Measuring beyond ISO 8000 with your Sekonic L-758DR Light Meter

With dSLR cameras acheiving higher and higher ISO ranges, like the Nikon D3s which goes up to 102400 ISO, I was left wondering if my top-of-the-line Sekonic L-758DR light meter is now obsolete (as it “only” goes up to ISO 8000).

Because the L-758DR has a nifty little USB port used when profiling your camera/film, I thought the first thing to check was the Sekonic website for a firmware update. No such luck. The meter’s specs are clearly stated as “ISO 3-8000 (in 1/3 steps)”. Well, that’s really too bad, I thought to myself thinking that I’m going to need to go out and buy a new meter…and who, by the way, is using ISO 3?!? The lowest ISO film I’ve ever used is Kodak Ektar ISO 25 and the lowest digital backs only go down to ISO 50.

But I digress…

Not taking “no” for an answer, I went on a hunt for a work-around, and a-ha!  My beloved meter has the ability to set Exposure Compensation…up to 9.9 FULL STOPS of +/- compensation! To access Exposure Comp. on the L-758DR, while pressing ISO 1 and ISO 2 at the same time, rotate the jog wheel to change the value.

So what does that mean? Well, simply put, there’s a lot of life left in this mighty little meter! I would also assume that other meters of similar quality and features have similar exposure compensation, so this may apply to your meter too, even if it’s not a L-758DR.

Warning, Science ahead!!!  If you just want to know how to set your meter for ISO 102400, set your ISO to 6400 and your Exposure Comp. to +4.  If you want to know how to calculate that (or any other value), read on…

Here’s a basic re-cap of ISOs, F-stops, and shutter-speeds. Remember that, for every increase in one FULL aperture value (smaller number = bigger aperture) or one FULL decrease in shutter speed, 2x as much light reaches the recording surface (digital sensor in this case). Also, for every FULL step up in ISO speed, 1/2x as much light is necessary for a proper exposure. So, when we talk about Exposure Compensation (in EV units), +1EV, twice as much light gets in vs. 0EV. So +1EV = 1 f-stop = 1/2x shutter speed = +1 ISO unit, OR +1EV = +1 ISO unit. Ergo, we can calculate the Exposure Compensation needed for any particular ISO.

Mathematically, we can calculate this by:

Where

ISO₁ is the starting ISO value
ISO₂ is the newly calculated ISO value we want to figure out
EV is the Exposure Value compensation

So if we plug in the meter maximum values we get:

Meaning that the maximum ISO value that the Sekonic L-758DR can effectively calculate is ISO 7,643,406…or 5.9 full stops higher than the highest value any dSLR is currently capable of!

But what if we want to calculate the EV required when shooting the D3s at ISO 102400? Easy, all we have to do is re-work the equation and solve for EV! Thusly…

Ta-da! But not so fast…because our calculators only calculate log in base 10 and we’re trying to calculate log in base 2, we have to do the following to convert our equation to log base 10:

And now we finally have a working equation we can calculate the required EV needed for any particular ISO!  Try it, it works! (But don’t forget your brackets and order of operations.)

So let’s try an easy one.  What’s the EV value needed to go from ISO 3200 to ISO 400? Just sub in your numbers and solve:

EV = -3!  Which we can prove by working it out manually.  That is, half of 3200 is 1600 (-1 EV), half of 1600 is 800 (-1 EV), and half of 800 is 400 (-1 EV).  So that’s 3 x -1 EV or -3 EV.

So if we want to calculate the EV needed for the Nikon D3s at ISO 102400, all we do is sub in the numbers ISO₁= 6400 (I’ll use ISO 6400 because it’s a full stop value and works out evenly…you could use 8000, but you’ll get a long decimal answer that’s hard to remember) and ISO₂= 102400 and solve:

So our answer is that we need +4 EV on our meters for ISO 6400 to be equivalent to ISO 102400.  And that’s it!

I hope you found this of use!
J R BERNSTEIN

Remember your ABC’s – Always Be Capturing!