Is it just me or is the physical camera a bit crazy with focal length / zoom / fov?

[joconnell]

Hi Folks,

I'm unfortunately doing a stereo job at the minute (I wanted to try and avoid that) and one of the things I'm looking at is ways to measure how much difference there is between the position of an object seen in both views of the camera so I was looking at the vray physical camera as it gives you maxscript access to the lens value and so on. We did a few simple tests and while our calculations of the percentage difference came out correct, our renders didn't when we tried to use that difference value in nuke to line things up, they were off. As a simple test to make sure that the aperture value in max was being respected, I made a plane object with the width of the 3dsmax aperture / film back and placed it the same distance from the camera as the camera's focal length. That should match up perfectly and will the frame edge to edge from my understanding of it, but the vray camera was quite far off. Using the same aperture value in the film back of the vray physical camera and the same focal length as my standard max camera it gave me totally different results in terms of view and also fov. I also noticed that if you're using the target of a camera, moving it in and out also changes the fov or the zoom of the camera. It won't change it's ui parameters but if you have the cone of the camera always visible, you can see the fov widening and narrowing, to the extent that as the target gets close to the surface of the camera it'll narrow to 0 degrees. At the minute I'm not sure how to set of the camera to get accurate values and likewise if I was animating the target of the camera so that I could track focus for example, the perspective of my shot would be changing rather wildly.

Am I doing something exceptionally dumb or not understanding the use of the physicam? I'm on max 2014 x64 base version, no service packs and vray 3.005. Here's a max file and a screen grab showing the measurements I was using and the settings of the objects. The max standard camera is a 50mm lens and the render dialog aperture is 36.0mm, display and scene units are both millimeters.

Cheers!

vray_physicam_weirdness_2014.zip

[DanielBrew]

I think the V-Ray camera is correct. The focal length of the lens is only 50mm for light coming from infinity. You are focusing on an object only 100mm away so the lens would have to move away from the backplate to keep the image in focus. This in turn would reduce the fov.

[joconnell]

Hmm, but if I want to use a 50mm lens and rack focus between two different objects so I can have a shifting focus point for dof? At the minute moving the target point totally changes the fov of the lens so it's no longer a 50mm. I'm also at a loss as to how I can make a lens with the test conditions I need to work out stereo bits using those rules where the film back is the focal length away from the optical centre of the camera - what you're suggesting does make sense in terms of how cameras work as in different lenses move this optical centre forwards and backwards to accommodate the fixed position of the film back in the camera, but in terms of the vray camera in the viewport is it doing this also without actually visually representing it? As in the vray camera is supposed to behave like a pinhole camera so I'd expect the optical centre of the camera to be at the pivot of the viewport icon seeing as it's field of view is drawing back into this point, and then the film back to be sitting behind it at whatever distance my lens is set to. On a real camera what I focus on on a fixed focal length won't massively change the field of view and perspective of the camera to the extent that the vray camera seems to in my file. I understand that on a stills lens there is some "breathing" that makes a slight difference as you rack focus on wide open settings but nothing to the extent I'm seeing here.

If I have a locked off camera with a small f number, I'd assume that I'd be able to leave my camera settings alone and then be able to just animate how far away the target is to rack focus from one depth in the scene to another without affecting my perspective at all, with my test scene above moving the target changes everything!

[DanielBrew]

As in the vray camera is supposed to behave like a pinhole camera so I'd expect the optical centre of the camera to be at the pivot of the viewport icon seeing as it's field of view is drawing back into this point, and then the film back to be sitting behind it at whatever distance my lens is set to.

I thought it was the Max camera that behaves like a pinhole camera. The V-Ray camera seems to behave as it it has a lens.

On a real camera what I focus on on a fixed focal length won't massively change the field of view and perspective of the camera to the extent that the vray camera seems to in my file. I understand that on a stills lens there is some "breathing" that makes a slight difference as you rack focus on wide open settings but nothing to the extent I'm seeing here.

The problem is exacerbated in your scene because the objects are so close to the lens. If you move the focus to a metre or more away from the lens the camera behaves more like you would expect. There is still some 'breathing' as you put it but the fov is much more stable. If you need focus on objects so close to the camera and need a stable dof then the only way I can think might work is to use an expression controller to change the focal length of the lens.

[joconnell]

You're totally right on the stability after looking at scene scale Daniel - much appreciated for the pointer. I find that the lens starts to travel at any distance below 10 metres and really dramatically around the 4 metre and below mark. I find this a tiny bit odd I must admit, I've got a 50mm lens for an slr and while it can't focus closer than 1 metre approximately, going around that or closer doesn't massively affect the perspective of the lens to the extent that I'm seeing here. I don't normally use the physical camera so it's not too much of an issue, it was mainly as it had a handy script accessible value for the lens length which the regular max camera doesn't. I can quite happily get by on the normal camera instead so I'll stick with that for the minute.

