Ursa mini 4.6 moire / aliasing better in 2k?

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Frank Henry

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Ursa mini 4.6 moire / aliasing better in 2k?

PostSat Feb 17, 2018 1:54 am

I've got to shoot a bunch of textiles close-ups at work next. We don't have the option to hire a different camera (love the UM4.6k - just tricky with this detailed stuff like this).

Would I be better off shooting the with the Ursa Mini in a different mode that 4.6k to get less moire / aliasing from the fibres? We're delivering to web so I thought shooting 2k or HD may be better. But I'm not sure if it's just downsampling the 4.6k sensor and baking in the weird moire prints.

I know it's not the ideal camera for fabrics - but unfortunately it's our only option (we're in a small town shooting). I'm using Contax lenses (28 f2, 21 2.8, 85 1.4) so it's not the most vintage looking glass, but should help things out a bit. We've also got a half HBM and a Tiffen Digital Diffusion filter.
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Denny Smith

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSat Feb 17, 2018 3:13 am

You can shoot a 2K or HD window mode, that is not down sampled.
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John Brawley

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSat Feb 17, 2018 5:39 am

Frank Henry wrote:I've got to shoot a bunch of textiles close-ups at work next. We don't have the option to hire a different camera (love the UM4.6k - just tricky with this detailed stuff like this).

Would I be better off shooting the with the Ursa Mini in a different mode that 4.6k to get less moire / aliasing from the fibres? We're delivering to web so I thought shooting 2k or HD may be better. But I'm not sure if it's just downsampling the 4.6k sensor and baking in the weird moire prints.

I know it's not the ideal camera for fabrics - but unfortunately it's our only option (we're in a small town shooting). I'm using Contax lenses (28 f2, 21 2.8, 85 1.4) so it's not the most vintage looking glass, but should help things out a bit. We've also got a half HBM and a Tiffen Digital Diffusion filter.



Textiles is tough. Really tough. It's almost impossible really.

I'd start with trying to shoot shallow depth. Shoot across the face/ plane if the shot design allows it, rather than square'd up to it.

Longer lenses should also help. Wide lenses, squared up and at deep stops is the worst scenario.

Shoot ProRes HD from the full 4.6K sensor, that can also tend to help mush some of the aliasing.

Peaking can help show up the aliasing for hunting it down. When you see it try changing the subject to camera distance slightly. Sometimes you can also get away with just every so slightly focusing in front of or behind what you're focussing. You can't really tell it's not in focus, but it can be enough to roll out the aliasing.

Hope that helps.

JB
John Brawley
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Frank Henry

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSat Feb 17, 2018 6:38 am

Thanks John, I'll have a go at shooting shallow in ProRes at work next week.

Also - as you're really far along in your career compared to the rest of us, is moire something you're eyeballing in everything you frame up? Even if you go through wardrobe and production design and get your head around the day, it still seems like you could easily get nasty surprises given how fast a production runs and how little time there is to review takes.

Or do you develop a six-sense for tightly-woven fabrics and certain brick patterns on buildings?!? Just seems like one of those never ending things - especially if you're shooting projects on the fly.

It's the one thing I get paranoid about. I wish there was some aliasing/moire playbook or primer that could give you the best practices and tell you what to look out for.
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Howard Roll

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSat Feb 17, 2018 6:47 am

Who is Moire? Well, Moire is an unforgiving and angry mistress. There are however a few ways to keep her from disturbing your happy home but it's important to know what she wants from you.

What she really wants is for your pixels and her pixels to align but not quite. Any fixed pattern can interact with a sensor's pattern to create Moire, OLPF or no. The trick is changing the sample frequency at the sensor level to mitigate the effect.

Filters are useless. I've tested it myself and never seen a single example of Moire mitigated through the use of an external filter.

Shooting wide open will help mostly because the lenses become softer and resolve less detail. The trick in this case is that the sensor out resolves the glass and helps to mitigate false detail generated by the sensor.

Downsampling may help, shooting Prores will definitely help. Because of the whole sample frequency though it's not a one size fits all scenario. Downsampling can just as easily make the problem worse, 5DMK2 anyone?

Your lenses aren't what I'd consider clinically sharp so that's a good start. If you can keep them about F2.8 you'll likely be OK. You may also find that it helps to fudge the focus a little. It's easy to focus on the Moire (literally) and think you're screwed. You may find that you can front or back focus a little and that will help you out, again it's all about sample frequency and how the pixels and textures interact.

Overall I think the key is having accurate monitoring so you can see when she's got you so you can take the appropriate steps to overcome.

Good Luck
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Frank Henry

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSun Feb 18, 2018 12:43 am

Sheesh - she sounds like my crazy ex!

Thanks for this. I guess eyeballing everything, shooting open, and and generally being super careful is the way forward.

Can i also ask - what is your experience with tartan? I've got a project coming up where one of the key costume pieces is a tartan dress. Nightmare world? I reckon I've got some room to negotiate with wardrobe to make sure it's not too tightly woven in stitching, but would it be better off advising them to use a plain type skirt instead?
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Howard Roll

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSun Feb 18, 2018 3:00 am

It is your crazy ex, showing up randomly and unexpected.

Tartan as in plaid? Sorry, I'm not fashionable, I have 22 of the same black t-shirt.

It can show up anywhere, knits, khaki, sweaters can be bad because of the visible/loose weave, I see aliasing consistently in hair. Again the key is catching it before you press record. Often it can be as simple as slightly changing the composition, moving a little closer or moving back.

Overall it's definitely more rare on the 4.6K sensor than it is the Pocket for example but shooting something challenging like textiles it's good that you're thinking about it beforehand rather than trying to fix it in post.

Good Luck
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John Brawley

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostSun Feb 18, 2018 4:12 am

Howard Roll wrote:Overall it's definitely more rare on the 4.6K sensor than it is the Pocket for example but shooting something challenging like textiles it's good that you're thinking about it beforehand rather than trying to fix it in post.

Good Luck


Howies's totally right, the larger the resolution of the sensor the less likely moire is to show up.

Tartan itself isn't a problem because the pattern isn't fine enough to trigger the visual harmonics, unless perhaps the tartes was very far from the camera and you had a very sharp lens.

But the fabric itself that the tartan is dyed from is what would be a problem.

As per what's already been discussed for minimisations. Try not to let it bite in the first place.

JB
John Brawley
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Frank Henry

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Re: Ursa mini 4.6 moire / aliasing better in 2k?

PostMon Feb 19, 2018 1:24 am

Thanks guys. Feel a lot more confident.

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