Digital Photography - The ASA Factor

Don Bradbury offers advice for squeezing performance from your digital camera.

Many amateur users of advanced digital cameras never stray beyond the standard settings. That would include the fully automatic exposure mode, automatic compensation for different types of lighting, automatic focusing, and also the standard 100 ASA CCD speed rating.

This latter variable is the equivalent of changing the film speed in a conventional camera. It gives you some control over the range of 'shutter' speeds and lens apertures available to you, and hence, for example, the freezing of subject movement or depth of field. But it also, generally, presents you with compromises in terms of image quality. Let me explain.

Film or digital

Just as with conventional film photography, if you load slow film you anticipate some restriction in the types of photography you can undertake, especially by way of lighting levels you can cope with. On the other hand, you expect better image quality. Conversely, if you pop in a fast film you can cope better with adverse lighting levels but expect to have to tolerate some loss of quality in the image.

In traditional photography, that's typically more visible grain, worsening colour balance, reduced colour saturation and image contrast, and so on. While recent years have seen considerable improvements in this direction in film production, they are still factors to consider for the serious photographer.

But in digital photography, you might be surprised just how inconsequential these effects can be on particular digital cameras. I'm not saying the final product can be just as good, whether you use a 25 ASA setting or a 400 ASA setting, but when your broadened photographic horizons are taken into account, some compromises may well be justified.

The evidence

I took a series of photographs with the Pentax EI-2000 digital SLR we reviewed recently, varying the ASA setting through the range 25, 100, to 400 ASA. On a consistently lit subject, with the camera on a tripod to hold everything steady (notably the subject area covered), and with manual control of the camera engaged, and the lens zoomed out to the maximum to reduce depth of field, the following exposure settings were suggested by the metering system:

 

ASA	Shutter speed @ min aperture f17	Shutter speed @ max aperture f4
25	1 sec	1/20 sec
100	0.25 sec	1/90 sec
400	1/15 sec	1/350 sec

 

So you can see that a wide range of shutter speeds and lens apertures was opened up by simply adjusting the CCD sensitivity over the available range. Some digital cameras let you make ASA adjustments over an even wider range than this, but if I was, for example, shooting for maximum depth of field on this Pentax, ie minimum lens aperture, the range of shutter speed available for freezing subject movement ranged from 1 sec to 1/15th sec on this particular subject, or four stops worth of variation.

If I was in a moving subject situation, on the other hand, bumping the CCD speed up to 400 ASA gave me access to shutter speeds right up to 1/350th sec, a very significant factor, as you can see.

Now you may well be thinking, yes, but what about image quality? Well I won't bore you by reproducing all six exposures I made - even if our Editor would let me. I'll just show you the 1 second exposure and the 1/350th second exposure, that is, those produced by the opposite ends of the ASA and f-stop control ranges. Just click on the two thumbnails to see the full image - then click your <back> button to get back here.

The evidence

Taken at ASA 25,  1 sec @ f17	Taken at ASA 400,  1/350th sec @ f3.9

See the differences? Not very great, are they? Colour balance and saturation are well preserved, and all-in-all both images would probably be considered satisfactory. At high magnification, additional artefacts were apparent in the shots takes at high ASA settings, as should be expected, and there's also rather less shadow detail preserved (one element of what the film photographer calls 'reciprocity failure'), but in smaller size prints, no very apparent difference could be detected. So use the ASA variable when you have to, it'll give you greater control.

Note that under more adverse lighting conditions, with longer exposure times, you may well see greater variations of colour balance than here, but such compromises may be tolerable when the chips are down and it's a case of upping the ASA rating or missing the shot altogether. If your usual subject matter is appropriate, eg action shots in poor lighting, you might even consider making a faster ASA setting than the default your standard. Have a little think about that.

Flash photography

Upping the ASA rating can also save your bacon when operating at the extremes of flash range. You may be surprised at the digital camera's ability to preserve adequate subject illumination if you exceed this distance by a few feet, but when really pushed, I'd increase the ASA setting if I had to. You may even be out of auto-control flash range, but every little helps when the chips are down and it's a case of throwing everything at the subject, including CCD sensitivity - the ASA factor.

Final words.

Not all digital cameras let you get away with increasing the ASA rating of the CCD so apparently penalty-free. On some models I've tried, the images were noticeably grainy at high ASA settings. Best to try this variable, if you can, before buying. The avoidance of introducing excessive 'noise' into images with increase in ASA setting is, after all, as much of an art as is avoiding image compression artefacts. Some manufacturers are better at it than others, and success is not always price related.

I've said nothing so far about reducing the ASA setting below the default (where the option is offered). That can, on some models, produce improved colour rendition, though it's rare in my experience. Give it a whirl sometime and see what difference you think it makes on your particular digital camera.

 

Don Bradbury