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Tuesday, July 27, 2004


ISO On the Fly...

Mall Carousel Exit
At first glance, a digital camera seems to be just a "filmless" equivalent of a film camera. And that's true enough, but it's not the whole story.

In a digital camera, film is replaced with a sensor to capture an image, electronics that processes the image, and some digital memory in which to record the image. And that changes the whole ballgame.

For you see, that combination of sensor, electronics, and memory can be a whole lot more flexible that a roll of film.

Let's talk just about the sensor and electronics today, and only about one aspect of that combination - the ability to change ISO rating on the fly.

In the good old days, which for me ended just about one year ago, one roll of film corresponded to exactly one ISO rating. Oh sure, you could push and pull the processing to raise or lower the rating if absolutely necessary. So, for example, if you found yourself with ISO 100 film in a shooting situation that demanded ISO 400, you could have your film push-processed to get the job done. But that special processing applied to the whole roll of film, not to individual exposures.

What did we do if we needed different ISO ratings at roughly the same time? Well, we carried two or more camera bodies, each loaded with different film, that's all. So in my two beloved Praktica MTL-5 bodies last year, I might have one loaded with ISO 50 Fuji Velvia and another with ISO 400 Fuji Provia 400F. And boy, did I look professional!

Of course, I also schvitzed a lot from schlepping those two heavy cameras, and I hoped like hell that ISO 50 and 400 were in fact what I needed.

So what are things like today?

With my Canon Digital Rebel SLR, like most digital cameras, I can set an ISO rating for any given shot. Now is that really something that's all that useful, you ask.

Yes, it is. And last Friday, I had a chance to prove it to myself again.
Metro Composition
I was in Washington, DC for the afternoon on some business that took me just a block away from the Smithsonian Institution Building, "the Castle." After my work was over, I ambled around the Castle, taking photos all the time, and over to the Mall on my way back to the Metro station.

As I passed by the old Carousel on the Mall, I thought some shots with motion blur might be interesting. It was still quite sunny out, so I set the camera's ISO to 100. This let me stop down my lens to the point where I could get exposure times in the range of one-half to one-fifteenth second. Lots of shots later , I ended up with the image above. One-eight second turned out to give me just enough blur to say "motion" while leaving horses and riders somewhat recognizable.

At the other extreme, about ten minutes later, I was standing on the platform of the Smithsonian station of the DC Metro. I glanced back up the escalator for a moment, and noticed an elegant sweep of concrete around the top of the escalator and above it, the Metro's trademark waffled vaulted ceiling. It was a beautiful "found" composition, ready for me to isolate... but did I have enough light to take the shot? I cranked the ISO up to 1600 .... f/5.6 at 1/4 sec, my viewfinder said. Just as my train arrived at the platform, I braced myself, prayed, and took one shot. And here it is.

One camera, two successful shots from bright sunlight to underground twilight, with ISO setting five stops apart! Not bad for a day's work.

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