The spec sheet on the Nokia N8 mentions ‘Active Hyper-focal Distance’ as the method used by it during video capture and unless you did some research on your own, I’m sure all of you are wondering how exactly does it work (I was). In part 2 of the making of the N8’s camera (part 1 was about photography), Damian Dinning explains exactly how that works, and why Nokia chose it over continuous auto focus and focus at the beginning of the exposure.
So instead of I saying the same thing in my own words, I thought it’d be better for you to read what the man had to say himself. It might seem long, but I assure you its more than worth a read.
‘The N8 uses a system we refer to as ‘Active Hyper-focal Distance’. Hyper-focal distance is a specific lens focus position which by using depth of field allows the greatest range from near to infinity for objects to appear in focus. The Nokia N8 uses a 28mm (35mm film equivalent) focal length Carl Zeiss lens. This provides us with huge depth of field. But by using the hyper focal distance means objects placed between roughly 60cm and infinity will appear sharply focused.
The reference to ‘Active’ relates to the nature of how we maintain this hyper-focal distance lens position. Most autofocus camera modules in camera phones rely to some degree on friction. The problem with this is that the lens which moves for focus adjustment can slip after being driven to the correct intended lens position. This isn’t normally a problem in the case of still photography, but in the case of video can create problems especially when recording with HD resolution. To combat this problem, for the first time we actively monitor the lens position and then adjust it if required. The result is simply, video that’s always in focus.
The alternatives today are as follows: 1. Focus at the beginning of the exposure and then lock it or 2. continuously focus during the video. Unfortunately both have disadvantages. The first means if your subject moves or you alter the composition it will be out of focus. The second means the image in screen needs to be always be in focus. However, maintaining this without seeing focus hunting is very difficult. Add to this, when your subject moves from it’s original position, or there is insufficient contrast or illumination it simply won’t be able to focus and unless you’re lucky, your video will be out of focus. Before I continue, yes our system does have the disadvantage of only being effective for subjects which are beyond 60cm. We felt however, this was a worthwhile trade-off given the benefit you get in return, always in focus video, regardless of where your subject is, how fast it moves, how much contrast there is or how much available light there is. And you’ll never see it focusing or hunting during filming either’.
So there you have it, film anything farther than about 60 cms and you’ll have great video. I personally don’t mind the trade-off but find myself wishing for continuous focus or even the ability to manually focus at the start, it would be great for filming all those unboxing videos. But Damain goes on to say something that gives me reason to be excited.
‘Of course the ultimate solution would be to combine both of these in some way. Earlier in development we trialled a continuous autofocus system for video, but concluded for the time being that the performance didn’t warrant its inclusion in the product at sales start. Having said that, we are continuing investigations to find a way to get the best of both worlds in the future. Let’s see…’
So if I interpret correctly, it is possible that continuous auto focus comes to the N8 via a firmware update later, the keywords being “didn’t warrant its inclusion in the product at sales start“. It is clear that Nokia’s really working hard to make the N8 the best camera phone on the market and if you take the time to go through the entire piece, you find a lot of insight into how the digital zoom works and how it is quite close to optical zoom when it comes to performance. The N8’s auto capture ability, with its dual microphones is also talked about.