Backfocus Problems - Help!

[Clark McCauley]

So I need some help with my backfocus. I'm on a scarlet with a tokina 11-16 f2.8. When I adjust the backfocus for the 11mm focal length at 6ft, everything looks fine, if I zoom in it's totally off. I made the adjustments at the higher focal length and went back to 11mm and that was totally messed up. Is it just a junk lens? I have the Canon 70-200mm f2.8 which is much nicer, is it safe to adjust back focus on such a telephoto lens?

Thanks

 

[Jacek Zakowicz]

You need to first check your Tokina lens on a collimator to make sure it's not out of whack.
If the lens checks out ok here is what you do:
with the lens on camera , camera on and 1:1 punched in on the screen
1: set up height resolution target (a dollar bill) square to the camera the same height as the lens center no more than 3ft away
Lens wide open.
Zoom in to16mm
adjust for best focus with the lens focus ring only
zoom out all the way to 11mm.
Adjust your image sharpness with the adjustment on the camera sensor only. do not touch the focus ring on the lens.
Zoom back in to16mm
Readjust the focus with the lens focus ring only
zoom back out
you may need to tweak focus again with the adjustment screw on camera only -do not touch the lens focus ring.
Zoom back in 16mm and double check.
Your focus should stay sharp now. If not adjust with the focus ring on the lens (it should need very little if any at this point) you can note the focus ring position, throw it out of focus and focus again to make sure you come back to the same spot.
Unless something is loose in camera or lens you should be done at this point.
That does not guarantee every lens would be in focus, if your other lenses are not focused correctly they will be out on the camera. That's why you need to check the Tokina on a collimaor first to make sure you are adjusting the camera sensor position to the correct standard...

 

[Josh Mitchell-Frey]

Your Scarlet's back focus (and Tokina 11-16mm) may be perfectly fine.

While cinema zoom lenses are designed to be parfocal (they hold focus when the focal length is changed), stills lenses are usually not parfocal. It's difficult to design a parfocal lens, so for stills where such a thing serves no real purpose, it's often not designed that way. Same deal with breathing, it's ideally minimised for cinema lenses, but not nearly as important for still shots.

Edit:

After a quick google search, it does look like there's a Tokina 11-16mm that's parfocal, but it's the cinema version marked as T3, rather than f2.8.

 

[Jacek Zakowicz]

Parfocality has nothing to do with the zoom sharpness on both wide and long end.
We've been through this like a million times on this forum ;-)

 

[Peter Lyons Collister ASC]

I have my camera collimated for PL lenses. When I use my Nikon mount and Nikon glass I always pick the focal length I want and then focus only for that focal length (using the magnify tool on the touch screen)

I am not sure about parafocal but I have never had any of my still zooms hold focus from wide to tight. I always thought it was because the still zooms are not parafocal but what do I know. Always happy to learn more stuff!

 

[Jacek Zakowicz]

Every lens can be adjusted for focus on both the long end with the front focus (focus ring) and the wide end with the back focus. That's how lenses are set in the factory. For still cameras it is safer to set the lenses "short " on the wide end because the environment and use tend to force the lenses out towards the long setup and if they do the infinity is out. Basically setting the lenses short on the wide end guaranties infinity no matter what. Since the lenses are auto focused the back focus does not make much difference.
You are absolutely doing the right thing to use still (zoom) lenses as variable primes because this was the intended use for them....

 

[Kennan Ward]

Brilliant, Thank you Jacek!

This is awesome coaching!

Kennan & Karen Ward

You need to first check your Tokina lens on a collimator to make sure it's not out of whack.
If the lens checks out ok here is what you do:
with the lens on camera , camera on and 1:1 punched in on the screen
1: set up height resolution target (a dollar bill) square to the camera the same height as the lens center no more than 3ft away
Lens wide open.
Zoom in to16mm
adjust for best focus with the lens focus ring only
zoom out all the way to 11mm.
Adjust your image sharpness with the adjustment on the camera sensor only. do not touch the focus ring on the lens.
Zoom back in to16mm
Readjust the focus with the lens focus ring only
zoom back out
you may need to tweak focus again with the adjustment screw on camera only -do not touch the lens focus ring.
Zoom back in 16mm and double check.
Your focus should stay sharp now. If not adjust with the focus ring on the lens (it should need very little if any at this point) you can note the focus ring position, throw it out of focus and focus again to make sure you come back to the same spot.
Unless something is loose in camera or lens you should be done at this point.
That does not guarantee every lens would be in focus, if your other lenses are not focused correctly they will be out on the camera. That's why you need to check the Tokina on a collimaor first to make sure you are adjusting the camera sensor position to the correct standard...

 

[Josh Mitchell-Frey]

Every lens can be adjusted for focus on both the long end with the front focus (focus ring) and the wide end with the back focus. That's how lenses are set in the factory. For still cameras it is safer to set the lenses "short " on the wide end because the environment and use tend to force the lenses out towards the long setup and if they do the infinity is out. Basically setting the lenses short on the wide end guaranties infinity no matter what. Since the lenses are auto focused the back focus does not make much difference.
You are absolutely doing the right thing to use still (zoom) lenses as variable primes because this was the intended use for them....

Jacek, are you saying that every zoom lens is capable of maintaining focus throughout the zoom range if the back focus of the camera is adjusted to a specific point?

Very interesting if so, although surely that's not practical, as you would need to adjust the back focus every time you changed lenses?

 

[Josh Mitchell-Frey]

Parfocality has nothing to do with the zoom sharpness on both wide and long end.
We've been through this like a million times on this forum ;-)

Hey Jacek, if you could go through this for the 1,000,000 and 1st time, it'd be greatly appreciated!

Are you saying that parfocality has nothing to do with the optical quality of a lens at the wide end vs the long end? If so, I completely understand that.

However if you're saying that parfocality has nothing to do with whether a lens holds its focus constantly while zooming in or out, I thought that was the entire definition of parfocality.

I thought the OP had a problem with his lens maintaining focus when zooming in or out, rather than having a lack of quality/sharpness

 

[Jacek Zakowicz]

Hey Jacek, if you could go through this for the 1,000,000 and 1st time, it'd be greatly appreciated!

Are you saying that parfocality has nothing to do with the optical quality of a lens at the wide end vs the long end? If so, I completely understand that.

However if you're saying that parfocality has nothing to do with whether a lens holds its focus constantly while zooming in or out, I thought that was the entire definition of parfocality.

I thought the OP had a problem with his lens maintaining focus when zooming in or out, rather than having a lack of quality/sharpness

Yes every LENS can be adjusted so it's sharp on both ends. It's not a good idea adjusting flange on camera to compensate- you're right.
Parfocal is a lens that maintains focus between the extremes of the zoom range....