Exposure expert needed!! Damn mrPhotographic Exposure

[Dave Buckley]

Right

 

This is really confusing me.

 

I am currently dialling in exposure values for SS, f-stop and ISO.

 

Here's an example for a daylight interior

 

ISO 100, f-stop 16, Shutter 1 = EV 8

ISO 50, f-stop 16, Shutter 2 = EV 8

 

Ok so theoretically these should both give the same exposure right??? Same EV Values.

 

However the one with ISO 100 is far brighter even though no other settings were changed??

 

Please can someone explain why??

[mattclinch]

i think i touched on this in another thread.

 

theoretically they should. but they dont. i dont know why.

 

its far easier to just use the eV value, unless you have depth of field (?) or are matching exposure in a known photo.

[Dave Buckley]

does the sunny 16 rule apply to interiors and exteriors???

 

i will use the EV value but wanna understand the other settings also

 

good for my mental strength

 

also are there any rules for night shots???

 

i'm currently using an exposure chart to help me so thats where i'm getting my values from

[mattclinch]

sunny 16 is just a rule of thumb to get an approximation of a good exposure without having to sit and fiddle and test for ages.

 

if you want to understand how ISO, SS and f/stop balance and affect each other then i'd do it on a camera, not in MR exposure control.

 

there are no 'rules' for night shots, only desired setups.

 

lower the ISO the less grain, but the longer it will take to expose.

higher the f/stop the less depth of field. ie. f/22 keeps everything pin sharp - usually a good bet.

quicker shutter speed for less motion blur.

 

what would i do for a night shot at night? low ISO, say 50 or 100 to reduce grain. f/22 for sharpness, and set the shutter speed to whatever will expose that iso and f/stop correctly. thats simple 'aperture priority' exposure, where your desire for sharpness (high f/stop) dictates your shutter speed.

but thats a camera.. MR is simply the eV value that exposes it correctly.

[Dave Buckley]

but thats a camera.. MR is simply the eV value that exposes it correctly.

 

 

techncially not though. As in the example. If i get two differently exposed images using from the settings i previously mentioned. if i just use the exposure value then when i get to ev8, i will have the exposure that mental ray wants to give me rather than the exposure i want.

 

if you understand me.

 

lets say i wanted the brighter of my two setting combos.

 

iso 100, f 16, ss 1

 

if use the ev spinner to get to a ev value of 8 then mr gives me values in my dials as iso 100, f8 and ss 4.

 

which is different again to the setting of 8 that i was happy with.

 

so in that case are you saying that i just lower the ev value even more and i will eventually get the same resulting image that i had with iso 100, f 16, ss 1

[mattclinch]

i think you are getting too hung up on the photographic numbers within MR. it appears to me that in MR they basically feed into an formula which gives an 'exposure value'. for EV 8, as you mention, there are multiple combinations of iso, f/stop and ss which should, theoretically, produce the exact same exposure. its not an exact science though, and MR obviously haven't perfected the formula yet. MR seems to pick one of these combinations at random, and whilst varying between these potential combinations should produce 'approximately' identically exposed renders for the same EV, in practice it doesn't.

 

get a camera. they've had centuries to perfect those

[Dave Buckley]

so what do you use down at HD???

 

EV

 

or do you dial in numbers

 

i can quite happily get nice results its was just confusing me as too how it works. as far as i'm concerned i just adjust values until the image looks how i want it to look, i don't car what the values are or what it is i'm adjusting.

 

the one i am struggling with is an interior night shot, with photometric lights IES files, and sky portals (with a blackbody shader) acting as area lights in the same scene.

 

with regards to the settings. in MR if i have a lower f-stop with the render be more blurred than if i use a high f-stop, or does this not differ in MR??? i guess that's why people use VRay and the physical cam in that right??

[mattclinch]

i personally prefer just using the EV value, but its a personal preference.

 

as far as im aware, f-stop will not narrow your depth of field (unless perhaps you have MR depth of field activated??)

 

i'd be interested to hear other peoples take on this function, Zap?, Brian Bradley?, as i wouldn't take my word as gospel!!

[Dave Buckley]

i guess we are just missing this physical camera in mental ray, would be great if there was one that worked like a normal camera and had exposure controls that worked like they should do

 

any tips on the night interior i mentioned

[pfbreton]

Here is how it works internally:

 

the mr Exposure control is based on documentation published by Kodak. It mimics the response curve of cameras by using the "right" weighting of ISO, Fstops and Shutter Speed.

 

However, if you look at the maths, it end up as a multiplier, which is your EV value.

 

As an extra color control layer, the other parameters (saturation, midtones, highlights) are applied on top of the calculated EV.

 

If all your lighting is within physical range (i/e/ correct photometric lighting with correct / typical light intensities), the EV used in the 3D world will match a Photographic workflow - its designed to be in line with the camera response, but to work correctly, your lighting need to be correct too.

 

Note: since its a 3ds Max exposure control plugin, there is no effect on the DOF and Motion Blur directly. Its purely affecting the color of the calculated pixel.

