How big will my gobo image be projected?

Gobo image size is a common concern. The Gobo image size has projected has nothing to do with the gobo size itself, but is determined by the lens degree and distance projected and gobo design. Each gobo fixture has a set lens degree, unless it is a zoom unit which incorporates many lens degrees in one fixture.
To make this easier to understand I will pick one of the most common gobo projection fixtures on the market the Source Four by ETC. The Source Four has many lenses to choose from but for this example we are going to only mention the 19, 26,36, 50 degree lenses. A 50 degree lenses is what you call a one for one. If you are using a 50 degree lens and are projecting a gobo image from a distance of 20 feet, you would have a 20 foot projected image, depending on the gobo design and if it uses the entire image area. So knowing that a 50 degree is one for one, we can assume that a 26 degree would roughly be half that size. So using a 26 degree lens from 20 feet would give you roiughly a 10 foot image, again depending on the amount of image area used in your gobo. Knowing the 50 degree is a one for one lenses makes estimating projection beam size a little easier. ETC doeas make much wider and narrower degree lenses for it’s line of Source Four lighting fixtures.