If you have photographed a subject with and without a grid modifier, you have probably noticed your power output is much lower with the grid on. This causes a less-exposed subject, forcing you to increase your light power to compensate.
What Are Grids?
For those who haven't used grids yet, these are also called "honeycomb grids." They are light modifiers put in front of an existing light modifier such as a reflector, softbox, or a beauty dish. They all look similar — a grid of black metal, plastic, or fabric stripes that block the light from going in different directions.
Depending on the depth of these stripes the grids "focus" more or less the light stream. They are categorized into degrees of light visibility. The less degrees the grid, the more "focused" or narrow the light beam is.
Sketch of how grid acts upon the light stream
The "Logical" Assumption
The grid is used to focus the light on the subject and separate it from the rest of the environment. It's logical to conclude more light will fall on the same spot, which would make the subject brighter. In reality, your exposure is even darker than without the grid.
The Explanation
The first time I saw this I was with a client. I played it cool and powered my light up to compensate for the grid, but I kept thinking, "Why is this happening." The reason was logical, straight forward, and not that difficult to understand.
I have used the word, "focus," several times in quotes. The reason for that is "focus" is not the correct term. It looks like the grids focus the light, but they actually block it. They act like many adjacent black flags that restrict light from going in certain direction.
Let's see how normal black flags work. Here is a drawing of one light using two black flags to limit its spread.
This is how a single grid cell works works. Now, multiply that and you've got a grid.
A stream of light contains of a vast number of beams that travel in all directions. Coming from the light source, some of them pass through the grid and hit the subject. Others hit the black stripes (or "flags") and get consumed by the black material. As we know, black subtracts or "eats" light, which means light is being lost and leads to underexposure of the subject.
Let's simplify the scenario by assuming there are only two beams of light — A and B. Here is what happens when there's no grid. A and B are both hitting the subject. The exposure on the subject is A + B.
Light beams (simplified)
Let's use a grid. Let B hit one of the stripes while A travels through them. B will be blocked, and light from this beam will subsequently be lost. The exposure will only be lit by A, or one beam of light. This is the reason for the underexposed result when using grids.
How grids work (simplified)
The more black honeycomb structures there are in the grid, the more light power is lost.
Blocking Light Without Draining More Power
If you are on location using battery-powered lights, you have to use grids with caution. If you want to have an easy-to-control, narrow beam of light, grids are a great way to go. However, if light power is a priority, it's better to use actual black flags to keep the light from hitting what you don't want brightened. This way you won't have to increase your light's power level. The drawback is that you have to have more light stands or assistants to hold the flags. But let's be honest: sometimes more gear in front of a client looks way cooler than just having a small, compact grid on your light.
This may be why cinematographers rarely use grids and opt for black flags instead. Video lights in general are less powerful than strobes; and because of that, every bit of power from a continuous light source is precious.
In your simplified light beams diagram, beams A and B are both traveling in the same direction. But then in the subsequent diagram, beam B somehow changes direction and instead heads towards the flag. Beam B should actually travel in a straight line and continue hitting the subject as it did in the first diagram. But what do I know, I failed Physics.
In real life beams travel in all directions. Think of A and B as groups of beams. Some travel in a straight line, some go to the side and hit the black flags. If "B" are the group that doesn't come from the center of the light but from the periphery of the reflector they will "see" the object without flags. However with flags they will not reach the object.
Indeed. And while the author still gets the concept across, it would perhaps be clearer to have the first diagram have ray B going off target as per the second diagram, and hitting a undesired part of the scene. That way it would clearly show how flags block undesired light without affecting the light on the subject.
Solid post though, which should hopefully help prepare some newcomers to grids.
If you know that focus is not the correct term, then don't use it. A grid is blocking a lot of random light rays and allowing a limited amout to go thru the grid holes, it's not focussing or concentrating the light. A spotlight with a fresnel lens will gather up a lot of those light rays going in random directions and will to some degree get 'em all going the same way in sort of a parallel fashion. To simplify, Cast a shadow with a piece of cardboard with a hole in it, and compare the light pattern to using a magnifying glass. the grid is blocking more light than it lets thru.
If you use a "strap on" grid on speed lights, don't forget to zoom all the way up for an even tighter spot and much more power. I'd guess more than a stop of light
I think the title of this is a bit too obvious, while the article itself is pretty helpful. Of course it requires more power, it's not a 2 dimensional object. It has 3 dimensions, as thin as it may be. So it will lower your output because it is blocking light.
This is a good article, I like it. My introduction to grids was my mentor telling me to grab a couple and use them and see what I thought... haha....
It's not a bad advice unless you paid for that :)
People pay for mentors? Unless you count beers I guess.... haha
A good read and informative. However a good analogy would be using LED lights. Let's say one bulb is 1W. So without GRIDS 500 bulbs will be 500W since all bulbs will contribute to lighting the subject. However, just imagine you have a subject that is very close to the light and you will be introducing a grid that is the size of one bulb only (extreme example). So even though there are 500 bulbs in your LED panel, the total power that reaches the subject will be only 1W since the grid blocks the other lights from contributing to the same spot on the subject. It doesn't matter if you have 1000 bulbs in your panel but the actual light that will hit the subject will only be 1W since you are blocking the other lights from hitting the same location in your subject (take note very close subject to light distance as an example). Then if you increase the size of the grid, say up to 5 bulbs, then 5W will hit the subject as long as the grid is long enough to limit other groups of light from hitting the same spot on the subject. Once you move the subject farther away from the light then other groups of light will start to contribute to hitting the same subject and thus increasing the power on the subject. So technically, the size of each grid and the distance to the subject is proportional to the amount of light hitting the subject. Meaning the closer the subject is to the light, the more power you will lose with the introduction of the grid.
That's a good analogy. Thanks