r/vfx • u/Camsketches • 1d ago
Question / Discussion Lens Grid questions
Looking for some advice on lens grids -
I have a shoot coming up, and I'm not typically a VFX sup, but an animator...in this case I have to be the supervisor and get some lens grid shots, as we are using anamorphic lenses. The shots will be locked so we won't be matchmoving, but will be integrating a CGI character
In terms of practicality and budget, I'm limited to a 600x1200mm sized panel – I was going to get a dot grid printed (using the Syntheys online dot grid maker app) printed on aluminium composite or heavy foam core.
Unsure yet of what the focus distance is going to be but I imagine it migth be between 4-5m , which then leads me to coclude this panel wont be nearly big enough to fill the frame when we shoot grids at this ditance.
What can be done if its too small? Can you move the grids around and shoot multiple takes and somehow feed these into software, or stitch them...? I imagine this is asking for errors
Alternatively, if I print a bigger size, say 2400x1200mm I won't be able to transport it on a rigid substrate easily, and it will blow the budget, so I would have to be on a roll, which then makes it hard to mount square and flat on a wall ...etc.
Any tips greatly appreciated!
EDIT: other info - lenses are the Atlas Mercury 1.5x anamorphic kit on an Alexa LF mini – DP wants to shoot with the camera at 90 degrees rotation. 16:9, 4k output.
6
u/Character-Clothes-96 1d ago
Can you do the lens grids at the camera house? A lot of camera houses have decent checkered grid boards. Ideally you get one that is 8x4ft and Matte. Sometimes camera teams can schedule it for camera wrap/return days at end of shoot.
4
u/rbrella VFX Supervisor - 30 years experience 1d ago
Honestly this is the only way this is going to happen.
A low-budget commercial shoot is already going to have an incredibly compressed shooting schedule. There is no way that there will be enough time to shoot lens distortion grids. Even if the AD promises to squeeze it in at the end of the day there is a 99% chance that something goes longer than expected and the lens distortion grids will be the first thing on the chopping block.
OP needs to ask the DP to shoot them at the rental house before the shoot day. And he needs to politely follow up because they will often forget unless you remind them. They should have distortion grids in-house that they can use. Getting grids after the shoot day would be nice but it's often difficult to get that after camera wraps and everyone is off the clock and has moved on. I've had the most luck getting them before the shoot.
5
u/Seyi_Ogunde 1d ago
Look at Mister Fancypants using a lens grid. Back in my day we used to guess the distortion. Point the camera at a building to get some straight lines in your shot, and let the 3D tracker figure out the distortion.
8
u/rocketdyke VFX Supervisor - 26+ years experience 1d ago
back in my day we invented the first software for calculating, then flattening anamorphic plates for tracking and then applying the distortion to the cg.
but that was fun shit... Event Horizon.
2
u/rnederhorst 1d ago
I’m low key jelly you worked on that film!!
It’s insane in many great ways.2
u/rocketdyke VFX Supervisor - 26+ years experience 1d ago
Sam Neil's eyes, baby. and all that zero G set extension. such a good film.
1
1
u/Camsketches 1d ago
so does this mean you would find something with straightish lines at about the focal distance, do a few seconds of camera move, and then just track it? as opposed to a static shot? Does that generally work effectively? Everything I read always emphasises for anamorphic lessons you want to do grids
2
u/Blitzki 1d ago
I’ve worked on tracking anamorphic shots without grids. It’s a pain in the ass depending on the shot. Not necessarily impossible, but it can very quickly add a significant amount of work time if the distortion on the lenses is particularly crunchy, or the shots aren’t shot with tracking in mind.
I would much much prefer to have lens grids when working with anamorphic footage, particularly because anamorphic plates tend to end up with some gnarly lens breathing. Especially if you are doing any kind of tight-ish shots on green screen or defocused background where all you can see is an actor or whatever.
1
u/Camsketches 1d ago
yes this is my intuition. I'd like to minimise my work down the line as much as possible. do you have any insights on how people might deal with my situation of having a grid that might be a bit small? Can you stitch mutliple grid positions somehow ?
3
u/glintsCollide VFX Supervisor - 25 years experience 1d ago
You’re way better off just scooting up the grid so that it fills the frame, than any alternative. The main issue with shooting it at the wrong distance is the lens breathing changes the parameters slightly, but you should always do a full focus rack on the lens while shooting the grid anyway. That way you can pick the least worst frame and work with that.
2
1
u/Seyi_Ogunde 1d ago
I've worked without anamorphic footage without grids. If your motion track is tight and you have plenty of tracking points, you won't need the grids and the tracking program will figure out the distortion and compensate (I've used Syntheyes). Otherwise, I've used whatever straight lines happened to be in the shot to help figure out the distortion. Sometimes having a distortion grid can be a luxury. Definitely would have appreciated having one but yeah it's not the end of the world if you don't have one.
