# When to use the LV formula?

```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

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.

• EV= log2(f^2/T) already has ISO applied to those settings, if about settings that the camera used or should use, or if about what light meters read as EV at an ISO. Any EV actually used necessarily already has ISO in it. But then we also have LV = EV + log2(ISO/100) which shows effect of ISO, but ISO has already affected the EV settings. I'm trying to learn any use for that LV formula? It seems just a trouble maker (because it adds ISO twice), worse than helpful. The LV formula seems only useful for converting ISO 100 to different ISO, but it is not described that way. What am I missing? Jan 30, 2016 at 15:35
• What am I missing? The fact that EV has nothing to do with metered light. EV is simply a combination of shutter speed and aperture. Period. Other combinations of Tv and Av that are equivalent all have the same EV. It doesn't matter if you are in direct sunlight or a light tight box with no light source. If you set your camera to f/5.6 and 1/60 second you have selected and EV value of 11. End of story. Jan 30, 2016 at 18:29
• There is no consideration of ISO or any amount of light in the first equation. It is simply a mathematical expression of the relationship between any particular aperture and any particular time value (shutter speed). Jan 30, 2016 at 18:31
• Once you use a light meter, it is computing what EV is appropriate for a given measured luminance level and a given sensitivity (ISO). But when someone says something like, "The scene is EV16" what they are really saying is the measured luminance of the scene will be properly exposed at a specific ISO with a combination of Tv and Av that correspond to EV16. Jan 30, 2016 at 19:18

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 EVISO (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.

• Comments are not for extended discussion; this conversation has been moved to chat. Jan 31, 2016 at 10:46