Exposure discrepency in VRay Camera

[vlado]

Re: Exposure discrepency in VRay Camera

Why is there such a discrepency? Why must I set the VRay camera to 1/100 instead of 1/10?

What are your scene units? Those are taken into account when calculating the actual aperture for the camera.

Best regards,
Vlado

[dlparisi]

Re: Exposure discrepency in VRay Camera

Why is there such a discrepency? Why must I set the VRay camera to 1/100 instead of 1/10?

What are your scene units? Those are taken into account when calculating the actual aperture for the camera.

Vlado: What should the units be set to? Is the matte plane problem above a bug with the physical camera?

[Dynedain]

Re: Exposure discrepency in VRay Camera

What are your scene units? Those are taken into account when calculating the actual aperture for the camera.

Best regards,
Vlado

My system units are set to Centimeters. My display units are Feet.

I tried setting my system units to Meters and converted the scene, but I got the same results:

[vlado]

Camera settings:
focal length: 37mm
shutter speed: 1/10 sec
ISO speed: ISO-400
F-stop: 5
Taken outside my office at 7:20pm (daylight savings time)

Looking at these numbers again, they look very weird. Normally, for a daylight scene you'd have ISO 100, shutter speed about 1/200 and f-stop about 8. However, with your settings, the film is four times more sensitive, the shutter speed is 20 times longer, and your f-stop is lower. I'd expect that the V-Ray camera would produce a blown-out image in this case. Are you sure your camera used exactly these settings (no auto-exposure etc)?

Best regards,
Vlado

[vance3d]

thats what I was thinking too. 1/10 of a second is a long exposure for 400 speed film in broad daylight.

V Miller

[Dynedain]

Looking at these numbers again, they look very weird. Normally, for a daylight scene you'd have ISO 100, shutter speed about 1/200 and f-stop about 8. However, with your settings, the film is four times more sensitive, the shutter speed is 20 times longer, and your f-stop is lower. I'd expect that the V-Ray camera would produce a blown-out image in this case. Are you sure your camera used exactly these settings (no auto-exposure etc)?

Best regards,
Vlado

Yep, the ISO speed is set to 400, and that's also what's recorded in the XMP data.


For comparison, here's two shots I just made with this camera... granted it's almost noon, not sunset. I also verified that the ISO setting being recorded in the XMP data is the same as what the photo is actually shot with:


**edit** updated the image with shots using the same fstop so the only difference is ISO and shutter speed

[vlado]

Right, but now it also says that the shutter speed is 1/160 and not 1/10; and you said that if you use 1/100 for the V-Ray camera you get a better result.

Best regards,
Vlado

[vance3d]

your new image shot at iso 400 looks brighter than your original (cement and umbrella top) yet the aperture is smaller (higher fstop) and the shutter speed is faster.

something doesn't seem right.

V Miller

[Dynedain]

Right, but now it also says that the shutter speed is 1/160 and not 1/10; and you said that if you use 1/100 for the V-Ray camera you get a better result.

Best regards,
Vlado

I'm showing a comparison of the real camera there. I'll do it again this evening to validate the results, but of course at sunset I'll need slower shutter speeds than 1/160 to capture the photograph.

My sunset photograph, with an ISO of 400 had a shutter speed of 1/10 a sec. But when I apply those same settings to the vray camera, I get the blown-out results.


What I'm trying to figure out is why when all the other values are set the same, in a scene with proper lighting setup and objects to mimmick the real-world condition, why am I having to set the Vray Camera shutter speed to 10x faster than what the real camera used.

[Dynedain]

your new image shot at iso 400 looks brighter than your original (cement and umbrella top) yet the aperture is smaller (higher fstop) and the shutter speed is faster.

something doesn't seem right.

V Miller

As I mentioned, the very first image (the one I'm trying to accurately match with a vray camera) was taken at about sunset (7:20pm). The camera setting match in my later post is taken about 11:45am.

[Dynedain]

For clarification:

Here is the shot I am trying to match, taken arround sunset, at 7:20pm. In this case, I have to set the VRay Camera's shutterspeed to be 10x faster than was captured with the real camera:





Now, since it's midday, here is a set of comparisons taken at 11:45am, comparing film speeds:

[vlado]

I'm doing some renders now, will post the results in a moment, but basically the only other thing that remains to be matched are the materials...

Best regards,
Vlado

[Dynedain]

I'm doing some renders now, will post the results in a moment, but basically the only other thing that remains to be matched are the materials...

Best regards,
Vlado

If it helps, here's the scenefile.

[vlado]

Here are my tests (using the notorious LWF) - I had the 3dsmax gamma set to 2.2 with affect mtl editor and color swatches; I also had 2.2 gamma in the Color mapping rollout and I used the V-Ray VFB (without its sRGB switch, as I already had 2.2 gamma applied to the rendering from the Color mapping rollout).

As you can see on the second noon image, the directly visible sky has a similar intensity to the one in your 11:45 am photos. That means that the sun/sky/camera combination actually works correctly. For the evening image, the background sky is burned in the same way as the corresponding photo. It follows that the only thing that remains in the equation then are the materials.

ISO 400 | shutter speed 1/10 | f-stop 5 | 19:20 pm | 28 August 2006:


ISO 400 | shutter speed 1/160 | f-stop 10 | 11:45 am | 28 August 2006:


Obviously, materials must be adjusted further to match those in the photo.

The scene that I used is here: http://www.spot3d.com/vray/images/su.../sunskycam.max

Best regards,
Vlado

[Morbid Angel]

if it is so, why is dynedeans images so different to yours ? is it scale related?