FS700 back focus/plane of focus issues...

C

Colinelves

Well-known member
I'm wondering if anyone has had experience with the following two issues I've come across.

1. 'Off' plane of focus: the right side if the frame falls off, out of focus, faster than the right - a bit like you're using a shift and tilt lens (but you're not).

2. Back focus off: shooting on wide fixed primes (14mm' 16mm), it's impossible to focus on a subject more than 4ft away (infinity focus cannot be reached).

These two issues were on separate cameras (the second was actually the replacement sent by Sony support while they tried to fix the first issue). The first is visible with both the kit lens and with lenses using the metabones speed booster.

the Second was only seen using the speed booster - but the lenses were perfect on the camera with the first issues - as in they hit their focus marks bang on on the first camera and wides focussed fine (excepting the off plane of focus).

I'm just wondering if various back focus/chip angle issues are common with the FS700, or if I'm just strangely cursed...
 
I can't say i've seen this but there are so many places for everything to go wrong and it highlights the weakest point of the FS700 which is the lens mount. I've noticed enormous amounts of flex between a ZA zoom and the mount. So much so that i anchor the LA-EA2 lens adapter (as well as other adapters with tripod mounts) to the rods. I also see play in some of the lens mounts as well. There's a good blog post on lensrentals.com about poor tolerances of various adapters. I've heard of issues with the metabones in the same way. All it takes is such a small amount of inaccuracy in the mount to add up.

I've also had lenses CLA'd when i've seen this tilted focus across the field. It's quite common for elements within the lens to not be seated correctly. I had a Zeiss 28mm f2 that took me ages to hunt down only to get a copy that showed this. After a rebuild though it's perfect.

If the two bodies exhibit the same problem, perhaps the kit lens you have is off? I have very little faith in the robustness of the kit lenses, i think they could go out of whack quite easily.

In answer to your last question, i've not seen universal issues with the 700 or 100, in that a problem with ALL lenses. It's been a combination of the factors outside of the camera. I would guess that the manufacturing tolerances are probably better controlled from emount to sensor then they are outside.

The question is whether you have a lens that you can rely on to be accurate as a testing point?

Of course i've also read of people with problems with sensor mounts not being aligned - across all cameras. So you might need to offer up a suitable sacrifice to the flange distance gods and hope that you can start with a solid body, then test...

cheers
Paul
 
It's not a lens issue as you can swap the lenses between the cameras and each camera exhibits it's own unique problem:

Camera A (shift tilt) has the problem with both the kit lens (no adaptor) and the metabones and the problem is not there on Camera B using the same lenses.

Likewise it's not an adaptor or lens issue with the back focus on Camera B as the same adaptor/lens combination is fine on camera A (you can focus them no probem - the centre is pin sharp).

I suspect the issue with Camera B (back focus) might well be a problem on the lens mount - easily fixable once Sony get it back (they seem to have perenial problems with their couriers in the UK: it's been sat at my house for over a week waiting to be collected!)

Those of us who started work when film was still dominant (or who are used to the old ENG cameras and lenses) are very familiar with back focus issues: if it's just a little off then then you'll have problems with wide angle lenses. With a wide angle the natural focal point is closer to the lens than the sensor or film plane itself, so an extra element needs to be added to throw the focal point back. If one of these elements (the lens mount or rear lens element) is off, you'll have problems focussing to infinity as the lens is trying to focus the image in the wrong place (not on the sensor).

It's not that uncommon for cameras to have the lens mounts off. Any decent rental house should have a flange clock and colimator to check their camera lense mounts are calbrated correctly before going out on a job (at least it used to be a standard check back when I started work in a rental house).

Sometimes the issue is with the shims on the lens itself - and you can check that with collimator or lens projector (something else that is standard equipment at most big/decent rental houses). Again, if they're off a lens engineer can adjust them

The problem with Camera A is a bit more of a mystery - I think the sensor block got shifted when it went in for the upgrade (or earlier when they removed some first from an internal ND (either way it was Sony engineers messing it up!). Now they're struggling to get it back where it should be (it only needs to be a fraction of a milimeter out of place to be problematic!)

When it came back the most recent time the issue had improved, but it hadn't quite gone away. I'm hoping if I send it back again they'll either be able to fix it, or finally give up and send me a replacement!
 
Colin, If you've seen the results change over a return then you're right. As i said i've not seen anything that screams out of whack with mine, but perhaps i've not looked hard enough.

What i would say is that Sony Prime have the most awful system when it comes to picking up. I bet they've sent couriers to a different address several times - the address of the retailer where the camera was bought - They did this 3 times in my case. It was only when some poor engineer had to override their local system to force an address in. Just saying!

Conversely their support has been excellent in my case, couldn't ask for more.

cheers
Paul
 
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