Ambient + continuous lighting - Calculating EV

[blopin]

If I have a light source that illuminates a subject giving me an EV value of 8 in-camera with zero ambient light, but have an environment that will for instance give me 8 EV without my light source, how many EV would my subject be? 

[AJG]

The easiest way to answer this is to use a gated flash meter, which is most of them from the last thirty years.  Used good condition Sekonic flash meters run around $100 and up and are well worth the money if you are regularly mixing ambient and flash lighting.

[BeBu Lamar]

In your example it's relatively easy as you give both the ambient and the light source EV8. The answer is EV9. But if you give them different values the calculation is much more involved. 

Let assume the EV value with all light source on is EVT, The first light source is EV1, second is EV2 and so on then the formula is

EVT= log(2^EV1 + 2^EV2 + 2^EVn...)/log(2)

Edited May 14, 2023 by BeBu Lamar

[paco_rosso]

9 EV.
 

Think this. The illuminance (lux) is:
E = k * 2^EV

Where k is a 270 / S and S is the ASA sensitivity (the firs number in the ISO sensitivity, the ISO value cameras adjust). 270 is for international units, and only god knows what it can be in those medieval units used in the US.
 

So you are adding a illumination E1 to another illumination E2. Let's go:
First, lets call n to difference beetween E1 and E2. So E2 = E1 * 2^n.
Now add both illumination:

Etotal = E1 + E2

Et = E1 + E1 * 2^n.

Et = E1 (1+2^n).

In the case you say, both E are the same, so n = 0. Then Et = E1 * 2. Et is 1 stop greater than E1. As E1 was the illuminance with EV1 = 8 then EVtotal = 9.

 

[paco_rosso]

Think this. The illuminance (lux) is:
E = k * 2^EV

Where k is a 270 / S and S is the ASA sensitivity (the firs number in the ISO sensitivity, the ISO value cameras adjust). 270 is for international units, and only god knows what it can be in those medieval units used in the US.

So you are adding an illuminance E1 to another illuminance E2. Let's go:
First, lets call n to difference beetween E1 and E2. So E2 = E1 * 2^n.
Now add both illuminance:

Etotal = E1 + E2

Et = E1 + E1 * 2^n.

Et = E1 (1+2^n).

In the case you say, both E are the same, so n = 0. Then Et = E1 * 2. Et is 1 stop greater than E1. As E1 was the illuminance with EV1 = 8 then EVtotal = 9.

We can build a sum table if we think in the difference in stops beetween both EV. This is:
dEV      EVt
0,0    1,0
0,3    1,2
0,5    1,3
0,7    1,4
1,0    1,6
1,3    1,8
1,5    1,9
1,7    2,1
2,0    2,3
2,3    2,6
2,5    2,7
2,7    2,9
3,0    3,2
3,3    3,5
3,5    3,6
3,7    3,8
4,0    4,1

 

 

Edited July 23, 2023 by paco_rosso

[rodeo_joe1]

Who the heck uses EVs anyway? Let alone converting the darn things to Lux. 

All you have to do is use a plain old light meter to get the F-stop readings at your chosen shutter speed for each light source. You then square each F-number, add them all up, then take the square root of the total as your working F-stop. 

Example A: Two light sources, one giving f/4 and another giving f/8 at the subject. 4 squared is 16, plus 8 squared is 64, giving 80 in total. The square root of 80 is 8.944272. So f/9 is your working aperture to the nearest 1/3rd stop. 

Example B: 3 light sources giving readings of f/11, f/6.3 and f/4.5. 

11^2 = 121, 6.3^2 = 39.69, 4.5^2 = 20.25. The total is 180.94 and its square root is 13.451394. Nearest preferred stop is then 13... or 14. Take your pick because both of them are only about 1/6th of a stop away from 'correct'.

A bit easier than converting everything to Lux or Foot-candles! 

Just remember that your working stop is always going to be a bigger number than your highest individual meter reading, as a double check on your calculations. 

Edited August 20, 2023 by rodeo_joe1

[AlanKlein]

So Joe, you're taking the square root of the sum of the squares of the f stops? 

Edited August 20, 2023 by AlanKlein

[AJG]

8 hours ago, rodeo_joe1 said:

Who the heck uses EVs anyway? Let alone converting the darn things to Lux. 

All you have to do is use a plain old light meter to get the F-stop readings at your chosen shutter speed for each light source. You then square each F-number, add them all up, then take the square root of the total as your working F-stop. 

Example A: Two light sources, one giving f/4 and another giving f/8 at the subject. 4 squared is 16, plus 8 squared is 64, giving 80 in total. The square root of 80 is 8.944272. So f/9 is your working aperture to the nearest 1/3rd stop. 

Example B: 3 light sources giving readings of f/11, f/6.3 and f/4.5. 

11^2 = 121, 6.3^2 = 39.69, 4.5^2 = 20.25. The total is 180.94 and its square root is 13.451394. Nearest preferred stop is then 13... or 14. Take your pick because both of them are only about 1/6th of a stop away from 'correct'.

A bit easier than converting everything to Lux or Foot-candles! 

Just remember that your working stop is always going to be a bigger number than your highest individual meter reading, as a double check on your calculations. 

Or just get a decent gated flash meter and skip the math.  Your calculations will work but my Sekonic L 718 from the 1990's has performed this work successfully many times for me  in a couple of seconds.

[rodeo_joe1]

17 hours ago, AlanKlein said:

So Joe, you're taking the square root of the sum of the squares of the f stops? 

Exactly, yes.

Works fine, since f-stops are based on the square root of lens area, and hence light-gathering power. Except the standard F-number series is inaccurate and highly rounded at every other stop. I.E. f/1.4 should be f/1.4142, f/2.8 should be f/2.8284, f/5.6 should be f/5.6569, etc.

Not an issue normally, but squaring those rounded numbers gives a larger discrepency when two or more of them are added. F/11, for example, should be SQRT(8^2+8^2) and not SQRT(121).

14 hours ago, AJG said:

Or just get a decent gated flash meter and skip the math.

Of course. That's what any sensible person would do practically. But that wasn't the OP's question - which was about ambient (presumably continuous) light addition. And working from logarithmic EV readings. 

The whole scenario is one that assumes individual light sources can be individually switched on/off and measured individually. When in most practical situations one would just stick an incident meter in front of the subject and take a single reading.