Question about GH2 DR

Tamir

Got my GH2 about a week ago and have been testing it with different hacks. I'm liking it, however, the dynamic range is a bit of a downer imo! After testing all the different color profiles, nostalgic seems to have the most latitude with everything set to -2. My tests were confirmed on this site http://www.dpreview.com/reviews/panasonicDMCGH2/page12.asp . It appears that GH2 has the least latitude of all the cameras tested. For me, to get the GH2 to bump up 3-4 steps (it's now rated at about 10 steps) on par with the canons with technicolors color profile would a BIG bonus. Just curious about the outlook of that happening? With that said, I want to give a big shout out to VK, in case I came across as ungrateful.

cbrandin

Here's a pretty good write-up I happened upon. He does a better job of explaining all this than I have - and it's specific to the GH2 to boot! he doesn't address pixel binning - but the rest of his explanations are quite good.

http://www.laesieworks.com/digicom/panasonic-lumix-gh2-noise.html

It looks like the best way to get another 1 - 1 1/2 stops of sensitivity would be to go to a Foveon sensor, at least theoretically. I wonder is anybody will do that...

Chris

cbrandin

It still comes down to how many photons hit a well in a given time - so that won't help for low light stuff. It could help with well light stuff, though.

Often people believe that better DR will improve badly lit scenes - generally it won't with modern cameras. The limiting factor with modern sensors is more the physics of light and sensor well size than anything else. When you are at that low a light level the only thing that will improve low light performance is a bigger sensor - whether you are binning several little pixels or using large ones. The greater the total area that the active sensor wells occupy the better. That, and of course using the lowest shutter speed possible. Lowering output resolution (e.g. going from 1080 to 720) will also give you DR (much like binning, or binning more pixels). I suppose downsizing in post would have a similar effect.

Chris

RRRR

Chris: going on a method like the one Vitaliy suggested (recording multiple exposures simultaneously) I guess it would be possible to virtually push the boundaries quite a bit into something like a HDR output? It would of course make more sense for manufacturers that use smaller sensors to develop this kind of technology..

cbrandin

@danyell

The issue isn't strictly sensor size - its pixel well size, so the tradeoff is resolution vs DR. A 16 megapixel image will have more detail than a 4 megapixel image and less DR given the same sensor size. A 4/3 video camera with a 4 megapixel sensor could achieve 15 stops, APS could achieve maybe around 16.5. I think that's approaching the theoretical limit.

If you look at the Clark Article (http://www.clarkvision.com/articles/does.pixel.size.matter/), almost exactly half way down you'll see a chart that plots dynamic range vs ISO vs pixel well size (pixel pitch). The dash lines are the theoretical maximums based on a perfect sensor. You'll see that the industry is getting fairly close to theoretically optimal limits.

a 12MP FF camera has a pixel pitch of about 8.5 uM. If you extrapolate on Clark's chart at ISO 160 that comes comes to a little over 14 stops - assuming the sensor is perfect. A 4/3 12MP camera will theoretically max out at one less stop. A 24MP camera FF camera will have the same spec as a 4/3 12 MP camera. Pixel binning has the same effect as having larger sensor wells (approximately). All this assumes one image capture (no HDR etc...). You can also improve DR by using lower ISO, or by combining multiple captures (same effect as lower ISO). My old Kodak SLR/n has an ISO 6 setting (it combines multiple images actually) that can produce an astronomical DR - better than any other camera I've seen. It's kind of a cheat, though.

Anyway, the upshot is that these camera sensors are getting close to theoretical perfection. The problem with lots of these specs is that they are not normalized - done at different ISOs, image sizes, etc... The physics of maximum dynamic range is incontrovertibly pixel well (virtual) size and available light (and indirectly ISO) limited. You could overlight (brighter than daylight) and do better than these specs indicate as they assume the brightest object is lit by the sun (at least I hope so) - they actually did this with some older movies.

Modern sensors typically have a noise level of 3-4 electrons per well - that means that they are able to differentiate light levels higher than that (photons are converted to electrons 1:1) - that's pretty impressive. There's not much room for improvement. Dark areas in poorly lit scenes involve 10's of photons per pixel.

There are so many different ways of doing these specs. True if it's Tuesday and you own a green umbrella you might get these high DR results. Under normal shooting conditions with a normal camera - nah.

Chris

cbrandin

@magnifice

You're right about pixel binning improving DR because the virtual pixel size is bigger.

DXO does some great analysis - but their numbers are absolutes, they don't factor S/N into their DR measurements. DPReview, on the other hand does - their DR measurements use either the lowest pixel value, or the level where low level detail is swamped with noise - whichever comes first. It's kind of a combination of DR and S/N - which translates to usable DR. Less technically correct, but more meaningful (IMO) if you simply want to know general sensor performance. The DXO numbers are more difficult to relate to final image quality for me. Look here:

http://www.imaging-resource.com/PRODS/D7000/D7000IMATEST.HTM

Down the page there is a chart that compares cameras. You'll notice that raw DR is nearly always considerably higher than JPEG DR (in the case of the D7000 it's 12 vs 9.7 stops). They do that because cramming too much DR into JPEG usually looks bad - and AVC compression is essentially very similar to JPEG. A few cameras do offer higher range JPEGs - but if you look at the result it doesn't look so good.

