shots (ie exactly the same settings)!
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Old Aug 20, 2003, 9:39 PM   #7
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Quote:
Originally Posted by marokero
they do flicker at 50-60Hz don't they?
If the electronics are symmetrical, as they may well be, you'll get a flash on each half-cycle not each cycle, so it could be 100 or 120Hz depending on which continent you're in.

Even with ordinary incandescent filament lamps, it's just possible in a darkened room to use the strobe markings on the sides of some old-fashioned LP turntables to adjust the rotation speed. I've seen it with my own eyes! The output rises and falls with each half-cycle at 100 or 120Hz, smoothed by the residual heat in the filament. With a neon or a standard fluorescent tube it's a stream of flashes, and the strobe markings work perfectly.

Either will interact horribly with a 1/105th second shutter speed. We need to see the same shot at slower and faster shutter speeds, or we could all try to reproduce the effect and post the results!
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Old Aug 20, 2003, 9:52 PM   #8
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I have to say I think I agree with everyone. Shene is technically correct. Metering in different places should cause different settings (Depending on what mode you are in... program, Av, Tv, whatever.)

But since the settings did not change, then some external source must be working here. I don't know enough about the light flicker to agree or disagree... but it sounds good to me.

Years ago I invented this phrase, and I stand by it:

"I know just enough to confuse myself!"

Eric
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Old Aug 21, 2003, 12:10 AM   #9
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Quote:
Originally Posted by eric s
But since the settings did not change, then some external source must be working here. I don't know enough about the light flicker to agree or disagree... but it sounds good to me.
Hmmm, I missed the flu light source portion. In fact, the answer should be all the above. The meter takes a reading that could under-expose or over-expose the image. Coupled with the flickering phenomenon, an even more under-exposed image is obtained (the left one). The right one is definitely due to over-exposure by metering at a wrong spot or it was metered between the flickering cycle or both.

A similar problem occurs when shooting TV or CRT monitor with slow shutter speed. Due to CRT monitor's refresh rate is fixed (about 1/60 sec), a faster shutter speed may only be able to take a portion of the monitor (i.e., the portion that is being refreshed). The following is an example of shooting my CRT monitor at 1/250 sec.


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Old Aug 21, 2003, 2:22 AM   #10
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A fine explanation Shene in that first post of yours, but I have to correct you on that 15% grey mentioned in your first post. It is actually 18% reflectance and not 15%.

A greycard consist of 18% grey - which is an standard settled by the Industry many years ago!
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