by Paul Schmitt
The digital photography has opened a new era in low light photography. In the film era, high ISO speeds meant grainy images. Not so now! Mike Goldstein and Paul Schmitt addressed the subject at our December meeting, first showing the increased noise in a scene as the ISO rose, and then using Lightroom to remove noise. Discussion re- vealed that many are unfamiliar with this and miss the opportunity to boost ISO enough to gain fast shutter speeds in low light. This is key to good subject sharpness. So, we will revisit this. The image at right has a wide range of tones from dead black to soft whites. Image noise is analogous to the static heard on a radio when the signal is weak. Dark tones have a weaker “photon signal” and are noisier.
Let’s look at the image noise in a small portion of this scene as the ISO steps up in a Nikon D300S camera. From left to right they are ISO200, 800, 1600 and 6400. (206×308 pixel samples).
Noise first becomes slightly apparent at ISO400, with it clearly growing in severity at ISO800. (Let’s use the highest ISO6400 so that the result is most apparent in the limited resolution of the present format.)
I’ve taken the ISO6400 sample into the Develop module in Lightroom5 as seen below:
Note the mottled texture on the borders of the black area. Noise! In the black, there are barely visible light speckles. Noise again. All are very visible on your computer screen. At the right, note the noise reduction sliders. The simplest approach is to advance that slider until the noise disappears but not so much as to soften details or real textures.
So, how does the image change as the slider is moved to the right? Compare the before on the left with the noise reduction slider at 37 on the right image.
Now, ISO 6400 is pretty extreme and only approached as a last resort. In your more typical usage there is more likely to be a good compromise at ISO800 or maybe ISO 1250 that allows a fast enough shutter speed to stop motion in the subject. The relation between ISO and noise depends upon the camera’s sensor and the internal noise reduction capability, so you need to test and learn how your images will react to increased ISO.
Be aware also of other software. Photoshop CS versions have noise reduction and third party software such as Nik Dfine (http://www.google.com/nikcollection/) work well, and often better than the above.