Tech Blog

[#1]

ISO (the sensor's light sensitivity) is sorta the 'pixie dust' of photography. It is not a mechanical process (such as shutter speed and aperture) and no longer a chemical process as when shooting with film. Now it is something as intangible as a pixel (whatever that really is) that catches a certain amount of light depending on the set ISO value.

The up-side

The up-side of ISO value is that even though it is not super-tangible, it adheres to the same simple principles of exposure (that determines how much light is let through the lens and captured by the sensor) as aperture and shutter speed.

↑ Exposing correctly.

To expose this scene properly we need a set amount of light. The photo was taken @ shutter speed 1/160 s, aperture f8 and ISO 400. If you change one of the three exposure settings, at least one of the other two will have to be adjusted to get the same exposure. Let's say you want more depth of field in your shot, so you set the aperture to f11 (equal to one stop of light), then you would either have to half the shutter speed (to 1/80 s) or double the ISO (to 800) to compensate for that one stop of light you took out by reducing the lens' aperture opening.

If you get in the habit of thinking in terms of stops of light (or f-stops), it soon becomes second nature and you can quickly make the necessary adjustments to get the right exposure.

The flip-side

Being able to adjust the light sensitivity of the sensor sounds great, but it's not a free lunch.

↓ Noise. Today's high-tech film graininess.

The fact of the matter is that any sensor at any ISO value will produce some amount of what's called noise. Again back in the good ole' film days this was a lot easier to grasp. If you shot with an ISO 200 film, it would have smaller grains (= less light sensitive) that to a large extent would not show up in your print. On the other hand, if you shot with an ISO 1600 film, the larger film grains (= more light sensitive) would show up in your final print making it grainy-looking.

So even though photography has moved into the digital age, you are still constrained by the correlation between light sensitivity of your 'film' (sensor) and the graininess of your images.

So when does ISO noise become a problem?

That's a tough question to answer, because it really depends on three facets: 1) The ISO-noise capabilities of your sensor; 2) the exposure level; and 3) your subjective opinion of how objectionable the noise is.

↑ Back alley beauty @ ISO 6400 (left) and ISO 25600 (right)

A good rule of thumb is that, the larger the sensor the better high ISO/low noise capabilities. New innovation though, is pulling in the other direction with Fuji's and Sony's outside-the-box approaches to sensor design. Properly exposing (even to the degree of slightly over-exposing) your image can also lower the apparent noise.

↓ Same scene slightly underexposed (left) and overexposed (right). Note the apparent noise difference between the two with the right image (overexposed) as a clear winner. 

↑ Histograms of same images. Note the bulk of the left (most noisy) image is to the left = underexposed.

So what can I do with a noisy image, chuck it?

Well, hang on to it just for now. There are some work-arounds that can turn an apparent too-noisy image into a usable image.

1 Give the noise reduction software built into your favorite image software a try.

↑ No noise reduction (top) and max noise reduction (bottom) in Adobe Lightroom. Beware that the noise reduction often will soften the image and lower the contrast, but that can be corrected to some degree after applying the noise reduction.

2 Convert the image to black and white. Noise consists of two elements: chroma noise (color blotches) and luminance noise (brightness blotches). By converting the image to b/w you get rid of the chroma noise in one easy step. This is my secret weapon, so let's keep it between you and me.

↓ Same image in color and b/w (ISO 25600)

3 Think small. A noisy image printed small in 4x6" or shared in a small size online (600 px on longest length) will not appear as noisy as the same photo printed 8x10" or shared in 1000 px size.

Got more questions about ISO? Fire away.

For the technically interested: All images shot with Nikon D700 and Nikkor 105mm f2.8 VR Micro lens, which by the way was not very easy. I shot the images for this blog on a sunny afternoon making it really difficult finding scenes that called for a high ISO value. Plus the D700 is one of the best performing sensors to this date in terms of high ISO/low noise capabilities, so I really had to squeeze it hard to get some objectionable noise out of the camera. A know, a luxury problem.

If you're looking for above-average noise capabilities in a smaller form-factor, I recommend trying Fuji's F200 EXR pocket camera. It has one of the best performing sensors in its class.