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Parfocal zoom lenses
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PostPosted: Sat Mar 15, 2014 1:29 pm    Post subject: Parfocal zoom lenses Reply with quote
I thought that it would be useful to start a thread about parfocal zooms; a parfocal lens being one that holds focus through the zoom range. This characteristic is particularly useful to run-and-gun situations for video shooters like me, but some still shooters might be interested as well.

Here are the manual focus lenses that I understand to be parfocal:

Minolta MD 35-70/3.5
Minolta MD 70-210/4
Olympus OM 35-70/3.6
Olympus OM 35-80/2.8
Nikkor AI-S 80-200/4

Of these, I have only used the Minolta MD 35-70/3.5 and I can verify that it is indeed parfocal.

Are there any other known parfocal manual zooms out there?


PostPosted: Sat Mar 15, 2014 2:29 pm    Post subject: Reply with quote
I would remark that parfocality is more the norm. I have kiron, Vivitar S1, and mostly tamron adaptall, they are all parfocal, however my earliest adaptalls, the 2 ring z250, qz250, visibly drift off focus when zooming. With single ring zooms its always a bit dificult to tell whether you are shifting focus a little bit when zooming.


PostPosted: Sat Mar 15, 2014 2:34 pm    Post subject: Reply with quote
Nikon Ai-S 35-70 f3.5


PostPosted: Sat Mar 15, 2014 2:57 pm    Post subject: Reply with quote
Vivitar 70-210/4.5 (Great lens!)


PostPosted: Sat Mar 15, 2014 3:02 pm    Post subject: Reply with quote
marcusBMG wrote:
I would remark that parfocality is more the norm. I have kiron, Vivitar S1, and mostly tamron adaptall, they are all parfocal, however my earliest adaptalls, the 2 ring z250, qz250, visibly drift off focus when zooming. With single ring zooms its always a bit dificult to tell whether you are shifting focus a little bit when zooming.


I would go further and say it is a bit academic whether a one touch zoom is par-focal, as you cannot really use them in that way anyway.


PostPosted: Sat Mar 15, 2014 4:42 pm    Post subject: Reply with quote
Basilisk wrote:
I would go further and say it is a bit academic whether a one touch zoom is par-focal, as you cannot really use them in that way anyway.

I agree. Perhaps we ought to stick to two-touch only.


PostPosted: Sat Mar 15, 2014 5:09 pm    Post subject: Reply with quote
these are non prime lenses. variable focal length. They fall into two categories. Zoom (parfocal) which hold focus when changing focal length, and Varifocal (non-zoom?) which do NOT hold focus when changing focal length. Of the two categories, the Zoom (parfocal) type is much more common but if optically/mechanically damaged they may sometimes lose the ability to hold focus while changing focal length.


PostPosted: Sat Mar 15, 2014 5:23 pm    Post subject: Reply with quote
I find that though many manual lenses hold focus more or less through the focus range, a truly dead-on parfocal lens is something of a rarity.

Here's a list that I found over at the Pentax Forums: http://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/201887-list-parfocal-zoom-lenses.html


PostPosted: Sat Mar 15, 2014 5:47 pm    Post subject: Reply with quote
QuickHitRecord wrote:
I find that though many manual lenses hold focus more or less through the focus range, a truly dead-on parfocal lens is something of a rarity.

Here's a list that I found over at the Pentax Forums: http://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/201887-list-parfocal-zoom-lenses.html

While I agree there is a minority of variable focal length lenses that hold focus truly dead-on, I do not believe it is by design only in those lenses. It is caused more likely by tolerances within
the mechanics and optics of the lens and camera and may not even be repeatable on a consistent basis from lens to lens of the same make and model.


PostPosted: Sat Mar 15, 2014 6:17 pm    Post subject: Reply with quote
Hi,
I will test tomorrow my 28-85 minolta zoom lens. I guess I focus at 28 mm and then I go to 85mm and I see if subject stayed in focus. Right?
Alex


PostPosted: Sat Mar 15, 2014 6:37 pm    Post subject: Reply with quote
hifisapi wrote:
It is caused more likely by tolerances within
the mechanics and optics of the lens and camera and may not even be repeatable on a consistent basis from lens to lens of the same make and model.

I see what you are saying. This makes it even trickier to come up with a list!

alaios wrote:
I will test tomorrow my 28-85 minolta zoom lens. I guess I focus at 28 mm and then I go to 85mm and I see if subject stayed in focus. Right?

I think that perhaps the best way to test this is to zoom all the way in, focus on a finely-detailed subject at max aperture, and then zoom all the way out without touching the focus ring and take a photo to see if your subject is still in focus.

Last edited by QuickHitRecord on Sat Mar 15, 2014 6:46 pm; edited 1 time in total


PostPosted: Sat Mar 15, 2014 6:40 pm    Post subject: Reply with quote
hifisapi wrote:
these are non prime lenses. variable focal length. They fall into two categories. Zoom (parfocal) which hold focus when changing focal length, and Varifocal (non-zoom?) which do NOT hold focus when changing focal length..


I was not aware of this being a technical definition of zoom and varifocal. As far as I am concerned, zoom is just a colloquial name for varifocal. Any varifocal lens can be called zoom whether or not it holds focus as the focal length is changed.

QuickHitRecord wrote:
I find that though many manual lenses hold focus more or less through the focus range, a truly dead-on parfocal lens is something of a rarity.


This is my experience also. Truly accurate focus can be held over only part of the zoom range.

hifisapi wrote:
While I agree there is a minority of variable focal length lenses that hold focus truly dead-on, […..]. It is caused more likely by tolerances within the mechanics and optics of the lens and camera ……..


I think this is probably correct and is especially true when using a lens on an adapter, since adapters are usually made a tad on the thin side to ensure the lens can reach infinity. the under-length adapter requires a different compensation from the focus ring as the focal length is changed.

For the record, the varifocal lenses that I have that come closest to par focal are:

OM Zuiko 35-70mm f3.6
OM Zuiko 85-250mm f5
Nikkor 100-300mm f5.6 (a one-touch lens)

I have also found that my Tamron SP 200-500mm holds focus quite well over small zoom ranges (+/- 75mm from focused FL say). I spent some time machining my adapter for this lens and this did improve its ability to hold focus, but it was clear it was never going to become truly par focal.

Mark


PostPosted: Sat Mar 15, 2014 6:40 pm    Post subject: Reply with quote
True zoom (perfectly parfocal) lenses are not that rare. I currently own about 15 zoom lenses and every one of them holds focus when zooming quite accurately. Why? Because
its the first thing I check when buying a zoom lens. If a zoom lens shifts focus when zooming, its very awkward in use from a manual focus standpoint and I dont want it.
I have seen many defective lenses that shifted focus while zooming and I know it was some sort of defect because I sought out other examples of the same make and model lens and those other examples did not always have the same problem.


PostPosted: Sun Mar 16, 2014 10:50 am    Post subject: Reply with quote
It's possible to set the parfocality of a parfocal zoom lens. At least it was with the zoom I tinkered with, Soligor 95-310, and was accualy pretty easy to do it. What I noticed was that the parfocality depens on registed distance from the lens to the sensor. Very small differences caused a huge focus shift, so maybe just by having an adapter that is too thin pafocality is not perfect anymore. In the case of that Soligor lens I varyed the position of the rear lens group until I got parfocality back.
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