Varifocal photographic lenses - Benefits for the user?

[AgX]

Likely we all know of these lenses out of the spring time of lenses with variable focal lenght. Later they were described as obsolete and I did not see any modern versions to show up.

However meanwhile I got a 1981 sample and I learned of another one. Both made by Kino Precision at about the same time.

Do lenses of such design have benefits for us users?

One may argue that due to the more simple mechanics plus for optical reasons lesser diameter they could be cheaper and lighter.

But what about optical performance?
If at similar performance the lenses can be smaller and lighter, what about the performance if not reduced in size?

Seemingly these two lenses were the only ones (or amongst very few) to resurrect this design. But why then remained this just an intermezzo?

 

[tedr1]

For bus driving I needed "bi-focal" lenses and the shark in the optician store sold me varifocals, cost a fortune. But when you have to read a vehicle instrument panel and distant road signs and everything in-between they are the right thing to have.

 

[AgX]

Sorry, but I did not mean glasses but photographic lenses.
Both are completely different things. (Except that both offer a range of focal lengths...)


(If others have been fooled too by the title and post, I might have to change the title.)

 

[pentaxuser]

Yes it fooled me as well AgX. The only varifocal lenses I had ever heard of before your post were the ones in spectacles

pentaxuser

 

[AgX]

I thought the term was known seen our average age... I asked modrators to insert the term "photographic" in the title.

Well, what I mean are "zoom" lenses that need refocusing with any change of focal length setting.
Strictly speaking the term "zoom lens" applies only for the more modern design where the focus setting remains constant.

 

[abruzzi]

As I understand it varifocal lenses are “zoom” lenses where changing focal length does not retain focus. I was under the impression that is was mostly a cost thing. Cinema lenses are not varifocal, and you are taught to zoom in, focus, then zoom back out to the focal length you want.

 

[ic-racer]

The only one I have familiarity with is the Konica. The student advisor for our 7th grade Photography Club had one. He used to let me use it to take pictures of the 8th grade basketball games. He let me use his T3 Auto too! The thing is that I preferred aperture priority, even at a young age. As I recall we used Tri-X pushed to 1200 so there was never any shadow detail . The lens was big, I used it on a tripod.

 

[John Koehrer]

FWIW, Vivitar 35-85/2.8 is a variable focus lens.

 

[AgX]

Konica and Vivitar, two lenses I did not know of...

 

[chip j]

I knew what you meant by Varifocal, AgX, shows our age. IMHO, they were just early forays into true zooms. Kino made really good lenses.

 

[Oren Grad]

Later they were described as obsolete and I did not see any modern versions to show up.

My impression is that in the years since autofocus became the norm for mass-market cameras, a substantial proportion of lenses sold as "zooms" have in fact been, strictly speaking, varifocals. So far from going away, this category has if anything grown substantially. But hardly anybody pays much attention to the distinction any more, because it no longer has much practical significance. It's assumed that most users under most circumstances will use AF linked to the shutter release, so that focus will be automatically adjusted just before exposure after the lens has been zoomed to the desired focal length, making the distinction moot.

Also keep in mind that as the resolution of digital sensors continues to increase, the demands on mechanical precision to achieve "parfocality" become more stringent as well:

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

 

[AgX]

Thank you for hinting at this aspect. I do not use seperate AF lenses, thus have not looked into their design, but yes, before starting this thread I read of them typically being varifocal only. As AF lenses seem typically be used in AF the focusing characteristics are not of effect to the user, but on the other issues it may be too.

 

[BMbikerider]

I have a 28/90 Vivitar Series 1 lens which is a varifocal. (F2.8/3.5)

This lens was claimed to be one of the best made bearing the Vivitar name when it came out perhaps in the 1980's. From the serial number comparing it to others on the web, this may well have actually been made by Cosina. The lens is for my manual Minoltas and will give me an image sharp enough to make a 12x16 print anytime.

At full aperture it is 'slightly' soft, but from 1 stop down it is sharp all the way through on all focal lengths. The standard of build is way above many lenses made by Camera manufacturers in the same period, all metal and quite heavy. The coating may not be cutting edge now, but I don't think I have had any serious problems with flare, even without a lens hood.

True zoom lenses from that era were not so good (as I recall) so by limiting the optical corrections needed to make this lens a true 'Zoom' and not a 'Varifocal', allowed the manufacturer to sell a lens that was way above it price and quality for an excellent performance. The successor to this lens was the 28/105 and not nearly so good. The close focussing of the 28/90 is a respectable 1meter or close to it.

However about the same time they also sold a 28/85 which was truly terrible. Barrel distortion that would almost make you think it was a Fisheye lens and a close focussing distance of as I remember about 2 meters. To add to this it wasn't very sharp either!

 

[AgX]

The basic design of zoom and varifocal lenses:

These lenses have a first group that forms the basic lens. It is followed by a second group acting as copy lens, thus taking up the aerial image (primary image). The idea behind this copy lens is to yield varying crops out of the primary image. (Think of a bellows with macro lens and in front of it a second bellows with a slide, for slide duplicating and cropping. The slide is what the primary image is above.) Of course varying the position of the second, the copy group, needs refocusing of the system.
This is basically what a varifocal lens is.