Now, if a camera provides pixel-binned raw - that is a different story. You can always trade off resolution for DR. The reason pixel binned high pixel count cameras can't quite reach the quality of bigger pixel wells (at least theoretically) is because the unused space between pixel wells (as a percentage of total sensor area) is higher with high pixel count sensors. Back light sensors largely mitigate this by essentially having no space between wells, but I don't know who is using those these days.

I've actually compared the low light and high ISO performance between a D3x and a D700. Specifications aside - the D700 does better under those circumstances (even if you downsample the D3x image to match the D700), requiring lower and less obtrusive NR. I haven't used a D7000, but I can see how performance could be similar if the D7000 sensor uses a higher percentage of silicon real estate for actual pixel wells, etc...

My original comment was that sensor performance is in large part related to pixel well size (or virtual pixel well size); and that, because of the physics involved, can't be improved much. Improvements have to come in the form of various tricks, like multi-image capture, pixel binning, etc... My other point was that if you map, say 14 stops, into 8-bit space the results won't look very good if any post processing is done. Also, raising ISO always decreases DR - again largely because of how many photons can make it into a pixel well space within a given time. If that weren't true how would you explain the effect of ISO on DR?

By the way, do you shoot exclusively at base ISO? If not, you'll never see the DR ranges you are talking about. Also, think about this: Many people prefer the look a GH2 gives, for example, at ISO 320 over ISO 160. Why is that? For sure it's not because ISO 320 has a higher DR than ISO160.

Chris

magnifico

many thanks to pana and to...VK :-)

danyyyel

@magnifico Yes I think the binning does it for the low light and DR. If you look at the dxomark test explanation, they do it toward a fix value of 8 megapixel. The d7000 at its 16 megapixel is about 13.5 going up to 13.9 in the 8 megapixel final reference size. The same is true for Noise performance.

I wrote a threat about the gh2 being king of low noise dslr (tittle change by vitaliy) with the hack, I thnk it is due to the down-scaling/binning that is narrowing the gap between the gh2 and the larger sensor Canon. In photo mode at 1to1 pixel the gh2 is clearly beaten in low light by the Canon and 5d mark 2 in particular, but in video with the binning and higher bitrate the outcome is different.

magnifico

I am not expert here. So what do you think about this :

My idea is that pixel binning in video afect, increase DR. It is like increasing pixel size.
So that pixel binning afects both S/N ration and DR for shadows.
You can not judge video DR of a camera only by analysing its DR performance for stills (DXo).

Reading danyyyel comfort me in these thoughts :-)

danyyyel

@cbrandin, I am no expert to discuss the size of the sensor relating to the dynamic range. From what I read it was the pixel size and the signal to noise ratio that determined the DR. There was a very interesting post here and someone summarized it as the fullwell capacity for the highlight and S/N ratio for the shadows. From a photo world perspective, dxomark has become more or less the defacto benchmarking standard in scientific/numerical/lab test. As technology has advanced even with smaller and smaller pixel the DR and low light has come up.

The d7000 and pentax k5 are considered the champion in DR even with there apsc sensor which is more recent than the D700. You said that your D700 has better DR than a D3x, but from DP review to DXOmark the 24 megapixel D3x beats the D700 12 megapixel handily. The D7000 and Pentax K5 have what some call ISO less sensor. That is you can shoot it at base ISO 100 and boost it in post to simulate ISO boost. They had pictures that where like completely black and people were boosting them in post and you could get usable image that where at least as good as the higher Iso ones because at 100 ISO the noise level is so low that you can like magically dig in them in underexposed images. At higher ISO it is a different matter.

The last example is the Alexa and even Sony F3 and possibly F65 that all have S35/apsc sensor size. The Alexa for example is widely regraded and tested as a 14 to 14.5 stop sensor. So personally I do not see a relation to sensor size as you said a full frame sensor could barely achieve 14 stop while the Apsc/S35 alexa is aleady exceeding it.

magnifico

@nomad
let ther be light and no bit to sensor :-)

magnifico

@cbrandin
thx for information :-)
So YUV model (SVC, JPEG) says DR for output is 8 stops (luminance).
I realised that I calculated luminance after rendering (3*2^8). 1 pixel=3 colors, 8 bist per color.

I did separate input an output and the calculation 3*2^8 was for output only.

I agree of course with your 3*2^12 bits for input. I just added 3 stops more (pixel binning :-).

DR is one thing, S/N ratio an other thing. And I like your analysis.