In terms of matching the cameras, how does the aperture width in the vray camera relate to the aperture in the main 3dsmax render dialog properties? Doe the vray camera completely ignore the value in the render dialog?

Cheers for the clarifications Daniel!

[Dave_Wortley]

You're totally right on the stability after looking at scene scale Daniel - much appreciated for the pointer. I find that the lens starts to travel at any distance below 10 metres and really dramatically around the 4 metre and below mark. I find this a tiny bit odd I must admit, I've got a 50mm lens for an slr and while it can't focus closer than 1 metre approximately, going around that or closer doesn't massively affect the perspective of the lens to the extent that I'm seeing here. I don't normally use the physical camera so it's not too much of an issue, it was mainly as it had a handy script accessible value for the lens length which the regular max camera doesn't. I can quite happily get by on the normal camera instead so I'll stick with that for the minute.

In terms of matching the cameras, how does the aperture width in the vray camera relate to the aperture in the main 3dsmax render dialog properties? Doe the vray camera completely ignore the value in the render dialog?

Cheers for the clarifications Daniel!

Put your camera on a tripod and try playing with the focus, it does crop quite a lot! We've got caught out with this before on small scale scenes, I know that it is 'correct' but would be nice to have an option to turn it off!

[Dave_Wortley]

In terms of matching the cameras, how does the aperture width in the vray camera relate to the aperture in the main 3dsmax render dialog properties? Doe the vray camera completely ignore the value in the render dialog?

V-Ray ignores the Aperture Width setting in the Render Scene Dialog, the camera FOV is calculated using the Vray Camera's Film_gate and Focal_length, so you can have multiple cameras in a scene with different film-gates, which we have quite often when playing with RED cameras and ARRIs with different backs/resolutions and setting up Medium format cameras in the same scene as well. We have a tool which also displays everything in 35mm DSLR terms so everyone can have a bit more of a feel for what the lens actually is. 18mm on a RED at 5k is near the same FOV of a 55mm on a IQ180, so sometimes you just need to know what sort of range the FOV is in.

[joconnell]

Put your camera on a tripod and try playing with the focus, it does crop quite a lot! We've got caught out with this before on small scale scenes, I know that it is 'correct' but would be nice to have an option to turn it off!

Much appreciated for the pointers on this and the aperture Dave.

Yeah I think you're totally right here, it'd be handy to knock off anything automatic that might happen as it'll trick users that aren't aware of it and lead to unexpected changes in renders.

[Dave_Wortley]

Much appreciated for the pointers on this and the aperture Dave.

Yeah I think you're totally right here, it'd be handy to knock off anything automatic that might happen as it'll trick users that aren't aware of it and lead to unexpected changes in renders.

My favourite one is if you're using a standard camera, set your focal length to say 50mm and then you change your render resolution using the presets to say 1080p in HDTV categorty in the render scene dialog it will adjust the aperture width to 20mm and therefore change your focal length value (to maintain your FOV), which can be very confusing for people who don't understand their cameras.

Much prefer how V-Ray does it! If only we could link render aspect to cameras as well, I should really finish my script which does this.... and oh adds presets for every single camera type as a custom attribute to each camera you make.

[joconnell]

Yeah - Maya's handling of cameras is far more impressive than max's, must give them a kick to do a few things like this!

[m_hinks]

Just reported this as a possible bug, and now I find this thread!

Its a very tricky thing as I'm camera matching plates with very accurate lens date, and surveyed points within the photos, and I have to go out to a target distance of over 2500m before the FOV is not effected, and the points match. An option to turn this effect off could be the best solution.

Mark

[joconnell]

Yeah, this to me seems only useful if every single camera lens in the world all react this way so you can use the camera for tracked or matched bits, if not it's better to have the fov / lens being totally constant and adding in the small changes yourself if needed.

[m_hinks]

Vlado has suggested entering the FOV in "specify FOV" and then the distance has no effect, as the FOV is locked. Interestingly the exposure might also change with target distance as he pointed me to this -

http://docs.chaosgroup.com/display/V...dFocusDistance

[joconnell]

Ah awesome - much easier to link the fov of a vray camera to a standard max camera then - handy for 3d tracks. Exposure automatically changing also seems like an unhelpful thing to me since it's not something you have to turn on and be aware of - similar to the fov change it's not going to match every possible camera so it's going to add in additional work for a compositor to counter animate brightness changes in a render.

[m_hinks]

I agree, maybe an option to remove that behavior would be good.

Edit - My photographer has just said the following "Does it have an infinity setting like a lens does? I've never taken an architectural shot at anything other than infinity."

Could we have an option to have the target distance set to infinity? I guess its the same as turning the effect off?