If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
His reply has nothing to do with your rude reply to the original poster.
Your reply was still RUDE. wrote in message ups.com... McLeod wrote: Does anyone understand what are the mechanics behind the ISO settings for RAW images? Yes, I understand them. At least for the Canon 1D Mark II: there are three buttons, a finger wheel, and a thumb wheel. Maybe the Nikon D70s has the Telepathic Adaptor which can lock onto the mind of the photographer, thus dispensing with these crude mechanical implements? I think the poster was asking a more generalized question than you responded to, but hey, who am I to stop you from making a jerk of yourself. You "think"? Usually the lowest ISO setting of your digital camera is going to be where you get the best picture with the lowest noise. Higher settings are amplified, I believe, so noise is increased. You "believe"? They are only in increments of 100, 200, 400 to remain reciprocal with shutter speed and aperture settings. Well, you have offered two speculations and one error of fact. And you accuse me of making a 'jerk of myself' for correctly answering his question as written? Nitwit. |
#12
|
|||
|
|||
|
#13
|
|||
|
|||
On Tue, 12 Jul 2005 01:29:30 -0000, Jeremy Nixon wrote: wrote: But how does it work for RAW in digital storage? "Approximation" algorithms are not supposed to be applied and whatever light reaches it should be recorded no matter what ISO is. Any ideas? The signal is amplified to reach the desired level. Got it now. Thank you. |
#14
|
|||
|
|||
In article ,
wrote: Sorry, I realize now that the original message was not "descriptive" enough. How ISO works on film is rather "material" - the grain of ISO 400 compared to let's say 50, has a different, more corse structure. But how does it work for RAW in digital storage? "Approximation" algorithms are not supposed to be applied and whatever light reaches it should be recorded no matter what ISO is. Any ideas? As I just got through typing (and thus, you have not yet had time to read it), and perhaps I can add a bit more detail this time, now that I know the focus of your question. 1) The sensor cells (CCD or CMOS) have capacitors which store a charge during the exposure. The maximum change which can be stored defines the low-end ISO for the sensor, as more exposure fills the cell to the maximum, and highlight detail is lost. 2) Once the exposure is done, the charge is moved from the sensor cells towards circuitry on the border of the sensor. 3) The first bit of circuitry will be a gain stage, so if the exposure was not sufficient to fill *any* of the cells, the range of voltage resulting from the charges which *were* stored is increased by applying amplification, to provide a reasonable range of voltage to the next stage. 4) The second bit of circuitry will be the A/D converter (Analog to Digital converter.) This converts the analog information from the sensor cell which came through the amplifier into a number which represents that level in a digital form Note that there is always *some* noise in the sensors, though it may be small enough so it is not noticed in the presence of the signal at low gains. However, if the gain is high, it is amplifying both the noise (which is approximately a constant level for a given exposure time) and the signal. This brings the noise up to a level where it is obvious (typically on darker areas of the photograph). There are at least three sources of noise: 1) "Thermal" noise, which is a function of the resistance (impedance) that the sensor and circuitry up to the amplifier sections and the temperature. The higher the temperature, the more thermal noise, and the higher the gain (ISO) the more visible the noise from a given temperature will be. Far Infrared sensors are often operated at very cold temperatures, such as 70K (the temperature of liquid nitrogen). You've probably seen far IR images in the video from missile strikes from aircraft from the Gulf War footage. 2) The digital "clocking" noise (which is left-overs from the signals used to move the information from the sensors to the A/D converters. 3) "Fixed Pattern" noise, resulting from minute variations in the sizes of the sensors, and slight variations in the accuracy of the A/D converters. This last one has the advantage of being somewhat predictable, and many cameras have a mechanism to subtract this out from the image data. This is commonly done (for long exposures) by closing the shutter, and collecting noise for the same time as the exposure just collected, and then digitally subtracting the noise from the signal data. The noise from these various sources all tends to be worse for long exposures or high ISO, and as a result, it behaves in a manner somewhat similar to film pushed to high ISOs. However, the film won't have the Fixed Pattern or the clocking noise sources, just the equivalent of the thermal noise. (There is a difference. As you push the ISO in a digital sensor, the brightness variation becomes greater, but the size remains that of a single pixel. As you push the ISO in film, however, individual grains become much larger than the pixel size from the digital image at the same ISO, so the noise (grain) from seriously pushed film is more obvious than that from the sensors at higher ISOs. Yes, you can see the noise when you push a Nikon D70 to 1600 ISO, but it is not nearly as obnoxious as the grain in film pushed to the same speed. Also, in digital images, there are various algorithms to reduce the noise. Some are built into the cameras (including that for correcting fixed-pattern noise), while other may be in the camera, or may be in the computer during post-processing. The D70 is not particularly aggressive in in-camera noise reduction processing, and (from what I have read) the Cannon is much more aggressive. One