[mgvh]

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What zoom lenses are parfocal?
I've tracked down a couple threads that deal with the matter somewhat (here and here), but I think it would be helpful to have a list of zoom lenses that are parfocal.

Quick summary:
Parfocal lenses are ones where you can focus at any focal length and changing the zoom won't affect the focus. With the advent of autofocus mechanisms, that became less important, so many (most?) newer zoom lenses are varifocal. (Apparently it is easier to make varifocal lenses, and they are also lighter.)

Why is this significant? If you want to shoot video on most (all?) of the Pentax bodies, the focus is locked once you start shooting video. On some of the newer bodies (e.g., the K-30), you can re-focus while shooting, but it does not offer continuous focus.
SO: if you want to shoot video with your Pentax, and you plan to do any zooming, you will most certainly want a parfocal lens. (It is nearly impossible to zoom and focus at the same time.)

In any case, the lens database here doesn't seem to indicate whether a zoom lens is parfocal or varifocal, so, as one consideration when buying a lens, a parfocal zoom lens list might be helpful.

In general (and here is where confirmation from your experiences would help), I think most (all?) of the film era zooms are parfocal: K, M, A, F, FA, and FA-J lenses.
From reading the other threads, it appears that some newer lenses may almost be parfocal. (DA 16-45; DA 50-135; DA 50-200 WR; Tamron 28-75)

UPDATE (and I'll try to keep this updated as something of a sticky): Thanks to beholder3, we have something of a list being established. (See his post on page 2)

After reading this here I started a similar discussion on another forum and this is what we got so far for parfocal lenses with PK mount.

UPDATED 2014.10.31 (w/ latest info)

1. M 28-50/3.5-4.5
2. Pentax M 80-200 mm f4.5 (push-pull)
3. Pentax F 100-300 mm f4.5-5.6
4. Pentax DA 16-45 F4
5. Pentax DA 18-55 mm F3.5-5.6 AL (v1)* - WR version
6. Pentax M 75-150mm f4 (push-pull)
7. Pentax M 35-70mm f2.8-3.5 (push-pull)
8. Pentax A 35-105 mmF3.5
9. Pentax FA 24-90 mm
10. Pentax F 28-80 mm F3.5-4.5
11. Pentax-F Zoom 1:3.5-4.5 28 - 80mm MACRO
12. Pentax F 35-70**
13. DA 50-200 WR
14. Pentax F 80-200 mm 1:4.7-5.6
15. Pentax DA 18-250*
16. Pentax FA 100-300mm F4.5-5.6
17. Sigma 18-200
18. Tamron 17-50/2.8
19. Tamron 70-150/3.5 (20A)
20. Tamron 28-75/f2,8
21. Tamron 28-80mm F:3.5-5.6 AF Aspherical**
22. Tamron SP 35-80/2.8-3.5 (01A)
23. Tamron 28-70/3.5-4.5 (44A)
24. Tamron 70-200/2.8
25. Tamron SP AF 24-135mm F/3.5-5.6 AD Aspherical [IF]
26. Tokina 19-35/3.5-4.5
27. Tokina RMC 25 - 50 F4
28. RMC Tokina 75-150 f/3.8*
29. Tokina SZ-X 60-300mm f4-5.6 (push-pull)
30. Vivitar Series 1, 35-85mm f2.8
31. Vivitar 70-150mm f3.8
32. Vivitar MC 75-205/f3.8 (1touch Kiron)
33. Sigma EX 10-20 f4-5.6**
34. Sigma 17-70/2.8-4.5*
35. Sigma 18-35 f/1.8 (NOT parfocal, but at f/4 and smaller the focus shift becomes trivial)
36. Sigma 18-50/2.8 EX Macro
37. Tamron Adaptall 70-150mm f3.5 Model QZ-150M (two touch, has manual/auto switch so can vary aperture manually whilst filming)
38. Tamron Adaptall-2 35-70mm f3.5 Model 17A (two touch)
39. Tamron Adaptall-2 70-150mm f3.5 Model 02A (two touch)
40. Tamron Adaptall-2 80-210mm f3.8-4 Model 03A (push-pull)
41. Tamron Adaptall-2 80-210mm f3.8-4 Model 103A (push-pull)
42. Tamron Adaptall-2 70-210mm f3.8-4 Model 46A (push-pull)
43. Tamron Adaptall-2 SP 70-210mm f3.5-4 Model 52A (two touch)
44. Tamron Adaptall SP 60-300mm f4-5.6 Model 23A (Push-pull)
45. Tamron Adaptall-2 SP 70-210mm f/3.5 (19AH)

