PL cine mount lenses: DoF & AoV???

[Michael B. McGee]

I'm sure this has been discussed numerous times, but I just woke up from a dead sleep and my brain won't let me go back to bed until I find an answer (or at least ask the question). So, if a 35mm adapter uses photographic 35mm full frame as the format in which we base on lens decisions on. Then how are we to calculate focal lengths, AoV, DoF, etc, with a PL Cine lens attached?

[Bob Hart]

There are published tables or charts for looking up different formats and the focal lengths to be chosen for fields of view. There might even be some for the new handheld computers by now.

The Guild of British Camera Technicians publishes circular slide rules. The US ASC and SOC may have their own or can recommend something.

[Charles Papert]

Don't lose sleep over this one! If you are an iPhone/iPod user, there are now a dozen or so DOF apps available, and the good ones cover still formats as well as cine. I use pCAM, which has a lot of other functions as well as DOF and is the most expensive of the bunch--the field of view previz is fantastic--but it has a standard full-frame 35mm as well as individual settings for all current DSLR's etc.

Do bear in mind that many PL mount cine lenses will not cover full-frame 35mm...

[Michael B. McGee]

Originally Posted by Charles Papert

Do bear in mind that many PL mount cine lenses will not cover full-frame 35mm...

that was gonna be my next question. I'm fully aware of all the apps, charts, and calculators. I might not be making myself very clear or the answer is so easy you along with Bob must think i'm nuts for asking it. here it goes again. I just want to know what the format would be using a PL Cine mount lens on a 35mm adapter? if its photographic 35mm full frame then what let's say would a PL mount 25mm lens become for calculating AoV, DoF, etc.?

[Charles Papert]

The answer is indeed easy, but it takes a moment to wrap one's head around it. A 25mm is ALWAYS a 25mm, whether it is mounted on a full-frame 35mm sensor or 35mm adaptor, an APS sensor, or a 1/3" DV camera. What changes is the field of view. The smaller the target, the less of the lens is used and thus the more restricted the field of view, which results in a more telephoto image. That same 25mm lens is considered wide when mounted on a full-frame target but considerably telephoto if used on a 1/3" camera (with a simple mounting adaptor, not a ground-glass adaptor). Lenses don't "become" different lenses, it's just a function of how much of the coverage of the lens do you end up using with a given sensor.

So for a cine PL mount lens which was designed to cover a Super35 frame, for instance, when you place it in front of a sensor that is substantially larger such as full-frame 35m, in most instances you will see vignetting or at least focus falloff as you get to the edges.

When you use the DOF or FOV (more generally used than AoV) calculations, the only constant factor you need to ensure is that you have selected the appropriate sized target size.

[Michael B. McGee]

so, if i'm trying to calculate DoF, AoV, etc. with a PL Cine 25mm lens on a 35mm adapter. My format will be 35mm FULL FRAME with a 25mm lens? correct?

thanks a lot in advance.

[Brian Drysdale]

If it's a cine lens, it more likely to be one of the 35mm motion picture formats than 35mm stills (35mm full frame).

P+S TECHNIK | Professional Cine Equipment Manufacture | PRO35 Image Converter - PRO35 Image Converter for 2/3" Cameras

[Bob Hart]

The PL-Mount, is a mount which serves both 16mm and 35mm motion picture film formats. You need to be a little careful in choosing your lens if it is coming off an auction site.

Example, the lens sets we are using here on a pair of SI2K cameras (Super16mm sized sensor approx) are CP Ultra T* in the focal lengths, 9mm, 12.5mm, 16mm, 25mm.

A similar Field Of View range 35mm lens set might start at say 18mm and finish at 85mm. Charles will likely correct me on this as I am muchly guessing here.

As for target size, the P+S 35mm adaptors were designed to faithfully render the standard 35mm motion picture frame or a widescreen crop of it to 1/3" camera sensors, 1/2" camera sensors for the Mini35-400 and 1/2" and 2/3" camera sensors for the Pro35. The target is in the ballpark of 22mm - 24mm wide.

The other groundglass adaptors are not so wedlocked to the 35mm motion picture frame. If you have a lens-in-camera style camcorder, you have some flexibility in how much of the groundglass area you use.

There are practical limits and it is prudent to stay close to the 24mm wide target. Greater "apparent" resolution can be had with a wider target but then some other issues can creep in like the soft corners and brightness falloff in corners or edges Charles describes.

Wider lenses will trend towards edge or corner faults.

It is prudent to select "fast" lenses of f1.8 or lower f number if you can. This is not to say you should routinely use them wide-open. However it gives you a greater chance of being within the "sweet" spot of the lens whilst remaining below f5.6 which is more or less the practical limit of groundglass relay systems.

A f3.5 or f4 lens is not going to give you as much headroom. For groundglass work you need as much sharpness out of the lens as you can get as it is in the nature of the groundglass to amplify any softness in the lens.

The same goes for poor focusing. I use the term "faith based focusing" for many who practice using the eye and LCD viewfinder alone. "It's looks sharp - roll camera". Later in the edit suite, heartbreak prevails when the chips in the paint on the doorframe a foot behind the softly focussed actor are as sharp as a tack. A good monitor is also beneficial but should not substitute for checking focus with a chart.

I drive people insane with this obsession.

Here is a trailer clip of a recent shoot I assisted with. It was shot with mostly Nikon primes on a Letus Extreme/Sony PMW-EX1 combination.

http://www.youtube.com/watch?v=Q5gsaFs-pDQ

If you look closely you will observe I was chasing a little too much of the groundglass in the quest for apparent sharpness by setting camcorder zoom at Z67 - Z69 ( about 38mm) and began to pick up the edges of the prism path. This Letus has been modified for x-y adjustment. Normally you would select about Z79 for a smaller target than the 28mm wide or so I was setting the camcorder at.

To achieve acceptable results, you must pay attention to getting focus absolutely spot on with either a focus chart in setting up every shot for near and far positions if pulling focus during the shot,


The obsession with maintaing absolutely perfect focus must extend to what I call relay focus as well, especially with lens-in-camera style cameras which can move slightly even if the focus is taped against movement when power is cycled. Interference by others on a loosely controlled set with vollie crew is not uncommon.

[Michael B. McGee]

Originally Posted by Brian Drysdale

If it's a cine lens, it more likely to be one of the 35mm motion picture formats than 35mm stills (35mm full frame).

P+S TECHNIK | Professional Cine Equipment Manufacture | PRO35 Image Converter - PRO35 Image Converter for 2/3" Cameras

are you 100% sure about this statement??? i'm pretty sure the lens doesn't dictate the format.

[Charles Papert]

It sure does, in the sense that all lenses are designed to cover a certain format, outside of which they will vignette, show focus falloff or both. Modern cine lenses are designed to cover Super35mm which is similar to APS-C. Older cine lenses may not even cover that. There exceptions such as lenses designed for large formats (Vistavision, Imax, Dalsa) but many of those are still lens conversions. We are starting to see cine lenses built for full frame formats, like the Zeiss CPII's, and there will surely be more to follow.

[Brian Drysdale]

Charles has more or less said it all.

You can also have PL mount cine lenses that only cover Super 16.

[Charles Papert]

Confirmed Mike, yes.

Thanks Brian for reminding me about the 16mm lenses also.