am I crazy or is this a back focus issue? (new FS100 user)

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dv416

Well-known member
just got an FS100 + stock 18-200mm zoom lens, and I'm trying to figure out if I have a legitimate problem or just a quirk of the camera...

basically, I notice that when I zoom in all the way to catch focus, then zoom back out, the focus slips a little on the way out. specifically, if I zoom in to 200mm and then zoom JUST A TINY BIT OUT, I noticably lose focus. I can even see by referring to the focus-zoom distance meter that the feet indicator has changed by .2 feet or so. so its a registered change by the focus-distance meter. and its enough of a change to slip out of focus. is this the same on every camera? is it just a quirk of the lens to slip when focusing at long focal lengths? or is this a legit back focus problem?

any help appreciated, THANKS!!
 
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Back focus problem is when the focus is out when you zoom all the way out to 18mm.
it seems that it goes out immediately when you start zooming out, so it could be something loose inside the lens.
Where is your indicator at when you zoom back to 18mm?
 
Just did some tests. Got some strange results.

I focused on a shelf approximately 12 feet away, throughout a variety of different focal lengths. I used the peaking focus assistant for one test and an eyeballed 8x focus expander for a second.


according to peaking
z99 = 13 (shelf measured as 13 feet away)
z91 = 11 (shelf measured at 11 feet away, etc)
z78 = 11
z55 = 17
z29 = 17
z18 = 17-20 (getting hard to tell)
z00 = 17 (hard to tell)


according to 8x expanded focus
z99 = 13
z91 = 13 (later, re-tested and got 11. strange)
z78 = 11
z55 = 16
z30 = 16-23 hard to tell
z18 = 18 (hard to tell)
z00 = 11-20 all looks to same. IN FOCUS


Due to the 13 vs 11 discrepency between the two tests, I re-tested at z91 on the 8x expander and got unequivocally different results.
z91 = 11, no question about it.

what do you make of all this? after re-testing, both methods seem to indicate the same results. a slip of the focus around z92 and again somewhere between z78 and z55. If I look at the shelf at z91 and focus it, the second I go to even z92 or higher, it slips out of focus. its maybe not a traditional back focus issue, but it seems like SOMETHING.
 
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The 18-200 is not a parfocal lens, so you can't zoom through the entire range and maintain focus, you'll typically have to focus after recomposing with the zoom. I found that to be the case with most of the super zooms I've tested. Some of Sony's pricier zooms like the 16-50, 24-70Z and 70-400G are parfocal - but those are pro grade lenses and price accordingly compared to a consumer zoom like the 18-200.

Back focus is when your focus confirm shows green and the focal plane is actually somewhere behind your point of focus, then you hopefully can tweak with AF micro adjust (if the camera is equipped) to fix align everything. What you're describing doesn't sound like a case of back focus to me, sounds normal with that lens.
 
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re: parfocal lenses - I didn't know about that, and I think you're right! Maybe I've just shot with parfocal lenses my whole life up until this lens. Because the only time I've ever had focus slips with zoom was on a larger ENG camera with an actual backfocus problem. so that's why I thought it was backfocus.

nice to know there's nothing wrong with my lens, but its still a bit disheartening that the focal distance meter is just an approximation. for some reason, I always thought it was more scientific than that.
 
The better ENG lenses are definitely parfocal, sadly the small superzooms are not. If your lens is sharp when you nail focus I'd guess its fine.
 
Both my 18200 that I've owned are similar.

If you zoom in, focus up, then zoom out there is a slight change of focus but only when there is torque applied to the zoom wheel. Once I leave the zoom wheel alone the focus point goes back to where I focussed up on. You can't do a creep zoom for sure and for obvious reasons the problem is worse at the long end.

I use the 18200 as a par focal lens and it's close enough not to embarrass you, or at least the two 18200s I've used have been.
 
The kit 18-200 is not sold as a parfocal. If you use it in autofocus mode (or in spot/face mode on an FS700) and zoom slowly enough for the autofocus to keep up, you'll 'get away with it'. Another anomaly is that this lens is 'fly by wire' - no manual connection, which makes manual focusing a little irksome AND there's no 'auto assist' like the EX1 lens. However, the rule of zooming in, focusing and then zooming out does not apply here. To a certain degree, the kit lens and those like it (Sony E-mount lenses) are designed more for a 'tap to focus' sort of operation.

FWIW, the EA50 demonstrates a new direction for cameras styled around >4K sensors - providing a 2x digital zoom by using the whole sensor down to a 1920x1080 crop, with a smooth zoom in between. It's not going to outperform a Red or Alexa (the EA50 isn't quite FS100 for that matter), but it will be enough for many videographers in its current form and I'd like to see it develop on the FS700 platform. And of course you're not relying on a parfocal nature of the lens.
 
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