16bl ground glass shimming

steve waschka · December 3, 2011

i had to shim the glass with increments of card stock to get it to match critical focus at the gate. can't check this lens on another camera but it seems correct based on the distance scale. film ive shot seems ok but its all been closed down beyond f11. i mean it works but my concern is that paper is gonna contract and swell with humidity. are there steel or alum shims avail? who sells old arri parts? arri?

Dom Jaeger · December 4, 2011

Aww Steve, you really should get that set by a tech with a collimator.

But you can buy plastic shim sets in various thicknesses, usually colour-coded down to 1/1000 of an inch, which you can cut and hole-punch to suit.

steve waschka · December 4, 2011

Aww Steve, you really should get that set by a tech with a collimator.

what you dont want me to collimate your a-cams via starlight or a distant neon sign at night!!?? dont blame you. what i need to do is get my own tools.

heres what happened... when you looked thru the vf, infinity subjects were passing thru focus before hitting infinity mark. so i threw a critical focuser in the gate and it was tack sharp on the infinity mark. so i shimmed the screen until it appeared to be dead nuts on viewing sharp edges at distance.

i wish doing it was just 10 times more tedious than typing this on a touch screen smart phone. but i needed to shoot and i wanted to run that camera. as it turned out the footage was sharp. but the mags were still throwing garbage in the gate. checking the gate on the beach between takes when takes are determined by the next set rolling in... eeehhh i smell things falling in the sand and misc other issues.

so i hope writing all this pointed out i agree with and respect your opinion. i was hoping arri has a precut set i could buy and keep on hand. cause i have ocd when it comes to keeping this stuff running. and i use a bunch of old cameras and am not real fond of sending them out at the cost of what it takes to get a pro tech do it. not that they dont deserve it. god knows i know its tedious tiny adjustments that make it right.

Tim Carroll · December 5, 2011

Steve,

You're gonna run into problems when you go wide on that zoom lens. Any telephoto shots will be relatively sharp because you have such a long depth of focus, but wide angle is going to be very soft. There's a reason we "techs" set the camera up a certain way, first making sure the FFD is spot on (within five thousandths of a millimeter (one tenth the diameter of a human hair)) and then use a calibrated and collimated test lens (usually in the 25mm range) to set the ground glass spot on.

Unfortunately there is no "just do it quickly in the field" way to set that up.

Best of luck.

-Tim

steve waschka · December 6, 2011

Steve,

You're gonna run into problems when you go wide on that zoom lens. Any telephoto shots will be relatively sharp because you have such a long depth of focus, but wide angle is going to be very soft. There's a reason we "techs" set the camera up a certain way, first making sure the FFD is spot on (within five thousandths of a millimeter (one tenth the diameter of a human hair)) and then use a calibrated and collimated test lens (usually in the 25mm range) to set the ground glass spot on.

Unfortunately there is no "just do it quickly in the field" way to set that up.

Best of luck.

-Tim

i am admittedly worried about the ffd. there's no way the ground glass should be off as much as it was. i havent needed wide angle yet. im gonna have to pull it out of use and take it in.

steve waschka · December 6, 2011

here's one that may not be new to you. i have a polarizer in the blimp. if you take it out the polarizer and use a long distant horizontal edged subject, it focuses properly at infinity. when you put the polarizer in, it focuses prior to hitting the infinity mark. i have reversed everything back to the way it was and am not using this filter with this lens.

that doesnt mean the camera doesnt need fine tuning. reading your web page now REALLY makes me want to get the registration checked and the ffd checked with really good tools. tools i just dont have right now.

Tim Carroll · December 7, 2011

Steve,

Not sure what that is in particular, but there are so many things that can cause focus and image issues with motion picture cameras, which is why we always go back to square one and start from there. Make sure the FFD is right (with a mechanical gauge) then make sure the mirror is plane (less likely but possibly an issue, again, mechanical gauges), then using a collimator and a calibrated, collimated test lens, set the ground glass. Then it doesn't matter what lens or filters you throw on the camera, if the image is sharp on the ground glass, it will be sharp on the film (of course this assumes the rest of the camera is set to spec, including the spring tension on the pressure plate, etc.).

Best,

-Tim

steve waschka · December 7, 2011

its just one of those things that prob was an issue with medium format too. im just over scrutinizing this rig right now. like you said a properly set up camera should be in focus at the gate if its in focus on the ground glass. but i blamed the camera for the result of a poor filter choice to the point i shimmed the ground glass. still... i want to get the body fine tuned. but im going to stop hunting for problems now. ive passed the point of diminishing returns with that.

your website also makes me want to buy a 16s. not going to though. i dont think. no im not.

thanks again