Since the subject of topic is GH2 DR, I rewrite here my conclusion :
"That means in fact that the impact of (input) DR for stills is not "so" important in video. "All" is in calculation "
It was adressed to OP and his:
http://www.dpreview.com/reviews/panasonicDMCGH2/page12.asp

nomad

Grin!

bwhitz

You're all forgetting one big thing here... the GH2 doesn't have XLR inputs. Which means that the dynamic range is only around 3-5 stops. Dynamic range is not about the S/N ratio of the sensor, nor the design. It's about the accessories the camera comes with and if it was intended to be a "video" camera or not. Only cameras with XLR inputs and built-in ND filters can hit the 9-10 stop range. ;)

nomad

Good explanation, thanks. Just one note: the sensor is analog, bit depth only applies to the A/D converter.

driftwood

@magnifico
GH2 Chrominance is 2x2, hence 4:2:0.

cbrandin

@magnifico

I think your math is theoretically correct except you haven't considered how data is rendered. Basically JPEG and AVC use a YUV model - that means luminance is a separate channel packed into 8 bits - which limits luminance to 2^8. In your bayer model there are four channels: 2xG, 1xR, and 1xB. In they YUV model there are three. Also, with AVC 4:2:0 the three channels aren't equal - the luma channel has 4x the resolution of the chroma channels. The math isn't strictly interchangeable.

The sensor isn't an 8-bit bayer array. I believe it is a 12-bit bayer array (maybe more), so the math of 3*2^8 doesn't apply, it would be 3*2^12. The bits aren't the limiting factor - the S/N ratio is and that is largely limited by sensor well size.

I think your analysis about pixel binning is true - which explains why the S/N ratio degrades in EXT mode. Actually it does still apply to still photos when they are down-sampled. True, pixel binning with video produces a better S/N ratio, which could improve what you are calling "input DR", output DR will still be stuck at 8 bits because the luma channel still limits luminance rendering. Now, you can increase the input DR, but the more you do that the worse the rendering looks - especially if there is much post processing. Like all things photographic it's a tradeoff - you trade DR for rendering smoothness. In fact, because of gamma - which skews rendering favoring dark areas - highlights can end up looking terrible if you try to bring them back in post because they are already so coarsely quantized.

Chris

magnifico

Hi all,
I am a rookie but try to understand by myself :-)
wikipedia:
http://en.wikipedia.org/wiki/Dynamic_range
"...Dynamic range, abbreviated DR or DNR,[1] is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 (decibel) or base-2 (doublings, bits or stops) logarithmic value..."

That means that our camera has 2 DR : one from input, second for output.
For output (video or stills), considering luminance, we have : 3 * 2^8. It is 8 + Ln(3)/Ln(2) = 9.6 stops.
It is all the same for all cameras ...

For input it depends of the camera. And input is important for the "piqué".
But for input, DR for stills is not the same that DR for video.
Considering gh2 : we can consider this ratio: 16 Mpix (full sensor)/2Mpix(video) = 8. That means that for 1 pixel, we have 8 surrounding pixels ( group of photosites) there participating for calculation of luminance. That is 3 théorical stops more here.

Considering this process for video:
input DR -> calculation (matrix bayer+compress+etc.) ->output DR.
The 3 stops in addition give "more importance" to quality of calculation, because it is like input is already over satutated with information considering "poor" output DR. That means in fact that the impact of input DR for stills is not "so" important in video. "All" is in calculation :-)

I dont know if all this is already known or...false.
Sorry if I did not explain well.

olegkalyan

bwhitz look forward to your posting, please post a link here!

brianluce

Interesting to note that the EX3 still sells full price. $8300,
http://www.bhphotovideo.com/bnh/controller/home?is=REG&sku=758893&Q=&O=&A=details

bwhitz

@brianluce

I'll just post them in sample topic thread then. Basically it's just what one would expect... EX3 is a bit softer with 4x the noise.

One of the shots is a really good example, where some bricks just turn into fuzzy noise on the EX... but are cleanly resolved on the GH2.

brianluce

@bwhitz
I'm a former EX3 user, I'd be interested to see your results.

bwhitz

@Vitaliy__Kiselev

"We recently had troll going about GH2 resolution, hope it is not the case."

Speaking of this... I did do the EX3 vs GH2 test with some long shots of an ally in downtown LA. It wasn't good for the EX3... ;) I can upload some stills or a quick video if you want to re-open that thread...

or I can just put it in sample topic thread...

cbrandin

Do you suppose you could achieve sort of a temporal anti aliasing the same way? You would have to vibrate the sensor during the capture itself, of course. Gee, variable AA filtering - that could be pretty cool too!

There would be the constant low level squeaking to contend with... But that could easily be differentiated out of the audio in the camera.

nomad

Well, if it's a piezo doing the vibration, it could be controlled pretty well, they are used for similar purposes.

cbrandin

Yup. Seems that they could do it if the ultrasonic vibrator can be controlled finely enough. Even random shifts - if they are small enough - would work, although not as well as being able to do a controlled shift. In fact, I think that sufficiently small random shifts would be approximately 66% as effective as controlled ones given enough captures.

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