Characterizing a Large Format Lens

Ok, ok, so I didn't do a full characterization on a lens, though we do have all the tools I'd need to do that at work. But I wasn't after optical data, I was after practical information for use with the lens in the field. And that's precisely what I got.

When choosing and buying a large format lens, there are a couple of values that matter. One is focal length, which can be used to get field of view. If you're after wide-angle, something in the 75-90mm range is nifty. For a "normal" lens, something closer to 150mm is good. For a long lens, something in the 200-300 is handy. (A 300mm lens roughly translates to a 100mm 35mm lens.)

Coverage circle is another good number to know. Large format negatives are, well, large. If your lens won't cover the whole negative with a usable image, it's not going to do you much good. Ideally you want your coverage circle to be a little larger than your negative so you can use the camera's movements to good advantage.

I won't go into the camera movements in this article since it's a whole article of its own. Trust me that one of the real charms with a large format camera is that you can shift, tilt, swing, and otherwise move the lens and the film with respect to each other, resulting in all sorts of photographic advantages you don't get with a rigid body lens/camera combination.

Finally, if you're shooting color film another good bit of information to have is how well corrected the lens is. Older lenses were optimized for the blue since older emulsions were blue-sensitive, but had little to no sensitivity in the green or red. (Which is one reason why older photographs often had completely featureless skies... there were clouds, but the film rendered it as a uniform shade of overexposed.) But if you're shooting color, you need a lens that focuses at least the blue and red at the same distance behind the lens (achromatic). Ideally you'd like it to be corrected at more than two colors, preferably red, green, and blue (apochromatic).

But these are all decisions you make before you ever buy a lens. Let's say you have a large format lens already. It's mounted in a shutter, it has its own iris, and it's ready to go. Time to go take pictures, right?

WRONG!

I was using a large format lens I'd had for years, but I was getting consistently over-exposed negatives. Since each large format lens has its own shutter, the likeliest place to look was either that the aperture was not closing correctly, or more likely the shutter had drifted. So I put a chronograph on my shutter and clocked it. Oh how enlightening! This is what the data came out like:

• 1 sec = 1610-1624ms
• 1/2 sec = 970-1006ms
• 1/4 sec = 460-475ms
• 1/8 sec = 256-267ms
• 1/15 sec = 102-106ms
• 1/30 sec = 44ms (it really didn't drift at all)
• 1/60 sec = 27-29ms
• 1/125 sec = 10-12ms
• 1/250 sec = 6-7ms
• 1/500 sec = 3.9-4.5ms

Yup. My shutter is off by a full stop at every position. Dang. So noted, and I'll add a laminated card to my lens board to that effect tomorrow.

Another number that matters is where the lens's "sweet spot" is. Lens aperture is a mixed bag: At wide-open apertures, you're using every bit of glass in the lens, which probably isn't all that well corrected for spherical aberration. As you stop down, you use less and less of the glass, more toward the center of the lens where the curvatures are more forgiving, and typically your lens performance improves. But as the apertures get smaller and smaller, your image begins to suffer from diffraction effects from the aperture itself. By the time you get down to f/64 or smaller, diffraction will be causing more fuzziness than lens aberrations.

Most of the time the mid-point between these two curves will lie in the mid-range of the aperture ring. On my Canon 100mm macro lens, the sweet spot is at f/11. On my Canon 50mm f/1.4 lens, the sweet spot is out at f/16. It's worthwhile checking this number. In the event that you're taking a picture where depth of field isn't an issue, set your aperture at the sweet spot, secure your camera to a tripod or stand, and shoot away.

Of course depth of field is often the bigger consideration, and smaller (or wider!) apertures are called for. In that case do what needs doing. I shoot at f/64 with this lens a lot when I'm doing macro work. I really don't have a choice. On a lens like this, your depth of field disappears in a hurry at close focus distances. Having the smaller apertures available is a necessity.

But for landscape use, I often shoot at wider apertures. So what's the sweet spot on the lens in question? Between f/16 and f/22 it's pretty flat in terms of image resolution. On either side of that things do start to go downhill.

So now I know: My shutter's slow by a stop, and my lens likes f/16 to f/22. So for aerial shots, I'd ideally set my shutter to 1/500 (1/250 in reality), and shoot around f/22. So it sounds like an ISO 400 film like Tmax 400 (TMY) would be my best bet.

Better pictures through mathematics!

Tom