In a zoom lens there is a third group which enables for the whole lens to be self-focusing on the primary image during cropping. There are different ways to achieve this.
In any case a zoom lens has this additional third group.



My question now is whether at a varifocal lens this lack of that third group not only has a disadvantage for the user, the need to refocus, but also advantages for the user.

 

[John51]

For bus driving I needed "bi-focal" lenses and the shark in the optician store sold me varifocals, cost a fortune. But when you have to read a vehicle instrument panel and distant road signs and everything in-between they are the right thing to have.

Our best bargains seemed overpriced on the day we bought them.

 

[Nodda Duma]

From a lens design perspective, both types described above — both “classic” design and “parfocal” design fall into the category of zoom lens.

The classic design has an objective group (static front positive lens or group) and a zoom group (the negative or negative group). Sometimes the rear positive is considered part of the zoom group and sometimes considered as the back half of a split objective, depending on what academic circle you come from and details of the zoom range. It is static in the classic zoom.

Parfocal zooms modify the classic design with the addition of a compensator — the rear positive lens/group moves on its own cam — to maintain parfocality.

The sole benefit of a parfocal zoom is maintaining parfocality over the zoom range. The classic design has the benefit of one less cam (less cost/complexity). There is not any other inherent performance advantage one vs the other. The choice is a trade-off based on other considerations.

That is it in a nutshell. There are other categories that overlap and/or more complex configurations, but these two categories are often considered the basic way to subdivide the zoom family.



Cheers,
Jason

 

[AgX]

Couldn't one argue that the sheer omitting of that third group enhances image qualty, or would that be mere academic, without practical effect?

And why then re-emerged these lenses for a short period? Or is that my misperception of the market back then?
Was it just a economic benefit in first instance for the manufacturer, producing cheaper and offering a cheaper alternative?

 

[Nodda Duma]

Couldn't one argue that the sheer omitting of that third group enhances image qualty, or would that be mere academic, without practical effect?

There’s no omitting groups. The difference is the rear group moves or it doesn’t. Positive - negative - positive configuration is required to be a zoom. It can be more complicated than that, but you must be able to identify those three groups.


Remove that rear positive group and it’s no longer a zoom lens...in fact no longer an imaging lens. A positive - negative is a telescope or beam expander.

 

[AgX]

But the addition of the third group is just how Schneider in a lecture on zoom lenses describe the difference between the two types of lenses (zoom lens vs. varifocal lens).

https://schneiderkreuznach.com/application/files/4515/0841/8897/variogon-zoomobjektive.pdf

 

[Nodda Duma]

I would suggest that you misinterpreted what was said in the lecture.

A zoom is a positive-negative-positive configuration or it’s not a zoom!

The concepts are described in any basic lens design book such as Kingslake...with more clarity and depth I’d presume than in a powerpoint.


Edit: I see the confusion. Schneider muddies the waters by describing a zoom as an afocal group followed by a positive...the afocal in their case being a positive-negative-positive. So they describe a positive-negative-positive-positive configuration which is a better starting point for them to talk about “advancement” by combining the two rear positives in this marketing piece. I suppose, at least, that they agree with wikipedia.

That’s an overcomplication of the basic premise of a zoom ... sort of like trying to describe a Cooke by starting with a Tessar when in fact the Tessar evolved from the Cooke.

 

[AgX]

I referred to the varifocal type of lens. (At least over here it is not called a zoom lens, and both terms are used here in opposition.)

And I looked through all text books at hand or available online and I did not find a better explaination than given by Schneider, let alone schematics depicting both types in comparison.

I shall have a deeper look at the design of the lens sample I got...

 

[Nodda Duma]

I can only give you the perspective of a lens designer.. “over here” or “over there” has nothing to do with that. These are all members of the zoom lens family in the eyes of a lens designer.

The variant you are discussing is a subset, as I described above

 

[AgX]

The lens design text books I looked at, including Kingslake, do differenciate the terms but do not go into detail on this matter.

 

[Nodda Duma]

I understand completely what you are discussing, just to be clear. There’s a lot of zoom design experience behind my words and I’ve dug down the same rathole many years ago that you are going down in this thread.

I’ve given you the further details that you’re missing above. The connection between what I’ve said and the Schneider pamphlet is creating an additional degree of design freedom by splitting the compensator into a fixed and variable rear positive. However, the reasons for doing so are unrelated to maintaining focus over zoom (more to do with maintaining constant overall length and, importantly, aberration correction). To see this evolution properly from a design perspective, which must include the ray tracing and design analysis, it helps to start with the fundamental zoom configuration rather than starting complex and going simple.

 

[abruzzi]

A slightly related question—it seems like a varifocal lens couldn’t have a focus scale without some way to move the scale when focusing, or am I wrong there? Has anyone made a varifocal lens with a focus scale?