18-85 zoom characteristics

[Juan Qi An]

Hi, my 18-85 zoom just came in and we have been doing some lens tests and found that it gets slightly soft upon zooming out from 85mm to somewhere around 40-50mm. does anybody have the same experience?

Juan Qi An

 

[Pawel Achtel]

Do a back focus adjustment procedure. :wink5:

 

[Alexander Ibrahim]

Do a back focus adjustment procedure. :wink5:

Yep, that'll almost certainly do it for you.

Can I suggest buying/renting a RED Null from Abel. It makes it easy and dead accurate.

 

[Jacek Zakowicz]

my experience

my experience

Hi, my 18-85 zoom just came in and we have been doing some lens tests and found that it gets slightly soft upon zooming out from 85mm to somewhere around 40-50mm. does anybody have the same experience?

Juan Qi An

Make sure it is tack sharp wide, but, in my experience- I've seen about half a dozen or so- that's normal- the contrast drops slightly- you still get the resolution. Plain and simply- it's not a prime lens...

 

[FocusOptics]

Hi, my 18-85 zoom just came in and we have been doing some lens tests and found that it gets slightly soft upon zooming out from 85mm to somewhere around 40-50mm. does anybody have the same experience?

Juan Qi An

JUAN,
If you focus at 85mm until image is sharp and zoom in to 40-50mm the image goes soft. Does the image came back in sharp at the wide end? If this is the case than you have a bad zoom curve send the lens back to red. If this is not the case and the image is not sharp at the wide end than you have to adjust the back focus by adding shim or removing shim from the rear mount. If this the only lens you are using you can adjust the back focus on the red camera. hope this helps
Stuart

 

[Juan Qi An]

Hi everyone,

thanks for all the help.

FOCUSOPTICS: It does get sharper back at 18mm, and we have already sent the lens back to red and they claimed that the softness was within their tolerance range and that there was nothing that they could do. Maybe a backfocus adjustment on the lens could fix this?

So using a Red Null would be the most accurate solution?

Juan

 

[Pawel Achtel]

...So using a Red Null would be the most accurate solution?

No, the most accurate will be a back focus adjustment procedure. In case you don't know what it is:

1) Set up a focus target.

2) Open the stop wide and use ND if the target blows out.

3) Zoom in and focus on the chart till in focus

4) Zoom out, if the target is in focus, you are done, if not continue.

5) Loosen the locking screws and turn the back focus ring back and forth, passing in and out of focus to find the place where it is most crispy.

6) Go to step 3)

Happy shooting.

 

[Brigham Edgar]

Hi everyone,

thanks for all the help.

FOCUSOPTICS: It does get sharper back at 18mm, and we have already sent the lens back to red and they claimed that the softness was within their tolerance range and that there was nothing that they could do. Maybe a backfocus adjustment on the lens could fix this?

So using a Red Null would be the most accurate solution?

Juan

I have exactly the same thing with our 18-85 and we have had two in our possession. I also had the lucky opportunity to take all of RED's lenses (even the RPP's) into Panavision and look at them on a lens projector with their tech. The 18-85 going slightly soft in the middle of the zoom range was within their tolerance as well and is as good as any zoom out there as it is a common theme with zooms. As long as it isn't too bad that is

A backfocus adjustment will NOT fix it.

 

[bert tan]

Hi, my 18-85 zoom just came in and we have been doing some lens tests and found that it gets slightly soft upon zooming out from 85mm to somewhere around 40-50mm. does anybody have the same experience?

Juan Qi An

Hi Everyone,

I just sent the 18 to 85mm zoom lens to Arri Dealer in singapore for a lens projector test.
Here is the result:

At 4ft distance on 85mm/ is 4ft focus
70mm/ is 4ft focus
60mm/ is 4ft focus
50mm/ is 4ft less by one line marking
40mm/ is 3ft 10" focus
32mm/ is 3ft 10" focus
25mm/ is 3ft 10" focus
18mm/ is 3ft 10" focus

At 8ft distance on 85mm/ is 8ft focus
70mm/ is 8ft focus
60mm/ is 8ft focus
50mm/ is 7ft 10" focus
40mm/ is 7ft 6" focus
32mm/ is 7ft 1" focus
25mm/ is 7ft focus
18mm/ is 8ft 3" focus


At 10ft distance on 85mm/ is 10ft focus
70mm/ is 10ft focus
60mm/ is 10ft focus
50mm/ is 9ft 11" focus
40mm/ is 9ft focus
32mm/ is 8ft 5" focus
25mm/ is 9ft focus
18mm/ is 9ft 5" focus


So is it normal for the Red Zoom lens to shift focus in this way from 85mm to 18mm?

Any advice on how to rectify the zoom lens?

 

[Pawel Achtel]

I have exactly the same thing with our 18-85 and we have had two in our possession. I also had the lucky opportunity to take all of RED's lenses (even the RPP's) into Panavision and look at them on a lens projector with their tech. The 18-85 going slightly soft in the middle of the zoom range was within their tolerance as well and is as good as any zoom out there as it is a common theme with zooms. As long as it isn't too bad that is

A backfocus adjustment will NOT fix it.

Correct. I didn't notice it was going soft in the middle.

In that case, as you say, it is the lens limitation or tolerance and no back focus adjustment is going to fix it whether with NULL or by zooming in/out procedure.

 

[chuck colburn]

ALL zoom lenses suffer from focus shift at different points in their focale length range. This is refered to as the "S" curve of the lens. While you can get front (telephoto) and back (wide angle) foci to reach a nominal "zero" at the film plane (F.P.) there will be focus shift to one side of the F.P. as you zoom from tele to wide and then go back to "zero" at about mid point. Then the focus will shift to the other side of the F.P. as you pass thru the mid range to the wide end. Some lenses are better then others.

 

[bert tan]

I have exactly the same thing with our 18-85 and we have had two in our possession. I also had the lucky opportunity to take all of RED's lenses (even the RPP's) into Panavision and look at them on a lens projector with their tech. The 18-85 going slightly soft in the middle of the zoom range was within their tolerance as well and is as good as any zoom out there as it is a common theme with zooms. As long as it isn't too bad that is

A backfocus adjustment will NOT fix it.

HI Brigham Edgar,

What was the results from the lens projector test you did in Panavision ?

 

[Juan Qi An]

I'm quite sure it isnt a backfocus issue, since the rest of our lenses are working perfectly fine. Anyone else out there with the same problem? Would love to hear about the results from your Panavision projection tests Brigham Edgar!