How to mix/match shutter speed / F-Stop and still end up with same exposure.

[nagboy]

I dont know if has been posted before, but here goes.

If you work with DOF/Motionblur and you find you have to change either the F-stop number or the Shutter speed to achieve just the right amount of DOF/MB, but still need to keep the exact same exposure prior to the change, then this will help:

The formula is simple enough:
EV = Exposure Value
n = Aperture / F-Number
t = Shutter speed (in absolute number)

EV=log2(n^2/t)

If you have a calculator with a solver function not supporting the log2 function, like mine this is the one for you:
EV=(ln(n^2/t))/ln(2)

Or if you don't have a calculator at all a spreadsheet with everything in it (only openoffice format):
http://www.adsign.no/doc/ExposureCalc.ods
The spreadsheet is very simple to use and will save you a ton of time of you are just doing the bruteforce method of trial and error (like me before i did the spreadsheet

Normally you would first find your scenes Exposure Value(EV) and then change either Aperture or Shutter speed and then get the corresponding Aperture / Shutter speed back which keeps the Exposure Value steady.

I hope this will help somebody

PS! when using the formula to calculate EV/F-stop/Shuttertime you will see that the numbers almost always are a little bit off from the reference tables (if you use those) the tables have just rounded off all the values to make them nice and even.

Vlado, if im wrong with anything here, please correct me before somebody gets angry with me...

[joconnell]

To be honest I use iso more than anything else - it doesn't have any repercussions like with a real life camera and won't change the motion blur or dof.

[nagboy]

You have the same basic problem using ISO to control it, if you change Aperture or Shutter speed the image will look brighter/darker with the same ISO. But i see your point, since it doesnt produce grain in vray, you can fiddle around more easy with it. Still when you want exactly the same exposure you will need to calculate the ISO as well.

[joconnell]

Log2 hurts my head.

[simmsimaging]

I know that an 'exposure lock' has been requested a few times and it makes this problem much easier to deal with. Hopefully that can be implemented for a future update.

b

[mmrender]

hi john, this formula is a bit easy to read. It is basicly the same of the first post but resolving the equation. Check the image attached.
n1=old Fnumber
n2=new Fnumber
t1=old shuter speed
t2=new shuter speed

Use the first formula if you change the shuter speed, you will get the new Fnumber required to have the same exposure.

Use the second formula if you chage the Fnumber, you will get the new shuter speed requierd to have the same exposure.

Hope this make sense.

Regards

[peterguthrie]

this is the calculator I use: http://www.giangrandi.ch/optics/lenses/expcalc.html

would be great if the vrayphysicalcamera got some kind of EV lock....

[Da_elf]

widows has a scientific calculator though