Lighting Ratios

[RandyMN]

Since I first learned photography I have always gone by the belief that each F-Stop cut light in half, therefore in doing lighting ratios for studio work I always thought a 2:1 ratio would be one F-Stop difference. 4:1 would be 2, and 3:1 would be 1 and 1/2.

I am reading a book by Norman Phillips named High Key Portrait Photography, and in this book he explians the following from page 30 on Calculating Light Ratios:

A ratio is calculated by measuring the difference in output between the main light(s) and the fill light(s). Each full f-stop equals two units of light.
If the main or key light is set for exposure at f-8 and the fill light is metered at f 5.6, there is a difference of one f-stop, or two units of light. The fill light is always calculated as one unit, and is added to the two units of key light for a total of three- thus the ratio here is 3:1

He then goes on to say that a common mistake of photographers is to count each f-stop as one full unit of light, and while acceptable, it may be misleading and can cause serious lack of detail in shadows.

So where have I been doing my calculations in light ratios all these years? I'm pretty certain that each f-stop does cut light in half or double it when going higher. So mathematics tells me that a 2:1 ratio is one light twice as bright as the other, thus meaning one f-stop diffrence.

Any one explain this to me in a way that mathematically makes sense?

[Yola]

I m thinking that bulbsin lighting photography have gotten a bit .. more sharp and advanced and the compensation of maybe 1.5 needs too allocated .


Cat scans .. angioplastics technology has change my bulb lighting .....as an EKG rate .

We see more in a post plasty in medical technology ...I think light meters may be changing ... Maybe ???

[breathless]

Perhaps an illustration will be helpful?

[RandyMN]

So my undertsanding of the f-stops being double or half is still correct, I guess the only part that still seems funny is why the 1:3? Seems if the f8 is 2X light then that would make it twice as bright and lighting ratio would be 1:2.

According to the drawing the f8 is three times brighter. Should I rethink this in being just a way of specifying light ratios? 1:3 actually means one unit twice the amount of light as the other?

[dlpasco]

I'm just trying to learn lighting ratios myself. I just read (and reread) a chapter on ratios and don't understand it yet. Art and mathematics don't always agree ;) The best I could come up with (math background) is that the ratio is 1:2**(f) where (f) is the difference between key and fill A 3 stop difference is 1:8 (2**3), a 2 stop difference is 1:4 (2**2) and a one stop difference is 1:2 (2**1). Where I got confused was when the author explained that a 1.5 stop difference is a 1:3 ratio. That doesn't fit precisely as 2**1.5 is (rounded) 2.83.

[RandyMN]

Let me look at what you just said;

1:8 = 3 stops- This appears correct as 2^3 = 8
1:4 = 2 stops- This appears correct as 2^2 = 4
1:3 = 1.5 stops- You say this is 2.83 and I'll take your word for it.

So now given what you said, I come up with the following:

1:8 = 3.5 stops or 2^3 plus 1
1:5 = 2 stops or 2^2 plus 1
1:4 = 1.5 stops or 2^1.5 plus 1 (maybe they are just rounding up as we have no partial f-stops)
1:3 = 1 stop or 2 plus 1

The author always takes the difference between the fill and the main light in units of light and adds the fill light to the ratio.

Therfore, I guess these mathematics do make sense to me as according to the author

1:8 = 7.5 units plus 1 | 3.5 f-stops which is 7 times the amount of light plus 1
1:4 = 3 units plus 1 | 1.5 f-stops which is 3 times the light plus 1
1:3 = 2 units plus 1 | 1 f-stop which is 2 times the light plus 1

[PacAce]

There are at least two ways to specify lighting ratios. The way you were originally thinking of it, eg. 2:1, reflects the ratio the main light intensity to the fill light intensity. In order words, the main light is two stop brighter than the fill light.

There is another way of specifying the lighting ratio for the same lighting scenario and that's the highlight to the shadow ratio. This, in the above example, would be expressed as 3:1 where the highlight is the total of the main light and the fill light (2 + 1) and the shadow is just the fill light by itself (1).

So, 2:1 (m:f) = 3:1 (h:s). ;)

[RandyMN]

There are at least two ways to specify lighting ratios. The way you were originally thinking of it, eg. 2:1, reflects the ratio the main light intensity to the fill light intensity. In order words, the main light is two stop brighter than the fill light.

There is another way of specifying the lighting ratio for the same lighting scenario and that's the highlight to the shadow ratio. This, in the above example, would be expressed as 3:1 where the highlight is the total of the main light and the fill light (2 + 1) and the shadow is just the fill light by itself (1).

So, 2:1 (m:f) = 3:1 (h:s). ;)

Thanks for that explanation. It's nice when every one can be right!;)

[dlpasco]

Now I'm really confused. I'm quoting "Master Lighting Guide" by Christopher Grey, Amherst Media, 2004:

"This f-stop number, based on the main light used in your lighting scenario, is always assigned a fixed value of 1."

He goes on to say that a fill light that is one stop darker than the key (main) light produces a 1:2 ratio. He shows several examples of varying ratios and how to specify them. He then describes the "highlight to key" ratio and specifies that at the left of the key value - still always 1. A highlight that is one stop bright than the main produces a 2:1 highlight to key ratio. Specifying a highlight:key:fill ratio of 2:1:4 for example shows highlight of one stop brighter than key and a fill that is two stops darker.

It seems his explanation is not consistent with most of what I found when I googled "light ratio". Most sites specify the main to the left of fill and fill has a fixed value of 1. However, Wikipedia does explain that 2**(f-stop difference) is correct.

I'm not trying to be argumentative or authoritative - I'm simply trying to understand. Why the different ways to specify the ratio? Is it that both are consistent - highlight:key:fill with key always equal to 1 is the same as key:fill with fill always equal to 1, just no provision for highlight? or...the main is the highlight?

Hmmm...mathematically it makes sense.

[ChadAndreo]

This still confuses me.
http://www.studiolighting.net/digital-photography-one-on-one-e010-light-ratios/

[AntonLargiader]

It may be how you're interpreting the significance of the ratio. The relative brightness of the lights is 2:1 but the illumination of the subject (ratio of dark to light) is 3:1 and what is the ratio supposed to represent? The drawing above shows that the subject has 1 unit of light on the fill-only side and three units of light on the key+fill side.

If you were looking for 3:1 with your old method, you'd end up with (I suppose) 4:1 using the 'correct' method which seems in agreement with the comment about lack of detail in the shadows.

[Wilt]

So my undertsanding of the f-stops being double or half is still correct, I guess the only part that still seems funny is why the 1:3? Seems if the f8 is 2X light then that would make it twice as bright and lighting ratio would be 1:2.

According to the drawing the f8 is three times brighter. Should I rethink this in being just a way of specifying light ratios? 1:3 actually means one unit twice the amount of light as the other?

This still confuses me

The issue is that lights can be compared in their relative intensity (incident) and also lighting can be stated in terms of subject contrast.

In relative intensity of light sources, 2:1 is where Key is one full f/stop brighter than the Fill. When the Fill is near camera position and falls upon the full mask of the face which is visible to the lens, and when the Key is off to the side and overlaps part of the face, the subject contrast is 3:1 (due to Fill + Main = 1 + 2 = 3) units of light falling upon one side of the face. Stated as subject contrast, 3:1 is low contrast, 5:1 is medium contrat, 7:1 is high contrast.