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I think my primes have focus drift?


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#1 Michael Ryan

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Posted 06 July 2006 - 06:45 PM

Hello All,

I mainly shoot in Super 8, but I have recently bought a 16mm camera. It is a reflex camera with a mirror shutter and I focus on ground glass. The camera has 7 primes lenses. It also has a spot on the side of the camera where you can hang a tape measure and set the focus on the lens (they have the markings in feet and inches).

So I set up the camera 12 feet from the wall and put a card up that has grid lines on it. I took each lens and focused on the card by my eye (through the lens) and focus mark the 12 foot mark on the lens. Here is the problem.

3 of the lenses matched my focus test. By that I mean I focused with my eye (though the lens) and then I looked at the side of the lens and it read 12 feet, so both agreed. However, when I focused my 150mm lens, when I focused by my eye the side of the lens read 15 feet. So if I filmed something with this lens at 12 feet would it be out of focus? I also realize that the F stop plays a role in this as well. At a higher F stop would my depth of field cover this...if indeed it is a problem?

The other lenses were only off by 5 or 6 inches. I wouldn't think that was much of a problem, but you tell me.

Thanks for the info.

Mike
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#2 Dan Goulder

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Posted 06 July 2006 - 07:21 PM

Trust your eye. Also, you should check the back focus on these lenses. When you focus to infinity, a far shot should be in the sharpest focus. If your focus knob goes past this point, you need to have a back focus adjustment done on the lens. If you can afford it, you should have a reputable lens tech check out the entire set. It's important to remove as many variables as possible, as it will give you greater confidence in your results.
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#3 Tim Carroll

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Posted 06 July 2006 - 08:18 PM

Trust your eye. Also, you should check the back focus on these lenses. When you focus to infinity, a far shot should be in the sharpest focus. If your focus knob goes past this point, you need to have a back focus adjustment done on the lens. If you can afford it, you should have a reputable lens tech check out the entire set. It's important to remove as many variables as possible, as it will give you greater confidence in your results.


You should also have a camera tech check to see that your flange focal distance and your ground glass are set properly. If both of those are exactly to spec, then anything that is in focus when you look through the viewfinder will be in focus on the film plane. It does not matter what the focus scale on the lens says.

-Tim
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#4 Michael Ryan

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Posted 06 July 2006 - 08:46 PM

It does not matter what the focus scale on the lens says.

-Tim
[/quote]

Hello Tim,

Thanks for the info. If you have the time, what do you mean by your quote exactly? Why do you see some DP's with a measuring tape setting the focus? Is this old school?

I can see the ground glass when I look into the lens opening and it appears to be set correctly. I guess I could also shoot some film to test the focus as well. Can you recommend any tests I can do with real film to check out the focus?

Thanks,

Mike
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#5 Tim Carroll

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Posted 06 July 2006 - 09:42 PM

Mike,

First, it might help if you could tell us which 16mm camera you are using. I can only speak from experience with Arriflex 16S and S/B cameras because I service them, and the Arriflex 16SR camera because I own and shoot with one.

The thing you must understand about motion picture cameras is that the flange focal distance on the camera, the distance between where the lens physically stops in the lens mount and the imaginary plane created by the surface of the gate, that dimension is critical for sharp focus.

So you start by setting that, and it needs to be set to a pretty tight tolerance, on the Arriflex 16S cameras I service, I set it to a tolerance of plus or minus .005 mm, or about one tenth the diameter of a human hair, so very exact.

Once you know that your flange focal distance is accurate, then you need to set your ground glass. On a reflex camera like the Arriflex, there is a rotating mirror that reflects the image to the ground glass. Using a specially calibrated test lens, you set the ground glass to precisely the right distance from the flange/mirror combination. This makes the ground glass optically the same distance from the flange as the film plane is from the flange.

Now with both of these set very accurately, you put the lens on the camera. You focus the lens until the image is sharp on the ground glass. Now since the ground glass is optically in the same place as the plane of the film, you know the image on the film is as sharp as the image in your ground glass.

The lenses that were made for the Arriflex 16S, 16M, 16BL, and other cameras from that period (1950's and 1960's), the lens distance scales were never that accurate to begin with. Now, forty or fifty years later, they ain't gettin' no better. So setting your FFD and GG accurately is the safest way to know that what you are seeing in your viewfinder is what you are getting on film.

Now, professional cameras with professional lenses, like the Arriflex 16SR, SRII, SR3, and Aaton XLR, LTR, and A-Minima, and the Zeiss Super Speeds or a good set of Cookes, those combinations are far more accurate. The distance scales on Zeiss Super Speeds better be accurate, same with the Cookes. And you will see a professional DP, or more commonly the camera operator, do a focus test at the beginning of the day with the camera and all the lenses, and if anything is off, he or she knows that the lens is out of calibration, or the ground glass is off. And for some shots, they will tape off the distance and match it to the lens, because on those professional cameras, which should be set up perfectly, measuring is a more accurate way to critical focus than depending on what you see in the viewfinder.