*= under discussion.
**= nearly parfocal


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I can confirm that my DA 18-135, DAL 18-55, and Pentax DAL 55-300 are NOT parfocal.

I can confirm that my F 35-70 is parfocal as is the Tamron SP AF 24-135mm F/3.5-5.6 AD Aspherical [IF].

Some lenses may be hard to determine, especially wide angle ones. My Sigma EX 10-20 f4-5.6 is nearly parfocal. (Since it's so wide, you have lots of room for error, but basically if you focus at 20mm, it will still be good at 10mm. If you focus at 10mm, it may be a bit off at 20mm.)

If anyone is interested, then, it would be helpful to confirm:

• If those older zooms ( K, M, A, F, FA, and FA-J) are indeed all parfocal
• Which of the DA and DA-L zooms are parfocal
• Which other non-Pentax lenses are parfocal

BTW, it might also be helpful to note which ones have push-pull zoom, since those seem to be easier for video.
Thanks!

NOT Parfocal

1. Pentax DA 18-135
2. Pentax DAL 18-55
3. Pentax DAL 55-300
4. Tamron Adaptall 35-80mm f2.8-3.5 Model QZ-35M
5. Tamron Adaptall 85-210mm f4.5 Model QZ-210M
6. Tamron Adaptall 85-250mm f3.8-4.5 Model QZ-250M
7. Tamron Adaptall-2 75-250mm f3.8-4.5 Model 104A

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Last edited by mgvh; 11-18-2014 at 05:03 PM.

[Clinton]

Would the power zoom lenses count since they refocus as you zoom?

[mgvh]

According to this thread, PowerZooms are not necessarily parfocal. (Isn't it still the camera controlling the focus, not the zoom? Since the camera fixes focus once you start videoing, it wouldn't adjust the focus.) I'm not sure how the PowerZooms work, so some may be parfocal, but at least one does not that someone tested.

[stevebrot]

By definition, zoom lenses are supposed to be parfocal. If not, the lens is referred to as vari-focal. At one time lenses of both types were available for sale. I remember quite a bit of discussion in the photo press back-in-the-day when zoom lenses were coming into greater prominence (early-mid 1070s) regarding focus shift while zooming and the conclusion at that time was that the issue was almost non-existent in name-brand designs of the time.

But to limit my input to that which I without a doubt know, the following pieces of zoom glass that I have actually used are parfocal:

• Tamron 70-150/3.5 (20A)
• Tamron SP 35-80/2.8-3.5 (01A)
• Tamron 28-70/3.5-4.5 (44A)
• Tokina 19-35/3.5-4.5
• Pentax-DA 18-55/3.5-5.6 AL (v1)

This is pretty easy to determine if you have a focus screen with a split image. Do critical focus at maximum zoom and go to minimum zoom and confirm that the split image is still aligned. Oh, and I suppose I should include the zoom lens on my Canon G-2. It is looks to be parfocal as well...at least it does not refocus if zoomed after focus is attained.


Steve


(You maybe should have asked if anyone knows of any zoom lens that is NOT parfocal Most legacy glass was designed for use on cameras with split-image finders and any weakness in this area is readily apparent to the user.)

