[RESOLVED] EV vs. Corona EV

[cjwidd]

It's been long time since that event but I would definitely remember all the people from beer afterwards :- )

That default intensity has some really funny name that only Ondra can write :- ).

But it works like this: You put simple white color into environment (255/255/255), create some object ( Teapot with 128/128/128 Grey color ), hit render, and the teapot at EV=0 will be exactly 128/128/128.
So it's more like 'no'-exposure giving you exactly what someone without photographical background might expect. I.e exact intensity of the HDRi how he sees it in material editor, or material to look exactly as well.

But I don't really want to defend it, not sure if this is truly practical in some kind of scenario...I would be perfectly happy with 100perc. photographic values. Currently the only difference is the real EV and Corona EV are shifted, though I never bothered to find out by how much.

This is a quote from a 2017 discussion regarding exposure settings.

For a camera at default settings (film width [mm]: 36mm, ISO: 100, F-stop: 16, Shutter speed: 1/50), what is the corresponding EV in the VFB?

Seems to be ~4.5?

What are the maths behind this calculation?
How many times has this already been answered on the forum?

[Juraj]

I had bit more complex post about this later somewhere but any such math would be pretty useless practically because the value is inverted anyway.

But "absolute" EV values are not used in photography either...for at least two decades, since inception of digital cameras.
EV is primarily used as "offset" ( i.e "+2 EV" meaning two stops more light ), not as absolute table value (i.e "2 EV" being exact number resulting from combination of shutter and focal length normalized per ISO).

Why would you even need this? The full settings (Sensor size, ISO, Shutter, Focal Length) are identical between Corona and cameras. None of my cameras (Sony Alpha 7r series) have option to input absolute EV value, it's long abandoned concept.

I am of opinion that Corona should go full way and allow EV modification even in full settings, so that is it absolutely identical to how digital cameras work. You set your settings (f-stop, shutter, iso..) and still have option to use EV as offset.
I'll make that a request when I'll remember.

[cjwidd]

@Juraj I recorded a short video in response to your question, but also to better illustrate why I posed this question in the first place (watch x1.25-1.5 speed).

EDIT: really sorry if I mispronounced your name(?)

[romullus]

If you want simple exposure to match photographic exposure by default, then set EV to -4.5 (or whatever you find appropriate) and save render settings as defaults. As Juraj already explained, there's very good reason why simple exposure is set as it is and i really don't see a reason why it should be changed. In fact i would be pretty upset if it would.

[cjwidd]

@Romullus Right, and your suggestion is a good one, although I am still curious what the conversion formula might be. I think it's clear enough that -4.5 approaches a visually similar result, but if we follow the typical exposure formula:

Code: [Select]

EV = log2 ( 100 * aperture² / ( ISO * time ) )
we arrive at 13.6; given default camera settings.

13.6/3 is approximately 4.5 (4.53)(?)

[Fluss]

I am of opinion that Corona should go full way and allow EV modification even in full settings, so that is it absolutely identical to how digital cameras work. You set your settings (f-stop, shutter, iso..) and still have option to use EV as offset.
I'll make that a request when I'll remember.

Definitely need this. Has it already been requested?

[Juraj]

I am of opinion that Corona should go full way and allow EV modification even in full settings, so that is it absolutely identical to how digital cameras work. You set your settings (f-stop, shutter, iso..) and still have option to use EV as offset.
I'll make that a request when I'll remember.

Definitely need this. Has it already been requested?

I think I or someone else might have asked for it long time ago, but I didn't make any request yet :- ). Go ahead!

[Fluss]

I am of opinion that Corona should go full way and allow EV modification even in full settings, so that is it absolutely identical to how digital cameras work. You set your settings (f-stop, shutter, iso..) and still have option to use EV as offset.
I'll make that a request when I'll remember.

Definitely need this. Has it already been requested?

I think I or someone else might have asked for it long time ago, but I didn't make any request yet :- ). Go ahead!

here : https://forum.corona-renderer.com/index.php?topic=29128.0

[cjwidd]

Just realized it's pronounced 'yoo-rye' - sorry man :/

[maru]

I understand cjwidd's request and the idea behind it. I will contact our devs to find out if there is some simple way to swap between EV and photographic values. My concern right now is that the photographic exposure is controlled by 3 parameters, so it appears to me that there will be actually an infinite number of possible combinations giving exactly the same EV equivalent. I guess providing a few examples, or locking two specific values at their defaults (e.g. shutter speed 1/50s and F 16) and providing the third one (ISO) would be enough here.

(Internal ID=503222427)

[maru]

I don't have any exact info yet, but here is how I started. Maybe someone has time and courage to take it further:
- Created a scene with 0 black background
- Placed a Corona Camera
- Placed a plane
- Painted the plane with Corona Color set to pure white plugged into self-illumination slot, self-illumination intensity set to 1
- Used a secret spell to remove gamma 2.2
- Rendered

This way we can read RGB values from the plane. Simple EV set to 0 gives Tonemapped sRGB 1, meaning that the surface has 1 Watt per steradian per square meter. Great.
So I tried with various simple EV values vs RGB and with various ISO settings vs RGB. I left shutter speed and f at 1 to simplify things.
I stored the results in Excel and drew a graph with extrapolation and showing the function which is used to draw the graphs.
Here is what I got:
EV = 1.4427*LN(RGB)
ISO = 0.1757*RGB

It is still guesswork, but it more or less works.

Now, with this, you can enter any RGB value and it will print the EV and ISO values for you. We would like to have EV<>ISO conversion, but I am simply too stupid for this.

I am sharing my XLS file "as is", without any instructions. ;)
The table you are interested in is the lowest one titled "ev and iso based on rgb".

Scratch that, unless you want to have a for fun exercise. See the post below.

[maru]

@cjwidd - Here is the formula used by Corona which defines the dependence between photographic settings and simple exposure:

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a = 78 * fStop^2 / (0.65 * shutterSpeed * ISO);
exposure = log2(683/a)
For some reason, I did not get it to work yet, but I will try again. Feel free to share your findings!

[maru]

Here is an XLS where you can enter the fstop, shutter speed, and ISO values, and it will print the corresponding Corona EV. I doesn't work the other way around, though.

[cjwidd]

this is amazing 😲

[cjwidd]

It actually was  -4.491148827 😆

This is so cool, thank you so much for sharing this Maru - thank you!