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Does NX1 lose focus on zoom?


Sekhar
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After mysteriously losing shots to bad focus, I tested my NX1 on how zoom is affecting focus. I found that with my Canon 70-200 and adapter, it's really bad. I guess that's to be expected because of the adapter and all. But then I found NX1 is also losing focus with the 16-50 Samsung lens! Try zooming in real tight, focus, and then zoom out. I never had an issue like this with any of my Canon cameras and lenses. Am I missing something? How are we supposed to zoom during video?

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Well, I never noticed this as an issue with Canon cameras and lenses. E.g., 70-200 is known to be really good even if not perfect. There's the auto-focus in video with NX1, but it seems to hunt while shooting (even when there's nothing moving in the scene), and I can't use it with the 70-200 anyway. A bit disappointed. I'd appreciate any workarounds you guys might have. I need to cover a small fashion shoot this afternoon for which I want to use the 70-200.

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Also the EF mount on lens and camera bodies is not as strictly accurate as lens mounts like PL are. They have some manufacturing tollerances and cannot be adjusted or shimmed to the correct flange focal distance. As a result this distance can vary somewhat from one body to the next which could make it closer to par-focal on some cameras compared to others but would be total random and no way of knowing util you try. Also I bellieve this lens is at best almost parfocal when zooming from tele to wide. The wider the focal length the more sensitive it is. 

A quick way to test if your FFD is correct is to see if the lens hits its witness marks. I have put the same lens on 3 camera bodies (5D mkII, c100, and A7s with metabones adapter) and each of them missed the witness marks by varying amounts... 

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Guest Ebrahim Saadawi

The camera/adapter has no effect on how focus is maintained throughout the zoom range, this is a strict optical lens specification. Photography lenses never maintained focus whilst zooming, that's why we always use digital zoom to check focus rather than do it the old fashion camcorder way of optically zooming to check focus. 

One exception to the rule, Canon's new STM equipped lenses. The cameras electronically use AF to maintain focus while zooming (even in manual mode) as an electronic substitute to parfocal lens designs, one the camera is turned off (no electronics) the focus goes way off while zooming. In case of STM lenses, they might have less focus shift on Canon bodies vs other bodies. 

I believe your problem here with the 70-200 and 16-50 is that you didn't notice the focus shift on Canon cameras due to their softness/low resolution while on the clear nx1 the slightest shift in focus is huge. 

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The camera/adapter has no effect on how focus is maintained throughout the zoom range, this is a strict optical lens specification.

​Camera's and adapters can do exactly what the OP is describing. Yes it is true that still lenses were never designed to be parfocal although some can be quite close. Still lenses are not manufactured to the standard that cinema lenses are there are much larger tollerances. Camera manufactures know this and make their still lens mounts a little short to guarentee focus on these more cheaply manufactured lenses. Just look what happened to blackmagic when they followed the exact EF spec of 44mm on their first run of cameras. They got a flood of complaints from customers who's lenses would no longer hit infinity so they reduced the flange depth out of spec to allow for better compatibility. 

The same is true of adapters, they are intentionally made short to guarantee compatibilty with a wide range of lenses, no one wants a lens that won't focus to infinity. So on a camera with an adapter chances are it is much more out of spec than on a camera with a native lens mount. In order for a parfocal lens to remain parfocal the flange focal depth must be correct.

I had a PL lomo foton which is a parfocal cinema zoom. Put that on a PL camera with a proper flange depth of 52mm and it was perfect. Put in on my canon with a EF to PL adapter and it was no longer parfocal because the flange depth was no longer correct. 

It is quite possible that the lens is be having worse on the NX1 with an adapter that it was on his other camera with a native EF mount. 

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Guest Ebrahim Saadawi

Could you explain why the parfocal behaviour would change with slightly longer/shorter distance to sensor? with a shorter/longer flange distance, the lens focuses on the sensor point (even if that point is slightly different from on a Canon body, it's a still a point within the focus range), and when zoom changes, focus should stay on that, because to the lens itself, it's just as if it's on a canon body and focused on a point slightly farther/closer point. 

Why would changing flange distance disturb lens focus consistency whilst zooming? Curious.

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I am have not researched the exact reason why that much so what I say may be a little off but here is my understanding as to why. Zoom lenses have both front focus and back focus. Front focus is for the tele end and back focus is more for the wide end. You have a parfocal zoom when both front and back focus are in sync focusing at the exact same distance so you can go through the zoom and the focus will remain constant throughout. However back focus is much more sensitive to the flange distance than front focus is and offseting the flange distance is essentially altering the focus distance of the back focus. So by having an incorrect flange distance the back focus has been affected and is not focusing at the same distance as the front focusing is as it is not as sensitive to the offset and the lens is no longer parfocal. 

As an example say you are focused at an object 10 feet away. On a properly collimated camera and lens a parfocal zoom will have its front focus at 10' and its back focus at 10' so you can zoom and hold focus. Move the lens foward a fraction of a millimeter and now the front focus is still 10' but the back focus has shifted and is now focusing at 8'. Zoom the lens and the focus will now vary from 8' to 10' throughout its range.

 

http://www.xdcam-user.com/2012/11/why-its-almost-impossible-to-have-true-par-focal-zooms-on-the-cheaper-35mm-camcorders/

 

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