Question about accurate focus

R

romy101

Active member
Here is what I am observing. At mid zoom, using the focus ring and a 50 inch plasma monitor I can achieve perfect focus on my subject on green screen. This is my benchmark.

When I zoom all the way in to the subject and perform the same focus again, same subject and distance, the setting on my focus ring is different than half zoom.

Keeping that focus setting, now when I pull back out to mid zoom, the image is a tad softer.

I can move the focus ring back to my initial benckmark setting and get perfect focus again.

Why is that. Backfocus problem? I backfocused about a gazillion times. Bad lens? I sent the camera back to Panasonic and they gave it an A ok.

Im puzzled.
 
romy101 said:
Here is what I am observing. At mid zoom, using the focus ring and a 50 inch plasma monitor I can achieve perfect focus on my subject on green screen. This is my benchmark.

When I zoom all the way in to the subject and perform the same focus again, same subject and distance, the setting on my focus ring is different than half zoom.

Keeping that focus setting, now when I pull back out to mid zoom, the image is a tad softer.

I can move the focus ring back to my initial benckmark setting and get perfect focus again.

Why is that. Backfocus problem? I backfocused about a gazillion times. Bad lens? I sent the camera back to Panasonic and they gave it an A ok.

Im puzzled.
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Hallo Roomy! Glad to ear that you still have the 301! Not glad to ear that your problem is still there! Ok, if the situation is the one described in your words...you have a backfocus issue! Have you the possibility to have a 7" or 10" calibrated monitor with the focus assist option? I have a Swit monitor and a SDI to HDMI adapter. With the focus assit you have red lines around objects, so it's easy to see if you are in focus or not. By the way... try again the backfocus and do it at 2 meters far away from the chart. Use the chart provided with the lens. Set the camera to 1080i ( give it a try ), set iris to open and unlock the backfocus screw. Now set the zoom in tele position and use the focus ring to set the focus. When you think that you're done...give other two or three little changes to the focus ring and see the difference. You are moving in minimum range so it's easy to see differences. Ok give a look to the result. Now set the lens in wide position and set the focus with the backfocus ring. If you are too far from the chart is almost impossible to set the backfocus in the right way. When you think that the backfocus is ok...give it another two or three little movement and see the difference. Now go back in tele, set the focus with the same technique ( two or three little movement ) and go back in wide position. Normally your backfocus should be fine. Lock the screw and make your tests!
Bests,
Roberto
 
Oh...i have another thing to tell! If you are in mid zoom and your focus is right, is normal that you have to change it when you zoom in. And if you leave the focus in the same position and go back to wide position you will have an out of focus picture. The way the backfocus works is from tele to wide position and not in the opposite direction. If for ex. you set your focus in tele,go back in wide position and return to tele your focus is gone!Not so much but maybe you have to check it again. Everytime you go back from tele to a wide position, if your focus is right and your backfocus is set in the right position as well, you will have a sharp picture. So the way to use the backfocus is this: set the focus in tele ( use subject's eyes for ex. ) and go back in the position that you like. Now, if you have to go back in wide position for another shot, your focus will be perfect, but if you have to go in tele again...you have to make your focus again!
Another thing...if you set your focus in tele and go back to a mid position but you don't like the frame, you can adjust the zoom position with minimum movements, but you probably (90%!) have to correct your focus again...the subject maybe is moving, so you have to consider that you are shooting with the iris wide open, so the depth of field is reduced and even little movement of the subject if you are in a mid or expecially in tele position makes the focus point change.
 
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Yes i'm still with the camera. Don't know if thats a good or bad thing though. I have a Marshal field monitor arriving at my doorstep tomorrow which does have the focus assist you are talking about. Hopefully I can nail it with that tool.
 
I hate to say this but, its just a limitation of the camera. I too tested a million times different scene file configurations. I set back focus with several different charts at different lengths always same result. The closeup shots look great, but wide shots look horrible if you have a lot of detail in the shot. If you move the camera the image blurs until it stops then it gets sharp....not to mention other issues like skew...etc

I finally decided to sell the HPX300 a few weeks after receiving my AF100. The AF100 was generating in my own opinion better looking video, that I could live with on an inferior codec. Yes, I would like to work with AVC-Intra, but what good does it do to record crappy video to a good codec?

Anyway I sold the HPX300 for $6200.00 with Body/Lens/Ikan Batteries not to bad when It cost $7400.00 after $800.00 rebate. I was also able to sell off all accessories for near what I paid except for a Brand New Black Porta Brace Carry on Bag.

