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LaesieWorks

Image Compression

The GH2 is a great photo/video camera, I love it, but it's not a "full frame" photo camera. Its micro four thirds image sensor size is about 1/4 of a full frame, and divided into about 15 mega pixels, so there will be noise, though much less than compact cameras with their tiny sensors. The M4/3 sensor is around 9 times larger than the 1/2.5" sensors typically used in compacts.

Every digital camera has noise, even on its lowest iso setting. This page shows what kind of noise is present in the GH2, in the absence of light. Please note that the images are filtered to make the noise clearly visible, this is NOT what you'll see on well exposed photos.

I wondered if the GH2 has one best ISO setting, and if there are certain ISO settings to avoid. You might expect the amount of noise to simply increase at higher ISO settings and longer exposure times, but that's not always the case.

Quick answers
- the GH2 is best at ISO 160,
- ISO 320 might be the "sweet setting". More sensitive, minimal loss of image quality.
- colors are good up to ISO 640
- sensitivity is useful up to ISO 1250
- use ISO 1600 to 3200 only if you have no better option
- avoid ISO 4000 to 12800
Further more: shoot in RAW, and don't under expose.
But judge yourself by looking at the test results on this page.

The GH2 image sensor
Max resolution: 4608 x 3456
Image ratios: 4:3, 3:2, 1:1, 16:9
Effective pixels: 16.1megapixels (the number of sensor pixel measurements used)
Sensor photo detectors: 18.3megapixels
Sensor size: 18.89 x 14.48 mm (2.73 cm2)
Pixel density: 5.9 MP/cm2
Sensor type: CMOS (Complementary Metal Oxide Semiconductor)

Pixel size
Larger sensor pixels have a better signal-to-noise ratio (less noise), more dynamic range and better colors. The size of the GH2's sensor pixels is 5.9 MP/cm2, which is pretty good. The bigger sensor pixels in full frame cameras are up to about 3 MP/cm2, and compact cameras usually have about 40 MP/cm2. (the lower the number of sensor pixels per square cm, the better the quality of the image pixels).

Missing pixels
Photodiodes are unable to tell the difference between different wavelengths of light. Therefore, a "mosaic" pattern of color filters, a color filter array (CFA), is positioned on top of the sensor to filter out the red, green, and blue components of light falling onto it.
Mosaic sensors with a GRGB Bayer Pattern CFA capture only 25% of the red and blue and just 50% of the green components of light. More than 66% of the light gathered by the lens is not used, wasted! The missing pixels in each color layer are estimated based on the values of the neighboring pixels and other color channels via the demosaicing algorithms in the camera. This is the case in almost all digital cameras.

Purple
Why is the average color purple, not neutral? Because 50% of the sensor pixels are sensitive for green, and just 25% for red and 25% for blue. Therefore the amount of noise in the green channel is less, and the resolution is higher. In the picture above, you can see that the green channel is much darker = less noise.

The effect of exposure time on noise
I expected the amount of noise to double each stop, but to my surprise there is no visible increase of noise from 1/4000s to 1/4s, but then suddenly BOOM at 1/2s the noise kicks in. Strange. So; don't worry about noise caused by a longer exposure time, up to a quarter of a second. This test is done at ISO 1250. Other ISO's may behave different.
The GH2 manuel says: when "long shtr nr" is set on, "The camera automatically removes noise that appears when the shutter speed becomes slower".
In the test above, it was not set off.

ISO 160-12800
The GH2 can offer 20 ISO ratings by applying some forms of amplification to the one signal coming off the sensor. This can be done by applying analog amplification to the signal before it hits the A/D converter, and by shifting the results after they have gone through the A/D converter. The first step is analog, and is probably not applied to every ISO step. So, while it may seem that there are 20 ISO settings, from an analog view there are less. The settings in between the analog outputs, are fabricated by the Analog-to-Digital converter. This is not a bad thing, unless it's done wrong, which is sometimes the case, causing "ISO settings to avoid". An alternative is to only shoot with the ideal ISO settings, and correct the exposure during RAW import. But what is the ideal ISO setting for your camera? It probably the setting with the most dynamic range.

