Are optical or electronic viewfinders better for shooting moving subjects? Bob Newman investigates

Prof Bob best for action

Any viewfinder delay introduces delay into the feedback loop by which a photographer tracks a subject

Proponents of traditional SLRs and the more recent mirrorless CSCs can discuss, seemingly endlessly, which type is more suited to different types of photography. One area where the DSLR seems still to maintain its lead, at least so far as press photographers are concerned, is sports and action photography. My own view is that camera choice is such a personal thing that talk of which is ‘better’ is rather futile, but at least it is possible to discuss advantages and disadvantages with respect to some technical design issues.

Contrary to some people’s opinion, the viewfinder in both types is non-operational while a photograph is being taken, the difference being that the DSLR’s viewfinder goes black, while the CSC’s continues to display the frame previous to the exposure. For a short exposure, this lapse may not even be noticeable so long as the mirrorless camera is using an electronic shutter. If it uses a conventional focal-plane shutter, the viewfinder freeze has to cover the closing of the shutter before the exposure and its opening after the exposure, which will usually be long enough to cause a hiccup in the action.

The traditional DSLR has a longer period of viewfinder inaction, since it also has to cover the flipping up and down of the reflex mirror, which might take a significant fraction of a second. On the other hand, the DSLR viewfinder relays the image formed to the photographer at the speed of light while the electronic one inevitably has a delay of one frame capture time. The difference is a delay of a few billionths of a second, to all intents and purposes zero, against a few hundredths of a second. This may seem insignificant but for the fact of the role of that delay when the photographer tries to track an erratically moving subject.

The combination of viewfinder, photographer’s eye, brain and hand/eye co-ordination, trying to keep the camera aimed correctly, forms what an engineer calls a ‘feedback loop’. To keep the camera directed at the subject, the photographer must register the change in its position in the viewfinder, his brain must calculate the amount of change and direct the muscles in his arms to move the camera to compensate.

Any engineer designing feedback loops knows that any delay within the loop is critical, and if excessive, can cause the loop to become unstable. It is at least possible that the additional delay due to the frame capture time in an EVF, although small, is enough to make such a viewfinder less suitable for capturing erratically moving subjects.