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Author Topic: Putting DR into perspective..  (Read 4306 times)

shadowblade

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Re: Putting DR into perspective..
« Reply #20 on: March 27, 2015, 12:09:17 pm »

Veiling glare contributes/adds mostly to the shadows where signal levels are low. Since the glare is a product of intra-lens and inter-lens element/group reflections (aggravated by dust and atmospheric deposits), it is not confined to the regions where light is (besides the lens receives all scene light everywhere on the lens before it is finally focused on the sensor).

If it were even, it wouldn't be a problem. If it added 200 photons to every photosite, it would greatly reduce the dynamic range of the scene and make it much easier to capture, and you could get it all back by adjusting levels/contrast in postprocessing.

Unfortunately it's not even across the frame and usually manifests itself as specific areas of lens flare. Fortunately, that can usually be blocked out with a hand.

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On the contrary, it works fine in most cases. It's often not the horizon line or other moving features that are contrasted with the brightest parts of the image. Most of the info is in a single shadow exposure shot, and only parts are in the ETTR highlight shot.

The usual culprit is branches/leaves on a tree sticking up above the horizon into the sky. When the camera is down low, long grass can also do it.

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Yes, photon shot noise gets reduced, but averaging also averaged read noise. It does it so well, that pattern noise will be better visible. That's where improved sensors (and/or black frame subtraction) will shine, that is by absence of pattern noise. The patterns become more noticeable because we humans are good at pattern recognition, even where there are none we see details (like shapes in clouds, or faces in moon rocks).

It averages the read noise and makes it smoother, but does it actually improve the DR?

For argument's sake, let's say the noise floor is 4 and the full well capacity is 16000. That's a 1:4000 contrast ratio. If you take 2 shots and average it, you have a total of 8 noise and 32000 maximum signal. That's still a 1:4000 ratio. Averaging it out will mean that the noise is much smoother and less noticeable (there will be more points closer to 8 noise in the combined file than there are points close to 4 noise in the single file) but it's still the same min-to-max ratio and, thus, the same DR.
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Bart_van_der_Wolf

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Re: Putting DR into perspective..
« Reply #21 on: March 27, 2015, 01:16:31 pm »

Unfortunately it's not even across the frame and usually manifests itself as specific areas of lens flare. Fortunately, that can usually be blocked out with a hand.

Just to make sure, there is a difference between flare (often colorful local reflected hotspots), and veiling glare. It's no just semantics. The veil is omni-present, not as strong everywhere but some of it is. The same happens as our eyes age and we develop some level of glaucoma. The image is not fully formed yet where it is diffused, so it acts as contrast reduction (worse where directly illuminated by a bright lightsource).

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The usual culprit is branches/leaves on a tree sticking up above the horizon into the sky. When the camera is down low, long grass can also do it.

Really, it is much more robust a method than what you give it credit for. Hans Kruse has also discovered that method and posted results in a number of threads. It is usually only small patches of the lightest areas that need to be blended in, and they only rarely coincide with moving detail. It can happen, but it's more rare that you suggest, it's the exception rather than the rule.

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It averages the read noise and makes it smoother, but does it actually improve the DR?

For argument's sake, let's say the noise floor is 4 and the full well capacity is 16000. That's a 1:4000 contrast ratio. If you take 2 shots and average it, you have a total of 8 noise and 32000 maximum signal. That's still a 1:4000 ratio.

That's (fortunately) not how it works. DR is defined as the number of photons at the saturation point, divided by the noise level at a low exposure or even no exposure level, just the read noise. What may seem like the full well capacity at 16000, actually took 4x as many photons if we shoot at base ISO (after all we want to avoid noise, we're not shooting action). Canon cameras can benefit from relatively lower read noise from boosting ISO a bit, but for the lowest noise they too should use base ISO if shutter speed is not an issue.

