Exposure Value (EV) and Camera UI

[zaar]

Hello!

I'm fairly new to Corona, and tried my best to search for information about how the exposure control under the camera tab in render settings is supposed to work. But I didn't find much.

We're running 1.7 DailyBuild Sep 20 2017 in the studio.

And I've found that when using "simple exposure" the term EV is used incorrectly. It doesn't at all correlate to actual EV-values that have been used by photographers for decades. For example. EV 8 should give the same result as 1/4 at f/8 at ISO 100. But EV 8 is way brighter. Is this a bug, or is it a Corona native way of adjusting exposure were higher numbers makes it brighter and lower makes it darker?

I assume a lot of beginners or people with low level of knowledge about cameras and exposure would like it to work that way. But you cannot write EV next to the value in that case. That would be like writing ISO or F-stop and not having it behave like ISO or F-stop. It's wrong.

My suggestion, if this isn't a bug in the version I'm running, is that you don't write "Exposure (EV)" under simple exposure, instead brightness, and make another value for actual EV values. Or just write "exposure", not EV. And maybe move EV to photgraphic settings?

As a photographer I also find it strange that White balance is only in Kelvin, and that there is no green-magenta shift. Or that you can't color pick from the framebuffer. But that's just me.

Also UI-wise there is some tidying that could be done. Some settings under the camera tab are global, no matter if you flip between simple exposure and basic photographic. But they are layed out as if the belong to either simple or photographic.


This is nit-picky I know, and I hope I don't come off as hysterical ;)

Kind regards!

[Juraj]

You are correct, but this doesn't belong to bug section :- ).

1.) The concept of EV as absolute measure isn't used in current cameras either. In form of parameter, it's used predominantly only for bias.
     I do presume that Corona used EV=0 as neutral state for the sake of simplicity. What it currently does (without even remembering the crazy name Ondra has for it), is that White (light) dome of intensity 1 will result in exactly that RGB value on scene.
     It still works in correct light stops, just that it doesn't correlate to the actual common table of values, therefore it's perfectly reasonable to use the unit of EV. The concept isn't predominantly tied to absolute table of values.

     The regular camera settings do match 100perc. though.

2.) As a photographer too I can tell you than minority of cameras feature anything else than Pure Kelvin. But ALL OF US (there is about 50 requests for it), have requested to have tint nonetheless :- ) Currently Corona feature tint...as color overlay...I have no idea why they have done it, no one has requested it and it's still there...
     It's not bug, it's just incorrect feature no one wanted. Hopefully we'll get (counter) tint and framebuffer picker soon.

     This will by the way, also incite the wrath of "nitpicky" photographers because there isn't single scale of white balance when other than blackbody temperature is used. The one used by Adobe in their ACR (Kelvin + Tint) is translated matrix from that of a camera. So 6500K happens to be 5xxx + Tint in ACR. If we'll get tint, it will be introduced as D65= K6502 + Zero tint, not the way Adobe uses the scale.

3.) UI re-work is long in order. I think 1.8.. It's definitely on Trello board. https://trello.com/b/EfPE4kPx/corona-road-map-3ds-max

[zaar]

Hello Juraj!

Nice to get such a long answer. We've met actually back in 2014 in Gothenburg on the Architectural Visualisation Day event! Don't expect you to remember but I enjoyed your talk and it was nice going out for beer with all the guys after.'

Yeah, wrong section. There was no "wrong terminology section", so I thought I'd give the bug one a try. Now I've been moved to I need help, hahaha. I don't, it's the UI that does ;) It's hard being new to a forum without making mistakes or taking ages catching up on what's already been said and so on. I'll just dive right in. I got another thing that I want to ask abuot too.

Anyway, interesting to know how it relates to a white dome at intensity 1 and exact RGB values as you write, I'll have to try that. Looking forward to reworked UI in 1.8. I don't mind the way the simple exposure works, only that it's called EV. Mean while I'll have to explain that to some collegues. And I'll go digging around to see where the proper place to make request is.

[Juraj]

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.

[maru]

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

You mean Watt per steradian per square meter?

This may be interesting for you (most of that is actually info provided by Ondra, and collected by me):
https://forum.corona-renderer.com/index.php?topic=13524.msg87291#msg87291
https://forum.corona-renderer.com/index.php?topic=10684.msg68213#msg68213

[Juraj]

"original RGB" 1,1,1 should mean 1 W/(sr.m^2)

Yeah that was it :- )

[Ondra]

"original RGB" 1,1,1 should mean 1 W/(sr.m^2)

Yeah that was it :- )

https://en.wikipedia.org/wiki/Radiance#SI_radiometry_units ;)

[pokoy]

And I've found that when using "simple exposure" the term EV is used incorrectly. It doesn't at all correlate to actual EV-values that have been used by photographers for decades. For example. EV 8 should give the same result as 1/4 at f/8 at ISO 100. But EV 8 is way brighter. Is this a bug, or is it a Corona native way of adjusting exposure were higher numbers makes it brighter and lower makes it darker?

I agree, if it's doesn't behave like EVs, don't use the term. I thought it's 1:1 comparable to EVs and wonder why it doesn't.

[Juraj]

It does 'behave' like EV though, and the name is perfectly correct for this :- ) They could have alternatively named it (f)Stops it wouldn't make any difference. EV use:

1) Absolute Exposure (for simple metering or setting equivalence). Last time EVs were used like this was film era decades ago. EV=0 is 1s at relative aperture of f/1. Higher luminance (like Sunny outdoor) correlates to higher EV value (15). Modern cameras don't allow for setting direct EV. This is very much a dead concept in practice.
2) Relative Exposure (for compensation). This is pure relative scale, inverted of the one above. +1 EV stop refers to one stop of light difference (takes Sunny 15 to 14).

Corona uses the relative exposure compensation in the same way as modern digital cameras use it in principle.
Although I could imagine improvements:

When using Physical camera exposure, instead of locking simple exposure, it would allow for compensation.
With that, Corona would be 100perc. identical in how modern digital cameras work when using f-stop/shutter/ISO.

Alternatively, Corona could offer both types of EV use, although how many people would use the Absolute ? Last time I saw it in CGI app is Physical Exposure in MentalRay. If people want to use direct values, then f-stop/shutter/ISO is much more relevant anyway. And relative EV is much more easy to use (and logical for 'simple exposure' ) and actually correlates to digital cameras (and cellphones) people use today.
It would just complicated the UI without any benefit at all.