Combining/Adding factors

[Bruce Osgood]

I remember reading in Adams' The Negative that if you have a need to use two contrast filters do not add the factors together but use only the larger. Okay!

Now if you need to add bellows compensation and a contrast filter, do you add these together or use the larger?

 

[Bruce Osgood]

AA's comment applies only when using two contrast filters of the same color group (e.g., light yellow + dark yellow) and says it may require just a bit of additional factor due to the combined density of the two filters. Since the bellows extension isn't a contrast correction, rather just for exposure control, its factor must be multiplied by the filter factor.

Thank you,

You say " Since the bellows extension isn't a contrast correction, rather just for exposure control, its factor must be multiplied by the filter factor." This confused me also. Can not both filter and bellows factors be ADDED and used as a single factor? Such as a 1 stop bellows factor and a .6 stop filter factor being 1.6 stop adjustment? Or am I mixing apples and oranges again?

 

[andrew.roos]

If the filter factors are expressed as multiplication factors, such as x2 for a yellow filter, then you multiply them, getting a result that is also a multiplication factor.

If they are an EV expressed in stops (e.g. 1 stop for a x2 filter) then you add them, getting a result that is also in stops.

 

[David Allen]

What you need to understand is that filter factors are based upon the manufacturer's determination of how their filters will effect an exposure in a given scene that corresponds to a given 'optimal' lighting situation. Well, this is not what happens in real life!

The very simple solution is that you need to meter the effect of the filter(s) yourself.

Except for the very early in-built camera meters, the responsively of most meters will be pretty close to that of your general panchromatic film.

You simply need to make a general meter reading of the scene that you wish to photograph and hold the meter still and introduce your chosen filter(s) into its light path to ascertain the effect of the filter on your exposure. (you will find that a Minus Blue filter will be less effected in a Uk landscape scene - i.e require less compensation in a general UK landscape scene than in a high altitude scene) - in comparison to up a mountain in Yosemite and then place your filter(s) in front of the meter and note the change in exposure - this will give you your exposure compensation factor for any given scene (which will also reflect the effect of the prevailing colour balance).

This is how much you need to correct your exposure to which you then add any bellows or similar light reduction influences on your exposure.

Bests,

David.
www.dsallen.de

 

[RalphLambrecht]

I remember reading in Adams' The Negative that if you have a need to use two contrast filters do not add the factors together but use only the larger. Okay!

Now if you need to add bellows compensation and a contrast filter, do you add these together or use the larger?

You need to add them logarithmically.in other words: add their log values.

 

[Bruce Osgood]

Thanks everyone, you all have been very helpful and I appreciate it.

 

[RalphLambrecht]

And then the sought answer would be the antilog of their sum. . My dad used to work with a guy who had memorized the log tables and could do all this in his head. Glad I'm not afflicted in that way!

filter logs are easy.0.3=1 stop
0.1=1/3 stopetc