STEVE PEARCE PHOTOGRAPHY

One of the great freedom's that changing to digital brings, is the ability to change ISO 'film' sensitivity from one frame to the next.  Having worked previously with an EOS3, changing films mid roll was always a pain and I regularly got the frame number one out and ended up with a double exposure or a wasted frame.

In this respect, changing to digital is very liberating and you feel that you have new freedom to quickly deal with changing situations.  With CMOS sensors such as the 10D, sensor noise is not such an issue, up to about ISO400.  At 800 the noise starts to become evident at full magnification and at 1600 and 3200 it becomes quite disturbing without extra processing such as neat image.

When first using the 10D, I found myself simply turning up the ISO when faced with a dark scene, or when using a longer lens handheld.  This works of course, but the quality of the resulting image is more dependant on post processing through neat image rather than the camera.  A better method I have discovered, involves simply underexposing the image one or two stops.  I generally err on the side of caution with the 10D and underexpose anyway - so as to not blow out the highlights (which are then unrecoverable), but taking this idea further can gain you the stop or two you need without turning up the sensitivity to high noise levels.

For example, if I shoot a scene using a 50mm lens (so with 1.6x focal length = 80mm on a 10D) I need a shutter speed of 1/125th to ensure a blur free image when hand holding (assuming for simplicity I can only choose full shutter speed steps).

If the camera is showing that at ISO100, using a minimum aperture (let's assume f4) a 1/8 second exposure will be needed, I could up the ISO to 1600 to give me 4 extra stops and the 1/125th shutter speed I need.

This is going to give me quite a noisy image of course.

A more attractive possibility is to make sure you are shooting in RAW mode, up the ISO to 400 (which is about the limit I find the 10D can work at without the noise becoming very obvious), then shoot the scene two further stops underexposed which will again result in the four stop gain needed to shoot at 1/125th.

The underexposed RAW image can be brought back to life in capture one (or at a push using Canon's software) resulting in a correctly exposed image exhibiting only the minimal ISO400 noise levels!  Not only has digital given us the freedom of quick ISO changes, but we additionally have the benefit of RAW which provides another 2 free stops!  (For a better idea of what capture one can do, see my review here).

Correct Exposure at f4:

It's all quite obvious really, but is nonetheless a new possibility brought by digital which needs some consideration until it becomes second nature when faced with situations in which it will be useful!

Here are a couple of examples where I put this into practice to get images which would have been of a lesser quality with higher ISO's:

This is a shot of St.Stephens dome in Budapest.  The light levels and contrast were extremely low in the Basilica, but by using ISO400 and shooting at 2 stops underexposed, I was able to use a 1/30th shutter speed.  Just enough to get a sharp image.

You can see from these 100% enlargements of the top image that the noise is not too bad.  The quality of the image is certainly no more reduced by using ISO400 than it is by my ability to shoot a sharp image at1/30th of a second.

Lastly, here is a comparison with a shot at ISO1600.  You can see that at this sensitivity, noise is much more of an issue.  Clearly the two stop underexposure route gives a much better result than increasing sensitivity by the same amount.

At this size it looks good:

but at 100%:

So if you didn't get a 10D yet, now you have another excuse!

(Or get a 20D on which I hear the noise levels are about the same).

Words and images copyright Steve Pearce, 2004.