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problem with focus on zoom lens - need help.

C

charles lim yi yong

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Hi everyone, I have this problem and was wondering how I can fix it. I have the RED canon mount and been using the tokina 16-11 and the canon 70-200mm . I have been using these lens happily, but I quickly realised that when I zoom in and focus and later zoom out , the point which I have focused on is no longer in focus.
I was thinking ok maybe this has to do with the backfocus, so I got my RED focus out and used it. OK this is the first time I am using the RED focus, and I went through the PDF. But it does write down anywhere when do I know the backfocus is correct. I presume that it's when the middle cross is in focus and sharp. So I tuned the backfocus to that.
However I am still having this problem.

thanks for reading this and I look forward to your helpful insights.
 
Your lens back focus or camera flange depth, or both, is out. Get it checked and corrected. It seems that you don't know how so leave it to the techs.
Red focus is only PL so that may not help you with Canon mount.
 
Thanks for the headsup. ok I am getting contridicting information here.
Optitek, the Canon mount backfocus has to be set differently? Wow that's a world of hurt there.

Anyway, what's the proper way to do this? get a laser range finder and set the distance to match the focus mark on the lens?
If not who do I send it to? Doesn't sound practical for folks which swap mounts in a middle of a shoot(not like I am thinking of doing that, but u never know).
 
It's absolutely practical. You need to set the back focus on your lenses correctly, your flange depth on the camera correctly and voila- mix and (always ) match!!!
It's worked on film cameras since the dawn of film making- I don't see why it wouldn't in digital acquisition realm....
 
I expect its the lens back focus.

Which means that most of the rest of this isn't applicable to your situation.

If your camera's back focus was off, then nothing would be in sharp focus at any point with any lens. That's why I think its the lens.

Jacek (Optitek) is telling you to check both just to cover all bases. It could be the camera ... but I think not.

Using Red Focus and setting Backfocus on the camera for PL lenses gives you a fairly accurate back focus for Canon mount lenses.

Technically yes you must set back focus differently for PL and Canon mount ... in practice its not a big deal.

If you do swap mounts a lot then I'd advise setting back focus on the camera whenever the PL mount goes on ... because things will get jostled. This will be overkill ... but its the kind of overkill I approve of.

I advise checking the Canon lenses with a focus chart. You may note a consistent difference between the Canon and PL mount ... if so you should account for it. I've only used the Ti Canon mount, and have noted no difference in back focus after mount swaps ... although I admit I didn't test that as thoroughly as I should have.
 
Alexander you making assumptions here:
In 25 years of doing tech support I've learned that making assumptions is your enemy- don't!!!!!
 
OptiTek said:
Alexander you making assumptions here:
In 25 years of doing tech support I've learned that making assumptions is your enemy- don't!!!!!
Click to expand...

Oh I agree!

Apparently I didn't make that clear enough.

I definitely think you should check both the camera and the lens.

I just was stating my expectation of where the problem is based on my rather limited experience.

If for some reason I could check one and only one possible cause, I'd check the lens. Thankfully that particular restriction hasn't happened to me yet!
 
Charles,

You may want to check that the particular lens you are using has consistent focus marks through its zoom range. Some of the Canon zooms have different lens sizes written on the barrel with a line to the witness marks (such as they are). If there is more than one line on there, it could be that they don't have a consistent focus mark for the entire lens.

That's not to say you don't have a backfocus issue as well or instead, that may be the case as well.
 
My guess is it boils down to using an SLR lens as a video lens. Video and cine zoom lenses are designed to hold focus throughout a zoom. That is not an issue when shooting stills, so the lenses weren't designed to do that. Sometimes you get lucky and find one that will. I have a Nikon 17-55 that doesn't hold focus throughout a zoom while a friend of mine has one that does. Same model of lens - different results. I use an SLR zoom as a variable prime, so I am not zooming during a shot. If that is something that is important to you, test the lens before you buy it.
 
To Larry's point, most SLR lenses are no parfocal lenses. Some are, and there's a list somewhere, but not many. The lens I use the most on my 5D isn't (24-70mm). That's why I use the zoom button to focus and not the zoom on the lens barrel. You can use the 1:1 monitor function to check this as well.
 
mine is a version 1 70-200mm canon 2.8 IS L , it's quite a well regarded lens in the stills world.
the other lens is the tokina 11-16mm 2.8 lens (wished that they made a short IS lens )
 
I'm surprised that Larry is the first person to bring this up but as he said, stills photographers do not care if their zooms hold focus throughout the range, nor do they need to. The majority of stills zoom lenses are not built with zooming in to check focus in mind. You may be lucky and find one that does or you may be able to get a lens tech to work on your lens and modify it to hold focus.
Stills zooms are a cheap way to get lots of focal lengths but this issue also underscores why cine lenses cost a lot more.

Before you paint me as a "pl elite lens " snob (terms in quotes from another reduser) , I've used stills glass for years on my camera. I bought one of the first examples of Jacek's red nikon mount and will be getting his epic nikon mount. It is a great way to get a lightweight, reasonably priced and sharp and fast lens set but it's not a replacement for a cine lens set. The zoom issue is unfortunately one of the compromises that you have to live with. There's no free lunch.
 
The issues he describes do not pertain to parfocal issue but rather to B/F- Flange issue. Parfocality matters but no matter what you should be able to have both extreme ends(wide and long) in focus by adjusting lens or/and camera.
Bringing parfocality (?) into this makes the issue unnecessarily more complicated...
 
charles lim yi yong said:
when I zoom in and focus and later zoom out , the point which I have focused on is no longer in focus.
Click to expand...

So are you able to refocus and get the chosen point back in focus? You are doing this with manual focus, not AF, correct?

If you can refocus after zooming and get your chosen point back in focus, it's a parfocal issue. That's part of the still lens design that stills shooters don't notice, especially if AF is on.

If you are can't focus at all at infinity, it's definitely a backfocus issue. You can adjust the backfocus to correct for that, but the parfocal issue with your stills zoom lens will remain.

If you are zooming in just to check focus, use focus aids instead.
 
That's not entirely correct- you may still be able to refocus zoomed out if you B/F or flange are off(short).
You may also not be able to reach infinity zoomed out if your lens B/F or camera flange(or both) is off(long).
Neither one of these scenarios has much to do with the lens being parfocal(or not)
 
OptiTek said:
The issues he describes do not pertain to parfocal issue but rather to B/F- Flange issue. Parfocality matters but no matter what you should be able to have both extreme ends(wide and long) in focus by adjusting lens or/and camera.
Bringing parfocality (?) into this makes the issue unnecessarily more complicated...
Click to expand...

I stand corrected. Good to know.
 
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