HPX170 - Interesting observation, normal or not?

[Leo Versola]

I was messing around with the HPX170 mounted on a tripod and I noticed something I haven't noticed with other video or still cams.

If you bring up the marker and center it on an object, the center spot in the marker at the far end of the zoom is not the same as the near end of the zoom.

This would mean that if (which I don't normally do) I were to zoom during a shot, I would need to compensate with a slight amount of vertical pan.

On my other cameras, the center spot does not change as the zoom changes.

Is this normal?

[Dino]

Yes, I have seen it with Sony 2/3" cameras. There is even a compensation menu item in my
Sony BVP-550's.

[Luis Caffesse]

Hm, interesting.
I just checked this and it does seem to fall just slightly lower than you would think when zooming in.
It's still in the center marker on my camera, but not dead center - it seems to consistently zoom in just slightly lower than center.

Odd.

I'll check my HVX in a moment and see if it does the same.

[Leo Versola]

Yes, I have seen it with Sony 2/3" cameras. There is even a compensation menu item in my
Sony BVP-550's.

Hmm, I suppose that's slightly comforting... Still, I'm wondering why that's so; shouldn't the focal point remain the same throughout the full range of the zoom?

[Leo Versola]

Hm, interesting.
I just checked this and it does seem to fall just slightly lower than you would think when zooming in.
It's still in the center marker on my camera, but not dead center - it seems to consistently zoom in just slightly lower than center.

Odd.

I'll check my HVX in a moment and see if it does the same.

Okay, keep us posted... Thanks Luis...

[Justin Kuhn]

Generally I have to adjust upwards when zooming in with my HVX.

[Leo Versola]

Generally I have to adjust upwards when zooming in with my HVX.

Same with the HPX170... I still think it's pretty odd, doesn't anyone else?

Is there a reasonable technical explanation of why this is so?

[Ron Rodenmeyer]

Maybe I'm misunderestimating what you're saying, but it sounds like you're talking about the normal behavior of any camera lens. For instance, when you frame someone in a medium shot and zoom into a closeup, you need to tilt up simultaneously or the person's face will be "cut off". Likewise, you must tilt down when zooming out to avoid a crazy amount of head room.

Is this what you're talking about?

[Mark Smith]

What is being discussed is telecentricity od the zoom lens. I don't think you'll see much telecentricity at this price level. I've seen plenty of zoom lenses that cost several multiples of an HVX that are not telecentric....

[frobozz]

I've not heard the term telecentricity, but aren't we talking about how well centered the zoom lens is over the CCD? (From my experience with the HVX200, the answer is "not very well.")

So the zoom lens could be equally diminishing the view angle in all directions as you zoomed in, but because the center of the CCD is not in the center of the lens' image circle, the effect is that of the image becoming progressively offset in one direction or another.

Duncan

[Leo Versola]

I've not heard the term telecentricity, but aren't we talking about how well centered the zoom lens is over the CCD? (From my experience with the HVX200, the answer is "not very well.")

So the zoom lens could be equally diminishing the view angle in all directions as you zoomed in, but because the center of the CCD is not in the center of the lens' image circle, the effect is that of the image becoming progressively offset in one direction or another.

Duncan

Exactly what I was thinking but waiting for someone else to give a different answer, as perhaps there may be some valid technical reason for it to be this way?

I researched telecentricity (great new .50 word BTW) and based on the explanation, it appears to be related more to the shifting of image perspective in 3D as opposed to the shift on the X-Y axis as observed on the HPX/HVX.

[frobozz]

OK, I also looked up telecentric lenses. Whoa, freaky stuff. But not what's being discussed here, I think. I've heard the term "zoom range eccentricity" before in relation to this. Google finds lots of patents talking about methods for making sure your zoom lens is precisely centered over your image sensor, but I guess none of those are owned by Panasonic :-)

Duncan

[KOZMO]

I believe the correct term is "lens tracking" and is seen in less expensive zoom lenses like the 170's. Spot on tracking is why motion picture zooms cost 25 to 50k. Honestly I've seen it to some degree even in those lenses. I don't think you'll find it's much of an issue for actual shooting.

Richard

[Leo Versola]

I believe the correct term is "lens tracking" and is seen in less expensive zoom lenses like the 170's. Spot on tracking is why motion picture zooms cost 25 to 50k. Honestly I've seen it to some degree even in those lenses. I don't think you'll find it's much of an issue for actual shooting.

Richard

Thanks. And true, it's not really an issue, at least for me, because I don't normally zoom during shots for narrative work.

[frobozz]

I believe the correct term is "lens tracking" and is seen in less expensive zoom lenses like the 170's. Spot on tracking is why motion picture zooms cost 25 to 50k. Honestly I've seen it to some degree even in those lenses. I don't think you'll find it's much of an issue for actual shooting.

Richard

Except that this problem has nothing to do with the zoom design per se, rather it has everything to do with how the lens is positioned in front of the sensor. One of the many reasons motion picture zooms cost so much is their design keeps stuff in perfect focus during a zoom, not sure if that's what you're referring to?

This whole thing really only becomes an issue if you screw something to the lens threads (thus perfectly centered to the lens) and expect it to be perfectly centered to the resulting image. A lens hood would be one example; a hard mounted 35mm lens adapter would be another.

Duncan

[KOZMO]

The focus is another advantage of the expensive zooms, tracking is another. It is in the lens, the centering changes often in relation to the film/sensor when you change aspect ratios and formats, i.e. academy centered, super 35 etc..

Richard