HDR – what on earth?

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Another “debunking” blog, clearing up all that is “HDR”.

HDR

Just another technology acronym

HDR is just another of those technology acronyms that is going to appear on those horrid sticky labels stuck to gadgets in the shop, as if the manufacturers really beleive that consumers will rate a product as “better” if it has more confusing labels splatted all over it.

But in the case of HDR, however, that little label might be worth watching out for.

HDR stands for “High Dynamic Range”. To explain HDR, we have to use a technology that is more familiar – the film camera. If you used film cameras, you will have picked up the photos from the chemist, and perhaps wondered why some photos had nicely exposed foreground but with a completely white sky. Or why some photos correctly show a bright blue shy with clouds but the foreground was too dark. Yet, you think back to when you took the photo and remember a loverly bright day with blue sky with some clouds and vibrant green grass. Why didn’t the photos accurately portray that scene?

The answer lies in the “dynamic range” of the film. The film can record a correct exposure with some variance either side. But in the above example, the sky was significantly brighter that the foreground, so the film could correctly record either the bright sky or the less bright foreground. Simply, the dynamic range of the film could not accommodate the large difference in exposure levels. Photographers often call such scenes “contrasty” because they have a wide exposure contrast (very dark and very light).

Sadly, the modern camera sensor has the same limitation – it can only correctly expose an image with a certain amount of brightness variance.

But, now imagine if you could have taken two images of the scene – one image correctly exposed for the sky and the other correctly exposed for the foreground. Then, you combine the correctly exposed parts of both images and you end up with a photo that is much closer to the actual scene you remembered.

In traditional photography, often three images are created with varied exposure by altering the shutter speed (altering the aperture can change the depth of field between the three images). The images are then combined (later during processing) to produce the HDR image.

This is, in essence, what HDR is all about. You take multiple versions of the same image, with various exposure levels, and combine the correctly exposed parts of each image to give an image with a bigger exposure range.

Why does this matter?

For cameras

For many (probably most) photography situations you will be happy with the standard exposure range. But there will be times when you want a higher range because the image is so contrasty. HDR will help with that. Cameras with HDR features will usually take three frames and combine them. Some cameras offer a variant of HDR called “low light” mode where they take multiple identical frames at the same exposure (where each frame is under-exposed) and combine them to create the final frame. That’s a clever adaptation of the function.

But when you get the hang of it, HDR is not just an exposure compensation issue; For example, perhaps you want to freeze parts of a frame but allow other parts to blur.

HDR can also be used to produce eye-watering stunning images – images that the human eye did not see but would be awesome if they could! Some may suggest that such stunning HDR images might not be “real photos”, but that’s an argument for another place. Personally, I like great images whether they are “unrealistic” or not.

But there may be occasions where HDR needs to be avoided – for example, where you deliberately want to isolate parts of a scene by precise exposure control.

But it’s not all done in the camera. Many of the so-called unrealistic images are created later in the digital darkroom. Inbuilt camera functions are limited (although that’s really the right word to use as having HDR is not a limitation) to correcting issues related to exposure range. The more “create” uses of HDR are for post-photo manipulation time. Time well spent though!

For TVs

For TVs, it is not about creating unrealistic images of course. It is about adding realism and depth to the images. Although the HDR specification has many aspects, for the viewer, HDR is achieved in three main aspects:

  • The range of brightness that the TV can render
  • The minimum and maximum brightness levels for each pixel. The minimum must be below a specified luminosity level, and the maximum must be above a specific level
  • The range of colours that the screen can create, which must be greater than 1 billion (as a reference point, Blu-Ray can define around 16 million colours). The much higher range of colours and shading in HDR creates a more realistic presentation by reducing the possible difference of colours between adjacent pixels.

What about LCD/LED and OLED

You can read about this article about the difference between these two technologies – and the difference matters.
In a nutshell, OLED can produce much deeper blacks (actually total blacks) where as LCD can, at least at the moment, produce greater brightness levels. So, in theory, it might seem that LCD might be better for HDR. But in my view, OLED is a more modern, better and more future proof technology. It is more adaptable, more environmental friendly, and can make thinner displays, curved displays and even flexible (roll up!) displays.

At the time of writing, LCD/LED TVs outnumbered OLED (by some margin), but this is an area of fast-moving technology and so me at least – I would buy into OLED.

Ultra HD Premium

In order to clarify and better define the standards needed for the best viewing experience, the new Ultra HD Premium standard is the benchmark for TVs that display exceptionally high quality HDR content, but it goes beyond HDR.

Ultimately, though, this is a certification standard and some manufacturers may decide not to meet; as of time of writing, it’s unclear if Sony will certify, appearing to stick with their “4K HDR” label. Given that Sony is a key player in the UHD Alliance (who drives adoption of Ultra HD Premium) it might indicate some diversity or parting of ways in the industry.

For the moment, when looking at those horrid sticky labels on the TVs, make sure the “Ultra HD Premium” is on there. That one is not so horrid.

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