Oddness in Iray EV Setting

Tobor

I've been noticing some discrepancies with how D|S calculates EV (exposure value) and ISO settings. Here's a page with a calculator that demonstrates the problem:

http://rechneronline.de/exposure/

Try changing the ISO in Studio, and the values appear to go in the wrong direction. At least that's how it looks to me.

ronmolina

For me in works in the right direction. Higher ISO the brighter the pic.

Tobor

Yes, I understand that. It's the EV that's going in the wrong direction. Take a look at Fred's chart B, near the bottom of the page:

http://www.fredparker.com/ultexp1.htm

For a given f/stop and shutter speed at ISO=100 as a baseline, increasing to a faster film should lower the EV, not raise it. 

D|S is behaving correctly when adjusting reciprosity values of f/stops, shutter, and ISO, so the exposure is technically correct when done this way. But the EV value is being incorrectly stated. This can throw people off if they are using exposure charts for reproducing physical scenes.

 

Tobor

Further to the above (before I hit the hay for the day):

1. Observe nominal EV at default settings is 13.0. This correctly corresponds to f/stop=8, and shutter=1/125 (128 in Iray's case). See, for example, 

https://en.wikipedia.org/wiki/Exposure_value

2. Set ISO to 400. That's two stops above nominal 100. EV now reads 15.0. (It should read 11.0, as per Fred's chart, the calculator I provided in my first post, etc.)

3. To compensate for the over-exposure, you have to change EV=17. That's now FOUR stops differenceaccording to the EV. (The shutter speed that is altered by changing EV is correct, however: 1/512s).

I'm guessing Daz had something specific in mind with how these settings work, but they don't jibe with real-world calculators and tables. As it is, you can't use the EV control for setting exposure to common lighting conditions if you've changed the ISO.

fastbike1

You are correct. From a photographic perspective, EV goes in the wrong direction. I mentioned this in a thread about Iray and tone mapping some time back. As you say., the effects of ISO and f stop are correct, but people just changing the EV value after observing Daz's results will not get the desired result.

barbult

I reported this as a bug months ago and was told that is was NOT an error and everything was working correctly. I still think it is wrong, though, in spite of the CS response I got.

Tobor

I'm also going to report this as a bug, as well, since it is clearly wrong, and demonstrably so with a number of authoritative sources showing how it works.