For those who worry about a half-stop or so, consider the fact that your LF shutter is likely slower than indicated, and processing introduces more variance, also flare. Most cannot even determine a stop difference in a negative. Take a deep breath, push away from the computer and work it out IRL.
It is likely that a modern light meter is more accurate than impressionistic second guesses.
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Hmm, you made it clear as mud. Could this help and take you out of your misery? - https://en.wikipedia.org/wiki/Filter_factorOriginally Posted by Havoc
I don't know if this screenshot will help or further confuse. It's a screenshot of an Excel curve that I plotted quite a while ago to show the relationship between the exposure factor and the number of stops. I plotted 3 points: Factor=1 (0 stops), Factor=2 (1 stop), Factor=4 (2 stops), then let Excel determine the equation for the exponential curve that passes through those points. With this equation, shown above the curve, one can accurately find the factor that relates to any number of stops. Comparing to your example above, you can see that a Factor=1.5 relates to .58 stops, rather than the 2/3 stop you found.
Using the equation, x=no. of stops, y=Exposure Factor, e=fixed mathematical value of 2.71828. Choose a value for x, multiply this by 0.6931 to get the exponent, then raise the e value to that exponent (power).
That might be the wisest thing to do, but be aware that some exposure meters have a bias towards the red end of the spectrum. Average yellow should be somewhere around 1 stop added, a yellow-green filter somewhere around 1.5 stop added and an orange filter somewhere around 2 stop or perhaps a bit more. All depending on the filter, the colour of the light, the film and your light meter. Also note that when in doubt, you should expose more liberal. Half a stop over exposure is well within the latitude of your film and doesn't do much harm.
Havoc,
Let's see if I can make this easier for you somehow, even though the whole filter thing is a real can of worms.
First filter factors: The first thing you have to realize is that every full f-stop or shutter speed changes the exposure by a factor of 2. For shutter speeds, this works just fine: 1/125 x 2 = 2/125 = ~1/60 = 1 stop more exposure. The problem here is that shutters don't have intermediate shutter speeds, so when you have a factor of, say 2.5, you can't use the shutter speed alone to make the adjustment.
For f-stops, however, it is more complicated due to the inverse square law, which says when you double the area of the aperture, you get 4 times as much light. In order to only get twice as much light, the real factor for multiplying the f-numbers ends up being the square root of 2. F-stops, therefore, progress in increments of approx 1.4. Not very easy to deal with together with the shutter speed factors.
That's why just about every filter manufacturer lists factors and the amount to change the f-stop for each filter.
So, here's an attempt at a table. I find it easy to just memorize (or list) the factors and the stops-to-change for each filter I own.
Factor ----- Stops-to-change
1.3 ----------- 1/3 stop
1.5 ----------- 2/3 stop
2 ------------- 1 full stop
2.5 ----------- 1 1/3 stop
3 ------------- 1 2/3 stop
4 ------------- 2 stops
6 ------------- 2 2/3 stops
8 ------------- 3 stops
16 ------------ 4 stops
There are theoretically more factors between these, but I don't know of any filters that use them. This should give you just about all the info you need to use just about every filter out there.
Now, on to filter designations: Wratten numbers are the bible as far as I'm concerned, but Hoya and B+W (and some other makers) use their own systems. Plus there are old systems used by Tiffen and others. It's good to know what equals what here. There are lots of resources on the net about this (the B+W catalog gives Wratten equivalents, for example), so let's just deal with the filters you have.
Yellow K1 (a Hoya designation) is the same as Wratten #6. Factor is 1.5 or 2/3-stop correction.
Yellow Y2 is an old Tiffen designation. It is a K2 filter = Wratten #8. Factor is 2 or 1-stop correction. (this is the most common yellow filter out there, so its a good bet that's what you have.)
YG (a Leitz designation) = G1 = X1 = Wratten #11, a "yellow-green" filter (looks light green to me) with a factor of 4 or 2-stops correction. Note that a Wratten #58 filter is really dark green and has a factor of 6 (2 2/3 stops). Secondary manufacturers rarely marketed these. Your Prinz filters are almost certainly not Wratten #58 filters.
