Lens Descolimation vs. Backfocus Issues

[Jordi Figueras]

Hi there!
Sorry if this has been treated before, but I couldn't find anything in the forums.
The thing is I had to send my RED 18-50 lens to the HQ because an issue with the /i lens data showing on the LCD. After 6 months they got it kind of sorted out (even it is neither accurate at the end marks of the focus ring nor the Iris ring). What I'm scared about is that when I send the lens the focus was spot on with the marks in the focus ring, but now it's not!(for example the lens has a minimum focus distance of 20 centimeters, in metric of course, and now it can't focus so close, the minimum focus distance seems to be 23 cm.)
So I can't really send it back and lose my lens for 6 more months.
As I understand, if the focus holds on when you are zooming is then a descolimation issue or is it backfocus?
Please, can someone clarify how can I know if it's a lens problem or a backfocus issue?
Thank you very much in advance!

Jordi Figueras

 

[J. Bernard Vallon]

If the focus does not hold through the zoom, then either the back focus is incorrect, or the lens is not collimated properly. It could be either.

However, the contrapositive might not be true for every lens. I.E., just because the focus holds through the range, doesnt necessarily mean the back focus/collimation are correct. The fact that the lens hits/misses its marks is a better indicator.

 

[Rob van Gelder]

When the zooming holds but the distance marking is not on the right mark, the lens is good but the scale is off.

put the camera on other distances, like 1 meter and 3 meter, check the mark on the lens for maximum focus. Zoom in and out and if the zoom holds, the scale is off.
If it is off for an equal amount on several distances, then the focus scale is needs to be re-adjusted. Not sure about the Red lens, but often there are very little screws on several places that hold the outer barrel to the inner focus unit and that can be adjusted.
Lens technician stuff.....

 

[Jordi Figueras]

Thank you for your fast responses!

The focus seems clearly to hold during all the zoom range in different distances, and when the AC goes to the end of the zoom to get a clearer focus mark, when going to more wide it seems to be ok. Also, after several trials it seems that the AC marks in the Follow Focus are ok for the same distances.

I really hope it is a backfocus issue, 'cause that is something I can recalibrate myself, but if it's the lens, as you said, it's a lens technician stuff (which after 6 months of having the lens in repair kind of angers me) and I would need to send it again (I'm not going to mess with the warranty there).

So, do you know something I can do, any kind of test, to know if it's the backfocus or the lens?

Thank you very much for your assistance!

Jordi Figueras.

 

[Rob van Gelder]

Jordi. I don't really understand:
You say that the focus holds well when zooming, so that means the back-focus is good.
You also say that the marks seem to be right, so 1 meter mark is really one meter and so on.

Then what's the problem?
If it is only about the closest marking (20 or 23 cm?)on the lens, just make a new mark on the lens and continue working with it.

 

[Jacek Zakowicz]

your options

your options

Thank you for your fast responses!

The focus seems clearly to hold during all the zoom range in different distances, and when the AC goes to the end of the zoom to get a clearer focus mark, when going to more wide it seems to be ok. Also, after several trials it seems that the AC marks in the Follow Focus are ok for the same distances.

I really hope it is a backfocus issue, 'cause that is something I can recalibrate myself, but if it's the lens, as you said, it's a lens technician stuff (which after 6 months of having the lens in repair kind of angers me) and I would need to send it again (I'm not going to mess with the warranty there).

So, do you know something I can do, any kind of test, to know if it's the backfocus or the lens?

Thank you very much for your assistance!
Jordi Figueras.

Jordi,
You basically have three options
1. your lens is off
2. your camera is off
3. both the lens and the camera are off.
I'd suggest to take the lens to the local rental facility and have it checked. That will tell you:
1.If your lens is off you at least have a second opinion to support your claim with Red.
2. If your lens is fine you know that the camera is off and you can adjust it.
It takes the tech 15 miutes to put the lens on the collimator and projector and tell you if your observations match his. this way you can even deduct
1.if your camera is off
2, roughly how much is off
3. which direction to change it to correct it.
As you can see you can make pretty good diagnostics with the lens checked properly by a qualified tech.
It's well worth the fee IMO.
Good luck
Jacek Zakowicz, optitek-dot-org:thumbup:

 

[Jordi Figueras]

Thank you all or your assistance!

Rob- Sorry if I didn't made myself clear. The focus holds on the marks the AC got by eye, but they don' match the distance stated in the focus ring. I mean, that when you focus to, let's say, 25 cm. the mark in the lens doesn't match that distance (it shows something closer to 28cm). This mismatch is always the same and is present in every mark on the lens, less notorious when you are getting to the infinity mark.

