[Leica] IMG: Roofers at 2500 ISO

[afirkin at afirkin.com (afirkin at afirkin.com)]

makes sense. I look forward to the examples and ideas on what can be done.

> Your point seems well taken, but that process would be in addition to what
> I was talking about.
>
> I believe that when the shutter button is pushed, what happens first, and
> without noticeable pause, is that the sensor is flushed of all thermal
> electrons (noise) accumulated since the last readout. Then during the
> exposure, both thermal electrons and the electrons freed by image photons
> accumulate at each photosite. Finally, when the shutter closes, the
> electrons are read out by the electronics and the corresponding numbers
> stored as the image file. This file contains contributions from thermal
> electrons (and other sources of noise, such as read noise). The longer the
> exposure (and therefore the interval between the pre-exposure flush and
> the post-exposure read), the more thermal electrons will accumulate at
> each photosite. Part of the thermal noise is random from one photosite to
> the next and so is not unambiguously removable, though statistical and
> contextual algorithms can be applied to make the picture look less noisy
> (Noise Ninja etc).
>
> So longer exposures produce more noise, and this is invariant with the
> number of "image" electrons that accumulate (unless a photosite
> saturates). If two images are made, one with a short exposure and low
> f/ratio and the other with long exposure and high f/ratio, both
> accumulating the same number of image electrons, you can see why the image
> made with a long exposure will contain more noise electrons than the short
> exposure. What I don't know is whether this difference is noticeable under
> usual working conditions, i.e. whether 1/60 or 1/30-sec exposures
> accumulate noticeably more noise than 1/1000 sec in an image that is
> adequately exposed?in the middle of the histogram. Certainly it's
> important as exposures get longer. I suspect it's noticeable in dark areas
> of subject matter in ordinary exposures at high ISOs. (Higher ISOs are
> obtained simply by amplifying the signal being read off the CCD, and some
> of the noise inherent in the process gets amplified too, so the image is
> noisier than one taken at low ISO?though not all of the noise, otherwise
> what's the point?)
>
> The random part of the thermal noise cannot be removed by the automatic
> dark-frame subtraction that digital cameras perform after a long exposure,
> because it's unpredictable and variable from one photosite to another and
> from exposure to exposure at each individual photosite. But there is
> another  component to thermal noise, one that depends on fixed, unique
> characteristics of each individual photosite, and is therefore consistent
> from one exposure to the next. These systematic aspects include almost
> atomic-level differences in the photosites acquired during manufacturing,
> differences in temperature across the sensor produced by the camera's
> electronics ("amplifier glow"), and other factors. This systematic thermal
> noise can be replicated by exposing a dark frame of the same duration and
> subtracting its values pixel-to-pixel from the image file. These things
> are well known to "astroimagers", who count practically every single
> photon from extraordinarily faint deep-sky objects using CCD cameras that
> are cooled to 30-60 degrees below ambient and apply dark frames, bias
> frames, flat-field frames, etc, to refine the result. Professional
> astronomers cool their CCDs for spectroscopy and imaging to the
> temperature of liquid nitrogen!
>
> And as you said, selectively bringing up the raw signal from
> under-represented pixels will selectively bring up noise of that color.
>
> I'll do some high-ISO comparison shots of short exposure at high f/ratio
> and longer exposure at low f/ratio and post them.
>
> ?howard
>
> On Oct 31, 2010, at 5:11 AM, Jeff Moore wrote:
>
>> 2010-10-30-22:32:57 Howard Ritter:
>>> In answer to your question about the low noise level in bright-light
>>> exposures at ISO 2500, here's my suspicion: The amount of noise in a
>>> given exposure depends on a number of variables, one of which is the
>>> duration of the exposure.
>>
>> Well, maybe.  But I also note that when there's bright light, it's
>> usually sunlight, while dim light is often something with a warmer
>> color balance like incandescent lights (or candles).  When I
>> particularly notice noise, it's often when I start with something with
>> very little blue energy in it naturally, and try to balance the color
>> temperature out to look not-entirely-orange.  That requires cranking
>> up the underexposed blue pixels, which respond by peppering the frame
>> with blue noise.
>>
>> Or is it some entirely diffrent phenomenon you're talking about?
>>
>> -Jeff
>>
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