There is a Hoya X0 filter, which is an even lighter green and has a factor of 2.5 (1 1/3 stops correction). Hoya designates this as (YG) I think just to tell you it's "yellow-green." It is weaker than the Wratten #11 (X1) filter, but I don't know of a Wratten equivalent for this. This may be the Prinz YG filter you have (especially if the factor is in the 2.5 area). Then your Prinz X1, with a factor of 4, would be the #11 filter.
G = Or = O = Wratten 15, is a "deep yellow" filter that looks orange. The factor is 2.5 or 1 2/3-stop correction. Don't get confused by Hoya telling your their X0 filter is green with a (G). X0 and X1 filters are green, a "G" filter is a Wratten #15 and is orange.
Take a look at the filter designation cross-reference here: https://unblinkingeye.com/Articles/BWFF/bwff.html for most of the common filters.
A final word about factors. The actual correction for a given filter depends on the specific film and the spectral content of the light being used. Kodak and others used to give separate factors for each film for both daylight and tungsten lighting. This practice is no longer common, but the laws of physics haven't changed. Published filter factors are only average values based on daylight. You need to be aware of the pitfalls inherent here and adjust as needed. When in doubt, a bit more exposure is your best insurance.
Hope this helps,
Doremus
Also it can be pointed that discusion: http://www.largeformatphotography.in...p/t-15599.html
It is explained why meter reading may need an additonal correction depending on the kind of meter we use, as meters may have different response to different colors:
I guess that one of best meters one can use for LF is an spare Nikon F5, it has a very refined RGB meter. Single problem, not lightweight.
While I feel fairly confident metering through filters with my Zone VI "color corrected" Pentax digital spot meter...I still need to maintain "spectral awareness" relative to subjects reflectance of color temps and how specific filters affect placements, a given films spectral response, and my own intentions for the image which also incorporate specific exposure/development scenarios. Not rocket science by any means...but just takes a bit of time. A bit of a pain reading through filter then mounting quickly while light is stable - and sometimes I cannot do this quickly enough (then usually simply use factors), and am thinking of just springing for a set of small filters for the meter itself (hmmm...maybe a small three-filter turret?)
https://www.pacificrimcamera.com/rl/00041/00041.pdf
Hello John,
The red curve shows sprectral sensitivity of the Pentax digital spot meter, it makes clear that metering a gray subject through filter with the pentax digital will lead to even one stop exposure difference on film, depending on filter, with a green filter you would underexpose one stop compared to a very deep red, and perhaps 2/3 stops compared to a very deep blue.
As very deep red and blue are perhaps not as common as yellow, cyan and central red then the practical difference is lower. What is clear is that a green filter on the Pentax D tends to meter for underexposure, as meter is very sensitive to green it points a higher EI than reality.
Of course subject color and film spectral sensitivity have also a share, an ortho or orthopan will underexpose a lot with a orange filter, an extreme situation: clearly we may obtain a black scene with ortho film and deep red filter, while the meter would say that there is light there that light won't expose the film.
So, perhaps aganist what it looks natural, if metering with a green filter on the Pentax D one should expose some 1/2 stop more than the meter says, at least if we don't want to block deep shadows, this is what I've personally concluded...
Again, I would insist in how good is the F5 rgb meter for those situations, as metering throught the filter is the common situation for an SLR then IMHO it has been considered in the F5 rgb meter.
An F5 is not a joke
Regards.
Pere...do keep in mind that I'm speaking of the Pentax spot meter version which was modified by the (now-defunct) Zone VI company...to specifically address what are otherwise some spectral biases of the factory version of this meter, as reflected in the chart which you've posted above. Not saying the modified version is perfect...but measurably better. Richard Ritter could give you more info on this modification. In the meantime...yes - I've heard amazing reports of the accuracy of the F-5's meter!
FWIW, here's a screenshot from the manual (Pentax Digital Spotmeter) that discusses metering of various subject colors. I have no idea what the Zone VI mod does. The Standard Index on my meter is located at EV 9-1/3; this info tells me that metering dark green will yield the correct exposure, whereas all other colors will yield varying amounts of underexposure:
Dark yellow = 1-1/3 stops under
Orange = 2-1/3 to 3-1/3 stops under
Bright red and Blue = 4-1/3 stops under
Indigo and Purple = 6-1/3 to 7-1/3 stops under
All this leads me to ask, for example, how to deal with metering a blue subject through a blue filter. Anyone? Guessing the only way to really know is to test it with a favorite film ... like Maris says.
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