Fixed Focus

Fixed Focus

When Panasonic and Olympus announced the Micro Four Thirds[a] system of mirrorless interchangeable lens cameras[b][1], the photography world was very excited for one big (or small) reason: We would finally have small cameras. No longer would we lug around cameras that were little more than 1960s technology with an image sensor stuffed into them.

"Wait," you say, "we've had compacts since, well, forever. Most people don't lug around a big DSLR - haven't you noticed?"

Yes, but not compacts like this. The compact digital camera suffers from three major shortcomings that makes it inferior to a DSLR - its inability to change lenses, its small sensor and its slow autofocus speed. While the autofocus speed is more or less a consequence of using the contrast-detection method as opposed to the phase-detection method used in DSLRs, and therefore almost unavoidable, the first two shortcomings would be removed with Micro 4/3rds.

So at the start, our expectations were for really small cameras with interchangeable lenses that may be a bit slow to focus.

When the first cameras were released, people realized that having a very small camera body didn't make much of a difference if you still had to attach a huge lens to it. In fact, if you were going to attach a huge lens, you might as well go with a small DSLR and get good autofocus speed. However, with an almost-flat lens - a so-called "pancake" lens[c] - we could finally have a really small camera, even when it had a lens attached.

But - if we're only going to use pancake lenses, it's not really an interchangeable lens camera, is it? Well, yes and no. You are still able to switch lenses - you just don't want to. We got the "really small camera" at the cost of "interchangeable lens".

The pancake lenses also had another downside - they wouldn't autofocus at all. To understand why that is, we must remember that autofocus requires the camera to move lens elements around. In order to move those elements, it needs a motor. Some systems, like Nikon's AF (not AF-S)[d], places the motor in the camera and exposes what looks like a little screwdriver head in the lens mount. The lens then provides the other half of the coupling. When the motor turns, the head turns the corresponding part in the lens and the focus elements move inside the lens. The disadvantage of this is that you need to fit the motor in the camera, and it has to be strong enough to turn the shaft fast enough to focus quick enough. In other words, not a good match for cameras that try to be as small and light as possible. Other systems, like the Micro 4/3rds, places the motor in the lens. The camera then controls the lens-mounted motor electronically. But in a pancake lens there is no space for the motor. No motor, no autofocus.

This wasn't as big a loss as one wold think. Pancake lenses also have a fixed focal length of about 50mm on a 35mm system, and decent depth of field, which meant that one could simply set a fixed focus distance and use the old sneaker zoom (moving the photographer instead of the focusing elements) to place the subject withing the field of focus.

What about the sensor, then? This is essentially the only promise these cameras fulfilled - we got a compact camera with a big sensor. So far, Sony and Samsung have crammed APS-C sensors into their cameras - quite an impressive feat. The Micro 4/3rds uses a smaller sensor than APS-C but larger than the sensors of most compacts.

At the end of the day, then, we got a small, fixed focus, fixed focal length camera perfect for street photography. Sounds a lot like a compact, modulo a couple of features, doesn't it?[2]

On a Compact

So why not just get a compact? Why bother with the ILCs? There are reasons - chiefly the sensor size - but given that they are mostly used for street photography in well-lit environments, I'd like to offer up a suggested feature for those manufacturers who do not yet field this type of camera: Put a fixed-focus option on your compacts. The best compact camera of all time, the Casio EX-Z850, had this feature[3] - but I haven't been able to find it anywhere else ever since.

My current compact is a Panasonic DMC-TZ7. The crop factor for the TZ7's sensor is roughly 6. With that in mind and some online DOF calculators, this is what a fixed-focus mode on the TZ7 would look like:

Focal Length in 35 mm f/ Near Width Height
4.1 mm25 mm3.40.55 m0.8 m0.5 m
7.7 mm47 mm3.41.8 m1.3 m0.9 m
16.0 mm97 mm3.47.5 m2.8 m1.9 m
22.2 mm135 mm3.415 m4.0 m2.6 m
48.5 mm295 mm3.470 m8.5 m5.6 m

The "Focal Length" and "in 35mm" are the focal length the lens is set to and the corresponding focal length in 35mm film, respectively. The "f/" number column is self-explanatory. The "Near" column shows where the near focus distance is, if the lens is set so that the far focus distance is at infinity. Basically, everything farther away from the camera than this distance will be in focus. The "Width" and "Height" columns show the width and height of the view at the near focus distance. What they show is the largest object that you can fit in the photo, if it is to be as close as possible and still in focus.

I think this would be a killer feature for street photography. Compacts do take a little while to start up, but once they are running, when it comes to single-picture taking it is only their autofocus speed that slow them down relative to DSLRs.


2010-06-13, updated 2014-11-16