Physical Camera exposure problems

[thebeals]

Hi everyone,

I'm not sure if this is expected behavior or not, but when I move my physical camera closer and further away from an object (or maybe the center point in the axis) my exposure changes. Take a look at the attached images of a white cyc lit above by a vray rectangular light. In each image I move the camera closer.

Why does the exposure drop the closer I get to the cyc, is this correct behavior or a bug?



Thanks for the help!

[tashko.zashev]

Hi,

This is an expected behavior. You can see the Vlado's post for more details in the thread below:
http://www.chaosgroup.com/forums/vbu...amera+interest

[VISUALMAN_TOKYO]

This is not a behavior common to all lenses though, with good quality prime lenses or better with cine lenses this behavior does not happens. I prefer to turn off exposure in physical camera.

[snivlem]

Is vignetting? Just can turn off the option.

[VISUALMAN_TOKYO]

Not vignetting but overall exposure.

[snivlem]

Oh I see, interesting phenomenon reading through the linked thread.

I wouldn't imagine it to be that severe difference in light though with your image above - that looks at least a couple of stops of difference there between the second last and last image.

[VISUALMAN_TOKYO]

I think the problem is the lens used as reference.

[t.petrov]

VISUALMAN_TOKYO:
Have you done the same experiment as we've done or are you just speculating?
Can you show examples?

[VISUALMAN_TOKYO]

Don't get it wrong I am just trying to help. It is not speculation, I work with lots of live action footage everyday (for backgrounds or references often I get the same shot in focus and out of focus) and I know that exposure does not change if the focus change (or at least is not noticeable) and the same for field of view in most of the lenses. If that happens, in movies it would be impossible to do rack focus and having a constant exposure. I have several 35mm lenses myself and I don't get this issue.

[t.petrov]

The real problem is that the units in Maya default to centimetres and thus if you enable the physical camera it works in something like a macro mode.
I've done a simple scene with sun/sky and a 1unit sphere. The focus distance changes from 6cm to 20cm.
Most real lenses can't focus this close, so there is no way to reproduce the effect.

On the other hand if you change the units to be meters in the preferences and make the same scene looking scene, then this time the focus distance changes from 6m to 20m (in my test).
There is no perceived difference in the exposure of the image when the distance is 6m and when it is 20m.

[VISUALMAN_TOKYO]

Thanks for your explanation, I understand, but even considering a Macro lens (it can focus that close) and changing the focus from 6cm to 20cm it does not cause any change in exposure. If you don't have a macro lens then check out some YouTube videos of lenses review and see it for yourself.
I am not trying to discredit the efforts of anyone here and I really love your renderer and appreciate all the support you guys do, I am just trying to give some constructive feedback on a tool that personally I think needs improvement, however since it can be disabled is a non issue for me.

[t.petrov]

If you don't have a macro lens then check out some YouTube videos of lenses review and see it for yourself.

Do you have a link? My search-fu is pretty bad today.

I am just trying to give some constructive feedback on a tool that personally I think needs improvement, however since it can be disabled is a non issue for me.

We're acknowledging the issue. If so many people are complaining, then something is not right.
The easiest solution is to add an option which enables/disables this effect.
But these days we prefer to removing options these instead of adding options.

[joconnell]

I found the same thing - for a cg lens you wouldn't expect it to give you different results at different points of focus, you'd expect it to be totally consistent regardless of whatever settings you chose. A long overnight render could be ruined with random bits of darkening in sections of it!

[t.petrov]

But this one is a physical cg lens So it tries to resemble the real world.

[VISUALMAN_TOKYO]

Do you have a link? My search-fu is pretty bad today.


We're acknowledging the issue. If so many people are complaining, then something is not right.
The easiest solution is to add an option which enables/disables this effect.
But these days we prefer to removing options these instead of adding options.

This one shows quite a big range of focus shifting with a macro lens and it seems quite consistent in the exposure https://www.youtube.com/watch?v=DKMrjD285IM
Due to really shallow DOF some areas can be perceived as darker or brighter but is an effect of the bokeh but the amount of light entering the lens should not change.

I work more with moving pictures and the lenses used in this field are measured by the real amount of light hitting the sensor/film, and they are very consistent (for example a 50mm and a 100mm with the same aperture, setting the same shutter speed/angle and iso they produce exactly the same exposure) to keep consistency of look when editing. Of course lenses for photography are more forgiving and theorical in measurements (just a math formula) and not considering the physical characteristics of glasses and construction, resulting in inconsistency of exposure between lenses using the same settings. However in most of all lenses the exposure is not supposed to change with focus.

As for physical camera exposure in my opinion it should not consider focus as a factor to affect exposure, because it would make it impossible to match a real lens.