The digital eye sees things differently

I recently attended a party with my son's playgroup.  After the party, one of the mothers generously gave everyone CDs of the photos her husband had taken with his new digital SLR (dSLR).  As I sat there scanning through the hundreds of photos, it slowly dawned on me that I would not have been able to take the equivalent photos with my digital camera.

The photos from the party were great.  The subjects, almost exclusively one year old babies, were in focus, caught in the moment of smiling, laughing, opening a present, stealing a cupcake, discovering a new toy.  The exposure was good, the colors were crisp, and the backgrounds were pleasingly fuzzy.  Like I said, I couldn't do this with my camera.

What's wrong with my camera.  The short answer is that it is about five years old.  Although has a pleasing feature set for an amateur photographer (also known as an enthusiast), including aperture priority, shutter priority, and manual exposure settings, manual focus, a zoom lens, a flash, a remote control, burst mode (with auto focus - which is increasingly uncommon these days), and spot metering, it is slow.

This is reason #1 why I couldn't have taken the photos; babies don't sit still for very long.  It's something I have been struggling with since my own baby started smiling.  It took weeks of trying before I was able to capture the moment for the grandparents to see.  So I started looking for a new camera (which is part of the reason the blog's been so quiet recently).  With everyone complaining about the slow speed of digital cameras, surely the designers had done something to improve it.

And they have.  I talked to some friends, asked strangers about the cameras they were using, found some reviews (if you're interested in really detailed reviews of digital cameras look at www.dpreview.com), and decided on one I liked.  It was smaller and lighter than my current camera, and it had a 12x optical zoom.  All I needed was to play with one a little to be sure it lived up to its hype, so I made a trip to the local camera shop and had a play.

It was peppy.  It was so much peppier than my current camera.  But my husband thought it could be better, so we asked to see a dSLR for comparison.  Actually, not any dSLR, but the same model used to take the photos at the party.  It was peppy.  It was very peppy.  And after taking a few photos with it, the compact camera felt just as slow the one I had at home.  As we walked out of the store, we concluded our next camera should be a dSLR.

Then we decided they were too expensive.

Since I was stuck with my old camera, I decided to revisit it's functions to make sure I was getting the most out of it.  After the shutter delay, my biggest gripe about my camera is the depth of field.  Imagine complaining about images being too focused, but that's what's been bugging me; I cannot take photos with a shallow depth of field.  This is reason #2 why I could not have taken the photos from the party with my camera.

Shallow depth of field: the subject is well focussed and everything else is blurry.

For those of you who aren't photography buffs, the depth of field of an image is the range of distances from the lens that appear crisp and focused.  It is commonly used to reduce the emphasis on parts of the image that are not relevant.  In the aforementioned pictures of the babies, it was the unfortunate background.  It is also commonly used to give a two dimensional image a three dimensional feel.

The traditional thought about depth of field is that it is inversely proportional to the size of the opening of the lens (aperture) so larger openings yield images with shallower depth of field and smaller openings produce images with more of the image in focus.  Aperture is measured as the ratio of the lens length to the diameter of the opening, therefore a longer lens has a larger opening than a shorter lens at the same aperture number.  The largest lens opening for a lens is always less than the length of the lens, and most lenses are three to five times as long as their largest aperture.

Using the aperture priority mode, I opened the lens as much as possible and took a few photos of my son.  Everything was in focus.  I tried getting closer, so the relative distance to the subject (my son) and the background was greater.  Everything was in focus.  I got even closer.  Now the background was slightly blurry, but still clearly distinguishable, and my son's face was starting to look distorted.  No matter what I did, I couldn't get a satisfyingly shallow depth of field.  Clearly there was more to it than just aperture.

The lens redirects scattered light onto the sensor, shown at left.  A shorter lens (top) makes a smaller image on the sensor, but a smaller sensor means the image can still be framed the same as with a larger lens and sensor combination (bottom).

When investigating digital cameras I came upon the term equivalent length, which means the length of lens on a 35mm camera that would produce the same field of view.  It's only natural that people would be interested in the equivalent length, since prior to the switch to digital cameras, nearly everyone used 35mm cameras and a large proportion of them had a sense of the field of view provided by a 28mm lens versus a 105mm lens.

One camera I looked at had a lens that was equivalent to 35mm at the wide end of its range, but it was actually only 6mm long.  This is accomplished by reducing the format size or sensor.  The image projected onto a small sensor does not need to be as large as the image projected onto a large sensor for the resulting picture to fill the same amount of the frame, so a smaller lens is used with a smaller sensor for equivalent framing. This camera's sensor was only 5.8mm across, or roughly 1/6 of 35mm.  But was this 6mm lens really equivalent to a 35mm lens?

I found a nice explanation of depth of field versus format size at www.wikipedia.com.  The smaller the sensor is, the deeper is the depth of field for a given aperture and equivalent lens length (where the camera is the same distance from the subject and the image is framed the same, assuming the same aspect ratio).

This is a really significant thing for photographers who want to use depth of field to their advantage to consider when selecting a camera.  My digital camera's sensor is 7.2mm across.  That means an image shot at 2 meters from the subject with a 35mm equivalent lens at F2.8 will have a total depth of field (from the near limit to the far limit of acceptable focus) of over 4 meters (according to the depth of field calculator found at www.dpreview.com), whereas with my film SLR it will be about 0.6 meters, and about a third of that with a 4"x5" format camera.  Let's put this into context, if the image is shot in my living room, the background would always be in focus on my digital camera, but nearly always fuzzy on my 35mm film camera.

So when you see a camera spec that states the lens length as 35-135mm equivalent, you should now know that it isn't.  As it turns out, only a few of the most expensive digital SLRs available on the market today have a sensor as large as 35mm film.  The vast majority of consumer digital cameras (as opposed to professional digital cameras) have sensors between 1/9th and 1/3rd as large.

Incidentally, I've just recently started getting prints of my digital photos and have discovered that they all have a blue cast (well, really it's cyan).  That's reason #3 I couldn't have taken the photos from the party.  Maybe I will get a new camera soon.