Monday, July 17, 2017

HDR high dynamic range, problem avantages and photo, raw, DRO, video, game, science, computer, screen, camera, web platform, understanding SONY auto-HDR


I will focus on the example of the Sony alpha 77 and 65 (the first with built-in auto-HDR).

Tone mapping

The method of rendering an HDR image to a standard monitor or printing device is called tone mapping. This method reduces the overall contrast of an HDR image to facilitate display on devices or printouts with lower dynamic range (LDR), and can be applied to produce images with preserved local contrast (or exaggerated for artistic effect).

The main problem is the dynamic ranges of common devices:
Dynamic ranges of common devices
LCD9.5700:1 (250:1 – 1750:1)
Negative film (Kodak VISION3)138000:1
Human eye (static)10–141000:1 – 15000:1
High-end DSLR camera 14.828500:1
Human eye (dynamic)20

In our case (camera), the CMOS sensor and its electronic is limited. For example:
The dynamic range is typically limited by the readout process of the CMOS imager pixel. Techniques have been developed in the past to cope with this, usually by non-linear compression of the signal.
ams Sensors Belgium developed a new CMOS image sensor pixels that allows readout of a photodiode with a wide dynamic range, which maintains a linear response to light. After exposure, the photodiode is read out via two transfer gates to two sense nodes. Two signals are then read from each pixel. The first signal only reads charge transferred to the first sense node, with maximal gain. This sample is used for small charge packets and is read with with low read noise. The second sample reads the total charge transferred to both sense nodes, with a lower gain. Pixels with a read noise of 3.3 electrons and a full well charge of 100,000 electrons have been demonstrated, resulting in a linear dynamic range of 90 dB.

DRO, auto-HDR, Raw

In fact you have 5 possibilities:

  • DRO (how DRO works:
  • auto-HDR
  • shooting a bracket 3-5 exposures (well within the camera's reach) and RAW then doing the HDR work on the computer (function "Merge to HDR"),
  • research domain with you own software (macro photo, fluorescent photo, photo with very high contrast, 3D photo)
  • one Raw and manual or semi-automatic processing with specialized software (raw converters):

see Merged HDR with many Preset

HDR auto from Sony

in french: Le mode HDR automatique des Sony Alpha 450, 500 et 550 (2010)

Auto HDR of alpha 65 and 77 (2012)
This excellent post shows the difference of DRO, HDR, and raw.
The best is to select HDR with you own contrast decision.

In-camera HDR ("high dynamic range") is a different way to solve the problem. Like on-the-computer HDR, in-camera HDR starts with several different exposures of the same scene, then combines them into a single output file in which the well-exposed bright areas from one shot have been combined with the best-exposed dark areas from another, and the composite file has been adjusted to make things look natural. Sony's programmers have written programs that seem to do a very good job — sometimes — of combining the exposures. But a key factor in getting good results, is providing the processor with good source images. The new fixed-mirror (SLT) cameras from Sony are especially well suited to gathering the multiple exposures because, lacking a moving mirror, these cameras can take more shots per second than their traditional reflex (moving) mirror competitors.

Raw file, unprocessed. and clipping region  (no contrast)
The red is where the scene is brighter than the camera could capture (with these settings) 
and the blue is where the scene is too dark for the camera (with these settings) to retain detail.
It's when you turn on Lightroom's "show clipping" feature.

 Raw file, unprocessed.

 Sony auto-HDR
The HDR AUTO has preserved detail outside well 
but surrendered detail in the shadows.
It's the choice of Sony's programmers.
 Sony auto-HDR 
with knowing that the dynamic range of this scene 
was fairly extreme, I set HDR to its max (6 EV). 

The raw data file has a lot more latitude than a jpeg. To compare what I can get from the raw file with what Sony's in-camera DRO and HDR offer, I reshot the scene, saved the raw file, and processed it myself in Lightroom or other software.
I used an adjustment brush to bring the bright areas (the windows) down 1.5 stops, and a separate brush to bring the shadow areas up 1.5 stops. Even so, the result was pretty good. See the original RAW file at the beginning.

Morever, because in-camera HDR takes multiple exposures and then processes them, achieving a single HDR result in the camera takes about five or six seconds. And you simply can't use it if anything in the scene is moving quickly. Finally, the A65 or A77's processor saves the HDR file as a jpeg, by necessity. The HDR file is a composite, a processed result. There is no raw original of the HDR result.

