UNDERSTANDING DIGITAL EXPOSURE:

Part 1: Exposure Reduction for Highlight Retention
Part 2: Digital Exposure & Noise
Part 3: When Blocked-up Shadows Aren’t Really
Part 4: Take a Balanced Approach to White Balance
Part 5: Dynamic Range
Part 6: Extending the Tonal Range
Part 7: What’s the Real Difference Between RAW and JPEG?

GENERAL:

1) COST-EFFECTIVE PHOTOGRAPHY
2) CONTROL THE RANGE OF FOCUS
3) IMAGE INTERPOLATION
4) LOSE THE DEAD SHOTS
5) SCANNING 35MM FILM
6) THE RAW vs. JPEG DEBATE…
7) IS PHOTOGRAPHY EASY?
8) MUST EVERYTHING HAVE AN ADOBE SLANT?
9) A CAMERA TO PAINT WITH
10) WHAT'S THIS COMPOSITION THING ABOUT?

Understanding Digital Exposure

2: DIGITAL EXPOSURE and NOISE

Thanks largely to a full-frame sensor (Canon EOS 5D with 50mm lens) and correct exposure, digital noise caused by ISO selection is barely visible at ISO 400 but might be a problem at ISO 1600. Smaller sensors increase the likelihood of noise (see below). Underexposure makes noise more noticeable. Shadow areas are particularly susceptible to noise so inappropriate ISO selection and software manipulation will make it look even worse. It makes sense therefore to shoot at the lowest ISO possible.

DSLR high ISO shadow noise, consumer sensor.

Compact sensor shadow noise at ISO 1600.

“The best method of dealing with noise is to avoid it in the first place. For this, a few things should be kept in mind. A DSLR should be used whenever possible. Point-and-shoot cameras have more noise. The lowest, practical ISO should be used. Underexposure should be avoided. In fact, for digital cameras, maximizing the exposure is often the best policy…” (Ron Bigelow).

When I come across very impressive images like Ron’s I can see what’s going on – tripods, lowest ISO settings or high quality slow films, filters, multiple images… That sort of thing. In this highly disciplined context underexposure is virtually ruled out. It’s a balanced and considered craft, but we don’t all shoot like that all the time. We may like to shoot from the hip, so to speak. So something has to give, otherwise potentially good shots will be ruined by blown highlights.

“Underexposure should be avoided.” At first this may seem to be at odds with this advice:

“…photographers should expose digital by biasing that exposure toward the highlights, common practice among slide shooters. Those who have worked with negative film and practice the tried-and-true maxim of exposing for the shadows and developing (or printing) for the highlights will find it simply does not work well with digital.

“…an image that might seem underexposed on preview in the camera’s monitor may have enough information to be useful when later ‘processed’ in an image-editing program. And digital sensors seem to outdo film in the amount of information they can gather in low light. This has been noticed in many situations and leads to the conclusion that digital sensors are much more light efficient in low light and/or handle underexposure in a better fashion than film” (George Schaub, Editorial Director of Shutterbug).

What are we to do? And at what point does a reduction in exposure become unhelpful underexposure? “Underexposure” is really a relative term.

It’s not always possible, but generally I check my histogram after a shot to see if any space is wasted to the right. I do that because I want to make the most of the tones in the scene. Spectacular highlights, like reflections, should be pure white, but as a general rule significant bright tones have to fall within the histogram’s range, otherwise the image will suffer and possibly be rejected. And digital sensors’ tendency to struggle with highlights means we may have to pull the exposure back further than a mid-tone reading would suggest.

But this in turn may plunge significant shadow detail into solid blackness. It's a bit like shooting slide film. What then is the solution? We can do whatever is possible to balance the tones before we shoot – polariser, graduated filter, shoot more than one image and use subtle yet powerful software techniques. Things like that.

But if we’re handholding and it's not possible to take a more methodical approach, we may have to reduce the exposure slightly and seek an answer in software.

Ironically, noise is much more likely in handy compact cameras, like Canon's G9 and G10. Light receptors (photosites) on smaller sensors are smaller too and more tightly packed than on larger sensors and this significantly increases the likelihood of noise.

ISO settings play a part as well. To get quality images from advanced compacts we must shoot at the lowest ISO possible. There are times when some will find the difference between ISO 80 and 100 unacceptable. To tell you the truth, I do! If your main camera for shooting quality images is an advanced compact like Canon's G-series models you may want to shoot RAW to avoid sharpened noise and to get slightly better highlight retention.

So in theory underexposure with one of these compacts is less desirable than with a consumer DSLR, or even better, a full-frame pro or prosumer DSLR. However, there will be situations when the very convenience of a compact will make noise far more likely simply because allowances will have to be made for the highlights. It’s odd then that more disciplined techniques are sometimes needed for cameras not often associated with them. As mentioned on the previous page, if you're using a compact camera it may be best to sacrifice some shadow detail to hold on to the more important brighter tones. Mid-tones can be carefully and selectively enhanced in software.

Slight DSLR underexposure at low ISO settings will be necessary in some situations (often when handholding the camera) to accommodate the brightest tones in the scene. In this context, targeting and sensibly boosting specific shadow detail will not result in visible noise. It’s always a wise approach if you’re planning on prints when the tonal range will be further compressed.

However, it’s a different story with advanced compacts like the G9, for example. In fact, you could say that it may well be worse with the 12 MP G9 given the small pixels that are packed onto its 1/1.7” sensor.

So noise levels hinge on camera types, sensor design, ISO choices and shooting restraints. The distribution of brighter tones in the scene may demand a slight exposure reduction and careful software fixes.

But we can’t overdo it. Reduced exposure will become damaging underexposure at some stage:

“Under-exposure always will increase noise, at least to a degree, and as underexposure increases, noise can increase dramatically. I’ve seen the same camera deliver a nearly noise-free image when exposed properly, yet when underexposed, the noise increases so much that you’d swear the ISO setting was changed” (Rob Sheppard).

“Nevertheless, slight underexposure is recommended to eliminate the risk of any highlight clipping, something those who have mastered slide film photography will recognize and appreciate” (Uwe Steinmueller).

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