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
|
|||
|
|||
David J Taylor wrote:
While we are talking about terms, I find JPS's use of the term "posterisation" confusing at best, and meaningless in a signal (or image) processing context. I think what he means may be "quantisation", the fact that an infinite range of analog values must be represented by a limited range of digital levels. Normally, sufficient digital levels are available and it is the accuracy of the analog signal which determines the signal-to-noise ratio of the system. However, if the quantisation steps are too large, it becomes the quantisation process itself which limits the signal-to-noise ratio. Yes, when a photo-geek starts talking about "posterization", they are referring to what anyone with a background in signal processing would call "quantization noise". http://en.wikipedia.org/wiki/Quantization_noise Many fields have a maddening "terminological thicket" to penetrate... |
#13
|
|||
|
|||
wrote:
If someone decides that "ISO 100 gives the best quality" and gets an image that utilizes only 1/16th of the RAW values available, they would have had a much better image if they had the camera set to ISO 1600 with the same aperture and shutter speed. I have a hard time saying that they "under-exposed" the image; it makes more sense to say that they under-digitized it (quantized it) by using too low of an ISO. The thing is, you're the only person I have *ever* seen talk in terms of "absolute exposure", or compare different ISOs at the same aperture and shutter speed. I'm not saying that's not a valid way to think about it, I just don't see how it could be useful to me. ISO 100 *does* give the best quality, as long as you don't underexpose it, and it's a given that a proper exposure at an elevated ISO rating is better than underexposing at ISO 100. Now, there is another angle to the whole thing, and that is the 12-bit A/D conversion. We've discussed previously how the dynamic range of current sensors is not limited by the sensor's capability, but rather by the A/D conversion. This presents an interesting situation. Imagine that a camera used 16-bit A/D conversion. Imagine that the extra range actually *did* use all of the data available from the sensor. You now have a situation where higher ISO settings are meaningless, and the camera would have to be marketed as (for example) ISO 100 with *no* higher settings. Imagine the outcry! The simple fact that higher ISO settings exist and are useful tells us that the A/D conversion is not using all of the data the sensor is providing. Higher ISOs are accomplished by amplifying the signal. If you can usefully amplify the signal to ISO 800, that means there was a signal there in the first place to amplify, one that *could* have been used at ISO 100, but was ignored at that setting. If no data from the sensor were ignored, there would be nothing left to amplify, and ISO 200 would just be ISO 100 with one stop less of range and no actual advantage whatsoever. That is, it *would* be better to underexpose at ISO 100 and then push it in processing, to avoid the amplification step. So, it seems that either 16-bit A/D conversion is more complicated to put into a camera than it sounds, or we are having our dynamic range artificially limited in order to allow camera manufacturers to say that their cameras can go to ISO 800 or whatever. Of course, if the sensor can provide more range than a 16-bit conversion would need, then there would still be room for higher ISO settings. -- Jeremy | |
#14
|
|||
|
|||
wrote:
I didn't say that the term exposure doesn't apply at all. I said that it wasn't a good way to describe the relative digitization at an ISO setting. And most photographers will say: "So what? Now let's meter the scene and set the exposure." You may be right about the term "digitization" but you draw strange looks becasue it is not the famillar term. And there really is nothing wrong with the term exposure. That word says is all: Time X aperture. This is what I'm talking about; this is what I think exposure really means (for a given subject intensity, of course). It is often used, however, for the relative brightness of an image converted with 0 exposure adjustment, which, IMO, is more appropriately called digitization. In engineering terms, sure. But few dedicated photographers are engineers. And most engineers who photograph as a hobby or interest do so as a pastime, not as an extension of their engineering life. Some nerds excepted (not all engineers are nerds, far from it in my experience). As to the 0 exposure adjustment, there is none, really. None of the sensors have an ISO 134.6767 (whatever) setting where they are unity gain. Few of the cameras record an ISO 100 (or other) sensitivity the same as their competitors (or even models from the same co.) You're raising a total non issue. Or at best an issue that is meaningful to you and very few others. Photographers make photography and think in photographic terms. I know engineers who paint as well, but they don't talk about "pigment carrying linum usitatissimum oils". They talk about oil paints. Cheers, Alan -- -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. |
#15
|
|||
|
|||
David J Taylor wrote:
