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Canon 30-300 Can't Hit Infinity

Canon 30-300

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#1 Craig Kovatch

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Posted 18 August 2018 - 12:04 PM

Hi everyone.

 

I have a canon CN-E 30-300 that can't focus infinity at the infinity mark. I'm able to get crisp infinity focus, but the mark on the lens is slightly before the infinity mark.   I'm trying to narrow down where the issue may lie, whether it's  a backfocus adjustment  or a flange depth issue with my F55.  Before I purchased the lens three years ago I had a 300mm Red Prime which had terrible CA so I tried adjusting the flange adjustment on my F55 thinking it might help the issue. Turns out it was just a terrible lens.   I made slight adjustments to the adjustment screw and put it back in it's original position when I was done.  But I can't help but think the flange is not in it's EXACT factory position.   Bottom line, is my infinity issue a backfocus or flange issue? 

 

Thank you.


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#2 Dom Jaeger

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Posted 18 August 2018 - 06:42 PM

The first thing is to isolate whether the zoom has an issue with back-focus or with the focus scale.

Does the lens focus scale line up at the 300mm end of the zoom? Does it line up at close focus distances as well as infinity?

At 300mm, back-focus can be quite a bit out before it affects the focus marks, so if the scale isn't lining up at 300mm, it's more likely that the scale needs adjustment rather than the back-focus needing a tweak. If it lines up at close distances but is out at infinity it could be that temperature is affecting the lens, or it may be an issue that a technician or the manufacturer needs to address.

If you focus at 300mm and then slowly zoom out to 30mm and find the lens is progressively losing focus then it's most likely a back-focus (or camera flange depth) problem.

You should be able to check the camera flange depth with a lens you know is correct, something around a 25mm works best. The wider the focal length, the more an error will throw out the scale, but too wide and it's hard to properly judge exact focus.

Otherwise (and this is the most reliable solution), take the camera and zoom to a lens technician or rental house service department and they should be able to check both in a fairly short space of time, and adjust them back to factory spec if needed.
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#3 Robin R Probyn

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Posted 20 August 2018 - 03:49 AM

Dom.. is there a reason Cine zooms may go slightly beyond infinity ..and go soft.. is it due to giving leeway for mounts/flange   to avoid I guess the worse situation of not being able to even get focus to infinity .. way back I was told as a focus puller.. always back a lens off slightly from infinity .. 

 

Thanks


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#4 Craig Kovatch

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Posted 20 August 2018 - 11:32 AM

Thanks for the replies.  I was out all weekend but I'll give all of your recommendations a try today or tomorrow and get back to you. 


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#5 Dom Jaeger

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Posted 20 August 2018 - 08:28 PM

Dom.. is there a reason Cine zooms may go slightly beyond infinity ..and go soft.. is it due to giving leeway for mounts/flange   to avoid I guess the worse situation of not being able to even get focus to infinity .. way back I was told as a focus puller.. always back a lens off slightly from infinity .. 

 

Thanks

 

Hi Robin.. I think the main reason is to allow for thermal variations which can slightly shift infinity focus. There may also be an accomodation for extenders and stacking filters. Most (if not all) cine zooms overshoot infinity, while only a few cine primes do.. it doesn't seem to be uniform. For example Panavision primes all overshoot infinity while Zeiss and Cooke tend to have a hard infinity stop (although the stop can be shifted by a technician). 

 

While low dispersion glasses and tight tolerances can certainly reduce the effects of thermal variations, I suspect it also comes down to the manufacturer's prefererence. Panavision for example also include a second index mark for focusing at full aperture, which is a more accurate way of allowing for the fact that focus can appear to shift when contrast is favoured over resolution, whereas other companies may not want to admit that their lenses might shift in apparent best focus at full aperture (though in many cases they most certainly do).


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#6 Robin R Probyn

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Posted 20 August 2018 - 09:54 PM

ok thanks for the info.. yes it what I always thought ..that cine zooms go beyond infinity .. having a discussion on another forum .. where a guy is saying only cheap/ re housed cine zooms do..  empty vessels making noise :)..


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#7 Craig Kovatch

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Posted 20 August 2018 - 11:54 PM

I've tested all three of my Canon zooms, the 15.7-47, 30-105 and 30-300 with a tape.  Bare in mind I don't have any resolution charts or a Siemans star, although I am going to order one.  I would run the tape, set the focus and then look at monitor.  I always had to make a "very" slight tweak to the lens and the peaking would pop right up. So I'm exactly certain how exact the marking are on the barrel.  Maybe this is why some of the Angenieux and Fuji Premier zooms are more than double the price of the Canon's.  The 30-300 is dead on at close distances.  Interestingly, the closest mark is 5ft, but the barrel turns a bit more closer, just like it turns a bit beyond infinity.  I went out to 28", the furthest I can get in my house, and it was almost dead on, again for a very minor tweak.   The 15.7-47 I found to be out by .5" at anything 3ft.  It's hard to tell anything else with the lens because it's fairly wide.   The 30-105 was out  by roughly an inch at 8ft.  None of the lenses seem to lose focus when I zoomed out.  But until I get a chart it's hard to say.

 

Same thing here Robyn.  I back off slighly from infinity and everything snaps into focus.  I'll use focus mag on the viewfinder at the same time just to make sure.


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#8 Dom Jaeger

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Posted 21 August 2018 - 02:21 AM

So it looks like the camera flange depth and lens back-focus is fine if all your zooms are holding focus, and the 30-300 focus scale is OK if the marks line up at most distances. 

 

If the 30-300 is only out at infinity it could be a temperature effect or the zoom might be a tiny bit out of tolerance. Are you using a true infinity object like the moon to check infinity? Is your shifted infinity mark the same at 300mm all the way down to 30mm? Any change if you stop down a bit?


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#9 Craig Kovatch

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Posted 21 August 2018 - 12:49 PM

Realistically how accurate should the marks be? There always seems to be a bit of an adjustment to nail focus although the marks are extremely close.   Obviously there's very little wiggle room at T2.8.

 

Most of my infinity shots have  been landscapes. Objects up to a mile away. It's possible my definition of infinity is not correct.  I hadn't thought of that.  If it's clear tonight I'll try focusing on the moon.  I'll also check infinity at 30mm and see if it holds.  It's tough to tell at 30mm as everything looks to be in focus.   I'll keep you posted.

 

 

Cheers.


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