Fabric Grids - An Interesting Discovery

[Hermes]

I was doing some calculations on the spread you get using fabric grids with squares of different sizes (preparing for an upcoming DIY project) and I noticed something odd - the figures I was getting were double those quoted by manufacturers such as lighttools (http://www.lighttools.com/content/lighttools/CustomPages/technical_data.htm) for squares of the same size. What I calculated to give a spread of 40 degrees was quoted on their website as giving a spread of 20 degrees, and so on.

I then spotted that they measured their grids' spread, not to the point where light from the softbox was completely cut-off, but to the point where it was reduced by two stops. In other words, when they sell you a grid with a quoted spread of 40 degrees, as far as I can tell, you're actually getting a grid that only fully restricts light 80 degrees.

I know some people might not consider this a big deal and as far as I know the practice might also extend to other types of grids, but I thought it was worth flagging up as there are some projects (shooting with rear-projection screens for example) where you really can't afford any spill of any kind. At the very least, It's something to keep in mind when you're buying or making grids of your own.

[TMR Design]

Wow Hermes, that's very interesting. When are we going to see your DIY grid? Are you going to make a real 40 degree grid?

[Hermes]

I'll hopefully have a grid finished in the next few days - I should have been started and finished by now but the snow in London has stopped all the couriers making deliveries and I'm still waiting for a couple of things.

As for a 'real' 40 degree grid (equivalent to a 20 degree Chimera or Lighttools grid), I'm fairly confident that even a grid this narrow can made pretty quickly and cheaply. The only potential problem is the weight - I know that such a grid would be very heavy (to halve the spread of a grid doubles the weight). Whether it's too heavy for a softbox to handle is something I'll have to find out.

[TMR Design]

The weight is something I would be concerned about as well. A few of the forum members that have been talking about DIY grids have some good ideas but I think weight of materials is going to be a prohibiting factor.

[Hermes]

Don't worry, it can definitely be done - I can easily switch to a lighter webbing if the current one is too heavy.

The only issue is that I wanted the grids to be as cheap and easy to make as possible - Polypropylene is perfect in that respect. Switching to a lighter webbing will add cost and decrease the rigidity of the finished grid so I'll only go there if it's absolutely necessary.

[PacAce]

Hermes, can you explain how you determine a grid's angle of coverage based on the physical geometry of the grids? I can make an educated guess as to how you get it using trigonometry but what I don't understand is how you can have an angle where the light can be 2 stops less at the edges than at the center. I'm thinking you'll either have light (when not blocked by the grid) or you don't (blocked by the grid) but that's obviously wrong based on what you said in your first post. :confused:

Thanks.

[Hermes]

Hermes, can you explain how you determine a grid's angle of coverage based on the physical geometry of the grids? I can make an educated guess as to how you get it using trigonometry but what I don't understand is how you can have an angle where the light can be 2 stops less at the edges than at the center. I'm thinking you'll either have light (when not blocked by the grid) or you don't (blocked by the grid) but that's obviously wrong based on what you said in your first post. :confused:

Thanks.

The reason the light falls off gradually instead of abruptly is side/corner shading. I did the diagram below in paint in about 30 seconds so forgive the scale and general lop-sidedness. Essentially it is meant to be a grid square with the lines representing light exiting the grid from the diffuser.

The gap between the red and blue lines outside the grid after they cross over is essentially the fall-off. that area is only being lit by a small percentage of the light and the wider the light spread goes (closer to the blue line) the weaker it gets.

Notice that the red lines create an angle of 45 degrees but the blue lines create an angle of 90 degrees. This is the disparity I mentioned. Grid manufacturers will generally count light that exits between the red lines (the hot spot) as the spread. To get the actual spread - the point where all light is cut-off - you need to pretty much double their figures to take into account the blue lines (the fall-off).

[PacAce]

Thanks, Hermes. Now that you've drawn me a diagram, it's one of those "Well, duh, of course! <tapping head>" moments for me. :rolleyes: It's crystal clear now. :)

Thanks again.

[WillMass]

Hi Hermes,

Any updates on the DIY fabric grid?

I'm looking for a grid, and am interested in saving (more than) a few bucks.

Thanks.

[form]

I made grids for my hot shoe flashes out of white corrugated posterboard. They work pretty well.