Relationship between camera exposure and vrimg exr files

[tricky]

When I render an unclamped VRimg file, I have much more latitude in post production to tweek the exposure of the image as the file is 32bit. How far can this go?

If I use a vraycamera with ISO 50 instead of ISO100, will the quality of the resultant VRimg/32bit EXR be the same as just adjusting the exposure ('doubling' the exposure) of the ISO100 version in Photoshop?

With photography, both film and digital, chosing a faster film with a higher ISO/ASA, such as 400, or setting your digital camera to ISO400 will result in a grainier image than using slower films or digital ISO settings. How does a vraycamera relate to this? Setting the vraycamera to ISO800, as far as I know, doesn't add grain to the rendered image. Is upping the ISO of the vraycamera essentially just upping the power of all the lights in the scene - so rendering at ISO50 is the same as doubling the power of all the lights in the scene (the sun, the sky, area lights, spot lights, IES lights etc)?

[tomtnt]

As far as I know. vrimg or exr by vrimg isn't 32 bit in defult. It is just 16 bit float. FOr proper 32 bit you need to write some magic script for prerender script. And about he second point. ISO. Every camera has different ISO noise level for same number. I mean. Old crap camera has noisy ISO 100 but the new one has ISO 100 nice smooth. How would you do that? ISO noise is practically bug of CCD or CMOS. The easiest way is just post-production. ISO is affecting the sensibility of CCD for light. And that is how Vray works now.

[tricky]

As far as I know. vrimg or exr by vrimg isn't 32 bit in defult. It is just 16 bit float. FOr proper 32 bit you need to write some magic script for prerender script. And about he second point. ISO. Every camera has different ISO noise level for same number. I mean. Old crap camera has noisy ISO 100 but the new one has ISO 100 nice smooth. How would you do that? ISO noise is practically bug of CCD or CMOS. The easiest way is just post-production. ISO is affecting the sensibility of CCD for light. And that is how Vray works now.

All I know is when I use the vrimg2exr to extract EXR files, the resultant EXRs open in Photoshop as 32bit. If these are 'true' 32bit or 16bit float, I do not know.

I know ISOs are different for every digital camera (not for film cameras though - a 400 ASA/ISO film in a compact will give same results as in a pro level SLR - that was/is the beauty of 'the good old days' of film photography!). The sensor of one camera is different to another and behaves differently - some much (much) better than others.

However, my other questions are not to reproduce grain - that is easy to do in post. I just want to know how vray is working 'under the hood' so I can tune my scenes accordingly.

For instance, if I render my scene with ISO100, will it take longer (I guess I can test this!) than rendering at ISO200 as 'more light is being let into the camera' (thinking in the same terms as how camera lenses work)? Is upping the ISO therefore just like increasing the value of all the lights in the scene by a factor of 2 each time I halve the ISO?

Also, is there a quality loss (contrast/noise etc) using higher ISO values for a vraycamera, or can I be happy to up the ISO whenever I need my image brighter?

[tricky]

Surprised there are so few responses. I remember asking similar questions of Chaosgroup a few years back at Siggraph, and nobody managed to give me a proper answer. Am I too geeky for you all ?

[voltron7]

good questions tricky, I hope you get some good answers

[3DMK]

I just want to know how vray is working 'under the hood' so I can tune my scenes accordingly.

For instance, if I render my scene with ISO100, will it take longer (I guess I can test this!) than rendering at ISO200 as 'more light is being let into the camera' (thinking in the same terms as how camera lenses work)? Is upping the ISO therefore just like increasing the value of all the lights in the scene by a factor of 2 each time I halve the ISO?

No it will not take any longer....I've tested.
Remember the VrayPhyscam does not work exactly like a Real world camera....just the parts of a real world cam that are relevant to 3D & vray
The ISO is simply a light multiplier & is also there to help you match real-world cam setting of a photo/video with regards Light values
It can also be very handy when you have an animation where you have adjusted your shutter speed to get just the amount of blur you want, then you can independantly adjust the ISO to affect light values without affecting the shutter speed & your MBlur steup

Also, is there a quality loss (contrast/noise etc) using higher ISO values for a vraycamera,
No ..as mentioned Vray does not care about ISO other than it is a light multiplier. It has NO effect on grain/quality/render time

or can I be happy to up the ISO whenever I need my image brighter?

Yes! ISO away

Hope this helps

http://www.spot3d.com/vray/help/200R...era_params.htm

Film speed (ISO) - determines the film power (i.e. sensitivity). Smaller values make the image darker, while larger values make it brighter.

[tricky]

I just want to know how vray is working 'under the hood' so I can tune my scenes accordingly.

For instance, if I render my scene with ISO100, will it take longer (I guess I can test this!) than rendering at ISO200 as 'more light is being let into the camera' (thinking in the same terms as how camera lenses work)? Is upping the ISO therefore just like increasing the value of all the lights in the scene by a factor of 2 each time I halve the ISO?

No it will not take any longer....I've tested.
Remember the VrayPhyscam does not work exactly like a Real world camera....just the parts of a real world cam that are relevant to 3D & vray
The ISO is simply a light multiplier & is also there to help you match real-world cam setting of a photo/video with regards Light values
It can also be very handy when you have an animation where you have adjusted your shutter speed to get just the amount of blur you want, then you can independantly adjust the ISO to affect light values without affecting the shutter speed & your MBlur steup

Also, is there a quality loss (contrast/noise etc) using higher ISO values for a vraycamera,
No ..as mentioned Vray does not care about ISO other than it is a light multiplier. It has NO effect on grain/quality/render time

or can I be happy to up the ISO whenever I need my image brighter?

Yes! ISO away

Hope this helps

http://www.spot3d.com/vray/help/200R...era_params.htm

Film speed (ISO) - determines the film power (i.e. sensitivity). Smaller values make the image darker, while larger values make it brighter.

So basically, the essensce of all this is that YES, Vray ISO behaves just like a global light multiplier. e.g. Choosing ISO50 over ISO100 doubles the power of all lights in the scene.
Ta!