While Wolf's post is very proper, I have to disagree with it's actual use.
On
bench focus, the 10' target idea is a good goal, I have also heard of placing a piece of paper in the
gel frame and the rest of it I forget, something about seeing how the light looks and being able to observe easily how the beam is centered. Think the first place I saw that was on Lighting
Network a few weeks ago. This is an actual alternative way to do a
bench focus with something in the
gel slot and it should work just as well. This if anyone is interest, I could dig up and post a link to it. The principal seems just as sound as focusing a light 10' away and might be of use in keeping the
gel cool in verifying that the convergance of the beam is not in the
gel frame holder area.
2 to 3 times a year on this 10' proper
bench focus, - ambitious and noble but un-realistic. Most people if the fixtures are hung just do it somewhat when they change lamps, or bump them hard at best and when there is time. Others if the fixtures always come down, do a quick focus test before they go back up but certainly don't have a proper
bench focus ever. Granted the S-4 has made it a little more rare that a lamp will be harshly out of
bench focus. For the most part, lamp quality and
base style these days has improved in any
fixture to the extent that the lamp you replace is going to be about the same
bench focus. Observing what your lamp looks like on a
flat surface should be fine to verify it without a proper
bench focus. Once a year for a proper
bench focus might be a better goal. Same time as you clean lenses and fixtures.
For now, Wolf has a good step by step detail in it from what I read of it in sounding like what I read and remember from text books. Sorry but I only skimmed it however. I don't think I have ever done a text
book proper
bench focus, but I’ll add my copy of how to do it from
ETC and
Altman below.
For me, two things come to mind that are supplementary notes to the
bench focus Wolf presents.
One is that a 6x16 focused at a target 10' away is going to be a
bit outside of it's
stage focus range, and much different in beam than a 6x12 much less 4.5x6. No matter how it looks at that range, at it's effective and used range, it just might not have the same proper
bench focus when given the same target range. Yes, the lamp will be centered and for the most part right, but you probably won't be able to get the required hard
edge on the beam. Also, given the
focal length, the beam might be a
bit more bright than is easily able to see what is dim and bright in it's even spread. A better idea that many
point out is to target your fixtures at optimum range at a source such as at a
scrim from it’s hang position, and observe what the beam looks like with a hard focus. I would assume that the hard fast 10' rule was written that way so as to keep it simple and easy to make an accurate accounting for what it’s doing because it really does need a target perpendicular to the source and other things about it, but realistically, not many people use the method.
Another small
point is that instead on a
bench focus of looking at one
point of aim, I perfer to swivel the beam up and down to truly see what the beam of light looks like. Your brain and eyes far too quickly adjust to and even out those little dark spots in a beam of light for you to perfectly adjust for them. Were you to swivel that beam up and down, since the dark spots are going light to darker, you do tend to notice them quicker and easier. It might be a test to add to the proper or any
bench focus. Than of course, if you go with the new style HPR lamp, you don't have those dark spots in the beam all other lamps will have. But that's another issue.
By the way, the
Altman 360Q series is superior in it's ability to adjust than that of the
ETC S-4
fixture in my opinion. Just had some this past week that I was on an off occasion trying to
bench focus - at least my working version of doing so. Couldn't get them properly done because it's not possible with a
ETC fixture to do as much with the focus. Sent the fixtures out with a noticeable bad
bench focus, because there was noting I or the person in charge of that area that makes her living off focusing such lights could do about a bad
bench focus or something else, given we endlessly attempted to adjust it's knob, changed both lamps and bases and nothing would correct the beam. Granted, the
360Q was also able to get way out of focus easier. For the most part still, a FLK lamp is going to have the same 2.3/8" Lamp Center Length as the one you took out. As long as it is seated properly, and there is nothing wrong with the lamp
base or the focus position of the last lamp, the next one should be fine. I don’t even think other than a major
fixture breaking bump is going to knock it out of focus given the center lock screw is down. I believe that
bench focus proper rules were written long before
halogen lamps came to market and for the most part are not of much value anymore as officially stated. That’s my opinion at least. Never had much use for a proper official
bench focus. Sorry Wolf.
Altman’s
Bench Focus notes and others on the subject of interest:-
Lens Cleaning: to
gain access to the lenses for cleaning, or other purposes, remove the
lens adjusting knob and slide the
lens holder out of the
unit. Remove the
lens retaining ring and carefully remove the front
lens. If necessary, remove the
lens spacer and remove the rear
lens. (It might be necessary to use rubbing alcohol or soaking the
lens to remove it) Reverse this process to install the
lens holder back into the
unit. NOTE: be sure to note the position and orientation of the lenses before removing them to assure correct reassembly and optical correctness of the
fixture. Clean the lenses with mild soap and water or a commercial glass cleaning solution and a soft, clean, lint-free cloth. Insure that the cleaning process does not leave film on the
lens, since the heat from the beam will bond the film onto the surface. (Vinegar or any other Coffee Pot cleaning chemicals also work well. Also, a freshly washed
lens will
smoke a little when used for the first time if not rinsed well enough. This smoking is also leaving a film on the
lens which will attract dirt necessitating more frequent washing.
