high/low sensitivity meter on the 7s

[stet]

the manual says that low sensitivity is for "ordinary brightness outdoors," while high is for "indoors or outdoors just before sunset." Yesterday I went out with a Minolta XG-M to compare readings, and I found that in total shade readings matched only if I switched the Canon to high sensitivity. But when it was on low, it would still give me a usable reading, just one or two stops off.

Is this an instance where I'd want to average, taking a reading in high and low? This happened to me in the shade when the sun was blocked by a building, but I'm also wondering about more varied light, like when I'm shooting a lit sidewalk with dark doorways (or from within the doorway/stoop, toward the street), or sidewalks partly covered by tree shade.

If it helps, I'm using a Wein cell.

 

[Sonnar Brian]

On my Canon 7 ('s, as in 3 of them) I use the low setting until it starts "pegging" out near the top of the chart. It is more accurate that the high setting at the bottom of the scale. Most meters are probably the same way. The high and low scale have some overlap, and often I get the same reading on both settings. I find that reassuring.

BTW: How did your camera do with the wind problem with the AR collar in the "locked" position. Could you find a zone where the advance stopped after one wind AND the release could not be pressed?

 

[Mark W]

Has your meter been calibrated? I wouldn't worry about how it reads until it was properly serviced by a quaified person. Even much newer meters need to be adjusted every so often. I 've had Canon F-1's meters as much as two full stops off at various parts of the range. I would assme the much older and earlier design of a 7s meter to require adjustment.

Also comparig meters with different angles of acceptance unless the light is carfully controlled can lead to the reading being different between the two meters.

 

[stet]

Brian:

Yeah, often my readings will match between high and low sensitivity, but sometimes they do come up different, again, usually by a stop. That's interesting about accuracy and the high sensitivity at the bottom of the scale, too; I'm probably gonna start muttering that to myslef in twilight when I'm unsure!

As for the film advance lever, I meant to check it out after I finished this roll today, but in the park I ran into a guy with a great brim hat and two cute Doberman pups. Totally forgot and popped in a new roll. But I haven't been advancing it automatically after I take a shot now. I hope I can train myself that way.

Mark: I did just get it CLA'd, so I'm hoping the meter is spot on now. I just had a problem with fogged film once, and the uniformly overexposed negs brought on this obsession with metering and accuracy (before I figured out I used a horribly X-rayed roll). I didn't, though, think about different angles of acceptance.

 

[jlw]

the manual says that low sensitivity is for "ordinary brightness outdoors," while high is for "indoors or outdoors just before sunset." Yesterday I went out with a Minolta XG-M to compare readings, and I found that in total shade readings matched only if I switched the Canon to high sensitivity. But when it was on low, it would still give me a usable reading, just one or two stops off.

Is this an instance where I'd want to average, taking a reading in high and low? This happened to me in the shade when the sun was blocked by a building, but I'm also wondering about more varied light, like when I'm shooting a lit sidewalk with dark doorways (or from within the doorway/stoop, toward the street), or sidewalks partly covered by tree shade.

If it helps, I'm using a Wein cell.

Because of the way CdS cells work, I'd say that you should use the high-sensitivity position until it "tops out," then switch to the low-sensitivity position.

Why I say this: a CdS cell has high "dark resistance" -- the darker it is, the more electrical resistance it exerts and the less current it allows to pass. If it's already exerting a lot of resistance, small changes in light are going to be proportionally less of the total resistance, and the meter will respond to them less effectively. On the other hand, if the light is bright, the cell is exerting less resistance, so small changes in light will be proportionally larger compared to the total resistance, and the meter will be more responsive to them.

The low/high control on a 7s is not electrical -- it just pivots a little flap back and forth over the meter cell. This flap has a tiny, precisely made pinhole in it, so that when it's in place (low sensitivity position) it cuts down the amount of light reaching the cell by a specific amount. When it's pivoted away (high sensitivity position) the full area of the cell is exposed to light. Naturally, when the flap is away, the cell sees "brighter light," exerts less resistance, and is more responsive to changes in illumination.

Make sense? I suppose I could get out my 7s and a handheld meter and make some comparisons -- but unless you have a standardized, calibrated light source of the type made for testing meter systems (expensive) this type of "test" doesn't really prove very much.

 

[stet]

Because of the way CdS cells work, I'd say that you should use the high-sensitivity position until it "tops out," then switch to the low-sensitivity position. [...]Make sense?

Good grief, I'm still digesting this. It does make sense, but I was around dawn today, and I was still confounded as which to use/listen to. Hope anyone doesn't hate me for saying this, but the time lag it's taking for me to learn these lessons (weekly class, alternating printing and processing) makes me wish for digital, just so I can see the results and learn quicker.

back to pen, paper and my calendar...