What aperture is 2.8.2?

[Roger Beck]

I noticed that lenses are marked with the aperture number and light meters are marked in 1/10 of a stop between whole stops. Does someone make a light meter that gives you the equivalent f stop number of say 2.8 and 2/10? On a meter that would read as 2.8.2. How would I know the decimal equivalent of say, f3? f12.5? Is f4.5 the same as 4 and 5/10?

[Eric Woodbury]

2.8 plus 20%. I commend you if your process is that accurate. Just give it f2.8 and 1/5 or a nudge. Then again, better over-exposed than under. If that other number is 1 second, you are in reciprocity territory anyway.

[BTW, I have this meter and the little ON switch was always sliding on and battery dead. Then I taped the switch off and the battery still seems to die, so now I remove the battery and tape it to the bottom of the meter for when I need it. You experience any of that?]

e

[MAubrey]

I noticed that lenses are marked with the aperture number and light meters are marked in 1/10 of a stop between whole stops. Does someone make a light meter that gives you the equivalent f stop number of say 2.8 and 2/10? On a meter that would read as 2.8.2. How would I know the decimal equivalent of say, f3? f12.5? Is f4.5 the same as 2.8.8?

The f-number changes based on the square root of 2 (1.41421...), so f/2.8 x 1.414 = ~4, which is why the stop are:

1, 1.4, 2, 2.8, 4, 5.6, etc.

Figuring out tenths between those two numbers would be math something like taking 1.414 and subtracting 1 (= .414) and then dividing .414 by 10 (=.0414). So .0414 is one tenth, so to calculate 2.8+.2 stops, you'd need to multiply 2.8 by 1.0828 (.0828 being .0414x2), which gives you f/3.03184...so basically f/3.

[Roger Beck]

Right, thank you, that helps, be nice to have that button on a light meter, to switch between decimals and working aperture.

[Roger Beck]

2.8 plus 20%. I commend you if your process is that accurate. Just give it f2.8 and 1/5 or a nudge. Then again, better over-exposed than under. If that other number is 1 second, you are in reciprocity territory anyway.

[BTW, I have this meter and the little ON switch was always sliding on and battery dead. Then I taped the switch off and the battery still seems to die, so now I remove the battery and tape it to the bottom of the meter for when I need it. You experience any of that?]

e

I have a couple and not that problem. I like them because they have the feature of self timing off, the battery never wears out, just turn it on and its on, it turns itself off after a while.

[ic-racer]

What you have is probably more useful. The indicator gives you a visual as to how far to move the aperture lever between the two numbers that are actually marked on the scale.
Other Sekonic meters, like the 558 I have, can show really crazy numbers. For example, when set for "T" and 1/3 increments, the readout can look like this as you scroll through the shutter speeds:

3.6(2), 4.5(0), 4.5(3), 5.0(2), 6.3(0), etc. (f-3.6 and 2/10, f-4.5 and 0/10, f-5.0 and 2/10, etc.)

[faberryman]

Given all the other variables, these are rounding errors and false precision.

[MAubrey]

Right, thank you, that helps, be nice to have that button on a light meter, to switch between decimals and working aperture.

I should add that other variations (f-stop vs. t-stop, tiny variations in film speed, shutter accuracy, etc.) are going to make 1/10th stop differences irrelevant for the most part. If you just round to the nearest third or half stop, you're probably going to be as accurate as you can get.

[Bob Salomon]

I should add that other variations (f-stop vs. t-stop, tiny variations in film speed, shutter accuracy, etc.) are going to make 1/10th stop differences irrelevant for the most part. If you just round to the nearest third or half stop, you're probably going to be as accurate as you can get.

To be a little more precise, a shutter’s speeds are accurate if they are +\- 30% of the marked speeds.

[LabRat]

The decimal setting is useful when lighting a set... If there is a trend where light fall-off is starting, 1/10th of a stop will not yet be seen by the film, but is starting (and you can start reading it)... Gives you a chance to even out the lights esp with strobes...

Steve K

[Jerry Bodine]

To be a little more precise, a shutter’s speeds are accurate if they are +\- 30% of the marked speeds.

Shutter speed tolerances according to Flutot's Camera Repair are shown here (http://www.flutotscamerarepair.com/Shutterspeed.htm).

[Bob Salomon]

Shutter speed tolerances according to Flutot's Camera Repair are shown here (http://www.flutotscamerarepair.com/Shutterspeed.htm).

Since we were the Rollei, Minox, Linhof and Rodenstock distributor we didn’t use Carol, we used the various factories and they all stated 30%.

[Sal Santamaura]

...I have this meter and the little ON switch was always sliding on and battery dead. Then I taped the switch off and the battery still seems to die, so now I remove the battery and tape it to the bottom of the meter for when I need it...

Eric, even you couldn't get a Pocket Spot? :)

[Roger Beck]

Y’all are right, I’m not trying to be super accurate. I tend to round up a lot for exposure and yes, lens specs are rounded up or down, the real focal length, the true aperture. Just wondered why there were two standards for lenses. Still, would be handy to have a list of actual aperture size for reference in 1/10 stop increments.

[LabRat]

Y’all are right, I’m not trying to be super accurate. I tend to round up a lot for exposure and yes, lens specs are rounded up or down, the real focal length, the true aperture. Just wondered why there were two standards for lenses. Still, would be handy to have a list of actual aperture size for reference in 1/10 stop increments.

Not rocket science... 5/10's is a half stop, but for something critical like on the edge of a white background, 1/10 would not be seen by LF chrome film, but 2/10's would start to be visible... Between 3 and 4 10's is a third of a stop, so that's a guide for you...

Happy metering!!!

Steve K

[Roger Beck]

Not rocket science indeed. Here are the multiply factors to get the aperture in 1/10 of a stop. After that are are the common maximum apertures of older lenses, and their decimal equivalent.

.0414 - .1 stop
.082 - .2 stop
.1242 - .3 stop
.1656 - .4 stop
.207 - .5 stop
.2484 - .6 stop
.2898 - .7 stop
.3312 - .8 stop
.3726 - .9 stop
.414 - 1 stop

3.5 - 2.8/6
4.5 - 4/3
4.7 - 4/4
6.3 - 5.6/3
6.8 - 5.6/5
7.7 - 5.6/9
9 - 8/3
12.5 - 11/3