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New to Cine Lenses (focus markings off?)


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I just bought a used set of cine primes (Canon CN-E 14, 24, 35, 50, 85, and 135) from LensAuthority.com. These are the first cine lenses I've owned. I've been playing around with these and think they may need some calibration, but am not sure.

How accurate are the focus distance scales supposed to be? I've heard very accurate... which makes me think these are off.
With the camera stationary, I am measuring from the sensor plane to a point in focus directly in front of the lens, and I'm finding the focus markings on the lenses to be off, sometimes by a few inches. The 50 seems to be dead on though.
Is it common for cine primes to go out of 'calibration'? Is it something that can be corrected? I'm a CPS Platinum member and am thinking about sending them in to be looked at, but figured I'd ask around first.
The lenses were advertised as being in 'excellent' condition, and they appear so - the glass is flawless, and the housings show no dents, dings, scrapes, or other signs of abuse. Being new to cine lenses, I guess I'm just not sure what to expect. Perhaps I'm not even testing them correctly.
Any insight or advice is appreciated!
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The focus scales on cine lenses should be as accurate as possible, usually down to within a couple of thicknesses of the witness line. (Since the same error will cause a lens to be an inch out at 2ft, or a foot out at 20 ft, saying a lens is 2 inches out doesn't really mean anything.. an error of 2 inches at 20 ft is within tolerance for instance.)

 

The most important setting that controls the accuracy of focus marks is the distance of the lens from the focal plane, known as the back-focus. Cine lenses are machined to within a close margin of the calculated distance setting and then shimmed the last tenth of a mm or so. This allows for slight variations in machining and optical components, since the final back-focus is usually set to within one hundredth of a mm (less than half a thousandth of an inch).

 

There are other factors at play however.

 

Most crucial is the camera flange depth, which is the distance of the lens mount to the sensor plane in a camera. This needs to be accurately set to match the back-focus of the lens. If this is not correct, all the lenses will be out. Because shorter focal lengths are more sensitive to back-focus variation, wider lenses will line up further away from the correct distance mark than longer lenses.

 

The faster the lens aperture (and some CN-E lenses are very fast at T1.3 or 1.5) the more critical the flange depth and back-focus settings are as well. So a T3.5 lens may not seem out even if the back-focus is off, but at T1.5 it would show an error.

 

Another consideration is the thickness of the filter stack (OLPF) that is in front of the sensor, essentially a glass block in the light path which can subtly alter the point where a lens will focus. A very fast lens will focus at slightly different points when fitted to cameras with different OLPF thicknesses. Most pro cameras have roughly similar OLPF thicknesses, but ideally fast lenses should be calibrated to the camera they will be used on.

 

One other consideration, which I have come across with Canon CN-E lenses, is that at full aperture a lens can exhibit aberrations that decrease the contrast, and so the lens appears sharper when the focus is shifted slightly to reduce edge flare (even though actual resolution is lower). Check how the focus marks line up when the lens is closed down by a stop.

 

Generally lenses don't go out of tolerance unless they are damaged or have begun to wear, but they can certainly be set incorrrectly. I often see lenses that have been offset to match a camera that is a little out for example.

 

Any experienced cine lens technician, either from a rental house or a repair shop, should be able to check and accurately set the back-focus of your lenses, and check your camera flange depth as well. If the vendor advertised them as properly callibrated you could ask about sending them back to be checked and adjusted.

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Thanks for the knowledge Dom! Very helpful.

 

I shoot on all sorts of cams from Red to Blackmagic to DSLR (even GH5S w/ Speed Booster)... so it's hard to say what these lenses might live on regularly.

 

I didn't realize there was as much variation in sensor-to-flange depth between different cams; figured it was a tight spec. So when you rent a camera and a set of lenses from a rental house, you may or may not have focus marks that line up? I guess the lenses can be adjusted on the spot by a technician prior to the whole package going out?

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The flange to sensor distance is mount specific, each mount has its own spec. If you use adapters from one mount type to another, you can introduce errors, especially if the adapter is cheap. Remember, the distance of both lens back-focus and camera flange depth needs to be within about 0.01mm to make the focus scale accurate, which is a tolerance cheap adapters rarely achieve. Stills mounts like Canon EF are not as sturdy as a PL mount and can flex or allow a lens to twist. Using speed boosters or doublers can throw focus marks out as well. DSLRs are typically not as accurately set as something like a RED or Alexa (because stills lenses are not expected to have accurate focus scales), and mirrorless cameras often have much thicker OLPFs, so there are plenty of scenarios in your set-ups where errors can creep in.

 

I would probably lower my expectations of focus marks lining up perfectly on DSLRs or prosumer cameras, even if the lens is a "cine lens" like a CN-E, but on a RED or Alexa they should be reliable.

 

If you rent a camera and lenses from a rental house, the camera and lenses should be calibrated correctly, but of course there is always the potential for something to be off. Camera Assistants ideally check that all the lenses line up during prep, and if a lens needs tweaking the rental house can do so. Some rental houses have better gear checking procedures than others - the more pro ones will have their own service departments. I work for Panavision as a lens technician in their service department for example, and spend most of my time checking, calibrating or repairing the rental fleet. During a feature prep where dozens of lenses are going out I might need to tweak a couple (depending on how fussy the Assistant is). We don't really deal with Blackmagics or DSLRs though.

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