 

Hope this helps,

Edited June 26, 2008 by pfbreton

[Fran]

Am I missing something? Doesn't the difference between ISO 50 and ISO 100 affect the brightness of the scene?

[Dave Buckley]

yes it does, but from what i gather you can compensate for that brightness increase by leaving the f-stop the same and bringing the shutter speed down a notch, like in my example. and what you get is an exposure value of 8 for both sets of numbers. so in reality, regardless of the settings of iso shutter and f-stop, an exposure value of 8, should result in the same exposure regardless of the other settings, but it doesn't. however, i am still learning myself

 

gotta admit its hard to get your head round. i think someone needs to run a test

 

jeff??? bri b??? zap??? you know you want to

 

now that i know that depth of field and motion blur aren't directly affected by these controls, then i guess it all can be controlled by the EV alone regardless of whether the other values work as they should.

 

i'l try and do a test myself with typical camera values for a typical sunny day and then i'l try and post it on my blog which is still in its early stages

 

www.3ds-arch-viz.blogspot.com

 

will try and get that updated after the weekend, as i have a hectic weekend planned. but i shall base my test on the fact that an interior with sunlight coming through the window should have an EV of 8. anyway you'll see the results after the weekend

 

its really gettin on my nerves now because i don't believe its workin as it should, but thats just my opinion

[Fran]

ISO 100, f-stop 16, Shutter 1 = EV 8

ISO 50, f-stop 16, Shutter 2 = EV 8 ...

However the one with ISO 100 is far brighter even though no other settings were changed??

 

Okay, but your settings are identical except for the ISO. Again, unless I'm missing something, the image with ISO 100 will be brighter than with ISO 50.

[mattclinch]

Okay, but your settings are identical except for the ISO. Again, unless I'm missing something, the image with ISO 100 will be brighter than with ISO 50.

 

he's trading off the drop in 1 stop of ISO (100 down to 50) with an up step in shutter speed (1 sec to 2 sec).

 

in theory 1 stop of ISO, shutter speed and f/stop should all be equal amounts of exposure.

Edited June 26, 2008 by mattclinch

[Dave Buckley]

matt is correct

 

fran, the point i'm getting at is regardless of iso shutter and f-stop, if the combination you dial in for those equals an exposure value of 8, then the renders should look the same in terms of exposure. but it doesn't. try it.

 

dial in a combination that equals a value of 8.

 

then dial in another combination that equals 8 on the EV and compare the results.

 

and the end of the day, what MR is saying is that the image is controlled by the exposure value. therefore if the EV is 8, the images should look the same regardless of the combination of numbers dialled in for the ISO, shutter and f-stop

[Crazy Homeless Guy]

I have been a bit skeptical of how closely the MR exposure represents a real world camera in terms of exposure in general. We should assemble a simple daylight scene for testing. A plane, a couple of boxes, and a daylight system should suffice. And someone should shoot outside on a sunny day, and compare the results.

[Dave Buckley]

i'm about to do just that but with an interior, i know it's wrong because i was testing today

[SandmanNinja]

There's a pretty good explanation here:

http://en.wikipedia.org/wiki/Film_speed

[MasterZap]

Hah

 

You guys are right, the "camera parameters to EV" calculation is wrong (it uses the ISO backwards). The "EV to camera parameters", however, is correct.

 

ISO 100, f/16 and shutter 1 should NOT be the same brightness as ISO 50, f/16 and shutter 2, it is shutter 0.5 that is the same.

 

Remember the "sunny 16" rule: same "shutter speed" as "film speed" for f/16.

 

So shutter and iso should move in the *same* direction (remember the shutter value is the 1/value measurement) to give "same value".

 

But the EV is actually calculated wrong for any iso other than 100!

 

But the EV isn't what is actually used, the "camera parameters" is what is actually used.

 

Also those wanting the "physical camera" shuold read here

 

/Z

[Dave Buckley]

Ok so in that case, the EV value in MR is well out.

 

So from what i understand camera values for ISO 100 are caluclated correctly. If i use any other ISO then i need to move the shutter speed in the same direction to get the same exposure.

 

For example, what you are saying is

 

Shutter Speed 1/1, f-stop f16, ISO 100 = EV 8.

 

IS THE SAME AS OR GIVES THE SAME EXPOSURE RESULT ON AN IMAGE AS

 

Shutter Speed 1/4, f-stop f16, ISO 400 = EV 12

 

The exposure values are so different yet they look exactly the same when dialled in and rendered. You would expect EV 12 to be much darker than EV 8 right??

 

But if the camera controls aren't contirbuting to DoF etc, then it would be safe to constantly stick with an ISO 100 and then adjust the other values to suit as this would give us a correct EV value for our desired scene??

 

ZAP i suggest you do a blog entry to clear this issue up

[mattclinch]

ISO 100, f/16 and shutter 1 should NOT be the same brightness as ISO 50, f/16 and shutter 2, it is shutter 0.5 that is the same.

 

*slaps forehead in disgust* what an obvious mistake!