Make sense?
-Tim
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#6 Charles MacDonald

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Posted 06 July 2006 - 09:43 PM

I can see the ground glass when I look into the lens opening and it appears to be set correctly. I guess I could also shoot some film to test the focus as well. Can you recommend any tests I can do with real film to check out the focus?

BEST TEST is to have a camera tech use their instruments ot get everything lined up.

Test with FIlm. Set up a test target, 1951 Air force, or simalar - Set the focus by eye, shoot a short sequence, (slated to say it was by eye. Measuree and set to scale,, slate and od another sequence. Get the negative processed and look at it with a microscope...

see if either is sharp. 8>
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#7 Hal Smith

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Posted 06 July 2006 - 09:50 PM

Here's my experience with used lenses:
I bought a 40mm Cooke Speed Panchro Series II on eBay from a camera store up north. When I popped it in my Arri IIB it was radically off, it wouldn't focus out to infinity. Sent it to Guy at ZGC, he serviced it (the barrel was slightly warped) and collimated it. Now it agrees with my other three II and III's. Repair price? $190 and result? a perfect lens. It was worth the money, the glass is flawless. He also did a small repair and collimated my 25mm Series III for $95, another eBay lens but at least its seller was upfront and sold it with the caveat that it had a problem.
The only Cooke lenses I've bought with no trouble came from Isaia. Sometimes it's just best to purchase from professional movie houses. The only reason I've been putting up with lenses off sleazyBay is that Cooke Series II and III's still in original Arri standard mounts are a bit hard to find.
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#8 Tim Carroll

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Posted 06 July 2006 - 10:04 PM

Hal,

I agree that once the camera is set up properly, which I'm sure your IIB is, and if you have the money, then by all means, have a good lens tech set them up. Also, I would put the Cooke Speed Panchro Series II lenses in the category of professional lenses.

I made an assumption (and you know what happens when you do that :P ) that since Mike has been shooting 8mm and just got a 16mm camera that came with seven prime lenses, that it probably wasn't an SR, or XTR or LTR, but something more along the lines of a Bolex, 16S, CP16 or the like. And I also made the assumption, which could be equally as wrong, that the lenses he was talking about were old Switars, Schneiders, Rodenstocks, or Angenieuxs. And though I have seen a few nice Switars, and I happen to own a cherry Angenieux, most of those forty and fifty year old lenses I have seen would definitely not fall into the same professional lens category as your Cookes.

-Tim

PS: On a totally different note, but pertaining to the title of this thread, I have a Schneider 25mm 1.4 that does have focus drift. I had never seen a lens do this before. I focus it sharp as a tack, and then don't touch it or the camera, and in thirty seconds the image is completely soft. So I focus it again, and thirty seconds later, soft again. I did this like ten times, never could get it to hold focus. It is just a lens that I took in with a package that I wanted, so I doubt that I will pay any money to have it fixed, but I was wondering if anyone else had ever experienced this. It is really strange, like something from the Outer Limits or Twilight Zone. You can't see the barrel of the lens or the focus scale moving at all as it goes out of focus. Must be a floating element somewhere inside.
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#9 Hal Smith

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Posted 06 July 2006 - 10:26 PM

Hal,
I agree that once the camera is set up properly, which I'm sure your IIB is, and if you have the money, then by all means, have a good lens tech set them up. Also, I would put the Cooke Speed Panchro Series II lenses in the category of professional lenses.
.......Schneiders, Rodenstocks, or Angenieuxs. And though I have seen a few nice Switars, and I happen to own a cherry Angenieux, most of those forty and fifty year old lenses I have seen would definitely not fall into the same professional lens category as your Cookes.
-Tim

I was adding into the thread the information that even with professional lenses you still need to double and triple check things. I haven't sent my Arri 2B off to be bench checked, it came to me from a trusted source. However you can bet your bottom dollar that if anyone ever hires me to shoot a whole bunch of film with it that it'll have a date with an Arri 2 specialist. I'll take chances with my own little short end projects but any camera for hire needs a thorough checkout (IMHO).
The expense of having equipment checked and serviced pales when compared with what it would cost to ruin a couple of thousand feet of new 35mm stock. To say nothing of having to bring cast and crew back for a reshoot. There's a very good reason why all the full-time working professionals in the movie business are very, very conservative when it comes to equipment and working practice.
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#10 Michael Ryan

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Posted 07 July 2006 - 02:10 PM

Hello All,

These lenses are from a fairly rare Kodak Reflex Special which looks like the Mitchell 16mm. It's a very well built and well thought out 16mm. Lots of great features. The 7 primes are 150mm, 100mm, 75mm, 50mm, 25mm, 15mm and 10mm. They are Kodak Ekton made in France by Angenieux. They are an R mount.

I believe this camera is from the late '60s or early '70s.

Mike
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