The slinky method proposed by Steve Shelley in A Practical Guide to Stage Lighting states:
The principle he advocates for achieving a smooth blend seems to be contradicted by his ground plan in Fig. 6.9 & 6.10 (for those of you who have the book), which shows that it is the edge of the field that should land in the middle of the adjacent beam. Assuming the fixture's photometrics adhere to the slinky formula, this results in the beams overlapping marginally. So firstly, which is it? Is it the beam or field edge that needs to bisect the adjacent field/beam?
Secondly, does the slinky formula still hold true, 20 years after the book was published? Having only been in the industry for a few years, I haven't noticed a change in field to beam ratio, but my understanding is that most modern fixtures are generally able to achieve a beam angle greater than half the field angle.
How is this field to beam ratio affected by focus? Does an ERS maintain the same ratio across its focus range? I.e. is the ratio the same for a sharp focus vs unfocused (softened edges)?
My takeaway from the book is that I don't need to be too concerned with the field overlap but just ensure that the beams themselves are at least slightly overlapping. Interested to hear what methods other people to take when it comes to focusing fixtures for an even wash.
Most theatrical lighting instruments are optically designed in such a way that the degree spread of the beam angle is almost always at least 50% of the overall field angle. This can be expressed in an equation known as the slinky formula:
field angle/2 = beam angle
To provide proper overlap between two lights and achieve a smooth blend, the edge of the second beam should land in the middle of the first beam
The principle he advocates for achieving a smooth blend seems to be contradicted by his ground plan in Fig. 6.9 & 6.10 (for those of you who have the book), which shows that it is the edge of the field that should land in the middle of the adjacent beam. Assuming the fixture's photometrics adhere to the slinky formula, this results in the beams overlapping marginally. So firstly, which is it? Is it the beam or field edge that needs to bisect the adjacent field/beam?
Secondly, does the slinky formula still hold true, 20 years after the book was published? Having only been in the industry for a few years, I haven't noticed a change in field to beam ratio, but my understanding is that most modern fixtures are generally able to achieve a beam angle greater than half the field angle.
How is this field to beam ratio affected by focus? Does an ERS maintain the same ratio across its focus range? I.e. is the ratio the same for a sharp focus vs unfocused (softened edges)?
My takeaway from the book is that I don't need to be too concerned with the field overlap but just ensure that the beams themselves are at least slightly overlapping. Interested to hear what methods other people to take when it comes to focusing fixtures for an even wash.