Camera Shift - correct offset to match real world camera?

[DPS]

I took a photo which I wish to match my vray cam to.

I was using a 17mm tilt-shift in portrait orientation. Full shift (12mm) vertically on a FF DSLR. Sensor size is 36x24mm.

I was expecting the vray cam offset to be 12/36 = 0.333 but I tried that, and using an offset value of -0.333 does not yield the correct result.

Can someone explain what vray offset I should use for the above scenario? A setting of around -0.47 is looking about right but I'd like to be precise.


Edit... I should add, the FOV that I used for my camera in portrait was 70.44 deg. Making it a 25.5mm lens equivalent.

[stef.thomas]

I find that for shift lens matching it's best not to use a vray camera with vertical shift function as it distorts the image height significantly. I prefer to use a standard camera, keep both camera and target level with horizon and apply a skew modifier set to 90° to shift up or down. This keeps the image distortion free and you can use vray exposure linked to a vray camera to control exposure and colour balance if you wish.

[DPS]

Is the distortion that you mention different to what you would experience real shift lens?

I tried using a regular max camera, with a skew modifier. The results are different to the vray physical cam, but it's hard to tell which is more accurate and the numerical value that I punch in to either of them doesn't seem related to anything.

It's my understanding that my real world shift lens extends the image frame in the same way as if you were extending a frame in a perspective drawing. So the shift parameter should be absolute. I'm ignoring any optical imperfections in the lens at this stage.

Can the Vray guys shed any light on how their offset parameter works and whether this can be related to a real lens?

[stef.thomas]

Actually there is a better way that removes this issue. Take your backplate into photoshop and determine where the horizon line is by tracing perspective lines back to vanishing point. Then make sure horizon is in the centre of your image, by increasing canvas and adding blank space to top or bottom. Now when you set up your vray camera all you need do is set camera and target level with horizon (i.e. height camera was above ground at time of shooting) and position it relative to your building. No need for shifting at all, just set your image output size to new expanded dimensions.

[DPS]

Hi I think you're right, that's probably the easiest method.

I also thought of using safe frames to set the limits of my particular real world lens, make the frames at the limits of where the horizon would be and then use Autocad to calculate the real world shift (offset) amount needed to shoot my backplate. It's a bit long winded.

I'm wondering is the vray camera intentionally meant to work like this?

Thanks stef