side effect is that the Cannon (at least the Digital Rebel cameras) appears to produce softer images, as sharpness is one of the casualties of aggressive noise reduction. I am rather glad that the Nikon leaves it to post-processing (in the computer), so I can make my own choices between sharpness and noise. I'm not sure how much of this noise reduction may be done in a Cannon camera in RAW mode. I do know that this last 4th of July, I opted to shoot with the in-camera long exposure noise reduction turned off, so I was not stuck waiting for the same time as the last exposure to gather the noise reduction information before I could shoot again. For most images, it came out rather nice, but for some (when I held the shutter open for longer than I expected, thanks to delays in the launching of the next fireworks), I wound up with serious fixed-pattern noise in those images. To see some selected examples, visit the following URL: http://www2.d-and-d.com/EXAMPLES/july-4th/index.html I hope that this helps, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
#15
|
|||
|
|||
DoN. Nichols wrote:
[] There are at least three sources of noise: 1) "Thermal" noise, which is a function of the resistance (impedance) that the sensor and circuitry up to the amplifier sections and the temperature. The higher the temperature, the more thermal noise, and the higher the gain (ISO) the more visible the noise from a given temperature will be. Far Infrared sensors are often operated at very cold temperatures, such as 70K (the temperature of liquid nitrogen). You've probably seen far IR images in the video from missile strikes from aircraft from the Gulf War footage. 2) The digital "clocking" noise (which is left-overs from the signals used to move the information from the sensors to the A/D converters. 3) "Fixed Pattern" noise, resulting from minute variations in the sizes of the sensors, and slight variations in the accuracy of the A/D converters. [] I hope that this helps, DoN Don, thanks for this, but you have missed one important noise contribution to the final image, which is photon-limited noise. In a nutshell, this is a characteristic of light which produces an uncertainly in the number of photons according to the square root of the number of photons collected. So if the sensor well can only hold 10,000 electrons, there will be an uncertainty of 100 electrons, and hence a best possible "signal-to-noise ratio" of 100:1 at each pixel. Hence the desire for a large well size to maximise SNR, and hence larger area sensors, and lower ISO settings. Cheers, David |
#16
|
|||
|
|||
In article ,
David J Taylor wrote: DoN. Nichols wrote: [] There are at least three sources of noise: 1) "Thermal" noise, which is a function of the resistance [ ... ] 2) The digital "clocking" noise (which is left-overs from the signals [ ... ] 3) "Fixed Pattern" noise, resulting from minute variations in the [ ... ] Don, thanks for this, but you have missed one important noise contribution to the final image, which is photon-limited noise. In a nutshell, this is a characteristic of light which produces an uncertainly in the number of photons according to the square root of the number of photons collected. So if the sensor well can only hold 10,000 electrons, there will be an uncertainty of 100 electrons, and hence a best possible "signal-to-noise ratio" of 100:1 at each pixel. Hence the desire for a large well size to maximise SNR, and hence larger area sensors, and lower ISO settings. You have a good point -- though I'm not sure that I would class this with "noise". It is at least an "uncertainty". And the fewer the electrons, the more effect the uncertainty has. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
#17
|
|||
|
|||
|
#18
|
|||
|
|||
In article ,
wrote: On 12 Jul 2005 20:11:12 -0400, (DoN. Nichols) wrote: [ ... all snipped at this point ... ] The information you gave is far beyond I would've been able to gather myself. The level of your understanding is amazing and you have my greatest appreciation for the willingless to share. Well ... it helps that I used to work with IR the development and testing of IR imaging sensors, so I had plenty of time to absorb the information. It was just a matter of re-describing them in terms of visible light cameras. Thank you all. You're welcome, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
#19
|
|||
|
|||
DoN. Nichols wrote:
In article , David J Taylor [] Don, thanks for this, but you have missed one important noise contribution to the final image, which is photon-limited noise. In a nutshell, this is a characteristic of light which produces an uncertainly in the number of photons according to the square root of the number of photons collected. So if the sensor well can only hold 10,000 electrons, there will be an uncertainty of 100 electrons, and hence a best possible "signal-to-noise ratio" of 100:1 at each pixel. Hence the desire for a large well size to maximise SNR, and hence larger area sensors, and lower ISO settings. You have a good point -- though I'm not sure that I would class this with "noise". It is at least an "uncertainty". And the fewer the electrons, the more effect the uncertainty has. DoN, it's noise in the sense that if you photograph a uniform field, the value of each pixel within the field will not be a single value, but will have the photon-noise uncertainty superimposed on top of the nominal value. Thus is looks just like the other noise sources you describe. I can't remember now if the statistics of the noise are Gaussian (Poisson rings a bell), and whether the square root is the RMS value of the noise. I also used to work with IR sensors, by the way, cooled to 77K as I recall. I modelled the entire vision system - target, atmosphere, sensor, monitor, eyeball, brain. Cheers, David |
|
Thread Tools | |
Display Modes | |
|
|