Question about F-Stops

Monty Gregory · May 4, 2015

First would would like to apologize for my ignorance on this subject, but I'm still new to this and would appreciate any help in understanding the matter.

My question is in regards to F-Stops or "Stops". I have been reading a couple of forums on the site and I noticed using those terms in regards to lighting i.e. gels (CTO or CTB) or filters. When someone says that you if you use this type of gel or filter it will be about 2 "stops" down what does that mean exactly.

I would appreciate any advice. Thanks.

Colin Elves_18850 · May 4, 2015

A lens f-stop represents the amount of light (theoretically) transmitted through the lens. Each full f-stop is half the light transmitted relative to the previous. So f2 is half as much light as f1.4. F2.8 is half again (1/4) etc.

When people talk about the 'stop loss' of a gel they're basically equating the drop in light output from a lamp caused by placing that gel in front (due to light being absorbed by the gel). So if a gel causes a 1 stop loss it means it abosrbs half the light put out.

In a more practical sense: say you have a lamp exposing a subject and your light meter (or camera) is reading an exposure of f2.8 - but you decide you want to change the colour of the lamp (tungsten to daylight via CTB for example) and you know the ge causes a two stop loss - then you'd expect your exposure (from that lamp) to drop from f2.8 to f1.4 (2 stops). In other words to have the same area lit by the lamp exposed the same with the gel in front as without, you'll need to open up the lens by two stops.

Colin Elves_18850 · May 4, 2015

In practice it's rarely simple as that as adding a gel can affect the characteristics of the lamp in terms of spread (ie it can soften it a bit) depending upon where it is positioned in the beam - which means your exposure will also be affected due to the inverse square law (which is basically that wider, softer sources tend to lose output over distances at a faster rate than smaller point sources).

Monty Gregory · May 5, 2015

Thank you for the information Colin. It has provided clarity on an issue I didn't fully understand. I will reinforce what I have learned by practicing it. I really do appreciate the information.