Help with Filter Factor - My Brain Hurts

[bmattock]

OK, I should be able to figure this out, but I guess the coffee hasn't kicked in yet. Besides, it's Sunday morning and I'm at work (waiting on server to be repaired), I have a broken toe that is throbbing, and I dunno what else. Oh yeah, it's raining.

If someone will take pity on me, I'd be most appreciative.

I've got my new/old Yashica Lynx 14e with me. No battery yet, hence no meter. No matter, I've got my trusty Sekonic L-358 Flash Master with me.

Arista.EDU 100 film. B+W 58ES Medium Yellow filter (marked 3X).

I set the ISO at 100 for the meter, but wait, I've got to factor in the filter.

OK, so 3x = 1.5 f-stops, right?

So, I set the secondary ISO on my meter to ISO 32, which is one-and-a-half times more exposure than ISO 100 film.

If I did that correctly, now when I hit the button on the meter, it should give me the correct exposure for any given f-stop both with and without the filter.

Unless the coffee has not yet seeped into my brain.

Any help would be appreciated!

Best Regards,

Bill Mattocks

 

[daveozzz]

That sounds reasonable to me and I've had plenty of caffine today.

 

[bmattock]

That sounds reasonable to me and I've had plenty of caffine today.

Wow, that was fast! Thank you! Just to make my brain stop hurting - is film ISO 32 1 1/2 times slower than 100? I mean, I know that 1/3 of 100 is 33, but does it work like that with ISO?

Best Regards,

Bill Mattocks

 

[VictorM.]

Most of the time, I just divide the film speed by the filter factor. So a 3X factor filter with Tri-X is 400/3 = 133. Or Delta 3200 with an 8X ND filter becomes 3200/8 = 400.

 

[RichardS]

Can't you just hold the filter in front of the meter and get a reading that way?

Dick

 

[daveozzz]

Now my head hurts as well.

I believe that the reason a lot of meters/cameras work in 1/3 stops is because that matches up exactly with the ISO scale. So 1.5 stops won't have an exact ISO match if that were the case. The correct ISO would be somewhere between 32 and 40 (I think).

Richard's way the headache free route. Take a reading without the filter and one with and adjust the ISO to make them match up.

 

[bmattock]

Can't you just hold the filter in front of the meter and get a reading that way?

Dick

Hmmm. Yes, I guess so! However, removing the lens hood and the filter for each shot could get a bit tiresome. I'll have to try it something, though. Thanks!

Best Regards,

Bill Mattocks

 

[richard_l]

VictorM is correct.

Divide the film speed by the filter factor and set that as the film speed on your meter. For ISO 100 film and a 3x filter factor, set the meter at 100/3 = 33.3 or the nearest film speed setting on the meter (usually 32).

That's why they use filter factors: to simplify the math. Otherwise one would need either to memorize the sequence of film speeds in 1/3 stop increments or else mess with base 2 logarithms and exponentials.

 

[bmattock]

Thanks, my friends! I actually WAS trying to do the math in my head...and it was so simple all along. Yikes.

Best Regards,

Bill Mattocks

 

[bmattock]

ManGo, you are so not nice. Now I'll have to carry an HP calculator into the field with me.

Why don't the external meters (Sekonic, Minolta, etc) include a FF calculator?

Best Regards,

Bill "Brain back to hurting again" Mattocks

 

[richard_l]

.....

FF (x) ------> Number of stops 👍

1 -----------------> 0
2 -----------------> 1
4 -----------------> 2
8 -----------------> 3
16 ---------------> 4

and so on.

Filter factors are not exact anyhow (since they really depend on the dominant wavelengtrh of the light being measured). So in the absence of a scientific calculator, one could just use linear interpolation on your little chart as a rough estimate.

For example, a filter factor of 11 is 3/8 of the distance between 8 and 16, so the f-number will be about 3/8 of the way from 3 to 4, or 3.4. (The exact value is 3.459...).

A filter factor of 7 is 3/4 of the way from 4 to 8, so the f-number should be about 2.8 (rounding to the nearest tenth). (Exact value is 2.807...).

Of course, you could just divide the asa by the filter factor and change the meter accordingly, but where is the fun in that? 😀

It's even more fun to do the log computations with a slide rule instead of a calculator, and you don't need batteries to make it work. 😀

 

[bmattock]

You guys are trying to make me scream, aren't you?

 

[Hektor]

Filter factors are not exact anyhow (since they really depend on the dominant wavelengtrh of the light being measured).

This is a relatively insignificant effect with most natural, or common illumination sources.

A much more significant effect is the spectral response of the film in use. e.g. if the film has an extended red response, and a red filter is used, then the filter factor might be something like 2 or 3, but the same filter used with blue sensitive film would have a filter factor of more like 8.

So the factor marked on the filter is not necessarily appropriate.