And if you really want to hedge your bets, shoot RAW + JPEG with DRO AUTO enabled. You may find that the raw file is badly exposed but the jpeg is usable and you won't have to fuss with the raw file on the computer.
 sony auto-DRO
sony DRO LV5;
Sensing that there was at least a five to six stop gap between the darks
and the lights here, I then changed the DRO setting from Auto to "Lv5".

 The feature is called "Auto HDR" and it has seven possible options: levels 1-6 plus an option called, confusingly, "auto." Sounds tautological to say "put the Auto HDR feature on auto," but it's not.

But a word or two further about raw might be pertinent here.

There is never a question about shooting raw or not. We all shoot raw always, willy nilly. Raw is how the camera works. The question is simply, where does the raw data get processed? Your choices are

  • (a) let the little processor with the no-choice software in your camera do it and hope you're happy with the results, because you lose the chance to do it over; or 
  • (b) keep the raw data, then process and reprocess as many times you like on a full-blown computer, using as many different raw converters as you like or as many different programs as you can afford. If you put it this way, it's not hard to see that, if you really need to get it right, you're better off shooting raw and shooting a bracket 3-5 exposures (well within the camera's reach) then doing the HDR work on the computer.

That said, while in-camera HDR might not produce the best results possible, I will readily admit that is that its results are pretty darned good. I'm still not quite sold, but I do acknowledge that Sony's done a tremendous job here and with MFNR multi frame noise reduction.
HDR image has less noise in the shadows.
One of the downsides to shooting and processing HDR in-camera is that you have no control over the tone-curve applied. 

See also
The Complete Guide to Sony's Alpha 65 and 77 SLT Cameras B&W Edition Volume II:


The 'HDR' mode can be set to Auto, or can be manually set from 1 stop to 6 stops EV. This is the mode that takes 3 frames quickly, and merges them together in camera to deliver a single HDR merged photo.

This is different than bracketing, and manually taking your own set of photos to merge in software after the fact, which is what most other folks on the web are talking about. For this, you need to see what the maximum bracketing range is for the camera in the exposure bracketing mode...with most Sony cameras, it is indeed only + -0.7 EV. The A77 being an exception. Also of note: you don't HAVE to use bracketing mode to take manual HDR exposures to blend - if you set up the camera on a tripod and take a series of photos where you manually adjust the exposure by a stop or two at a time, you can take any EV range you want and any number of photos you want - bracketing is what people use when they are trying to eliminate motion blur between a series of shots from handheld action, as it takes 3 photos relatively quickly. Sony's HDR mode does the same, but also does the blending of the HDR in the camera so the output is a single, final HDR photo.

Page 129 of the A65 Handbook states, You cannot use the Auto HDR function on RAW images. And, when the exposure mode is set to

  • AUTO, 
  • AUTO+, 
  • Sweep Panaroma, 
  • 3D Sweep Panaroma, 
  • Continuous Advance Priority AE 
  • Scent Selection, 
  • when Multi Frame Noise Reduct. is selected...

you cannot select Auto HDR.

I think that the delay between the 3 photos is around 150ms (frequency 1.6 Hz; or "speed process"=1/6.6), "speed of all the shooting process"=1/3.3then be careful when you have a moving part in your photo. The sound between the 3 photos indicates this tempo...
After the computer processing is around 5s and your camera is blocked.

JPEG, RAW and high iso.

List of all cameras and Auto Exposure Bracketing option

Auto Exposure Bracketing Settings by Camera Model
list of all cameras
Camera Model Auto-bracketed frames Max EV step increment Max EV range in AEB Max burst rate
Sony Alpha A65 3 0.7 1.4 10 fps
Sony Alpha A77 3 or 5 3 (3 frames), 0.7 (5 frames) 6 12 fps

Many digital cameras include an Auto Exposure Bracketing (AEB) option. When AEB is selected, the camera automatically takes three or more shots, each at a different exposure.
Auto Exposure Bracketing is very useful for capturing high contrast scenes for HDR. However, AEB wasn't intended for HDR initially, but for ensuring that one of the shots taken is correctly exposed. This means that some camera models only offer a maximum of 1 EV spacing, or even less, in just three auto bracketed shots.
Unfortunately, three shots spaced by one EV are often not sufficient for capturing high contrast scenes.

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