Someone who understands the dynamics of this sensor (because I don't). These are tests I conducted this morning (I've done the same before, just wanted to double check).

I mount a vintage Canon 12-120 Floutite lens I bought new in 1974 on my Pocket camera. At T2.8 I zoom in, focus, zoom back and it holds focus through the range.

I mount the same lens on the LS300. Zoom in, focus, zoom back and the focus shifts wildly - I have to crank focus a full turn (clockwise from the rear) closer when wide @ 12mm.

This lens has been parfocal on a dozen cameras over the years from an Eclair NPR, three Eclair ACLs through my Aaton - and now the Pockets.

I would think this would be a flange to sensor issue with my LS300 - However...

I mount the Canon 24-105 with Metabones adapter. Zoom in, focus, zoom back - and I need to crank focus a 1/4 turn counter-clockwise toward infinity to hit focus... That's the opposite direction from the 16 lens.

I mount an Olympus 12-40. Same exercise - when zoomed back I have to turn it clockwise to hit focus.

The 24-105 and the Olympus 12-40 are nearly parfocal on the Pocket Camera (the 24-105 requires a minor turn clockwise, opposite from when on the LS300).

Can anyone explain this behavior?

(BTW, my Leica-R primes focus at infinity correctly... they're on Novoflex adapters)

I haven't seen any focus shift issues when using the following zooms on the LS300:

Lumix 45-200mm f4-5.6
Lumix 45-175mm f4-5.6
Olympus 12-50mm f3.5-6.3
Olympus 9-18mm f4-5.6, Four Thirds mount
Olympus 14-35mm f2.0, Four Thirds mount
Sigma 18-50mm f2.8, Four Thirds mount
Tamron 24-70mm f2.8, Nikon mount
Nikon 12-24mm f4
Nikon 70-200mm f4

Of these lenses, only the Sigma 18-50mm f2.8 appears to be parfocal. The rest appear to focus on the LS300 as they do on their native mounts.

I really hope this isn't another issue with my camera... Anyone else try a vintage zoom?

I would think this would be a flange to sensor issue with my LS300 - However...



Steve,

you correctly mentioned the reason for this behavior. It is a "backfocus" issue. Professional ENG-lenses, for example, have a "backfocus" adjustment ring. When mounting such a lens to a camera body, one has to adjust the lens to the camera body's distance between flange and sensor. This is done by full zoom into a "Siemens Star" at a distance of about 3 - 4 meter, set focus, zoom out and set focus with the backfocus ring. This procedure is repeated until there is no difference of focus when zooming in and out. The backfocus ring is then fixed with a srew.

Your photo lenses do not have a backfocus ring, Therefor an adjustment between camera flange and sensor surface is not directly possible and zooming in and out requires focus adjustments. If the adapter between lens and camera body allows backfocus adjustment, this could overcpme the focus issue. However, you would have to adjust backfocus everytime you change the lens against another one.

I hope this clarifies your issue. It is definitely not a camera problem.

Joachim

Thanks Joachim: Actually the CA-1 to MFT adapter (on the Canon 12-120 Flourite - the one with the most dramatic variation) has shims. Although I'm not a lens technician by any means - and don't have testing tools, I tried eyeball testing several different shim combinations, assuming first that the rear element was too far from the camera sensor. Nothing I did improved the problem. I then tried using the thicker shims to move the rear element away from the element - no luck. I put it back together the way it was and tried it on my Pocket Camera again...Perfect.

But the weird thing is that with the LS300, while the Canon shifts dramatically (a full 360 degree rotation) when zoomed out, my Angenieux 15-150 (my least favorite lens, but works well on the Pocket), only has a slight shift, about 15 degrees, when moving from telephoto to wide... and it's in the opposite direction!

Also, my prime lenses, particularly the native MFT Nokton, focuses perfectly at infinity at f2. So it doesn't seem to be the flange - If I lived near someone like Duclos I'd take the camera to him and have the flange to sensor distance measured.. unfortunately I live on the Monterey Peninsula and don't have time for shipping.