So that's 64000 photons for each shot we want to average, which stays 64000 on average then. The read noise of e.g. 8 (no photons, just standard deviation of noise) is reduced as we average more and more shots. Two shots have 1/Sqrt(2) of the noise so 8/Sqrt(2)=5.67, 8 shots would have 8/Sqrt(8)=2.8. So that would be log(64000/2.8)/log(2)=14.5 stops of DR, if we want to go through the trouble of averaging instead of blending (the best parts of) images.

Cheers,
Bart
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shadowblade

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Re: Putting DR into perspective..
« Reply #22 on: March 27, 2015, 02:07:10 pm »

Just to make sure, there is a difference between flare (often colorful local reflected hotspots), and veiling glare. It's no just semantics. The veil is omni-present, not as strong everywhere but some of it is. The same happens as our eyes age and we develop some level of glaucoma. The image is not fully formed yet where it is diffused, so it acts as contrast reduction (worse where directly illuminated by a bright lightsource).

From a mathematical point of view, even, sensor-wide glare isn't a problem and can even help you deal with limited dynamic range, so long as the sampling (i.e. bit depth) is great enough that you don't run into problems with posterisation.

Consider this, for argument's sake. You have a scene that, at 1s exposure, gives you 16384 photons in its brightest pixel, and 2 photons in its darkest, for a scene DR of 13 stops. Your sensor has a full well capacity of 18000 photons and a noise floor of 8 pixels, for a sensor DR of 11-and-a-bit stops. Naturally, you can't capture the entire scene in one shot.

Let's say that you have glare that adds 200 photons to each photosite. Your brightest pixel now receives 16584 photons and your darkest one 202 photons. The dynamic range of the scene, as seen by the sensor, is now around 6.5 stops - easily capturable by the sensor. Since your sensor has 14-bit output, the output is now distributed over around 16336 luminosity levels instead of 16535 levels - hardly a significant decrease in levels and unlikely to cause posterisation. This is because the brightest stop contains half the luminosity levels, the next brightest half of the remaining, and so on. The top six luminosity levels, therefore, contain 98.44% of the total levels available; the rest of the levels, the shadows, are all crammed into the remaining 1.46%.

Of course, glare isn't completely even across the frame, which is the problem. But this is considering the hypothetical perfect glare - it wouldn't actually be a problem.

Also, I think you mean cataracts rather than glaucoma.

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Really, it is much more robust a method than what you give it credit for. Hans Kruse has also discovered that method and posted results in a number of threads. It is usually only small patches of the lightest areas that need to be blended in, and they only rarely coincide with moving detail. It can happen, but it's more rare that you suggest, it's the exception rather than the rule.

It certainly doesn't happen in every frame. But, when it does happen (which, while not the majority of shots, is certainly common enough to cause problems), it's one of the most annoying things to try to deal with.

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That's (fortunately) not how it works. DR is defined as the number of photons at the saturation point, divided by the noise level at a low exposure or even no exposure level, just the read noise. What may seem like the full well capacity at 16000, actually took 4x as many photons if we shoot at base ISO (after all we want to avoid noise, we're not shooting action). Canon cameras can benefit from relatively lower read noise from boosting ISO a bit, but for the lowest noise they too should use base ISO if shutter speed is not an issue.

So that's 64000 photons for each shot we want to average, which stays 64000 on average then. The read noise of e.g. 8 (no photons, just standard deviation of noise) is reduced as we average more and more shots. Two shots have 1/Sqrt(2) of the noise so 8/Sqrt(2)=5.67, 8 shots would have 8/Sqrt(8)=2.8. So that would be log(64000/2.8)/log(2)=14.5 stops of DR, if we want to go through the trouble of averaging instead of blending (the best parts of) images.

Let's say one shot has a maximum of 16384 photons per photosite, with an average 8 photons added by electronic noise, with a distribution of 8 (i.e. the equivalent of 0-16 photons added per pixel). This puts the saturation point (16384) 11 stops above the noise floor (8). Let's just say that the distribution of noise is equal within that range - that is, the same number of pixels receive 1 'photon' of read noise as receives 6, 8 or 16 (in reality, it would approximate a normal distribution curve, but that would just complicate the mathematics and this will serve just as well for argument).

