Quantum Efficiency Might Be More Important Than Image Resolution

What causes blur? It seems a simple enough question, but in pursuing the answer to this, I've ended up becoming a firm believer in quantum efficiency. So where did this journey begin?

When we talk about blur in images our immediate thought is usually related to the lens system. We might want blur for artistic effect, for example with bokeh, but when it's unwanted it's probably because we haven't focused the lens correctly. That problem is compounded when we have a small depth of field and can be particularly noticeable in portraits.

Is anyone in focus?

A second source of blur is from diffraction, when we stop the lens down. Whilst the depth of field might widen, diffraction causes the spot size formed on the focal plane to increase. This might not be as severe as blurring, but it does lead to a softening of the image.

The first two reasons are lens based, however the second two are related to movement. Most obviously, if we move the camera when we are taking an image then we are likely to blur the resulting photo. Photographers use this creatively with intentional camera movement (ICM), but usually it causes us problems! Unless the shutter speed is fast enough relative to the movement of the camera, blur occurs. This is the reason that many manufacturers incorporate in-body image stabilization (IBIS), with the class-leading cameras offering up to 6-stops of compensation. A tripod is the other solution.

The last problem is when the subject is moving relative to the camera. No amount of IBIS will solve the problem of a person moving when you are taking their photo. In this scenario a faster shutter speed is needed. The special case here, common in sports photography, is where you are trying to track a subject through your frame, such as a racing car. It's here that you need good panning technique which will allow you to keep the subject sharp (even though it's moving) whilst blurring the background.

It's a real problem when the camera and the subject is moving!

Technical Setup

So what techniques can you use to get tack sharp focus? Firstly, to get correct focus many pros will set their camera to a single focus point with continuous focusing, combined with back button focus. This ensures focus is where you want it and gives you ultimate control by decoupling focus from the shutter release. It's not ideal for all scenarios, but works well in most. Secondly, watch your f-stop. Your lens is likely sharpest at mid-apertures and, if you don't need to use them, avoid small apertures.

The two movement related problems both reduce to using a faster shutter speed. If you can't get correct exposure at the aperture and shutter speed of your choice, then boosting the ISO is the best option. I switch on Nikon's AutoISO, limiting it to 3200 with a minimum shutter speed based upon the lens I'm using.

Quantum Efficiency

What are the toughest situations to shoot in? Given that the currency of photography is light, then it's when this is in short supply. Low light is the bane of every photographer from gigs, to weddings, to astrophotography. They all involve scenarios where light can be limited, but I'd like to widen that to anything that is light limited for a given shutter speed. That can involve portraiture, sports, and street to name a few. Heck any genre can be light limited. Common solutions are to use faster lenses and higher ISOs and these work well. It pays to have that 50mm f/1.8 lens permanently in your bag for just such an occasion.

On an overcast day at f/9 and ISO1600 and the shutter speed drops to 1/60s.

However there is one more variable you can choose to target: quantum efficiency or, more simply, the sensitivity of your camera's sensor. It's measured as the percentage of photons hitting the sensor that produce an electron (electrons per photon). You only need to look at smartphones to see that, with every iteration of a sensor, quantum efficiency improves. When I qualitatively compared the noise levels of Nikon's 24MP D610 with my aging 12MP D700 they were about the same. The higher resolution of the D610 means that the sensor photosites are smaller, yet the improved quantum efficiency results in noise levels that are similar.

The alternative to upgrading to the latest and greatest camera is more simple. Increase the size of the photosites, which means reducing the resolution of the sensor relative to contemporary cameras. This is the approach Nikon takes with its top level pro cameras such as the D5. Contrast it's 21MP sensor (6.5 micron photosites) with that of the 42MP (4.3 micron photosites) D850 where the photosites are half the area. However it's Sony's Alpha 7S M2 that epitomizes this approach - its 12MP sensor has 8.4 micron photosites which makes it ideally suited to light limited photography.

Marginal Gains

Photography is often about marginal gains — a method or technology is improved that enables you to perform slightly better. For example, each iteration of IBIS has increased the ability to hand hold shots. As photographers we take gains in quantum efficiency for granted and it's impact is only reported indirectly through better noise levels and dynamic range. However it's benefits are felt much more widely and no more so than in those areas of photography that dip in to light limiting situations. These occur far more often than we might think, so the next time you are considering upgrading you camera body pause to think about quantum efficiency and the types of scenes you photograph. Might it actually be better to decrease the resolution of your new purchase? Remember, the number of pixels required for a job is more nuanced. You need less than you might think. Better quantum efficiency might well let you nail the shot and that's a whole lot easier than going back and shooting your job all over again.

Of course, there remains a fifth reason for blur: resolution! It's salutary to remember that you need enough pixels to cover your subject. When shooting a job that means trying to achieve it, where possible, in-camera. Where you can't, extra resolution will allow you crop in.

Lead image courtesy of Alexander Andrews via Unsplash, used under Creative Commons.