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Last edited by stevebrot; 10-12-2012 at 09:59 PM.

[SyncGuy]

And I believe the Pentax A35-105/3.5 is parfocal...

[Giklab]

The off-brand 75-150/3.8 (also rebranded as the e-series Nikon 75-150/3.5 are parfocal. I find this mega-useful for portraits.

[Lowell Goudge]

My tamer on 200-500/5.6 and viviparous series 1 70-210/3.5 version 1 also do not change focus when zooming

You can also add my sigma APO 70-200/2.8 EX (non DG non macro)

[Just1MoreDave]

This is pretty easy to determine if you have a focus screen with a split image. Do critical focus at maximum zoom and go to minimum zoom and confirm that the split image is still aligned.

Both copies I have of the SMC Pentax-F 70-210mm f4-5.6 fail this test. So does my SMC Pentax-FA 28-105mm f4-5.6 Power Zoom.

Passes: Vivitar/Kiron Tele-Zoom 85-205mm f3.8 M42, Takumar-A 70-200mm f4, Pentax DA 16-45mm f4.

Kind of a motley zoom collection, isn't it?

[Clinton]

It would be interesting to try a video with the K-30 or K-01 and see if the FA PZ zooms still maintain focus while zooming. It might bring a whole new value to PZ.

[top-quark]

Are you sure that the 55-300 isn't parfocal? I've just tried out the DA variant. Focused at 55mm, zoomed out to 300, half-pressed the shutter and got an instant focus confirmation in the viewfinder. Sounds parfocal to me.

[vonBaloney]

In general, if you focus on the wide end and then zoom in, it could be a bit off, parafocal or not. Pretty sure my DA 18-250 is parafocal -- I often zoom in to focus, especially when what I want is going to remain in the center (otherwise I'm recomposing when I zoom back out which changes the plane anyway).

[boriscleto]

It would be interesting to try a video with the K-30 or K-01 and see if the FA PZ zooms still maintain focus while zooming. It might bring a whole new value to PZ.

Do the K-30 or K-01 power zoom? Because the K-x and K-r didn't.

[audiobomber]

I can confirm that my DA 18-135, DAL 18-55, and Pentax DAL 55-300 are NOT parfocal.

But to limit my input to that which I without a doubt know, the following pieces of zoom glass that I have actually used are parfocal:[*]Pentax-DA 18-55/3.5-5.6 AL (v1)

I find it hard to believe that the original 18-55 kit lens is, but the DA L isn't. Surely both are, or neither are?

Also, the 16-45 seems to be claimed as parfocal and almost parfocal above. It should be confirmed one way or another. Is there a standard test you would suggest?

I haven't paid much attention to this lens parameter, but I know for certain that the DA 18-250mm is varifocal.

[vonBaloney]

I haven't paid much attention to this lens parameter, but I know for certain that the DA 18-250mm is varifocal.

Of course I could be wrong. Still works better to zoom in to focus. If it is varifocal, and I zoom in and out on the same thing and refocus at those different zoom levels, shouldn't it *do* something, i.e. adjust focus? It will of course if I start on the wide end but if I start at the long end and back up all I will get is a confirmation I'm already in-focus -- no adjustment. Also, if a lens is varifocal, doesn't the automatically make the distance scale on the lens wrong or meaningless since it stays stationary as you zoom in and out, indicating that it is focused on the same distance as you do? I'm not sure I've ever used a zoom that if it was in-fact varifocal, was "varifocal enough" to actually matter in practice. But then other than the 18-250 and the kit lens (which I never really used and then sold), I've only used older film era zooms...

[audiobomber]

I just tested the 18-250 again to ensure my memory was correct. No matter whether I go from 18 to 250 or back again, it must refocus after each change, thus varifocal. I found the same with the original DA 18-55, the DA L 18-55 and the DA 16-45mm, all varifocal. The 16-45 was tricky to test because it's so wide.