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I have a Sekonic L308X. At the bottom right, next to aperture value, there is a number. See below image:

enter image description here

The manual only said it's a 0.1 step display.

What is it? Hand how can it help us to make exposure?

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I don't have this particular model (I have an L-308S-U), but after a bit of experimentation with a flashlight...

That means that the currently recommended f-stop is f/4 plus 2/10 of a stop - towards 5.6, meaning the light is slightly brighter than what would cause it to recommend exactly f/4.

You would use that number to determine when to move up to the next stop, especially if your camera supports 1/3 or 1/2 stops. If it doesn't, a measurement of 4.0/9 might indicate you would be better choosing f/5.6 instead of f/4.

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    @scamander And it is NOT to be confused as f/4.2. That is NOT what it says. It is read as f/4 plus 2 tenths EV, which is 2/10 of way to f/5.6. Cameras don't use tenth stops, but one way tenths is extremely useful is to meter multiple flash units. Then their difference (lighting ratio) is immediately obvious, not hidden in the fstop math.
    – WayneF
    Aug 9, 2019 at 14:50
  • @WayneF in other words that value only useful when we use flash?
    – neversaint
    Aug 9, 2019 at 21:48
  • It's an accurate reading in tenths for why ever you want it. Which perhaps may not be very often, but one use where it is certainly particularly useful (even indispensable) is for metering two flashes to easily know the lighting ratio between Main and Fill. For example, how many stops between f/4.5 and f/8? But it metered in tenths, then this is obvious as (f/4 + 3/10 stop) to (f/8 + 0/10 stop) = 1.7 stops
    – WayneF
    Aug 9, 2019 at 22:43
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How fine an increment can the camera exposure be adjusted?

For more years than I want to remember, I managed a department that made process control materials. We exposed test films and made test prints; the goal was uniform precision day-to-day. The best we could do in the laboratory was an end product controlled to 1/3 f-stop. No easy task; everything that touches film, printer and process must be inspected and calibrated daily. I would challenge anyone who claimed control to ± 1/6 f-stop. That includes flash photography. This is more difficult because control is trickier.

I don’t think a meter display or metadata does anybody any good displaying a 1/10 f-stop increment. To accomplish this on the camera requires minuscule adjustments to the diameter of iris diaphragm / shutter speed. Suppose a camera with a 50mm focal length lens is set to f/11. The diameter of the iris is approximately 50 ÷ 11 = 4.55mm in diameter. Change this setting to f-16, and the revised diameter is 50 ÷ 16 = 3.13mm. Making a 1/3 f-stop iris change from f-11 towards f-16 dictates setting the iris to f/12.7. The revised diameter is approximately 3.94mm.

Making such setting changes in 1/3 f-stop increments is mechanically possible. If the goal is predictability of setting, my experience is, 1/3 f-stop is about the limit. To accomplish with 1/6 f-stop increments, you are forced to use precision neutral density filters.

Another point – Making control materials using photo film also requires precision film processing. The best that can be done with due diligence in the laboratory is ± 1/6 f-stop uniformity day-to-day.

Again, I think 1/10 f-stop increment control is wishful thinking.

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  • Very good point!
    – user82065
    Aug 11, 2019 at 10:18

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