F Stops

Started Aug 11, 2012 | Discussions thread
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Shorthand Senior Member • Posts: 2,972
Re: Um, no.

I will say that this was much easier to understand when each click (stop) was actually a factor of 2 . Now, each click is 1/3rd of a factor of two, except ISO on Rebels which is a full factor of two.

I am fully aware that digital sensors need the extra fine tuning (whereas film is much more forgiving of over-exposure), but it definitely complicates learning the fundamentals.

Joyless wrote:

Right, but that was my original question:

"Just out of curiosity, is there a piece of software that can guesstimate how much light was captured in order to make the picture? It could perhaps tell me how much light is removed when I put on an ND, if it's really close to 10 stops, or if it's nearer 10 1/3 stops for instance."

Has anyone posted a link to the Exposure Value wikipedia page in this thread yet?

It is very much possible to use the RAW file from your camera to measure the number of photons captured or to convert to any other measure of luminance ... the equations are further down in the Wikipedia article.

(If you want to go crazy, you have to factor in lens transmittance / T-Stop ( http://en.wikipedia.org/wiki/T-stop#T-stop ), but its not hard. DxOMark.com has T-Stop information for most common lenses.)

With this, I meant: Take one picture at some camera setting. Put on a filter, take picture at same camera setting. How much does light level vary between the two?

A 3-stop ND filter will let through 2^(-3)=1/8=0.125=12.5% as much light. A 6-stop one will let through 2^(-6)=1/64=.015625=1.5625% as much light. However, I encourage you to think in geometric terms (number of Evs) instead of think of it arithmetically ... it will drive you crazy otherwise and only becomes important in certain shutter speed situations.

Yes its possible to measure this with what's in the RAW file as well.

Indeed, so I discovered, and that wasn't quite what I was after. I just wondered if there was a piece of software that could put a number on the amount of light in my picture, however far away from, or close to a good exposure it might be, no matter the camera settings. I imagine something like that could be calculated.

  • First: exposure is subjective ... there is a place for both low key and high key in scenes.

  • Then there is HDR tonemapping, which blows the concept out of the water and makes it not only subjective but also relative to its context in a scene. (Your eyes adjust as you scan a scene and commit it to memory, HDR tonemapping simulates this process.)

  • The tool you're describing is your camera's histogram function. (It also is there in most RAW converters.)

  • The most comprehensive scientific method of "correct" exposure remains Ansel Adams' Zone System ( http://en.wikipedia.org/wiki/Zone_system ).

  • However in a digital world, I would recommend exposing to the right instead: http://www.luminous-landscape.com/tutorials/expose-right.shtml - That will make the best possible use of the tonal range of your camera's sensor. (This means over-exposing some scenes and then adjusting the exposure back in RAW-to-JPEG conversion.)

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