Putting drama into images with HDR effects

HDR: The HIGH DYNAMIC RANGE Filter

While we’re talking resolution and color range, let’s consider the High Dynamic Range (HDR) toning filter in Photoshop (found under the IMAGE menu). Every day someone invents something new that makes ever higher res images possible and audiences are getting to the point where they expect every photo to be dynamically detailed. HDR images are a hot trend that enables photographers to record a greater range of tonal detail and broader contrast while maintaining detail than a camera could capture in a single photo. This opens up a whole new set of lighting possibilities.

The new "merge to HDR" feature of Photoshop accomplishes this by combining a series of bracketed exposures into a single image, which encompasses the tonal detail of the entire series.

There is no free lunch however; trying to broaden the tonal range will inevitably come at the expense of decreased contrast in some tones. Learning to use HDR software can help you make the most of your dynamic range under tricky lighting — while still balancing this trade-off with contrast.

Single exposure.

With HDR toning.

THE DYNAMIC RANGE in Camera

As digital sensors attain progressively higher resolutions, and thereby successively smaller pixel sizes, the one quality of an image which does not benefit is its dynamic range. This is particularly apparent in modern compact cameras with 8 or more megapixels, as these are more susceptible than ever to blown highlights or noisy shadow detail. Furthermore, some scenes simply contain a greater brightness range than can be captured by current digital cameras — of any type.

The "bright side" is that nearly any camera can actually capture a vast dynamic range — just not in a single photo. By varying the shutter speed alone, most digital cameras can change how much light they let in by a factor of 50,000 or more. High dynamic range imaging utilizes this characteristic by creating images composed of multiple exposures.

INNER WORKINGS OF AN HDR FILE

Photoshop creates an HDR file by using the EXIF information from each of your bracketed images to determine their shutter speed, aperture and ISO settings. This tells Photoshop how much light actually came from each image region. Since this light may vary greatly in its intensity, Photoshop creates the HDR file using 32-bits to describe each color channel.

HDR files use these extra bits to create a relatively open-ended brightness scale, which can adjust to fit the needs of your image. However, the key benefit is that these extra bits are also used more efficiently than typical 8 or 16-bit images (which we'll refer to as "low dynamic range" or LDR images from here on out).

The 32-bit HDR file format is able to record a greater dynamic range using floating point numbers. A floating point number is composed of a decimal number between 1 and 10 multiplied by some power of 10, such as 5.467x10³, as opposed to the usual integers of 0-255 (for 8-bit) or 0-65535 (for 16-bit). This way, an image file can specify a brightness of 4,300,000,000 simply as 4.3x10⁹, which is more concise and uses less memory in coding. But how does this help a computer? Why not just keep adding more bits to specify successively larger numbers, and therefore a larger dynamic range? It's a problem of diminishing returns. As more bits are added to ordinary LDR files, an exponentially greater fraction is just used to specify color more precisely — not to extend dynamic range. This results in far more bits being used to describe the darker tones than the lighter ones:

HDR files get around this LDR dilemma by using tonal values proportional to the actual brightness of the subject matter. This causes bits to be equally spaced throughout the dynamic range — allowing for greater bit efficiency. Floating point numbers also ensure that all tones are recorded with the same relative precision.

All of these extra bits provided by the HDR format are great, and effectively allow for a nearly infinite brightness range to be recorded. However, your computer display and prints can only show a fixed, narrow range. Useful tutorials therefore not only focus on creating 32-bit HDR files, but also on converting these into images which can be displayed on your computer, or will look great as a photographic print. This conversion step is commonly referred to as "tonal mapping."

Creating a true HDR image requires capturing a series of identically-positioned exposures, with a sturdy tripod. Take at least three exposures, although five or more is recommended for optimum accuracy. More exposures allow the HDR algorithm to better approximate how your camera translates light into digital values (a.k.a. the digital sensor's response curve) — creating a more even tonal distribution.

HDR images require relatively static subject matter, due to the necessity of several separate exposures.

CREATING A 32-BIT HDR FILE IN PHOTOSHOP

Many photoshop users like to get what they call an HDR “look” with ordinary tiff or raw files. You can use Adobe Photoshop to convert a sequence of exposures into a single image, which uses tonal mapping to approximate what we would see with our eye. First, you combine all exposures into a single 32-bit HDR file:

For a tutorial on accomplishing this, see:

http://www.cambridgeincolour.com/tutorials/high-dynamic-range.htm

How to get the HDR effect from one image

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When you’re faced with a subject that has a high dynamic range – that is, one that has high contrast, with both very bright highlights and very dark shadows – one technique you can use to capture the full tonal range is high dynamic range imaging. With Photoshop there is a simple way to get an HDR effect from just one raw format picture.

To produce an HDR effect, you typically shoot a range of bracketed exposures (find out how to use your camera’s auto-exposure bracketing) to capture detail across the tonal range, and if you want the image to retain the utmost quality then this is the best way to do it.

But while HDR is great for landscapes and cityscapes, you can’t use the technique for sports or other action shots, because if there’s any movement between exposures you won’t be able to align and blend the images effectively.

However, it is possible to produce an HDR effect from a single raw file, by producing three different versions of the same image – one ‘standard’ exposure, one ‘under’ and one ‘over’ – and then merging these images using the HDR filter.

Click here to see step-by-step how to create an HDR effect from just one picture.

Other Photoshop experimenters suggest using a variety of tools including the HDR toning filter under the image menu to bring up darks and lights without losing detail to give photos a cinematic appearance. Check out http://www.tipsquirrel.com/single-image-hdr-toning-in-photoshop-cs5/ and https://www.youtube.com/watch?v=Rd_rXkvTDBk

for a couple video demos.