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Old Aug 20, 2005, 8:55 PM   #1
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Please see attached photograph by my Canon Rebel XT.* There appears to be noise in the shadows and on this guy's black shirt.* I thought noise is only prevalent at ISO speeds higher than 400.* This picture was at ISO 200.* It was shot at L quality JPEG fine.* A circular polarizer filter was placed on the kit lens. This picture has only been cropped.* No other adjustment.




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Old Aug 20, 2005, 8:56 PM   #2
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Is this typical of all Canon Rebel XT's? Or is it just mine?**
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Old Aug 20, 2005, 9:02 PM   #3
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Another.* This time at ISO100.* Unmodified.**
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Old Aug 20, 2005, 9:27 PM   #4
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the problem is that you underexposed your shots.. when you underexpose a digital camera, you will get noise no matter what ISO you are shooting with.. and that goes for all digital cameras, no matter what the price.. i have seen similar results from new 1d/1ds users.. they can't understand why there is noise in the 7000 dollar camera's images.. so don't worry, your XT is functioning quite properly... and i might add that it handles noise quite a bit better than most dslrs..

if you get a properly exposed shot, or nearly properly exposed shot, the noise will be gone...
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Old Aug 21, 2005, 12:33 AM   #5
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I feel so much better now.* Thank you, Sir!
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Old Aug 21, 2005, 12:55 AM   #6
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You constantly hear people say that it's ok to underexpose with digital and correct in post processing ... but as Hards80 stated, you need to get your exposure right to minimize noise. A bit of underexposure is ok to try to ensure you don't blow highlights, but grossly underexposed photos will look muddy and noisey after post processing.
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Old Aug 21, 2005, 9:27 AM   #7
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Obtaining the proper exposure was definitely tougher after the addition of my new circular polarizer.* Now, it's not the best brand of filter, but I see how it's making everything darker.* I was told a polarizer is the single best addition after getting my camera.* Do I ALWAYS have to use it outside?* Or are there situations outdoors where not best to use the filter?
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Old Aug 21, 2005, 10:03 AM   #8
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Deliberately underexposing an image to get faster shutter speeds than the ISO speed selected would normally provide is also a "trick" way to set your ISO speed higher.

Some digicam owners with cameras that have a fixed ISO speed like 100use this trick to simulate ISO 200 by underexposing a stop using a -EV value with Exposure Compensation (but, I wouldn't push it more than a stop). ;-)

I use this trick on a little pocket camera when I want to set ISO speeds "in between" allowed values (50, 100, 200, 400). For example, I'll set the camera at ISO 200 and underexpose 1/2 stop to simulate ISO 300. ;-) That way, the noise is usually about half way in between ISO 200 and 400 (with shutter speeds 50% faster than ISO 200) when I brighten the image later using software.

So, if you underxpose an image by a stop at ISO 400, it's more like shooting at ISO 800 from a noise perspective. 2 stops is more like shooting at ISO 1600.

Since you have a wide dynamic range in a typical image anyway, noise ishigher in shadow areas (since these areas are underexposed compared to the rest of the image).


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Old Aug 21, 2005, 10:16 AM   #9
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radiohead wrote:
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Obtaining the proper exposure was definitely tougher after the addition of my new circular polarizer. Now, it's not the best brand of filter, but I see how it's making everything darker. I was told a polarizer is the single best addition after getting my camera. Do I ALWAYS have to use it outside? Or are there situations outdoors where not best to use the filter?
You're going to lose 2 to 3 stops of light with most polarizers, depending on how it's rotated.But, the metering should be compensating. Was there something much brighter in the image you posted that could have thrown off the metering (I assume that the underexposed area is just a crop)?

I guess it could have been looking at the light reflections on some of the vehicles. Center Weighted metering may have been a better bet here (although you may have overexposed that way from a black shirt without some compensation). ;-)

As for filters, there are pros and cons. You are going to lose two or three stops of light with a polarizer (depending on how it's rotated). So, you need to keep things like shutter speeds in mind. Also, anytime you add another piece of glass, you introduce the potential for degradation (internal reflections, flare, etc.). I personally wouldn't use one unless shooting landscape type photos with lots of sky.

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Old Aug 21, 2005, 3:00 PM   #10
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I only use a polarizer to deepen the sky and increase the contrast of the clouds. Otherwise I don't see much change in a picture. I inadvertently left a circular polarizer on my lens and it increased the underexposure of flash pictures normally seen on a 20D. Not sure why the meter didn't compensate, but it didn't...or couldn't. I also found that the polarization of flash bouncing off certain objects introduces some weird artifacts ie polarized noise! So I would not use one when you are taking flash pictures...

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