Focussing Digital SLRs At Night

In my experience focussing digital SLR cameras at night can be surprisingly difficult.   To an extent this is to be expected, night photography is a niche interest, and camera designers cannot be expected to cater specifically for our needs.   However, getting sharp digital images at night is possible with current kit, but it requires care and new technical assumptions, particularly concerning the accuracy of "infinity" focussing.  

1. Auto focussing.   Digital auto focus has come along way in recent years, but lack of light is clearly an issue for this technology.   In the real dark auto focus gives up, its role can often be resurrected if you have a bright torch which can be shone on the object you wish to focus on.

2. Focussing using the lens scale.   When I came to digital I assumed that this was the technique of choice, after all it worked so well with film, and I am predominantly a wide-angle photographer where depth of focus should be relatively forgiving.   However since I have started using digital seriously (and have been testing lenses by a wide range of different lens makers producing equipment for different camera mounts) I have been routinely staggered how inaccurate or ambiguous so many lens scales actually are, particularly in relation to the sacred "infinity" mark .   That is a great pity, because focussing by scale and the related technique of hyperfocal focussing are both basic and powerful photographic tools.  I have seen more than one modern lens by major manufacturers where there is no focussing scale whatsoever!  Admittedly these are budget models but the trend is clear.   Digital just isn't forgiving enough for this kind of sloppiness.

In terms of my own recent experiments an honourable mention goes to Tokina for their 12-24mm f4 AT-X Pro, a lens that I can focus accurately at infinity by scale without any thought at all.   Just goes to show its still possible, and not necessarily a problem inherent in digital imaging.

3. Focussing using "Live View" mode.   So far I found the situation a little depressing, and until the autumn of 2007 the focussing problem was seriously putting me off committing to digital.   However, the day has been saved (for me at least) by the new technology called "Live View".  

Using a DSLR with a Live View mode allows you to focus accurately using the large LCD panel on the back of the camera.   The viewfinder is ignored, you are focussing using the camera sensor itself, not a focussing screen.   The key point is that you can then magnify the image in the LCD screen to obtain perfect focus, in the case of my Canon 40D by a power of 10x.   Doesn't sound much but the effect is eerily reminiscent of focussing on the ground glass of a view camera with a powerful loupe, or focussing an astronomical telescope.    I routinely focus my wide and telephoto lenses with this mode, and find distant street lights and the brighter stars good focussing targets.   If your subject is quite close a good method is to place a small torch next to it and then focus on the light.

Live View, predictably, works best with fast lenses, not just because the image is brighter, but also because images from fast lenses appear to "snap" in and out of focus decisively.

[Incidentally in my opinion Live View is also a powerful way of quickly assessing lens quality.   Slightly defocusing a Live View image of a bright star is a very informative test, and will spot many common lens design and assembly problems in about 2 seconds flat.   Its the cheapest optical bench I know of].