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Old Jun 23, 2003, 11:59 AM   #1
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Default varifocal zooms

Does anyone know how common a varifocal zoom design is? I was used to the traditional zoom design, which allows you to zoom, focus and then zoom out and the focus won't change.

This was all with old, all manual equipment. Now that I have a newer camera which has AF and other automatic things, I've got my first varifocal design lens. So this got me thinking...

Can they do things that the old design can't? Are they just cheaper? Any ideas?

Eric
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Old Jun 23, 2003, 2:39 PM   #2
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Varifocal are more common with all-in-one camera, since they allow a more compact design and high zoom ratio. ie a high 5~6x or higher is usually a varifocal design! The EF28-135 f3.5-5.6 IS USM is only 4.8x but remember they also have to cram IS in there as well... 8)

It's still a relative minor beachhead in dSLR land where most zooms there are still true zoom since most folks here still value that manual control, and varifocal are a pain to use in the manual mode. A true zoom is not as compact, usually require more glass, hence will cost more to make and few compact camera buyers will want to pay for since the camera's AF will compensate for the design shortcoming anyway.

Actually the D7's 28-200 zoom are varifocal as well, but as you turn the manual zoom ring, the focus motor is working in the background faking the whole thing out like a true zoom hence the distance scale in the EVF is rather inaccurate! :P

BTW the same thing is true for constant aperture as well since it put another constraint on the lens maker. Remember all the cameras now have AE, but changing f-stop while zooming is also a boo-boo not too long ago! Unless of course one works in the studio with the camera on manual...(This is also where that 24-70mm constant f2.8 will come in handy!) :lol: :lol: :lol:
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Old Jun 23, 2003, 3:27 PM   #3
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NHL... you seem to suggest varifocal zooms are not a bad thing. Before I found out about this, I was frustrated that everytime I tried to use my zoom in manual, it wouldn't track focus as in a traditional SLR and pro TV cams. I wouldn't mind a bit of tracking error if DOF took care of changing zoom fl, but it doesn't.

I then realised that these cam varifocal zooms really are fly by wire focusing and if you are lucky your cam has a centre image magnifier - but not a patch on the zoom quality as a focus aid, and if the AF is not able to work, it's down to earth with a bump. It must be cheaper to put in a varifocal lens and AF controlled stepper motor. I find this restricting, but perhaps others have just got used to it. VOX
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Old Jun 23, 2003, 9:23 PM   #4
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NHL... you seem to suggest varifocal zooms are not a bad thing.
On the contrary... while it's OK on P/S where the big DOF can compensate for poor AF in the camera, it could be a potential problem with dSLR especially @ large aperture, ie one can be focusing on the eye and when upon zooming out the focus point can change to the nose or the ear instead! :?

The other problem with changing aperture is more subtle, and can occur in studio shoot for example when the camera is on manual and sync to the studio slaves. You can set your camera with a flashmeter, but then your aperture changes as you zoom in or out! Hence the need to use constant aperture or fixed focal lenses...

In the end manufacturers make what sell, so as long as people ask for 6~12x or more, and the camera's electronics can compensate for all other shortcomings who cares? I bet you Canon sell a lot of theses lenses since it's so a popular, plus the Image Stabilization catchword and all, I know because I'm one of thoses owners! :lol: :lol: :lol:

Would you buy a costlier more bulky constant aperture true zoom with less reach (or a camera equipped with one)? A marketing suicide I would say... at least for the consumer market.
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Old Jun 23, 2003, 11:07 PM   #5
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Default Re: varifocal zooms

Quote:
Originally Posted by eric s
Does anyone know how common a varifocal zoom design is? I was used to the traditional zoom design, which allows you to zoom, focus and then zoom out and the focus won't change.
Depending on which definition you wish to use, VARIFOCAL lenses can be simply zoom lenses or lenses whose sharp focus drift as the focal length changes. Zooms appeared in the 30's and were developed significantly after WWII. In the 60's and 70's, zoom lenses that cannot maintain focusing were popular. Nowadays, virtually all zoom lenses made by well-known lens manufactures are the true zooms that the focus will be maintained to a very high degree while zooming.

A varifocal lens (according to your meaning) is easier to design with fewer glass elements and easier to correct optical deficiencies (e.g., spherical aberration, color aberration, etc). It may not be a zoom. For example, many considered Leica's three-fixed-focal-length lens is varifocal. In fact, all well-known camera makers started their new generation of manual zoom lenses as true zoom lenses with constant aperture. As optical design advances in the 70's, varifocal zoom design disappeared quickly. However, the constant aperture feature became variable aperture in lower level zoom lens because people started to accept this type.

Quote:
Originally Posted by eric s
This was all with old, all manual equipment. Now that I have a newer camera which has AF and other automatic things, I've got my first varifocal design lens. So this got me thinking...
I am surprised to learn that varifocal zoom lenses are still available on an AF camera? What lens is this? It would be very interesting to know.

Quote:
Originally Posted by eric s
Can they do things that the old design can't? Are they just cheaper? Any ideas?
In fact, varifocal lens designs are older than the true zoom designs. A good varifocal lens can do everything a true zoom can do with one penalty: one must refocus and re-adjust aperture because most varifocal lenses do not maintain constant aperture.

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Old Jun 24, 2003, 7:02 AM   #6
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Quote:
I am surprised to learn that varifocal zoom lenses are still available on an AF camera? What lens is this? It would be very interesting to know.
We're talking about the above EF28-135 f3.5-5.6 IS USM:
http://www.stevesforums.com/phpBB2/v...191&highlight=

Quote:
Nowadays, virtually all zoom lenses made by well-known lens manufactures are the true zooms that the focus will be maintained to a very high degree while zooming.
Zooms with high ratio usually are varifocal to remain compact, and I suspect the EF35-350mm to be a varifocal as well... (most true zooms are only 3-4x) This is really a common practice among non-dSLR cameras though (at least the ones that span wide to long tele like the CP5700 also)
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Old Jul 1, 2003, 12:54 AM   #7
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Originally Posted by NHL
We're talking about the above EF28-135 f3.5-5.6 IS USM
After some communication with Canon, Nikon, Sigma and Tamron, here is what I have.

Canon: None of Canon lenses is a true zoom. That is, all Canon zoom lenses cannot maintain focus while zooming.

Nikon: All AF zoom Nikkors will probably require a slight adjustment. It is your interpretation! IMO, AF zoom Nikkors may be varifocal (i.e., cannot maintain focus while zooming) but focus shift may be insignificant.

Sigma: All Sigma zoom lenses are true zooms. Exceptions are, for example, 28-200mm, 28-300mm or 50-500mm etc. They are varifocal at closer focus distance.

Tamron: All Tamron zoom lenses are varifocal.

It appears to me that lens manufactures are bet on the benefits of autofocus speed to compensate focusing shift.


Quote:
at least the ones that span wide to long tele like the CP5700 also
As far as I know, all Nikon Coolpix cameras are varifocal. This can easily be verified by entering manual focus and then zoom. The focus will not be maintained. In fact, Nikon's manuals make it very explicit that in manual focusing mode, refocus is required after zooming.

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