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10th Aug 2009

Low light photography is about more than High ISO

Since posting our Nikon D200, D90, D300 and D700 High ISO Comparison last week, we’ve had plenty of feedback. I touched on a very critical point in the comparison, which has sparked a lot of the feedback: it takes more than good high-ISO ability to make a great low-light camera.

Most ISO tests on the web are done in lab conditions, where everything is tightly controlled – lighting, focus (manual focusing to ensure sharp focus isn’t uncommon), lens stopped down a little for maximum sharpness etc: basically all the variables are tightly controlled, so the only thing you are really varying is the ISO (and shutter speed). This is great, it gives the reader a very good indication of the output of the camera at High-ISO under optimal conditions. However what it doesn’t do is tell you how well your camera is going to perform when you are shooting a concert, your kids school gym sports, or any other real world lighting condition – conditions that are typically far from optimal. If you are in a studio in a controlled environment you don’t care about high ISO anyway, you should be shooting at the base ISO and cranking up the lights instead.

To make a great low light camera, a clean image at a high ISO value is one part of the equation – a very important one, but just one part. The cameras metering and autofocus need to perform too – a camera is a complex system made up of a sensor, autofocus system, metering system, and a bunch of other stuff that works together to capture and store the image.

Going back to our test results, we shot wide open, because it is usually better to open up the lens before pushing ISO values too high, especially if you are using pro glass. Metering worked fine on all the cameras tested – the D200 came out a little darker, but that is not uncommon in our experience, I always find myself using the exposure comp button a little more on the D200 than on any other DSLR we’ve owned. However the difference in AF systems really became apparent.

As you’d expect, the Nikon D300 and D700 nailed the autofocus every time. The D200 with the same lens did OK; a few times it hunted back and forth, but always eventually found focus (if you are shooting indoor sports, it may cause you to miss the shot though, you may be better off anticipating the action and pre-focusing).

The disappointment however was the Nikon D90. To start with it just wouldn’t focus in the available light. The AF illuminator came on, but we had the lens hood on the 24-70mm. The lens hood is big enough to stop the light from the AF illuminator from falling on the target, so it just couldn’t get focus. Removing the hood solved the problem, and when the AF illuminator could light up the subject, it performed marginally better than the D200.

I’ve had problems with the AF on the D90 before. Late last year I was trying to get a shot of the full moon with a 200-400mm f/4 lens (a nice big, bright subject). The Nikon D90 simply wouldn’t focus, just sat there hunting back and forth going through the focal range, with its little AF illuminator light hopelessly glowing away. Switched over to the Nikon D300, virtually instantaneous focus lock.

Another problem I’ve had with the D90’s autofocus system revealed itself when we first got the camera last year. I tried taking some test shots of my then 3-year old daughter, who loves to pose for the camera. I pointed the camera at her, half pressed the shutter, and she’s standing there trying to put on her best smile while squinting and blinking. Took a few seconds to figure out that the AF illuminator was too bright (this was indoors in poor light), it took a while for here eyes to adjust. Less than ideal for those spur of the moment candid shots.

While the D90’s illuminator helps compensate for what is a poor (compared to the D300/D700) autofocus system over short distances, it doesn’t work over larger distances and introduces other potential issues. Even thought the Nikon D90 has a slight edge in high ISO performance over the Nikon D300, if you are shooting in a school gym, poorly lit church, or at a concert where flash is prohibited, the Nikon D300 makes a far better low light camera than the D90, and will give you a much higher keeper rate.

Bottom line, low light photography is about more than just high-ISO ability. The D300 and especially the D700 give you an amazing combination of high ISO performance mated to a truly great autofocus system. The D90 has the ISO ability to match or even slightly exceed the D300, but it’s badly let down by its AF system in low light conditions in comparison.

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