Does your zoom stay in focus when you change focal lengths?

[David Lyga]

I have a tip that I learned some time ago that might be of help to those experiencing this 'focus-shift' problem with his/her zoom lens.

When you carefully focus at the widest setting, then switch to the tele setting, does your focus shift slightly? (Or even more than slightly?)

I have found that the distance that that rear element-set is from the film plane is the determining factor in rectifying this malady. On zooms, the rear element-set usually has two circular threaded-nuts for which there are two different functions. The INNER one guides the element set into the lens body. That is common sense and is found on almost every lens, zoom or fixed focal length. The OUTER one guides how far that rear element-set is supposed to go into that lens body. In other words, on zooms, the rear element-set is not supposed to go all the way into the lens body; the amount it is screwed in determines how accurately the tele focus and wide focus will match (as they should on zooms, rather than on varifocal lenses).

Here is the rule of thumb that seems to work for me. First, focus as accurately as you can at the wide setting. Choose something very bright and well defined so that you can be ACCURATE. Then, without moving that focus even a bit (tape the focus in place in order to secure it), zoom to the full tele position. IF, AT THAT POINT, YOU HAVE TO MOVE THE FOCUS RING TOWARDS INFINITY IN ORDER TO BE IN PERFECT FOCUS, THE REAR ELEMENT-SET MUST BE POSITIONED A BIT MORE INWARD (TOWARD THE APERTURE) IN ORDER TO CORRECT THIS. (Of course, the opposite is true if you need to go the other way, but I won't repeat what is obvious.)

By trial and error, mounting and remounting the lens and carefully focusing upon the same spot, you will find the exact position that that rear element set must be positioned and all this should use no film and take about twenty minutes of time.

Honestly, there might be some lenses which adhere to a different way of doing this, but I have found that most will adhere to what I have just explained. - David Lyga

 

[wiltw]

Some lenses are 'parfocal' and some are NOT! Parfocal lenses remain in focus even when you change FL. The Canon EOS EF lenses of today are often not 'parfocal'. Somebody wrote 10 years ago about a list of parfocal Canon lenses,

"A (non exhaustive) list of parfocal lenses provided by Chuck Westfall a few years ago:

EF 16-35mm f/2.8L USM
EF 17-40mm f/4L USM
EF 24-70mm f/2.8L USM
EF 70-200mm f/2.8L USM
EF 70-200mm f/2.8L IS USM
EF 70-200mm f/4L USM
EF 75-300mm f/4-5.6 IS USM
EF 75-300mm f/4-5.6 III USM
EF 75-300mm f/4-5.6 III
EF 90-300mm f/4.5-5.6 USM
EF 90-300mm f/4.5-5.6"
​

In 2010 someone wrote that while the 70-200 f/2.8L was parfocal, he discovered that his 70-200 f/2.8L II was not. Roger Cicala at Lensrentals.com has something on this topic. In fact, it varies from copy to copy of the SAME lens design!

https://www.lensrentals.com/blog/2016/03/mythbusting-parfocal-photo-zooms/

 

[Ian Grant]

As above some hold focus others don't. My two Vivitar S1 70-210 lenses did earlt f3,5 version then later f2.8/f4others like a similar Sigma, well 5 different Sigma's, didn't but the build quality was so poor at that point (1970's), one wasn't sharp, one kept focussing until it fell apart, another couldn't focus at infinity, the dealer stopped selling Sigma lenses.

Cheaper Zooms don't hold focus but then I don't use them. Having said that Vivitar made a superb (I think)28-90) vari focal S1 lens and refocussing was not an issue.

You get what you pay for.

Ian

 

[Andrew K]

Hi Ian

cheaper lenses were not always the offenders. The Canon New FD 35-70/3.5-45 (the plastic one sold with T50's) could easily be adjusted to maintain focus through the zoom range.

The NFD 35-105/ one touch zoom was almost impossible to adjust. On the other hand the NDF 35-105 two touch zoom could be adjusted - as David says above you needed to adjust the rear of the lens first (a pain as you had to dismantle the lens to get to the shims) - and often you had to make a shim out of paper, or grind a thicker shim down to get focus correct.

It also depends on the condition of the zoom guides and zoom cams - if they wear you can't adjust focus.

Cheers

Andrew (one time Canon Australia camera tech)

 

[Europan]

Having my first varifocal lens disassembled before me, a Pan-Cinor 8 through 40, I am entering the subject.
This lens consist of three distinct parts, the basic image forming (strong positive power) part, the middle afocal, and the front focusing part which consists of a cemented doublet or achromat. True zoom lenses have mechanical or optical compensation for a number of geometrical things that change upon shifting the afocal elements. I should say that zoom lenses that stay in focus are complicated and therefore expensive products. Everything else is amateur gear.

That Pan-Cinor also has second best iris mechanics.

 

[mgb74]

As above some hold focus others don't. My two Vivitar S1 70-210 lenses did earlt f3,5 version then later f2.8/f4others like a similar Sigma, well 5 different Sigma's, didn't but the build quality was so poor at that point (1970's), one wasn't sharp, one kept focussing until it fell apart, another couldn't focus at infinity, the dealer stopped selling Sigma lenses.

Cheaper Zooms don't hold focus but then I don't use them. Having said that Vivitar made a superb (I think)28-90) vari focal S1 lens and refocussing was not an issue.

You get what you pay for.

Ian

The Vivitar Series 1 28-90 I had was a very good lens, but did require refocusing.

In general, with zooms, I was "taught" that it's best to focus at longest setting (where limited dof makes focusing easier) and zoom to desired focal length.

 

[AgX]

Cheaper Zooms don't hold focus but then I don't use them. Having said that Vivitar made a superb (I think)28-90) vari focal S1 lens and refocussing was not an issue.

But "varifocal" means that focus will not be kept during zooming, "parfocal" is the designation for the steady type.

 

[AgX]

I have a tip that I learned some time ago that might be of help to those experiencing this 'focus-shift' problem with his/her zoom lens.

Here is the rule of thumb that seems to work for me. First, focus as accurately as you can at the wide setting...

The common rule with a (parfocal) zoom lens is to focus at the tele setting and then to zoom.

This avoids apparent focus shift due to imprecise focusing at the wide setting

 

[Sirius Glass]

Try focusing at the longest focal length and then go wider, not the other way.

 

[ME Super]

+1. This is precisely the trick I use. It won't work with a varifocal lens, but if you have a true zoom lens, this absolutely works.

Here's one possible explanation as to why this technique works: If you focus at the wide end, any focusing error is not readily apparent. If you now zoom to the tele end, the tiny focus error you had at the wide end is now magnified. If you focus at the tele end, and then go to the wide end (without disturbing the focus), any focus error is shrunk even further.

Note that if you're using a varifocal lens, focus at the tele end and then go to the wide end, your focus may very well shift!

 

[benjiboy]

Try focusing at the longest focal length and then go wider, not the other way.

This has always worked for me too.