Correcting focal measurements on non-cine lenses

jwing

jwing

Active member
Sorry if this has been asked here before, but I did some searching and I couldn't really find anything.

Anyway, it goes without saying that standard 35mm lenses are not terribly accurate in terms of their focal measurements. At times, they are ridiculously off. I have measured from a subject to the lens, and to the back of the lens (to the front of the GG on my adapter), and to the back of the adapter, figuring I would cover all bases of the focal plane. Regardless, it's all way off. It seems accurate for the first 1.5-2 feet, but thereon it gets off by several feet. (While I'm at it, could I get a final word on what I should measure to? I've heard that it is to the front of the adapter, back of the lens. Right?)

For example, I once measured a subject that was 3 feet away, but it wouldn't come into focus until I set the lens (in this case, a Nikkor 28mm f/2) at about 8 feet! That's a 5 foot discrepancy!

That said, for those of you who measure and set marks, how have you gone about "calibrating" or adjusting for this discrepancy? I suppose I could get a fine tip marker and use thin white paper tape to cover the lens and do a much more accurate measurement; or, I could produce a chart of the discrepancy and refer to it as necessary. Both ideas seem a bit absurd, but maybe the first one is a little more practical.

Any other ideas?
 
What adapter are you using? Have you checked the lens collimation (distance between the rear of the lens and the GG)?

Also to measure the distance, you'll want to measure from the subject to the GG.
 
I'm using the SGPro. But like I said above, I checked all possibilities, including from the subject to the GG. What would lens collimation tell me?

In either case, I've heard that 35mm lenses (At least Nikkor in my case) have especially inaccurate markings. Is that not correct?
 
Well if your collimation is off then it will throw the focus marking on the lenses off. Do you have infinity focus on the lenses?
 
Hmm. I guess I meant, how do I check the lens collimation? Check for the distance between the rear of the lens and the GG? What do I do with that measurement?

Yes, I have infinity focus on all the lenses. They are: Nikkor 28mm f/2, Nikkor 50mm f/1.8, Nikkor 85mm f/1.8. All pretty standard.

Just yesterday I had the 50mm up, and I focused on an object that was 5 feet away, but the lens marking was reading about 2.5 feet when in focus. I mean, that's a huge difference, and won't account for where I am measuring to on the focal plane in the first place. Otherwise, everything seems to be in good condition. These are nice lenses, and when I set everything up properly, I'm getting good, sharp images. It's just that the focal measurements are totally useless...

What's the deal?
 
jwing said:
a Nikkor 28mm f/2) at about 8 feet! That's a 5 foot discrepancy!
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Could you tell me exactly what Nikon lens it is and from what year?

35mm photography in SLR alway relied on another method other than distance marks to get your subject into focus (like ground glass prizms), but it should not be "that" off the mark, just a tad, enough to mess up a 16mm shoot on a cam w/o GG or VF when fully open. And that GG trick worked with the 8 perf sideways ground glass for 35mm stills, did not work too well with DSLR's. Most shoot those in AF at low f-stops.

On some lenses the focus ring is a dial that can spin, if the lens has ever been cleaned or repaired it may just simply be mis-aligned.

But I would like to know what exact Nikkor lens this is. - These problems are more evident either with-in 1 meter of the lens, or very far away between say 20ft and infinity. You got one right in the middle of the ring.

ALSO: Have you tried taking measurements at different intervals 1ft 3ft 5ft 8ft etc.. on the same lens, distance vs. what the ring reports?
 
jwing said:
Hmm. I guess I meant, how do I check the lens collimation? Check for the distance between the rear of the lens and the GG? What do I do with that measurement?
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If the adapter doesn't let you adjust lens collimation, there's not much that you can do. This is the distance between the lens mount and the GG such that the focus mark on the lens will line up with actual subject distance. All the newest adapters does it (SGBlade, Brevis, Letus Ultimate/Elite, Redrock M2E) but the feature wasn't standard in older adapters like the SGPro and the Letus Extreme.
 
The lenses themselves aren't going to be calibrated and marked accurately enough to allow you to do true focus measuring anyway. Those markings are for ballpark, not for precision.

Cinema lenses are precisely calibrated in terms of the focus distances, which is why they have "witness marks" on them, and why the focus rings have so much more travel.

In short, trying to measure focus on an SLR lens is going to be a crapshoot.
 
Absolutely right about the inaccuracy of distance marks on stills lenses. After all they were designed for photographers to focus by eye in viewfinders or use AF where the marks don't matter.
You either use a follow focus unit and make marks or go the low budget paper tape on the lens method as you suggested.
 
jwing said:
Sorry if this has been asked here before, but I did some searching and I couldn't really find anything.

Anyway, it goes without saying that standard 35mm lenses are not terribly accurate in terms of their focal measurements.
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For most 35mm SLR work, using the focusing ground glass was the method to focus. While for some copy work, perhaps some one would measure the distence from the focal plane mark on the camera body, then set the lens to the distance markings... but that was rare, and probably as you found, inaccurate, so people who needed accuracy would make their own marks as required.

