Silverbased

I’ve been having a pang of guilt lately: So far, this blog been a little heavy on the tech-talk, and a little light on showing photographic examples. So lets look at a pair of images. (Click to see larger versions, which will be easiest to compare opening each in a new browser tab.)

Cemetery sculpture: camera ‘E’

Cemetery sculpture: camera ‘M’

Both show the same subject, from the same camera position, and include the same angle of view. The tonal ranges may not match perfectly, but I’ve adjusted them them to be pretty close. So what’s the difference?

Well obviously, the upper image shows much more detail in the background. The zone of sharp focus—aka depth of field or DOF—seems to be much deeper. This is particularly obvious with the twigs at the right edge: The lower image isolates a few; the upper one shows a whole thicket. So that first image must have been shot at a much smaller f/stop, right?

Heh, heh. Gotcha. Would you believe me if I told you both of these photos were taken at the same aperture—f/4.0?

So what on earth created such a huge discrepancy in the depth of field? Simple: The bottom image was shot on 6×4.5 format film, using a 110mm lens. The upper one was shot on my digital compact, where the corresponding focal length is about 11mm. (The digital image has been very slightly cropped to match the film frame.)

Let’s take a brief optical detour to understand why this happens.

Focal Length Changes with Format

Old-school photographers who jumped between 35mm, 120 and sheet-film cameras learned that as you switched formats, you also needed to shift gears thinking about focal lengths.

For any desired lens coverage (say a “portrait” lens, or a moderate wide-angle), the corresponding focal length scales up in proportion to the film’s dimensions. For example, one well-known guideline is that a “normal” lens is one whose focal length equals the diagonal of the image.

Thus, a normal lens for 4×5 sheet film has a focal length of roughly 160mm. But the tiny sensor chip in my digital compact could fit atop a pencil eraser. So the zoom setting corresponding to a “normal” lens for this format is about 7mm.

And those exceptionally short focal lengths lead to extreme depth of field (just as switching to shorter focal lengths does with any camera).

Fuzzy about Focal Lengths

By the end of the 20th century, most serious amateurs shot 35mm exclusively—so, awareness of how focal lengths related to film size began to fade. It became an easy shorthand to quote 35mm-format focal lengths to express how wide or narrow a lens’s angle of view was. If you said “I’m shooting with a 28 today,” everybody understood what kind of perspective you were talking about.

But the digital-camera explosion spawned a plethora of different chip formats—all much smaller than 35mm. Focal length confusion roared back with a vengeance. Camera marketers despaired that publicizing this crazy-quilt mix of new focal lengths would cause chaos and confusion. What did a “12mm lens” even mean in this brave new world?

So they latched onto the well-intentioned—but wholly fictitious—crutch of specifying “35mm equivalent” focal lengths.

These bogus 35mm equivalents will only serve to confuse us. So please, forget they ever existed! It’s the actual, optical focal length we are talking about here. (If not labeled on the lens, it might take some rooting around in your owner’s manual or the image EXIF data to find out the true f.l. numbers.)

Whither Depth of Field?

Meanwhile, the DLSR world had its own confusion. Photographers could now put their old film-body lenses onto new digital models, with “crop factor” sensors. But if a 50mm lens “acts like” an 80mm on your DLSR, then what happens to the DOF? It’s still the same lens, right? To this day, whenever the issue comes up in online forums, discussions can get quite agitated, with dogmatic assertions and misleading generalizations.

Once again, the DOF issue has nothing to do with digital—it’s simply another effect from scaling between one format size and another.

But confusingly, there are two factors to consider:

As I mentioned, smaller image formats require shorter focal length lenses to capture an equivalent angle of view. And a shorter focal length lens gives deeper DOF.

However, a smaller format must be enlarged further to give an equal-sized image to view (whether printed or onscreen). This higher magnification means any fuzziness becomes more detectable—i.e. the limits on how much blur is acceptable become more stringent. So paradoxically, you’d expect depth of field to get shallower with smaller formats.

in this Optical Steel-Cage Death Match, I’m sure you’re breathlessly wondering—who wins? Well, I won’t keep you in suspense: The focal-length effect trumps the magnification one. The essential rule is:

When f/stop, distance, and lens angle-of-view are all held constant, the larger the image format, the shallower the depth of field.

Of course, shallower depth of field is a double-edged sword. Here’s another pair of photos—also both taken at f/4, again differing only in image format:

Depth of field: small-sensor digital camera

Depth of field: 645 film format

The lower, 645 version helpfully takes all that distracting background junk and throws it out of focus. Yet even with this flat-ish subject, only the raised hand is in good focus; the eyes and the right edge of the sign drift off into fuzziness, just because it was at a slight angle to the camera. Shallow DOF places more demands on the photographer (and here, I blew it).

But an equally vital observation is this: Any camera based on a tiny image format (and this includes virtually all digital point-and-shoots) gives you no chance to use selective focus creatively.

All the digital P&S images shown here use the longest available focal length, the widest aperture, and are shot fairly close to their subjects. Thus, they represent the most favorable combination for achieving shallow, selective focus. Yet in all cases, the backgrounds appear sharp enough to be cluttered and distracting.

If this still seems all rather unintuitive, it helps to try plugging some representative numbers into a depth-of-field calculator, like this nifty online one. Remember—you must change the popup for image format, not just the focal length. (Depth of field calculators make certain simplifying assumptions, and are not the last word on the subject; but they’re a great way to compare what happens as you change one factor at a time.)

But, is this just a minor geeky technicality, or does it matter… artistically? Well, here is my final photo comparison. Same ground-rules as before:

Depth of field: digital point & shoot

Depth of field: 645 film format

Scout’s honor here: I did not move the tripod an inch. Now I’m hardly claiming the second image is timeless art. But the upper (digital compact) version is just a tangled, impenetrable mess. With no way to blur the background, and pull the viewer’s eye towards the cattails, there is simply no photograph at all.

Yet today, millions of people are doomed to use cameras where this effect is unavoidable. Again, this is not a “digital-versus-film” issue per se; it’s simply an inevitable result of the smaller image format.

But is it really fair to compare a cheesy digital point-and-shoot to a serious medium-format camera? Well, there’s one last little detail I haven’t mentioned yet.

The film camera I used here is a Mamiya m645—a recent purchase off eBay. While well-worn and probably 30 years old, its sturdy construction seems ready to keep kicking for a few more decades. The 110mm lens was “bargain” grade from KEH.com. The price? Actually—together they cost $50 less than what I paid for my digital point & shoot.