When to use the LV formula?

Question

EV = log2(f^2/T)   
LV = EV + log2(ISO/100)

Question is to those that this is already clear. It is not a math question, it is a photography question about using Light Value.

My puzzlement is "what is the useful use of this LV = formula?"

Clearly it adds effect on EV of ISO relative to ISO 100. But when would we need to use this LV = formula? (specifically, by adding it to EV?) In use, what EV would we add it to?

If we calculate the top EV = part for any existing camera settings (for any exposure assumed useful), fstop and shutter speed used already have the ISO effect in it. Basically, EV is LV, EV at an ISO. But then adding any LV ISO effect would repeat it, incorrectly doubling ISO effect. So "EV" already has the result that I might incorrectly imagine "LV" would add.

If we meter a scene at ISO 800, then we look up that result EV in the standard EV chart, to find the row of Equivalent Exposure settings to be correct in that light at that ISO. Basically, EV is LV, EV at an ISO. Using those settings (which now have ISO in them), we cannot recalculate those settings again (EV we can, but the LV part would repeat ISO, doubling the proper ISO effect.)

What is a realistic example of when we would actually use the LV = formula ? (and not see an incorrect doubled ISO effect?)

Thanks

Answer 1

EV, Exposure Value, is used to summarise the two camera settings, exposure time and aperture, in one value. Cementing the idea that there are many pairs of exposure time and aperture that yield a given exposure.

If the ISO used is known, or assumed, typically ISO 100, then the EV is directly related to the lighting conditions. Sometimes this is shown as EV₁₀₀ (with the 100 as a subscript). So people sometimes say "it's EV 6 in here", without bothering to mention the ISO, which is assumed to be 100.

Light Value is the EV with ISO taken into account, but almost no-one uses this, or calls it LV. Some early light meters marked in LV were referring to an arbitrary scale used to transfer a light reading from the meter to a dial calculator.

Answer 2

There is a looseness or imprecision in the general use of terms like EV and LV that are leading to some confusion here. So first we need to use consistent terminology and labels to unravel this.

EV is a relative number, independent of any concept of ISO. There is no ISO anywhere in the EV equation. This agrees with the Wikipedia article on Exposure Value:

EV corresponds simply to a combination of a shutter speed and an aperture setting, independent of any ISO setting.

As for LV, quoting from the "Meter indication in EV" section of the same article,

Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100. However, this term does not derive from a standards body, and has had several conflicting definitions.

The definition of LV you provided is mathematically equivalent to "adjusted EV when used at a particular ISO", which is usually written EV_ISO (i.e., EV₄₀₀).

Returning to your question you said,

If we calculate the top EV = part for any existing camera settings (for any exposure assumed useful), fstop and shutter speed used already have the ISO effect in it.

The part in bold is incorrect, which leads to your confusion that "LV ISO would repeat [ISO], incorrectly double ISO effect."

If we meter a scene at ISO 800, then we look up that result EV in the standard EV chart, to find the row of Equivalent Exposure settings to be correct in that light at that ISO.

Then you have determined EV₈₀₀. Or as defined above, "LV" when ISO = 800.

What is a realistic example of when we would actually use the LV = formula?

Effectively, you already are, when you use relative EV when applied (adjusted) for a particular ISO. Using the term "LV" is redundant, and ill-defined.