Dynamic Range
How much information can be gathered from middle gray towards the shadows and highlights. As a general rule: the higher the ISO, the less dynamic range.
At ISO 160, the dynamic range of the GH2 is 10.8 stops (Ev), but at ISO 12800 it has droped down to only 4.7 stops.
160=10.8
200=10.7
400=9.8
800=9
1600=8
3200=7
6400=6)
12800=4.7

Software corrections
The GH2 writes RAW files that are lens corrected. Panasonic has integrated software correction of distortion and lateral chromatic aberration into the system. That is easy for the user, but all pixels become altered, and there's no way back. Every lens setting causes a different noise profile because of this, making it harder to remove the noise with dedicated noise reducing software. It is possible to shoot without a lens, with the body cap on (digg deep in the menu, there is a "shoot without lens" option).

About the test images
The most noise is present in blackest blacks. That's why these images are of black only. If this camera had no noise, all images would be 100% black.
All test photos below:
- RAW of course.
- 1/60sec, 16mm.
- Adobe camera RAW import; everything set on zerro/normal, except for the exposure which was set on +4 to make the noise painfully clear.
- In Photoshop used levels and brightness to improve visibility (same settings on all).

100% crops of all 20 ISO settings
Two pictures. The first shows all 3 channels, and the second picture only the Red + Blue channels, because the Green channel is actually a whole a different story, because there are twice as much green sensor pixels. I didn't please the values in the picture because that distracts. The ISO values are:
160, 200, 250, 320,
400, 500, 640, 800,
1000, 1250, 1600, 2000,
2500, 3200, 4000, 5000,
6400, 8000, 10000, 12800.

Next: the whole frame at all iso settings
Note: The horizontal lines are curved. The lens was at set at 16mm. This shows that the GH2 does a lens correction before writing the RAW file!
Note 2: Why are there horizontal lines??

From these 20, I conclude that the GH2 is good up to ISO 320, all right up to 640, useful up to iso 1250, in case of emergency up to 3200, and pretty useless from iso 4000 to 12800. This is of course not a real word test, but as black contains the most noise, it is a good indication.


The best ISO of the GH2
GH2 RAW is 12 bits, meaning that the digital values of lightness per color channel run from 1 up to 2^12, in decimal terms that is: from 0 up to 4095 (not 4096 because 0 is one combination also). The largest dynamic range the GH2 can produce, is 10.8 Ev. Let's keep it simple and safe by stating that the GH2 has a maximum dynamic range of 10 Ev, or 10 stops. 1 EV (Exposure value) is corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.

Each sensor pixel is a little photo-sensor which frees electrons as photons of light are absorbed. One sensor pixels is like a bucket. The bucket can be empty, giving the blackest black with a value of 1, or the bucket can be maximum full, giving the whitest white with a value of 4096. When you over expose, the sensor pixels receive more light than they can handle, and then instead of more information in the highlights you'll get washed-out areas of maximum white, "blown highlights", and all kinds of trouble in neighboring sensor pixels. The maximum number of electrons a sensor pixel can hold is known as "full well capacity".

Each type of image sensor has its own optimal exposure, and that determines its real sensitivity, its "base ISO" "native sensitivity" or "native ISO". But as an image sensor is a composition of 3 different sensor pixels (RG and B), there is no one perfect ISO setting. There is an ideal setting though, the best compromise. When you take a photograph at this ideal ISO number, you'll get the best quality exposure your camera can offer. It's no surprise that a higher ISO setting degrades the image quality, but did you know that a lower than native ISO setting also degrades the image quality?

So with the GH2 we have maximum 10.8 stops (Ev) of dynamic range, at ISO 160, the lowest setting. Note: most often, a scene has more than 10 stops of contrast. On a sunny day for example, 16 stops can be the case. That's one of two reasons why photographers are so obsessed with soft light.
Don't divide the 4096 combinations into 10 here, because each stop is half or double the amount of light. So, the range of values goes something like this:
Stop #10: from 2049 to 4096
Stop #9: from 1025 to 2048
Stop #8: from 513 to 1024
Stop #7: from 257 to 512
Stop #6: from 129 to 256
Stop #5: from 65 to 128
Stop #4: from 33 to 64
Stop #3: from 17 to 32
Stop #2: from 9 to 16
Stop #1: from 1 to 8
Note: this is why the highlights are of higher quality. Much more light = much more information.
I can image though, that in the RAW file, more bits are being used to describe the shadows, and less for the highlights.

Every image sensor has noise, which pollutes the darkest areas most. To get clean blacks, it is good to discard at least the first stop, leaving you with only nine stops dynamic range.