For movie film cameras, the lenses were calibrated for more accuracy, not only in the focus distance marks, but also the 'f-stop' settings, as in T values... that is the actual measured light transmitted through the lense, rather than some 'guestimate' marking that most 35mm still (or any other still format) lens had.

If your distances measured are 'way' off, as you indicate like many feet, then I'd look at how you are 'determining focus', that is when focusing, the lens should be wide open, and further, how the 35mm lens is in relationship to the apparent plane of focus in the adapter... perhaps the adapter is either defective, or out of adjustment.

Back in the way back olden days, pre-SLR, lens markings on 35mm lenses did have perhaps better accuracy, but then the f-stop was such that the depth of field covered any inaccuracies... But then in the way back pre-SLR days, one had parallax problems with the view finder being offset from the lens...

All the more reason for me not to like the 'adapter' setup for fixed lens digital video cameras...

Sort of like, if you can't have the lens you want, live with the one you got...


As an additional note, my 'lens check' procedure goes like this.

Take a standard newspaper, tack it to a wall. Have the camera on a tripod, and with the lens wide open, focus for sharpest letters, etc., and take a shot.
Adjust the f-stop in steps down, and take a shot at each step.

Do this for several distances.

In your case, you would measure each distance from the 'focal plane'. If the adaptor does not have an equivalent 'focal plane' mark... I'd worry...
 
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As soon as I get a chance, I'm going to run some tests with all three of my lenses, and focusing at different distances fully open. I will record these and report back to you guys sometime in the next few days.
 
Honestly, as an AC myself, just look at those distance markings and then just forget that they ever existed. Sure, it's helpful to know where things are in terms of distance and how much of a turn gets you how much distance, but really, the distance markings are so inaccurate that they just give you this false sense of security.

Of course all lenses are different, but the ones I've worked with recently are just a waste. If you really want, I suggest doing what someone suggested earlier, and putting up a focus chart (a siemens star is a good thing to have). From there put your camera at all the distances you would like to have on your lens and then remark it. You can find some very thin striping tape and use that to cover up the previous distances and write in your new ones. If you're detailed enough at this, I'm sure your AC will appreciate the effort and not hate you quite so much. Also, once you do this, you should be checking it every day to make sure nothing has slipped.

The other thing you could do is adjust the distance between the ground glass and the lens, but since a lot of adapters don't have a good way to adjust this, I would just suggest the previous method. Even if your adapter does have a good way to do this, I would still be adding more inbetween distance markings with a grease pencil.

Even when you do this though, with any lens over maybe a 35 I would suggest just getting eye focus. It's way more accurate and you won't end up with soft footage.
 
RyanT said:
Honestly, as an AC myself, just look at those distance markings and then just forget that they ever existed. Sure, it's helpful to know where things are in terms of distance and how much of a turn gets you how much distance, but really, the distance markings are so inaccurate that they just give you this false sense of security.
...
Even when you do this though, with any lens over maybe a 35 I would suggest just getting eye focus. It's way more accurate and you won't end up with soft footage.
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Well, actually, one of my reasons for doing this is as an insurance. I (and my ACs) use our eyes first and foremost, but I think it's always good to have that second insurance for the details that we can't capture, or for characters that are a little shadowy and thus difficult to focus by eye even on the sharpest monitor. On a recent shoot, I had a director complain that the focus seemed a bit soft on a subject, but I adjusted and readjusted by eye, and it seemed to me that the focus was as correct as it could be. With measuring, at least I could double check and also give the director a sense of security. There are times for me when an actor seems too dark to focus by eye (but there is still enough light to expose properly), and there are times when I feel like it might be in focus, but even using my eyes I'm not always 100% certain. So for me, measuring would be only secondary, but nice to at least be able to do.
 
Yeah, I understand.

Just a helpful hint if you're having trouble focusing in dark areas. Sometimes I'll have my 2nd go out their with a maglite and unscrew the top half, leaving just the bare bulb. Putting that at the point where we need to focus makes things as a breeze, as it's just a light source that goes pretty in or out hard and fast. That's just in case you're on a tight lens and still feeling a little iffy about focus...
 
Barry_Green said:
The lenses themselves aren't going to be calibrated and marked accurately enough to allow you to do true focus measuring anyway. Those markings are for ballpark, not for precision.

Cinema lenses are precisely calibrated in terms of the focus distances, which is why they have "witness marks" on them, and why the focus rings have so much more travel.

In short, trying to measure focus on an SLR lens is going to be a crapshoot.
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If you were to measure them yourself and remark the barrel would they remain accurate? The markings are off, but they're consistently off, right?
 
They should stay reasonably accurate for a while, but I wouldn't count on it. If the lenses are taken to a hot or cold environment, the shrinking/expanding of the metal might throw the marks off again. Cheap SLR lenses aren't made to be that accurate, and frankly they'll never be accurate enough to rely on for measuring distance for critical focus, you'll always be better off focusing by eye.
 
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