October 14, 2011

Great Strategies Using Adobe Camera RAW To Remove Excess Noise With High ISO Exposures

I often shoot where it's difficult or impossible to use a tripod for twilight exposures. While the sensors of high-end digital cameras keep improving, most photographers prefer to use settings less than ISO 400 for the sharpest results and least noise. But when using small apertures for greater depth of field plus reduced ISO settings, tripods are typically required in many low light environments. Higher ISO settings might eliminate the need for a tripod but at the expense of increased image noise.

When shooting RAW exposures it's possible to reduce the effects of noise even with very high ISO settings of 1600 or 3200 by using the 2010 Adobe Camera RAW version, which has nine different adjustment sliders for noise and sharpness control.

To illustrate my noise reduction strategies I've chosen a hand-held twilight image of the Golden Temple (click the link below to view) in Amritsar, India, where no tripods are allowed. The image was exposed at ISO 3200 with a shutter speed of 1/15th second at an f/8 aperture and a focal length setting of 50mm using a Canon 24-135mm lens with built-in image stabilization. Normally a 50mm lens position would require a shutter speed not longer than 1/50th second to avoid camera shake but the image stabilization feature kept the image sharp even at 1/15th second hand-held.

Using ISO 3200 results in extreme digital noise but taking advantage of the full range of controls with Adobe Camera RAW I was able to dial in settings that resulted in the effect of a tack sharp, grainless image. I imported the RAW photo from the camera's CF card straight into Adobe Lightroom for Camera RAW adjustments prior to exporting the photo into Photoshop. For the sharpest result I exported three copies of the same image into Photoshop as separate layers with increasing amounts of luminance noise control applied separately to the highlights, mid-tones and shadows respectively.

Using a high luminance global setting across the entire photo would have eliminated noise in the shadows but at the expense of overall image sharpness. Using less noise reduction for the mid-tones and even less for the highlights kept the image looking sharp. The other eight Camera RAW slider settings listed below enabled me to finesse the final result to regain the effect of overall image sharpness with no noise.

Using feathered Color Range selections for highlights, mid-tones and shadows I created layer masks for each of the three layers in Photoshop--one that revealed only highlights with the luminance set at 30 and the sharpness set to 10, one with a layer mask that revealed only the mid-tones with the luminance value increased to 50 and the sharpness moved up to 20, and finally one copy with a layer mask that only revealed the shadows with the luminance set to an even higher value of 70 and the sharpness set at 30. These are my nine Camera RAW sharpness/noise control settings for the shadow areas of this image:

Sharpening: Amount = 30, Radius = 1.0, Detail = 100, Masking = 100

Noise Reduction: Luminance = 70, Detail = 100, Contrast = 100, Color = 100, Detail = 100

Typically, even with high ISO images, noise is hardly visible in the highlights so they don't need much luminance noise reduction. A bit more noise is seen in the mid-tones but the most is visible in the shadows where I've applied the highest luminance noise control. Every low light level image will require different amounts of correction since increased time exposure can also increase noise. Noise reduction must be tailored accordingly and viewed at 100% magnification for examination. The key is to use as little luminance noise reduction as possible so as not to de-sharpen the image. Using the least amount of luminance noise control in the highlights with slightly more in the mid-tones will keep the image looking sharp. Noise is most noticeable in the shadows where luminance noise reduction is most important. The darker areas of the image can handle more de-sharpening without noticeable degradation of the overall image sharpness.

Copyright Glen Allison ALL RIGHTS RESERVED

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