Jacek- Thank you very much,. I think you are right, but I want to know if there is some test I could do by myself to know what's wrong. But I guess I should go to a lens technician, in the end (which is hard to achieve here in Spain, 'cause almost all the renting houses have an agreement for not renting material or helping people who owns a Red One).

So, is there something I can do to know it or it's out of any test I could do?

Thanks to everyone!

 

[Brigham Edgar]

To me it sounds as though the cameras backfocus is out.

If your lens holds focus throughout the range that is a good thing. BUT, whether the actual markings on the lens are accurate to the actual distance it is focusing is another. You could adjust the backfocus on the camera and this will fix your scenario completely.

BUT, if your lens is not collimated to it's markings correctly you will find when you put on another lens it will most likely be out, as you have adjusted the backfocus on the camera to that specific lens. BY FAR what you need to do as mentioned in a previous post is get that lens in to a tech and get them to check the markings line up with the lens and that it truly holds focus through it's zoom range. If it doesn't hold focus your gonna be in a world of pain...well your focus puller is anyway.

Two extra things.

1. To correctly collimate your RED you need a correctly collimated lens (maybe a control lens for just this purpose) or get it on a collimator itself. By using off lenses you are going to get an immediate fix on that lens but will most probably be readjusting every change as it will not be correctly 'backfocused'.

2. In my experience with the budget line of RED zooms if you can get a lens that ACTUALLY HOLDS focus i would hang onto it, they are rare. If the distances do not line up with the physical markings on the lens i would make new markings in some way (after getting your RED backfocused correctly, or get the lens re-marked on the collimator). Either this or send your lens back again A zoom lens that holds focus is WAY better than one that does not.

 

[Blaire Johnson]

First off let's get the vocab right here guys, what you are calling backfocus is actually the FLANGE FOCAL DISTANCE.

To trouble shoot your problem you're going to have to use another lens that you KNOW is accurate/properly collimated. Get out your focus chart and do a collimation test with this perfect lens (zoom in focus, zoom out adjust the focal flange).

When you get the FLANGE on the camera set correctly put the questionable lens on the camera. Test the collimation WITHOUT adjusting the FLANGE (zoom in and focus then zoom out to see if the focus holds). Do this at several distances checking to see if the marks on the lens also line up with the actual distance. This will be kinda difficult to precisely determine because there are no witness marks on the 18-50mm so use your best judgement. If the distance does not match up with the lens that means the lens is bad.

3cm might not be worth losing another 6months of your life due to poor engineering and lens design... Red- making our lives easier with completely thought through products... oh wait...

 

[Rob van Gelder]

Jordi, it has all been said here, in several different posts.

Reading your remarks, I suspect that the barrel with the focus marks is not on the right place, but the lens is in fact well collimated.

So, as Brigham already mentioned: make new mark on your lens and go with that.

Also, it might be wiser to find a lens technician nearby that wants to re-adjust the barrel, instead of sending it out to RED for repair, for a long time. The guarantee is probably over already....

And yes, the Back focus adjustment on the RED body should be checked and adjusted with a known good fixed lens, not a zoom lens. Then, the zoom lens should be adjusted to the camera, by adding or removing shims between the PL-mount and barrel.

If you find that your zoom lens is not correct with your RED, you should not re-adjust your camera again as that will give you bad results with other (fixed) lenses.
Fix the zoom lens!
BTW, I have not seen any RED 18-50 that really holds the focus all through the range, it is in my opinion a "vari-focal" lens, not a cinema quality zoom lens.
Therefore it needs to be re-checked every time you adjust the zoom size.

 

[Jordi Figueras]

Thanks to everyone!!!!

Thanks to everyone!!!!

Hi there!

Sorry to bring this thread up again, but I'm shooting on location during the week and we don't have access to Internet over there.

I just wanted to thank everyone for helping me figure this out. I've been talking to my 1st AC about all your posts and they make a lot of sense to us both. She has been doing great focus pulling in difficult shots and we are both quite confident working with the lens now.

As it seems, I've been quite lucky with my RED 18-50 zoom, as some of you said. So I'll try to take it to a lens technician as soon as I have time to do it (and if I can find one here in Barcelona, Spain, who will help a Red One owner).

Anyway, this post was to thank everyone who stepped out helping us!

Cheers!:cheers2:

Jordi Figueras
DoP