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06-20-2011, 01:15 AM   #1
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Noise and Over Exposure

So I was woken in the night last night, and ended up seeing what my K-7 was like in low light and at high ISO by taking pictures in the front room at 4am.

Not surprisingly, ISO 6400 is horrible. Really, really bad.

However ... I found that by exposing way above what the meter suggested I was able to get reasonable results. I don't have the card to hand to check (or upload pictures) but from memory the meter was saying 1/8th f5.6 and I was using 1sec f5.6 (3 stops over the meter reading)

This made pictures of my dark front room look like they were taken in much better lighting, but it also meant that the noise was much reduced.

Is this just an optical illusion, or is there an explanation as to why it happens?

I must experiment to see if, say, overexposing by 2 stops at ISO 6400 gives less noise than exposing 'correctly' at ISO 1600. Had anyone else tried this?

Incidentally I was quite impressed with how well the SR was able to cope with my hand holding at 1 second, even if I did have a sofa to lean on.

Will upload images when I get chance, but any thoughts on the above much appreciated.

06-20-2011, 01:30 AM   #2
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Have you looked at the aperture and shutter speeds also?

Having said that, underexposure and/or shadow regions is where noise is at its worst.
And so if we push the scene to the right(eV+) then we can create a more favorable setting.

However... there is a trade-off too.
When we push exposures, the shutter time also goes up.
And so the question arises... are we gaining by pushing to the right over that of lowering sensitivities?

The answer might be worth experimenting with.
06-20-2011, 01:49 AM   #3
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Well, as long as the overexposure doesn't lead to exceeding any headroom restrictions somewhere along the line, that's pretty much what's happening - you're effectively changing the ISO from 6400 to 1600. You may need to compensate in PP though, and even then you may find you're blowing highlights (because you have exceeded headroom somewhere). But the shadows will look much cleaner.
06-20-2011, 01:50 AM   #4
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These are all JPGs with max noise reduction on, at ISO 6400 on a k-7. The darkest one is the truest representation of how much light was in the room.

1/6th, f4.5, ISO6400


1/3rd, f4.5, ISO6400


1s, f4.5, ISO6400


All handheld, so excuse the camera shake.

Next step, when I have time, is to PS them to see if I can get the 1 second exposure to look as dark as the first one, as that is what the scene actually looked like.

06-20-2011, 02:58 AM   #5
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IMHO its kind of "expose right" case.

A.
06-20-2011, 05:10 AM   #6
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Intuitively, the more the exposure time, the less noise due to less need to boosting image exposure in PP. If less noisy images are desired, at least for static subjects, it would be prudent to shoot with lower ISO and longer shutter speeds using a tripod to stabilise the camera for the exposure.
06-20-2011, 05:22 AM   #7
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QuoteOriginally posted by Northern Soul Quote
However ... I found that by exposing way above what the meter suggested I was able to get reasonable results.
In my experience the light meter tries to give you a darkish image in low light. I've heard of reciprocity failure but that's probably not it in your case. Could it be that the Pentax engineers rigged the meter to yield the dark image to make it true to life?

I routinely dial in +0.7 to +1 in low light and it conceals the noise somewhat. But then it doesn't "look dark" anymore.

Sincerely,
--Anders.
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