The range of color lightness values in the text above, can be kind of translated into photos. The maximum amount of photons a sensor can handle is 4096 per x surface area. This x is not equal to one sensor pixel, but the ratio can be compared.
At ISO 160, one sensor pixel can receive & process 4096 photons.
At ISO 320, one sensor pixel can receive 4096 photons, but process only the first 2048.
@ ISO 640, it can receive 4096 photons, but process only the first 1024.
@ ISO 1280, it can receive 4096 photons, but process only the first 512.
@ ISO 2560, it can receive 4096 photons, but process only the first 256.
@ ISO 5120, it can receive 4096 photons, but process only the first 128.
@ ISO 10240, it can receive 4096 photons, but process only the first 64.
@ ISO 20480, it can receive 4096 photons, but process only the first 32.
With so few photons at high ISOs, it's amazing that a camera can produce a photo of it, no wonder that the quality degrades. Shooting at ISO 10000 is like under exposing a photo 6 stops, and then pushing the limits of the RAW importer and photo editng software to its extremes.

But the GH2 has 20 ISO settings! How is that possible? The answer is that this is NOT really possible. Every digital camera has just one ideal ISO setting, the question is which. It's probably the setting with the highest dynamic range.

What tricks are used to generate all these different ISO settings?

ADC
During an exposure, electrons are stored at each sensor pixel until they are read out, sensed and transformed into an analog voltage by an amplifier, and next an Analog-to-Digital Converter converts the voltage into a digital number. At a higher ISO, you don't get more photons, nor does the sensitivity of the image-sensor really change. What does change is the amount of amplification given to the voltage from the image-sensor before it goes to the A/D converter, called "gain". Input * gain = output. The higher the gain, the higher your ISO seems to be... you'll just get a brighter picture, with less dynamic range and more noise!
In some cameras, the lowest ISO setting is less than its native sensitivity. A negative gain is used, causing a darker picture, with less dynamic range, and less noise. I don't think the GH2 has a negative gain. If it has, that would only be a bit at ISO 160.

Post ADC processing
Another trick is to alter the ADC's output. Bit shifting to make ISO adjustments, causing gaps in the histogram (before RAW conversion). The Lumix GH2 does a lot of corrections before writing the RAW file: lens corrections, which have their effect on noise and sharpness.

Not 1 native ISO, but a mixture of 3
Each color channel has its own native sensitivity, and its own gain to get the color balance right. In the GH2, Blue sensor pixels are most sensitive, then Green, then Red. Ratio R=0.43 G=0.5 B=0.56. But as there are twice as many sensor pixels sensitive for green, on average green sensitivity is double, with 50% less noise, and 50% sharper. But as the sensor pixels size is equal, the point of "full well capacity" stays the same.
So, there is a mixture of 3 different native sensitivities, not 1 perfect ISO setting :-(
When making monochrome reproductions of grayscale originals, it's best to only use the green channel, and also use a green filter in front of the lens, because the green sensor pixels are a little bit sensitive for the other colors.

Do you think the GH2 has one sweet setting?
ISO 160, 320, 640, and 1250 seem pretty good, which happen to be all multiples of the lowest setting 160. Some big amplifying gain clearly happens at ISO 1600 which makes me not wanna go higher than ISO 1250.
The Red and Blue channel have less noise at ISO 320 than at ISO 250. That doesn't necessarily proof that this is the best setting, it might just show that there is a transition between 250 and 320 where the 320 output is reduced a bit.

My advice
- ISO 160 will give the best image quality, but beware not to over expose.
- ISO 320 might be the sweet one. More sensitive, with a minimum loss of image quality.
- If you care for realistic colors, don't go beyond ISO 640.
- Sensitivity is useful up to ISO 1250.
- Use ISO 1600 to 3200 only if you have no better option.
- Avoid ISO 4000 to 12800, they're crap. In the GH2 you can set an ISO limit. Set it.
Further more:
- Always shoot in RAW. Tip: don't save a JPG, to safe memory and energy.
- Remember that a higher ISO has less dynamic range, and there's not much to start with.
- Don't under expose. Under exposing will cause a lot of noise.


I hope this helps you to find the optimal settings for your Lumix GH2. It of course is possible that not all GH2 bodies are the same, and that new firmware will change things. Optimal settings for photography might be a bit different relative to shooting a movie.
In this test I only looked at what the sensor does in darkness. If want to discover the image quality of your camera, it is best to set your camera on a tripod, use a constant light source and shoot one picture on every ISO setting.

This test is done on 2011-02-22.