An example from audio might be that when sounds are digitised to 8-bits rather than 16-bits, there is an added roughness to the sound. An error is introduced which depends on signal level. In an image, too few bits would show as a contouring effect where what should be a smooth transition of brightness levels instead appears as a finite series of perceptibly different brightness bands. I would call this effect quantisation errors, or more specifically errors due to using too few bits to represent the signal. Is this what you mean by posterisation? I brought up the issue of quantization noise many months (over a year?) ago on rpd. It seemed to sail over the heads of just about everyone. I too did some signal processing work 10 or so years ago synthesizing complex radar wave forms in realtime. Quantization noise was not a problem in the synthesis (16 bit DAC) but for the system under test with a very high dyncamic range, it was a serious issue when the SNR was very low. For photography, the quantization noise is the noise we typically see at high ISO settings in the shaddow areas of the image. Some liken this (erroneously) to film grain. However film grain has dimension across the image (x,y), as well as in color error (z), whereas quantization noise is dynamic (z) (color) only in digital cameras. Cheers, Alan. -- -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. |
#16
|
|||
|
|||
wrote:
The analog to slide film exposure is actually the analog exposure on the sensor; the ISO settings of the digital camera are like setting different ranges of exposure in a slide to be digitized by a scanner. Why then, do we call utilizing the specified range "exposure". Because when taking a picture, the sensor is exposed to light. When not taking the picture, the sensor isn't exposed to light, particularly when it comes to DSLRs. |
#17
|
|||
|
|||
Alan Browne wrote:
[] I brought up the issue of quantization noise many months (over a year?) ago on rpd. It seemed to sail over the heads of just about everyone. I too did some signal processing work 10 or so years ago synthesizing complex radar wave forms in realtime. Quantization noise was not a problem in the synthesis (16 bit DAC) but for the system under test with a very high dyncamic range, it was a serious issue when the SNR was very low. For photography, the quantization noise is the noise we typically see at high ISO settings in the shaddow areas of the image. Some liken this (erroneously) to film grain. However film grain has dimension across the image (x,y), as well as in color error (z), whereas quantization noise is dynamic (z) (color) only in digital cameras. Cheers, Alan. Thanks, Alan. I would be surprised if quantization noise were an issue at low SNR (i.e. high ISO settings) in a digital camera, though, as the signal is amplified before the ADC, so that the photon noise should swamp the quantisation noise. However, I haven't sat down and done the sums.... Cheers, David |
#18
|
|||
|
|||
In message ,
"David J Taylor" wrote: wrote: [] This is what I'm talking about; this is what I think exposure really means (for a given subject intensity, of course). It is often used, however, for the relative brightness of an image converted with 0 exposure adjustment, which, IMO, is more appropriately called digitization. In signal processing we might use the term "headroom". If the sound level a system could capture was 8, and the loudest sound to be recorded was 4, then we might say the headroom was 6dB (an engineering term for a factor of 2 in linear voltage or current terms). Similarly, if the maximum value from your image sensor is 4095, and the white level of a particular image were 2047, then you also have a factor of two headroom. Were any part of the image to exceed the maximum value - specular highlight for example - we would say the value was clipped. "Digitisation" is simply the process of converting analog to digital. Implicit in everything I say here about digitization is the quality factor; "well-digitized", "poorly digitized", etc. A RAW file with the highest value as 600 in a specular highlight is poorly digitized. A maximum of 2048 is not as well-digitized as a maximum of 3800. -- John P Sheehy |
#19
|
|||
|
|||
In message ,
"David J Taylor" wrote: While we are talking about terms, I find JPS's use of the term "posterisation" confusing at best, and meaningless in a signal (or image) processing context. I've understood "quantization" to be a specific kind of "posterization"; one where the values are integers or multiples of integers, whereas posterization can include things like 1 2 4 5 7 8 10 11, etc. "Quantization" would be clearer, I suppose, even if this is correct. I see the word posterized much more than I do quantized, nowadays, and assumed the former was more in common usage. People almost always refer to an image with too few color levels to represent smooth gradients as "posterized". -- John P Sheehy |
#20
|
|||
|
|||
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
COMM: Australia only- film prices | Karl | General Equipment For Sale | 1 | February 9th 05 01:25 AM |
What densities at which zones? | ~BitPump | Large Format Photography Equipment | 24 | August 13th 04 04:15 AM |
Kodak on Variable Film Development: NO! | Michael Scarpitti | In The Darkroom | 276 | August 12th 04 10:42 PM |
Digital Exposure Question -- Middle Gray vs Exposure At Highlights | MikeS | Digital Photography | 1 | June 24th 04 08:04 AM |
Develper for Delta-100 | Frank Pittel | In The Darkroom | 8 | March 1st 04 04:36 PM |