Monday, 6 April 2009

On the virtual tripod

Wall art and lamp, London, March 2009

UPDATE 06/04/09: Added the missing figure

You may have seen already the new image stabilising testing at Imaging Resource (posted at TOP yesterday). Alongside the reviews is their white paper on testing methodology. Being of the geeky persuasion in these things, I read the whole thing. I've noticed some very interesting things, besides the straight results that I wanted to air.

Overall, I think it is a good effort. They've fairly well achieved the goal of repeatable, comparable results. Highlighting the statistical nature of such things is pretty important - no one will get perfectly sharp images every time when hand holding, IS or no. Being able to compensate for the human factor, in a manner that has meaning for real results is quite important in this context.

The second big thing to take away is the importance of focus in achieving good photographic results. They talk about it at length. The testing method completely eliminates focus problems but for real shooting, that will be an issue. reading through the rest of the white paper, it's pretty clear that photographer technique is (as to be expected) as important as any of the technical assistance, be that in AF or IS. We would all do well to keep that in mind.

The big thing that came to my mind, however, was rather technical and concerns type of motion. Something briefly mentioned but not addressed in the testing protocol. Let me explain.

For the purposes of a discussion on IS, there are three directions of movement that we are interested in: vertical translation, horizontal translation (left-right) and rotation about the lens axis. (I realise that vertical and horizontal motion isn't pure translation but for the purposes of this discussion, and practically, it can be approximated to such. It is also a little easier to conceptualise than rotation about the other axes.) The Imaging Resource results seem to lump the translational results and don't mention rotation at all.

Why is it important to distinguish? It goes to how stabilisation systems work, how photographers move and how one relates to the other. I reckon there are 4 major movement effects going on that cause blur: high frequency "jitter" vertically or horizontally due to holding the camera, low frequency sway due to how one stands (try standing perfectly still without any kind of movement), and last some rotation of the camera as the shutter is released due to the force applied to the button.
Mechanical stabilisation systems are really designed to only address only the first two. And with those, the relative effect of each will be determined by how steady the photographer is in general and how the camera is held. The swaying effect is unlikely to cause problems except for long exposures.
But what about rotation? How important is it and can anything be done about it? As to importance, I would rate it pretty highly given what I see in photographs. When friends ask me about blur in their photos, I reckon the biggest issue is rotation. It's fairly easy to spot: look across the frame and you'll see vertical blur that varies markedly from left to right. As stabilisation is normally designed to work vertically or horizontally, rotation won't get corrected. For lens-based systems it is impossible to correct. But for in-body stabilisers it could be possible. See the figures below. The first shows the sensor and its possible range of motion. The second and third show vertical and horizontal shifts, the fourth shows a possible rotational shift.
And in the great debate of which system is better, this isn't ever mentioned: in-body stabilisation has the potential to cure a mode of movement that in-lens cannot.

1 comment:

  1. You provided a rather nice argument here. In practise, it seems that the effectiviness of image stabilization varies a lot and depends on the user.

    Some days I feel that I can take handheld photographs in low light much better than on others. Last autumn I managed to take reasonable sharp shots even at 1/2 second exposures with the LX3 (24 mm equiv.), but not lately any more. Now even 1/10 second is sometimes challenging. Something has changed but I don't know what.


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