One other last piece of advice you might want to call ZGC maker of the Putora Chart. The tech will walk you through his setup procedure for setting back focus on HPX300. His suggestion was that the camera did not provide enough resolution to be set back 3 meters from chart and suggest setting up really close to chart then performing back focus.

Good Luck
 
Hey Chance, you just described everything I experienced. I got my monitor today and pulled accurate focus. Was still a mushy picture. At closeup on a face it looked super good. When I am about half body it falls apart. No detail in hair. When the actor moves slightly the hair goes into a mush. When he stays completely still then detail comes back. Frame grab side by side to an EX1, vastly inferior image. So there you have it. I have a buyer wanting $6200 for mine so I will go ahead and let go of it.
 
Well that is not my experience of my HPX301 it is pin sharp!

Here is a shot from an indie feature that I did last year, it used the full zoom ratio of the stock lens and it is sharp all the way through, this is just a You Tube upload but the full 1920x1080 25p master shows how good this camera can be: http://www.youtube.com/watch?v=rSlA-2HOuBs
 
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Hallo Gary! I've seen the trailer and i like it very much! Nice photography and very cinematografic look! Is this short movie finished? I'm courios to see it!
Bests,
Roberto
 
Thank you I now have all the final pro res HQ masters, can you send me your address and I will post a QT file to you so you can see how well the 301 graded for the master.
 
If it would be a back focus issue, you would never be in good focus no matter what zoom distance you have?

What you are describing is something that I test every time I want to buy a camera, and if the focus dosen't stay the same after I zoomed out, than in and out again, there's something wrong with your lens.
 
Admir said:
If it would be a back focus issue, you would never be in good focus no matter what zoom distance you have?

What you are describing is something that I test every time I want to buy a camera, and if the focus dosen't stay the same after I zoomed out, than in and out again, there's something wrong with your lens.
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Yes,that's right. But the stange thing is that at Panasonic lab's they don't have find something wrong with the lens... i think that they have repeat the backfocus procedure...i don't know...
 
How hard is it to backfocus though, really. You can wiggle the backfocus dial only so many time and even by accident one of those time I have to nail it. My setting is pretty much close to the factory preset on the lens but no matter what I can never get sharp focus in midrange when I zoom in, focus then zoom out. The act of zooming out put it out of focus slightly. With my sharpness chart I move the focus barrel and snap it into focus and I am all set.

Panasonic does not answer questions. I sent my camera back to them and they took it for 3 weeks. When I called to ask a question they said they're looking at it and will send it back when done. I cannot talk to a tech or anyone working on the camera. And the lady in the New Jersey facility is rude as beans, and even hung up on me once.

If I send it back again I am sure they will just send it back and say it is within spec, again.
 
Back focus is most critical at the shortest focal length of the zoom and wide open aperture. Best practice is to use a large chart and back off enough to get true infinity. If you can't do that, measure chart to sensor plane distance and set the focus ring on that witness mark. Zoom all the way in to full tele, do initial back focus adjustment, zoom all the way out to full wide and fine adjust for sharpest wide focus. If the lens is properly collimated it should remain in focus at full tele and full wide. All zooms, even the best, will shift focus slightly over the range between these two points, but your full tele and full wide should be sharp at the same focus setting if the lens is adjusted properly and should be nearly perfect at infinity throughout the range, The closer your focus distance the more variance you are likely to see.

If your back focus changes at full tele and full wide after a couple of zoom in/out cycles there is a mechanical problem with the lens.
 
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All zooms, even the best, will shift focus slightly over the range between these two points, but your full tele and full wide should be sharp at the same focus setting if the lens is adjusted properly and should be nearly perfect at infinity throughout the range, The closer your focus distance the more variance you are likely to see.
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Right and clear!
 
All zooms, even the best, will shift focus slightly over the range between these two points, but your full tele and full wide should be sharp at the same focus setting if the lens is adjusted properly and should be nearly perfect at infinity throughout the range, The closer your focus distance the more variance you are likely to see.
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Right and clear!
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Ditto. At close focus distances all kinds of things tend to happen. Video lenses are actually pretty good in this regard, the problem is worse with photographic lenses, where focus can shift even if the aperture changes.
That said, this probably isn't as much a problem with the 3xx series cameras as it is with the lens itself, or rather the fact that after buying a $9k or so camera, the idea of getting a proper lens that will cost at least as much the camera itself may sound a little absurd to most people. The stock zoom lens, from testing and practical experience, is simply not very good at anything other than the long end of the zoom; it's probably $1,500 worth of glass that you pretty much get for free.
 
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