Beam Adjustment and Lamp Centering: The beam may be altered in appearance utilizing the three
flat head machine screws in conjunction with the round head
machine screw which acts as a tension mechanism for the three screw mounted lamp
base. In some cases this fourth screw has been replaced by a spring. The “hot spot” may be increased (peak field) or deceased (
flat field) by moving the
socket mounting plate towards the front or rear of the
fixture respectively. This is accomplished by loosening the round head
machine screw a few turns, then by moving the
socket mounting plate towards the front or rear of the
fixture respectively. This is accomplished by loosening the round head
machine screw a few turns than be alternately adjusting the three
flat head screws, the field can be adjusted to the desired look. The lamp should also be centered during this procedure, again using the three
flat head screws to “fine turn” the field until the hot spot is centered in the beam. Once the desired field has been achieved, tighten the round head machine serew to lock the
socket mounting plate inot position, then tighten the two nuts on the round head screw onto the
socket cap to prevent the screw from vibrating loose. Caution: always insure that the lamp is centered when using the
iris. A lamp imaged onto an
iris will cause premature
iris failure. Also when using an
Iris, close the shutters down as tight as possible on the cut of the
Iris to absorb some of the heat off the
iris.
Lenses - Focal Points: The relationship between the positions of
the light source or the object (slide,) and the
point where the transmitted rays from an
image is determined by the “
Lens Formula.” When P = the distance from the object to the principal plane of the
lens, Q = The distance from the principal plane to the
image, and F = The
focal length of the
lens; than 1/P+1/Q=1/F.
Focus of Focal Center: when the source (given the lamp is out of focus with its
reflector,) is moved forward from the principal focus, the reflected rays converge. When moved back from the principal focus, they diverge with a dark spot in the center of the beam. Also a bright half or partial moon like figure hanging around the center of the beam, denotes a lamp horizontally out of focus with the
reflector and its lenses in the opposite direction as is shown
Radius and
Focal Length: Lenses are about .52 times their
focal length by radius of curvature of the
lens. For
ellipsoidal lenses.
The
Iris of a Spotlight, when the spotlight is flooded, (by moving the
filament towards the
lens.) The
Intensity at the center of the beam will
drop to 1/10 of its original focused amount, and if at the same time the
iris is closed to keep the spread of the beam or the diameter of the area lighted the same. Dimming the light can be effected by
manual mechanical manipulation.
ETC’s
Bench Focus
The Difference between
ETC Source Four ERS Fixtures and Standard
ERS Fixtures:
Measuring Cosine Field Lumens: (The Proper
Bench Focus) If carried out correctly, using a calibrated lamp, this test should have a margin of error of about ±5%. To make this easier use the form below and a calculator with a square root function, a light
meter and a tape measure.
1) Set the
fixture up and focus it perpendicular to the wall. Adjust for
cosine distribution, centered within the field, with sharp focus. For best results, the
fixture should be at least ten feet from the wall.
2) Draw a
line on the wall, bisecting the beam
mark beam center.
3) Measure the distance from the front
lens to the wall, in feet. Enter the
center line distance in the box provided.
4) You need to take readings at center, at 1° and then every 2° off center, to one side only. To find where those points lie on your
line, multiply the
center line distance by the tangent of each angle. Enter the result in column A and make a
mark on the wall for each angle.
5) To find the
throw distance, measure the distance from the
lens to the
mark for each angle (apart from the center.) For each
point, add the
center line distance squared to the distance off center squared, and take the square root of the total. Enter the result in column B.
6) Place your light
meter, with its back
flat against the wall, and take a footcandle reading for each
point. Enter the result in the footcandles (measured) column.
7) To find Candelas, enter the
throw distance from column B and enter it in C. Multiply by footcandles, and enter the result in column D.
8) To find Beam Lumens, multiply candelas by the zonal constant for that angle. Calculate only the values where the footcandles figure is 50% or more than the footcandle reading for the center of the beam. Sum the result.
9) To find Field Lumens, multiply candelas by the zonal constant for that angle. Calculate only the values where the footcandles figure is 10% or more than the footcandle reading for the center of the beam. Sum the result.
10) To find efficiency, calculate the total field lumens as a percentage of initial lamp lumens, if known.
11) Individual lamp
lumen output can vary by up to ±10%. If you used a calibrated lamp, you should now scale the readings taken to correspond with the lamp’s Nominal Lumens, as stated in the manufacturer’s catalog.
By the way, I had actual reference data on these sources with publishing dates and other info such as would be in a bibliography. They are all gone in my lamp notes file now for some reason, be it from the upgrade, or from some simple stupid mistake. I can no longer find them - just a bunch of error lines running across the bottom of the last
page. Something like 600 sources with
page numbers in lamp notes are now gone - at least for the moment.
I am reminding such a thing to all of you because a back up on anything you want to keep is crucial. Everyone knows this, but how many people actually do this and as much as recommended? How pissed do you think I would be now had after 5 years of research into lamps, I was suddenly missing my entire biblography for a
hand typed 2,386KB file. Now, I'm concerned as to what happened, but know I have a good backup.
For me, it means Monday, I’ll go to work and E-Mail myself a fresh copy of the lamp notes since they have not changed since the last time I sent a backup out. Had I not a computer at work to store stuff on, I would be using a tape backup. Everyone that want’s to save stuff needs some kind of backup and to use it. If you have something you can't loose, you need to get into a force of habit in backing it up either automatically or manually such as I do.
I made a change to some lamp prices, and to a lamp chart used for some
projector lamps today. At the end of the day, I shipped the files home so that there is another copy not only at work but at home and in both E-Mail systems for backup. Even if you only E-Mail yourself stuff, anything to save important info can help. Writing a term paper, how about E-Mailing yourself a copy each night you progress with it to ensure that if something crashes, you can get that info off another computer by accessing your E-Mail.
Just a reminder to backup what you want to keep or you just might loose it. One side note on E-Mailing stuff back to another computer. If it has a different printer, you must shut off the reformatting sequence that will happen each time you open the file or it will add a few hidden bits of memory to your file that after time will eat up your file's space. That's a bad thing that I learned about the hard way.