Monday, September 09, 2013

De-mystifying HDR

Everything I shoot with my Nikon D200 these days is photographed with the potential to create HDR (High Dynamic Range) images.
An HDR image is a composite of three or more versions of the same image with different exposure values. When I started shooting HDR, I set my camera to make three bracketed frames of my subject on high-speed continuous shooting – one properly exposed (according to the built-in light meter), one underexposed by 2EV (exposure value, aka f-stops), and one overexposed by the same ratio. The overexposed and underexposed frames capture details that are outside the tonal range of a “properly” exposed frame.
When combined with good HDR software – I use Photomatix Pro – you get an image with a much wider range of detail from the deep shadows to the bright cloud-strewn sky.
The problem with shooting multiple frames is that you have to hold the camera perfectly still. Even then, people or things that move create ghost images in your picture.
Here’s an example of ghost images – the cross-traffic in this HDR image of an approaching storm in Crawfordsville, Ind.
But I’d seen HDR images of people and things in motion that were perfectly sharp and came to the realization that I could simulate a variety of EV levels starting with a picture shot in camera RAW.
Most people set their cameras to generate a jpg file for each photo, which is adequate for most purposes. A jpg arbitrarily discards a lot of data that isn’t needed to produce an acceptable photograph.
When you set the camera to shoot RAW, the resulting files contain all of the data the camera is capable of recording, far outside the limited range of a jpg. The result is a substantially larger file – something on the order of 16 megabytes, versus about 5 megabytes for a jpg – that you can manipulate with much greater precision when you import it into Photoshop.
Here’s a RAW image viewed in Photoshop. You can create multiple jpg versions of it at varying EV levels by adjusting the EV settings in the circled box and clicking on Save Image. I typically make five jpgs with settings at –4EV, –2EV, 0EV, +2EV and +4EV.
strip of 5
Then I import those five jpg images into Photomatix Pro, which combines them and their wide range of detail and offers me several choices of HDR effects:
I choose one – in this case it was Surreal 2 – and the program creates a jpg ranging in size from 5 to 11 megabytes that I can open in Photoshop to crop and edit to suit my taste.
If I decide not to go the HDR route with a RAW image, all I have to do is click the Open Copy button at the bottom of the Photoshop window and I get a jpg version of the photo, just as if I had set my camera to shoot jpgs only.
The only drawback to shooting RAW versus jpg is the loss of memory card capacity. But my 16 gigabyte memory card still holds more than 800 RAW images – more than I am likely to shoot at any one time unless I’m shooting a wedding or a similarly protracted event.
The Nikon D200 and subsequent models gives you the option of shooting RAW or jpg, or both – creating a RAW and a jpg version of each picture.

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