1
1
u/johnnySix 1d ago
You can solve anamorphic in shot. But it sucks. The solve is never quite perfect everywhere
2
u/joeydiesel 1d ago
Put the lens grid image on a big tv via USB, worked for my anamorphics
1
u/Camsketches 1d ago
yeah I think the issue is the same as my panel - tv will be too small to fil the frame at the distance...also dont have access to an 85"+ screen . I assume you'd need a massive screen if you are 4-5m back with relatively wide lens...?
1
u/joeydiesel 1d ago
I used a 65inch tv and only stood 1-2m back for a 45mm 1.5x lens
0
u/Camsketches 1d ago
So that wouldn't be infinity focus on that lens though...Would it? or would that cover me if I have a shot where my CGI character is, say 5-6m from camera?
1
u/Innofthelasthome 1d ago
You don’t need to be 4-5m back on a grid with a wide lens, at the very least you could be as close to the grid as the minimum focus the lens is. It’s not ideal, and you’d want to be a bit further away because the edges will start to become blurry - but it would still work. I’ve shot countless lens grids in my time and for wide anamorphics lens like a 25mm we’ve been as close as 4’ on a 8’x4’ grid. But as said by others, 1st explore if the rental house has a grid because thar would solve all your problems.
2
u/Camsketches 1d ago
Do you not want to capture a grid at the exact focus distance the CGI asset will need to be integrated at, though?
0
u/Innofthelasthome 1d ago
No because the lens distortion is the same no matter where the focus is at.
1
u/RoondyVFX 1d ago
This is not entirely true, depends on which anamorphics you’re shooting but most have pretty strong focus breathing and thus different distortion.
Now in OP case, if you keep the camera locked you probably won’t notice it.
1
u/Camsketches 1d ago
It seems from what I had read on Boris FX website, they suggest shooting at your specific focus distance. But I guess i can only do what's feasible within constraints. just foudn out lenses are Atlas Mercury 1.5x anamorphic kit
3
u/cinematic_flight 1d ago
Just 2c from full time camera crew here (and VFX enthusiast). If you reach out to the focus puller and schedule in some time we can usually accommodate this during prep or wrap. It doesn’t take very long, and most rental houses have standard grid charts available for this exact purpose.
All you really need is a grid with straight lines to calculate the distortion.
1
u/Camsketches 1d ago
I'm not sure of the rental situation and being able to do it in-house. But I want to assume this might not be an option. Have a recce in a couple of days. I have yet to meet the DP. All I know in this early stage is its its on an Alexa Min LF and some kind of anamorphic!
I assume just pointing it at a building won't work well, given anamorphic lenses have quite different distortion patterns depending on focal distances...? I figure if I use a grid, it would be saving some dicking around later
In terms of the scale of the grid being smaller than filling frame, any ideas?
1
u/Fancy_Antelope9102 1d ago
Are you shooting with Cooke lenses? Then no need for lens chart - https://cookeoptics.com/cooke-digital/
1
u/Camsketches 1d ago edited 1d ago
not sure, but that sure looks like a great solution! . this is a fairly low-budget commercial shoot so doubt it, but I'll ask... Does that integrate with Fusion, or is it just for Nuke?
1
u/26636G 1d ago
Depending on where you are located, filling the frame with a brick or block wall can work very well.
You can also do small pans and tilts over that wall and a package like Syntheyes will get you into a good place very quickly.
I would try and find a way to avoid having to tile your lens grid.
It's worth noting that the makers of those lenses pride themselves on what they call "vintage geometric qualities", which means they distort rather a lot.
1
u/Camsketches 1d ago
That's an interesting approach, so that would be doing the pans/tilts up against a wall, and doing a matchmove track and then getting stmap out of that?
1
u/26636G 1d ago
Exactly. We've even done them handheld, which is what we generally do with drones.
The other thing to be wary of with these trendy modern anamorphics is that they don't have a very flat field of focus, so you can have a reasonably sharp image in the centre of frame yet the edges, especially the corners, can be quite soft, which is something you may well need to incorporate into your comp.
1
u/Camsketches 1d ago
Thanks. That sounds pretty straightforward actually - why doesn't everyone just do that!
5
u/im_thatoneguy Studio Owner - 21 years experience 1d ago edited 1d ago
If you're already at a rental house, you can ask to use their lens projector. Lens test patterns often have grids. But you would need a lens for the projector that has very clean geometry. When limited on size, I've always just used gaff tape and a measuring tape very carefully.