Filter Factor

Can someone explain to me precisely how the filter factor works. Today was the first day I've ever used a filter. I used hoya 25a and the filter factor is 3 (2 stops). When I put the filter on, the light meter picked up the 2 stops automatically. I don't need to open up more stops do I?

Also, with my polarizer, a hoya linear, the filter factor is between 2.3 and 2.8 (1.3 stops). I'm not really sure what to do here. How do I open up 1.3 stops on camera that only has full stops?

Thanks -

If you are using an in-camera meter, filter factor does not matter. Your camera will be measuring the light coming through the filter.

If you are using a hand-held meter, the light reaching the meter is different from light reaching the film through your camera lens and through your filter. In that case, you take the meter reading and OPEN whatever number of stops necessary.

The polariser filter factor varies, depending on the setting of the filter on the lens. The camera meter sees the difference and adjusts the reading accordingly. This peculiar feature of polarisers makes it difficult to use them with a seperate exposure meter. Whether or not you can dial in 0.3 or 1/3 stop of a difference depends on the camera. If it has aperture priority auto, the shutter speed is likely to be continuously variable, and will therefore set the exact speed for the aperture you have set. The readout may show the closest marked speed. In manual, you are usually stuck with set speeds, but the aperture can be tweaked to 1/3 or 2/3 between settings. I hope that makes sense!

A 3x filter factor is a 1 2/3 stop compensation.

Several things, There's nothing to stop you from setting the aperture between stops. There just aren't clicks and it's not precise. It is "close enough for government work" though.
The factors are suggestions from the manufacturer. just like film you may find you need to apply a little "windage"
Often reading through the filter will not give what you need because of the spectral response of the metering cell.
Try a few test exposures using the ttl method and the using the marked recommendation.

i agree with all of the above, ignore the filter factors if you use a built-in meter or adjust the meter recommendation with experience. some hand-held meters allow you to measure a filter's filter factor

I would like to second the previous stateement. Using the camera's built in light meter assumes that its sensor is equally senbsitive across the visible spectrum. This may or may not be true especially for dark colored filters. Testing may be required.

Firstly, filter-factor is a gross approximation. If you take a photo of a mostly-red scene through a red filter, the change in exposure will be small whereas if you shoot a blue scene through a red filter, the required exposure change is quite large.

A 25A has a filter factor of 3 stops, which is 8x. In theory if you use a TTL meter, it will read the already-corrected exposure but that makes the assumption that the meter and film have the exact same spectral response, which in general they will not. I've found that many meters are quite red-sensitive, so they will report only +2 stops with a 25A whereas that filter has a factor of 3 stops with most films in daylight conditions. If your film lacks red sensitivity (e.g. Acros), then it's even worse. But if the film has extended-red sensitivity (e.g. SFX200), then the filter-factor will be lower!

So you need to be aware of the interaction between lighting spectrum (red/orange filters have little effect under tungsten light because it is already missing a lot of blue), scene spectrum, filter absorption spectrum and film sensitivity spectrum. They all matter, and bundling them into a single number is just a handy oversimplification that works maybe 80% of the time.

mporter012,

Your Hoya 25a has an 8x filter factor. They also say "3" which is simpler to think - 3 f/stops.

So with a Red filter you open up 3 stops normally (from a meter reading without filter).

I may have opened the can of worms in your mind when I described the different effects you can get with a filter and the different exposures that would lead to those results. I said if you want to make green normal while really brightening red you would open more. Strictly speaking you can perform any of the exposure variations in the darkroom when printing. You don't have to wrangle with the crazy thought I threw out there.

For your example, the in-camera meter told you 2 stops. Well it was off the filter factor because it was about to make you underexpose 1 stop. You probably should open up at least one more stop.

There is a refined "Hutchings" method in "Steve Simmons Using the View Camera" - Amazon will serve the specific page if you are lucky but avoid untrustworthy spyware download sites in Google results. In the Hutchings method you meter through a filter and then compensate according to a short chart.

In that chart a 25a red filter says open up yet additional two stops after reading through the filter.

Reason: shadows have a lot of blue light - and the blue light is severely cut by red filter. So to get sufficient shadow detail you need much more light than the meter indicates.

So a factor quoted as 3 meant 3 f/stops. Your meter was a little off. Steve Simmons would say a lot off.

Reality: You are fine following the meter reading but the Hutchings method is easy to use (for many filters it says use reading through the meter).

Strictly speaking: I think it was for Tri-X, different factors might need to be "worked up" to be accurate with tabular grain film.

Back to reality: I carried the Hutchings chart in the wilderness and when I needed it, I was too lazy to turn to the page I had written it on. If you like the concept, tape it to the back of your camera.