Now, let's say you averaged out 4 frames. You now have a maximum of 65536 photons per photosite. But you've also added an average of 32 photons of noise per photosite, with a distribution of 32 (although the actual distribution curve would be much tighter - there would be far more pixels close to 32 noise in the combined image than there would be pixels close to 8 noise in the single image and you'd have a bell-shaped curve rather than the equal distribution of the single frame; in an actual situation, where the distribution of read noise in the single frame is also a bell curve, you'd have a much tighter bell). Your ceiling is still only 11 stops above the average noise floor.

Of course, this all changes if you set the black point at the average noise floor, i.e.  produce the image based on 'white' being full well capacity, and 'black' being the noise floor. This would mean subtracting 8 from each image, or 32 from the four combined images. In other words, your scale would go from 0 to 16376 for a single image (with noise present from 0-8, with 50% of pixels receiving 0 and the rest evenly distributed between 1-8), or 0 to 65528 in the combined image (with noise present from 0-32, with 50% of pixels receiving 0 and the vast majority receiving just 1-8, with occasional pixels receiving more, due to the tighter bell curve). Therefore, the saturation point in the single image would be around 11 stops above the noise floor, while the saturation point in the combined image would be almost 13 stops above the noise floor, due to the tighter bell curve.

OK, I just shot that part of my own argument. But I was merely speculating whether there would actually be an improvement in DR - hadn't actually done the calculations to prove or disprove it, until forced to! Looks like it comes down to the fact that the 'zero' point is set at the average noise floor rather than an absolute 'zero' signal - when done that way, there is indeed an improvement in DR.
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Jack Hogan

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Re: Putting DR into perspective..
« Reply #23 on: March 27, 2015, 07:06:08 pm »

Consider this, for argument's sake. You have a scene that, at 1s exposure, gives you 16384 photons in its brightest pixel, and 2 photons in its darkest, for a scene DR of 13 stops. Your sensor has a full well capacity of 18000 photons and a noise floor of 8 pixels, for a sensor DR of 11-and-a-bit stops. Naturally, you can't capture the entire scene in one shot.

Not that it matters to the substance of the argument, but in the last couple of posts it would be more physically correct to talk about photoelectrons rather than photons.  If we want to talk about photons we need to take into consideration effective QE, which these days tends to be around 15-30%.

Jack
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Iliah

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Re: Putting DR into perspective..
« Reply #24 on: March 27, 2015, 07:49:20 pm »

The flare and glare tend to be non-uniform, and even with uniform field the response linearity is sacrificed and thus white balance becomes problematic. But yes, adding flare artificially can be used as one of last resorts. I use such filters in front of the lens (similar to Tiffen Ultra Contrast, but custom-made in Germany).
« Last Edit: March 27, 2015, 08:08:21 pm by Iliah »
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Phil Indeblanc

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Re: Putting DR into perspective..
« Reply #25 on: March 27, 2015, 08:09:56 pm »

Photons shmotons...where is that building?!

(nice Erik)
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ErikKaffehr

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Re: Putting DR into perspective..
« Reply #26 on: March 27, 2015, 10:54:29 pm »

:-)

47°24'8" N 16°25'30" E

:-) Erik :-)

Photons shmotons...where is that building?!

(nice Erik)
« Last Edit: March 28, 2015, 07:13:47 am by ErikKaffehr »
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Iliah

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Re: Putting DR into perspective..
« Reply #27 on: March 27, 2015, 11:12:21 pm »

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shadowblade

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Re: Putting DR into perspective..
« Reply #28 on: March 28, 2015, 10:42:41 am »

Not that it matters to the substance of the argument, but in the last couple of posts it would be more physically correct to talk about photoelectrons rather than photons.  If we want to talk about photons we need to take into consideration effective QE, which these days tends to be around 15-30%.

