Lens Tests

[Peter Smuts]

Hi All,

 

 

I'm working as 1st AC for the first time on a very small indie shoot. I realise that during the gear check I am expected to run various lens tests and I am not too familiar with this.

I know of the Harp test, and to test for calibration - what other tests are there? What should I look for?

 

 

Thanks in advance

 

Peter Smuts

[Nick G Smith]

II think you mean collimation not calibration. Check each lens against its distance marks to see if they are accurate- start at close focus, 3ft/ 5ft/10ft etc to infinity. If any lens is out get it collimated and check it again. Check that the iris leaves open and close accurately. Check for scratches or any marks on the lens glass. If you are using older lens see if there are any color discrepancies. If you have a long prep you can do color matching tests, focus fall off tests etc. Make notes.

[K Borowski]

Make notes.

[Gregory Irwin]

What is the Harp Test?

 

G

[K Borowski]

Am embarrassed to say this, but I'v never heard of this either, at least not by this name.

[Brian Drysdale]

Am embarrassed to say this, but I'v never heard of this either, at least not by this name.

 

I've never done one, but did a quick check to see if it's what I remembered the test being. Yes it is - this consists of strings on a frame that allow you to work out depth of field and I'd imagine that would also allow you to see if your focus is forward or behind where you think it should be.

 

Of course, there's a Kubrick story, in this case from "The Shining" - "He resurrected the "harp test" for his lenses and then went beyond that to invent a bizarre variation on the harp test which positions one focus chart every inch for fifteen feet out from the lens."

[K Borowski]

Never heard or seen it done, even under another name. Assumed I was just unfamiliar with that particular brand of slang. I guess the strings resemble strings on the musical instrument? Haven't really done DOF tests, either, as an F/stop and the circle of confusion formulae I work with are always accurate if the stop is accurately set. YOu could measure the light loss with a lens meter, then extrapolate back to the mathematical F/stop, or measure the size of the iris and work it all out mathematically without blowing film.

 

 

I heard another Kubrick story at ACVL; this is hearsay but it came from a person of importance for the British company Photomec, a manufacturer of motion picture processors: Supposedly Stanley Kubrick used to, in addition to personally testing each camera for steadiness, used to PERSONALLY come in and watch the film come off the processors, PERSONALLY make sure all the densitometers (color density checkers for film stock) were CALIBRATED. I know the man was a classic type A personality, but when the hell did he sleep if he was checking the lab at night, shooting during the day, inspecting cameras etc etc etc. Did he have a "Prestige" style living arrangement? ;-)

[Brian Drysdale]

I know the man was a classic type A personality, but when the hell did he sleep if he was checking the lab at night, shooting during the day, inspecting cameras etc etc etc. Did he have a "Prestige" style living arrangement? ;-)

 

Seemingly only one printer at Rank Labs was capable of producing steady prints and they shot on matte perf stock. I don't think he needed much sleep, Kubrick was always phoning people in the middle of the night.

 

The "harp test" is listed on the UK film industy's training organisation Skillset's knowledge requirements for (I assume) the 1st A.C.

http://www.skillset.org/uploads/pdf/asset_13097.pdf?1

[Tom Jensen]

The first thing I do is look at the lens for dents, cuts, scratches missing paint on the marks. I physically move the iris ring and the focus ring to see if there are any tight spots, grinding of sand or debris, or anything out of the ordinary like a loose focus ring. I look through both sides for dust, water and grease, then I open and close the aperture to make sure the iris opens and closes or if there is a rogue leaf. I will then shake the lens and listen for loose elements. Sometimes the front element is loose and the retaining ring needs to be tightened. You will often hear the leafs when you shake it but loose glass sounds different. Check to make sure that a filter ring or an adapter ring screws onto the lens. Examine the mount to see if it is clean or gouged. Take a small screwdriver and check to see if the mounting screws are tight and or missing.

 

Place the lens in the camera mount and make sure that the camera mount tightens and loosens freely. Set up a focus chart and tape off your distances like has already been mentioned. Fill the frame with the focus chart and make sure the chart is square to the camera and flat. focus on the center and make sure it is sharp. Now look in all four corners or the four quadrants and make sure they are the same or similar. They should be in focus but not always depending on the focal length. On a wide lens if the corners are out of focus they should be out evenly. If one corner is softer than the others, that is not a good sign. Have a tech look at it. Check for lens shift. If you change focus and the image moves, that is also a bad sign.

 

Check for color aberrations. Look where black meets white and rack through the focus. If you see a green tinge on one side of sharp and a blue tinge on the other side of sharp, you have a color aberration. This is inherent in the lens and cannot normally be fixed but if it is bad, swap out the lens. The best way to test the lens is on a projector. If the lens tech will allow it, go for it. This is how you learn. Ask him if he will show you how to collimate the lens as well. Wide angle lenses are the most problematic. They have the greatest depth of filed but the least depth of focus. If the lenses are not spot on or within tolerances the whole image is soft. If the flange focal depth is off, it's the wide lenses that go bad first. Telephoto lenses are the opposite. Telephoto lenses have a smaller depth of field but have a larger depth of focus. If the flange focal distance on the camera is slightly out, you might still get acceptable images with a telephoto lens.

 

Make sure the follow focus unit works with all the lenses. If the lens binds at all, move the follow focus arm slightly away from the lens and try it again. Sometimes the ring gets out of round. I'm sure there is more and If I think of anything, I will repost.