Vray physical camera focal length question

[seandunderdale]

Could someone let me know how these red and green values interact with each other? If I make to cameras looking at the same object, one using physical cam focal length and the other using Maya focal length, but set them both to be 135mm for example, the result when looking at the same object is different...I would have hoped the camera views are identical. Which one can be relied on as being "true" focal length?

[piotrus3333]

lens breathing? VRayPhysicalCamera in max does that - maybe maya cam does not model this or just models it differently?

[hermit.crab]

There seems to be some link between the two focal lengths and may not represent the same thing. I'll ask around. The resulting FOV from the internal formula is correct, though.

[seandunderdale]

My issue is, if Im being asked to use a certain lens by a client, I need to know which lens value is correct. It seems very confusing to have two different values showing, and they be called the same thing.

[piotrus3333]

My issue is, if Im being asked to use a certain lens by a client, I need to know which lens value is correct. It seems very confusing to have two different values showing, and they be called the same thing.

as in a specific model of a lens or just specified focal length?

[seandunderdale]

client wants to see their product, a car, at multiple heights and angles through various lenses, so they know if they give that data to a photographer, or another cgi artist, they can create a shot that looks they way they want. It basically a PDF of a few hundred angles of the car, to show how it looks from anywhere with various lenses. Since Vray can give a conflicting focal length, Im not sure if the info Im giving the client is correct.

[seandunderdale]

I also did not expect Vray to change my Maya camera aperture values when I toggle the "specify FOV" settings...and it doesnt revert the values back if I set it back to "from maya camera". This is something I had not checked, and did not expect. I had been intending to set all my cams up with "default" full frame sensors / apertures, and did not expect to Vray to change those settings. If I change "from Maya camera" to anything else, it changes my camera aperture from 36 x 24mm to 25.4 x 25.4mm.

[SonyBoy]

I'd really like to see better management of the conflicting Maya and Vray camera attributes, it gets very tiring having to remember which Vray settings override which Maya settings when it comes to things such as motion blur and depth of field. Some are on cameras, some are in the global overrides. It's a bit of a mess, TBH.

[Wilyman]

client wants to see their product, a car, at multiple heights and angles through various lenses, so they know if they give that data to a photographer, or another cgi artist, they can create a shot that looks they way they want. It basically a PDF of a few hundred angles of the car, to show how it looks from anywhere with various lenses. Since Vray can give a conflicting focal length, Im not sure if the info Im giving the client is correct.

Hi Sean,

Thankfully, focal length is practically irrelevant in matching heights and angles.
Not sure which company you're working for at the moment, but a decent H&A document should have the camera height from the ground and then a couple of distance measurements (camera to front badge, camera to rear wheel, etc.).
As long as you've matched those, the view will be correct. The only effect the focal length will have is how large the car is in shot. It's essentially just a crop factor.

I appreciate it's still pretty unhelpful for two seemingly identical cameras to show different figures, but if your heights and your angles are correct, your view of the car will be correct, regardless of whether you're using a 10mm lens or a 500mm lens.

Cheers,

John

[seandunderdale]

Hi John. I appreciate the input. The way Im differentiating between cameras is keeping the car the same size in view by moving the cameras closer or further away. So if you pull up a Maya viewport number grid overlay, Im keeping the car consistently filling the same screen space, regardless of which camera you are using...and say on a profile, the front and rear bumpers will land on the +8 and -8 of the grid. The assumption being that the client would want the car to fill the same screen space in a given shot, but would like to see the results of different lenses. So different lenses are closer or further away from the car, depending on which lens is used...otherwise, as you say, the car just gets bigger, or smaller in frame.

Interestingly we have explained this to the client, and they've not been too interested in the distance from car to camera, which is kind of a key ingredient in this. Theyve only wanted camera lens, and height of cam from floor.

[Wilyman]

Sorry - Hadn't realised you were creating the H&A doc rather than matching an existing one.

But basing it on lens is really going to come back to bite them!
I fell out with a photographer on this a few years ago, who didn't understand that it's the camera position that affects the view of the car, and insisted we used the same focal length as an H&A doc, even when the car was only taking up 10% of a location shot, rather than 90% of a studio shot.
So you end up with a camera 50m away from the car, which looks practically orthographic.

But if you've already tried to educate the client, there's not a great deal you can do until they end up in a similar situation!

[matthew999]

totally agree as you say lensing is lrrelevant and just adjusts the FOV, distance to subject is what matters. This reminds me of when a client states a dpi but with no hight or width when we ask what res you want this rendering out at.

I have found vray phys cam focal to be pretty accurate to a regaular 35mm digital back in landscape. I find if i need to match to medium format the easiest for me is to leave the back as 35mm on vray phys cam and use the cropping factor on medium format camera website, we made a small spread sheet for conversions

[Wilyman]

Exactly. Which raises another problem for the client if they only have focal length info.
Photographers are all going to be shooting on different medium format backs, and any lens they own is only going to be in the ballpark of the 35mm equivalent they need. Whereas a tape measure will give them the same result every time.

[seandunderdale]

Yeh, I find its best to keep to a default full frame cam and let them figure out any lens conversions...so I just use the 36mm x 24mm maya default camera aperture...which is annoying when diving into the vray phys cam settings as when you toggle the "specify FOV" to anything other than "from maya camera" it changes the maya camera aperture from 36mm x 24mm TO 25.4mm x 25.4mm. Reverting the setting back to "from Maya camera" does NOT revert the maya cam aperature back to 36mm x 24mm as I would expect. This has messed up my cameras in the past without me realising.

Im yet to find out a reason why Vray changes the maya cam aperature to 25.4mm and does not change back if you revert back to "from maya camera"...And as mentioned in previous posts, when using any of the "specify FOV" settings, the focal length result in Vray phys cam is always different to the Maya focal length, which is confusing to me.

Im sure there is a reasonable explanation for it, its just very counter-intuitive to my smooth brain.