Hoods ‘n the boyz

Lens hoods – a good thing all round. There are those clever petal-shaped ones, square or rectangular ones to match the film format, the odd but sexy Leica ones with the cut-aways plus various rubber offerings. But what if you need one for a particular lens, perhaps only for a short time? How about making one?

Hoods

We’re not talking 3d printing here. With a bit of calculation and the kind of drawing set you had at school, you can make a custom lens hood from black paper.

What you are going to make is a frustum, a cone with the top cut off. The narrow end of the frustum will fit over the front of your lens. The angle of the sides will match the angle of view of your lens. The depth of the hood will be what you want to make it, limited only by the size of your sheet of paper. There are some calculations involved, but I did this once in a spreadsheet so I only need to enter the key measurements to make a new hood.

The first key measurement is the diameter of the front of your lens. The lens hood will need to be a sliding fit over the lens. This measurement is not the filter thread size – measure the actual diameter of the lens.

Next is how deep you want the hood to be. This is handy if you are making a hood for a special situation, like using a long lens in strong side or frontal light.

Lastly, the difficult measurement: the lens’ angle of view. This can be difficult because it’s not just a function of the lens focal length: it’s the relationship between the focal length and the size of the film or sensor. This is why a 50mm lens is considered standard on 35mm film, but a 6×6 negative on 120 film uses a 75mm or 80mm lens.

What could be a right chore is made simple by looking-up your lens and film/sensor combination in previously published data. The BJP had a great article plus graphs in the 116th edition in 1976, but I bet you didn’t keep yours… There is also a useful calculator here.
I’ve got a 55mm lens for a 35mm camera. Looking-up the angle of view from the calculator I’ve linked above, the diagonal angle for a 35mm frame is 43 degrees. I use the diagonal angle because anything less than this is likely to vignette the corners. So this means my lens hood should be a cone with an angle of 43 degrees. I fancy making the lens hood 50mm deep – for no other reason than it feels like a useful compromise between no use and too big.

The last key measurement is the diameter of the lens; in this case 57mm.

So I want to do the trigonometry to calculate a frustum with a 57mm wide hole at the top, the sides sloping at an included angle of 43 degrees and with a vertical height of 50mm. The first time I did this I did all the calculations myself. Then I discovered a dedicated website with the sole intention of providing the calculations needed to effectively open-up a frustum and lay it out as a shape that can be drawn on paper. The formulae look difficult, but it’s simple to put them in a spreadsheet. If you provide cells to put the specific values into – lens diameter, angle of view, depth of hood – the same spreadsheet will give the drawing measurements for any hood that takes your fancy.

I made the calculations for my hood and got three key measurements: two radii needed to draw the pie-slice on the paper and an angle of arc for which I need to draw the curves. In the case of my special lens hood, I need to draw two arcs with radii 78 and 132mm and to draw the arcs with a sweep of 132 degrees.
Here’s a picture.

Hood 1

I used plain black art card. For the pictures I marked it out using a white pen, but for normal use I would use a pencil. I marked a tab at the end of the arc to give myself an overlap that I could hold down with tape. If I was going to use this hood a lot, I would use the tab and a slot to let me disassemble the hood when I wasn’t using it (or I would hold it together with masking tape, which peels off). I also left some extra material on the inside of the smaller radius. This is cut into a number of tabs that will go over the lens and hold the hood in position, particularly if you put a bit of tape on them.

Hood 2

It’s easy to find a protractor, but a large set of compasses is more difficult. I use a bit of card with holes for the tip of the pencil and a pin to pivot on.

Hood 3

Cut out and fitted it works just fine.

Hood 4

This is an excellent way to make a hood for short-term use or for a lens that you would not otherwise be able to fit. Unstick the masking tape and the hood can be stored flat.

There you go – something useful for a change in place of the usual grumbling.
If anyone wants to use my spreadsheet rather than do your own calculations, drop me a comment and I will post it somewhere accessible.

Author: fupduckphoto

Still wishing I knew what was going on.

One thought on “Hoods ‘n the boyz”

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