6 x 9 etc.

Older ERS instruments were classified by their lens size and focal length.
6x9 is a 36 degree instrument
6x12 is a 26
6x16 is a 19

 

Altman still makes and sells a lot of 360Q's. They list for about $120 less than a Source 4 so it's a good choice when money is tight. There's nothing wrong with them, it's solid older technology. Grog's numbers are a close approximate but they aren't exactly correct. A 4.5X6.5 is 47 degrees, 6X9 is 34.3 degrees, a 6X12 is 24 degrees, 6X16 is 16.7 degrees, 6X22 is 11 degrees

This is a link to the download for their spec sheet. It has a nice little photometric chart in it to help you out.

Altman 360Q cheat sheet to calculate the size of the area of light:
For a 4.5X6.5 multiply throw distance by: .88
For a 6X9 multiply throw distance by: .62
For a 6X12 multiply throw distance by: .41
For a 6X16 multiply throw distance by: .29
For a 6X22 multiply throw distance by: .19

 

These are the usual substitutions if you make a plot for S4 and it goes into a 6xX house or vice versa. You will see some places out there actually right on the yokes the deg of the unit and what its altman brother is, those houses being places that hang shows very quickly and tend to have older shows coming in.