Okay, now it really gets bazaar - I pulled the CA-1 adapter off the Canon 12-120 and put it on my very rare and very terrible Angenieux T1.3 16-44. This is one of the worst zooms I've ever seen - shame on Angenieux, even if it is faster than anything that was available at the time...

Anyway, it's nearly parfocal on the LS300... which is strange enough, but it's NOT near parfocal on my Aaton... Twilight Zone...

So it doesn't seem to be the flange - If I lived near someone like Duclos I'd take the camera to him and have the flange to sensor distance measured.. unfortunately I live on the Monterey Peninsula and don't have time for shipping.

Steve,
it is not a matter of the absolute distance of flange and sensor, it is always the combination with the mounted lens. If a lens is designed for MFT, it should have a well defined focal distance from its flange. However there will always be slight tolerances which require adjustments. This is even true for exorbitant expensive studio cameras and lenses.

With photo lenses the issue may even be more relevant, as they do not have the need to stay in focus when zooming in or out. You always take a single shot with a defined zoom position. That is one reason why video lenses are usually more expensive. They require much more engineering efforts to stay in focus when zooming through.

Joachim

Joachim: I appreciate your responses... however, the anomaly that I'm intrigued by doesn't fit any scenario, or combinations of scenarios that I've ever experienced... To reiterate...

I own three vintage professional 16 and S16 cine zooms with native CA-1 (Eclair) mounts. An Angenieux 15-150, an Angenieux 16-44 and a Canon 12-120.

Those lenses have all behaved identically over the decades on disparate cameras - an Eclair NPR, Arriflex 16S (w/ CA-1 to Arri adapter), three Eclair ACLs, an Aaton LTR7 (w/ CA-1 - Aaton adapter) and two BM Pocket Cinema Cameras (w/ CA-1 to MFT adapter) as follows... (And on the AF100, which I forgot to mention before because they didn't cover the sensor so I never used them).

Both the 15-150 and Canon 12-120 are parfocal throughout the zoom range.

The Angenieux 16-44 requires refocusing when zooming.

But on the LS300, the 12-120 is so non-parfocal that it's impossible to focus at infinity when wide; the 15-150 requires a 1/4 turn (in the opposite direction!) to refocus when pulled back; and the 16-44 is closer to parfocal than with past cameras.

While I appreciate that there may be differences between cameras, I would expect those differences to cause related problems, not widely differing ones.

Like I said, Twilight Zone.

Although this problem is curious, I would't assume that the distance from the mount to the image sensor is the same between the BMC Pocket Camera and the JVC LS300. I believe that the two cameras use different image sensors and would not have the same construction.

Well, I've pretty much given up hope that I could use my old Canon 12-120 anyway - which is a shame because I always liked the look of that lens (and I shot my first feature length documentary with it in 1973-4).

I've decided that working with the Olympus 12-40, with the VSM set at 92%, is giving me close to the same throw, and I can always stick the Lumix 35-100 in my vest pocket.

For faster, I've ordered the Rokinon 3 lens set that's on sale now - I know they're not the best lenses in the world (at least the 24mm isn't) but they're dirt cheap and give me a lot of T1.5 options. I often shoot in places like homeless shelters and farm labor camps, so I don't want anything too expensive out in front.

I wanted to post some test stills from the Olympus, using the full range of the lens + Prime Zoom, but they just won't upload - they're jpeg,s but they must be too big...

For faster, I've ordered the Rokinon 3 lens set that's on sale now - I know they're not the best lenses in the world (at least the 24mm isn't) but they're dirt cheap and give me a lot of T1.5 options.
WRT the Rokinon T1.5's, I found the 35mm and 85mm noticeably sharper than the 24mm.

That's what I've read - I bought them on a whim - I was researching lens repair options (I usually use ZGC, but was looking for something closer) and I ran across a review, and the info on the sale (and I like T stops)...

The 24 might not hold up with the Cine Zoom though... I have the Nokton 17.5 as well, so we'll see which is best.