Jack

That's the term I was looking for - had a mind blank while I was writing it.

Although a recent development in solar cell technology, which allows one photon to produce two photoelectrons instead of just one, has the potential to increase this to 60%, making for better high-ISO performance.

Personally, I'd be more interested in increased well capacity, though.
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Bart_van_der_Wolf

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Re: Putting DR into perspective..
« Reply #29 on: March 28, 2015, 11:06:54 am »

Not that it matters to the substance of the argument, but in the last couple of posts it would be more physically correct to talk about photoelectrons rather than photons.

Yes, that would be more accurate, however ...

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If we want to talk about photons we need to take into consideration effective QE, which these days tends to be around 15-30%.

However, only photons that got converted into electrons are relevant for DR discussions. The QE has no bearing on DR performance as such, it just gives an idea about how the exposure times may differ to collect an average number of photons for conversion.

Cheers,
Bart
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Jack Hogan

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Re: Putting DR into perspective..
« Reply #30 on: March 28, 2015, 05:55:27 pm »

Yes, that would be more accurate, however ...

However, only photons that got converted into electrons are relevant for DR discussions. The QE has no bearing on DR performance as such, it just gives an idea about how the exposure times may differ to collect an average number of photons for conversion.

Right.  When camera DR is involved what counts is the ratio of the maximum to the minimum recordable signal as defined, both expressed in units of either output-referred raw values or input-referred physical units.  Physical units are photoelectrons, and this is what my comment was aimed at.

If one wants to relate DR to a specific Exposure, one needs effective Quantum Efficiency to be able to reach back into photons and photometric units.  For instance, what's the captured DR when we want the maximum signal to be 1 lux-second?  Unless your sensor saturates at 1 lx-s or higher it will be less than expected.  A bit more on this here.

Jack
« Last Edit: March 28, 2015, 06:00:37 pm by Jack Hogan »
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Ajoy Roy

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Re: Putting DR into perspective..
« Reply #31 on: March 31, 2015, 09:43:09 am »

Higher DR helps a lot if you cannot nail the exposure perfectly (or are a bit lazy like me). In my case with Nikon D3300 which has a DR of about 12, I can recover 1EV highlight and at least 3EV of shadows, before noise rears its head. For most of my daylight shots the DR of 12 is sufficient to recover all but deepest shadows, and a bit of blown highlights. This is in contrast with the older DSLR where the DR was much less and shadow noise was more, and you had to be more careful in exposure.

With full frame DR approaching 15 things become even easier. You can expose for the setting sun and recover rest of the scene perfectly.
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dwswager

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Re: Putting DR into perspective.. (part 2)
« Reply #32 on: March 31, 2015, 12:44:57 pm »

Hi,

This is another image, with wider dynamic range, first let's look at an HDR exposure (P45+ exposures from 1s to 30s), fused in Lumariver HDR.



The whole luminance range is impressive, perhaps 14 stops

Best regards
Erik

What this whole post shows though is that it is no only the DR, but the ability to recover highlights and shadows and how well.  Unfotunately, the cameras I have seen that have limited DR also present the shadow recovery problem. It is amazing to be as an amateur with limited opportunities for photography, when the DR is needed and when not.  Some situations are obvious and other again, are not. 

All I know is that I got up at 5:30am to take sunrise photos while with my kid at the coast for a softball tournament.  A cloud wall blocked the sun and there was no sun and no color at all, just a hazy grey.  So I spent my time doing some long exposures.  I was using 10 stops of ND and the exposure times were around 14 minutes.  I forgot to take into account the rapidly increasing light level.  At about 11 minutes it dawned on me and I closed the shutter.  Ended up with an image exposed to the right, but the D810 hadn't reached saturation yet!  Whew!
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barryfitzgerald

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Re: Putting DR into perspective..
« Reply #33 on: April 01, 2015, 06:20:57 pm »

I don't disagree with the op DR has improved massively over the last decade, on the other hand if I were shooting Canon I'd be pretty unhappy
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