Metering through filters

[aporodagon]

Trying to come to terms with regard to what exposure to use when using filters, I assembled 4 meters, some filters and several film types. Each of these represents a variable to be tested.

This test was run with 120 film because the electronically controlled shutter on a
Rollei 6008AF would be more accurate and precise than any LF mechanical shutter.

The meters tested were a Sekonic 778 spot meter, a Sekonic 608 multifunction meter, a Pocket Spot meter and the meter housed by a Rollei 6008AF.

Films were 120 format 320 Tri X, HP5+ and 100TMX (the films I use the most).

Filters were my most commonly used Heliopans, a Wratten 8 medium yellow (ff 3.0X), a Wratten 13 green (ff 3.0X), a Wratten 22 orange (ff 4.0X), and A Wratten 25 red (ff 8.0X).

Target was an 18% grey card. Film was developed N (to 0.75 DU for Zone V) and read on a densitometer. The meters were secured to a tripod immediately beside the camera such that the reading angle was fixed. Meter readings were were transferred to the Rollei which has manual selection of shutter speeds to 1/3 step. For meters with 1/10 stop display, the closest approximation in thirds of a step were made.

After a reading though a filter was transferred to the camera and an exposure made, a reading without a filter was exposed on the next frame. Therefore, the difference in an exposure through a filter and without a filter should show the amount of correction that would have to be applied if one metered through a filter.

Differences of 1/3 of a stop are probably insignificant (e.g. the Pocket Spot only reads in 1/3 steps - a reading of , for example EV 13 * * could be between 13 * and 14 EV).

The results (see thumbnails) suggest that a meter may be keyed to a particular film (e.g. the Sekonic 608 to 320TRX, and the Sekonic 778 and Rollei to HP5+). The Pocket Spot seems to cover both these films equally well. 100TMX , on the other hand would require a correction factor to be used, regardless of which of these 4 meters was used.

This may be justification for having more than one meter if more than one film type is used. This doesn't represent a real world situation unless you specialize in photographing grey cards but hopefully, this will give some insight and a starting point to metering through filters. As in most things, YMMV. Proceed with caution and do some testing with your own combinations, especially with coloured objects in addition to neutral targets.

John

[Sevo]

You won't find any meters keyed to black and white film later than a Norwood. They are tuned to match daylight reversal film in response. The factors printed on the rim or in the spec sheet of black-and-white filters are indeed matched to some film, at the discretion of the manufacturer - usually films of Pan X/Tri X vintage, unless the manufacturers have reformulated their filters and run new tests since the seventies.

Sevo

[Eric Woodbury]

John,

A good test. Thanks for the work.

As you have discovered, T-grain films are typically more red sensitive than 'normal BW films' such as Tri-X or HP5+. Other films that will throw off the meters are Tech-pan type films (very red sensitive) and odd-balls such as ortho, IR, and UV films. It is hard to design a meter for all purposes.

When designing the spectral for my meter, I found it best to compromise on one film -- a 'normal BW film'. Other spectral filtrations are possible, but this may be confusing to the user.

[Glenn Thoreson]

I understand that meters have varying sensitivity to the different colors of the spectrum, as do films. It would probably be impossible to get all the variables to match up exactly. That's why we test and bracket.

[Andrew O'Neill]

Real men don't metre through their filters....
I've never tested how my metre (Minolta digital spot) would respond while reading through different filters. I've done all my testing with filters over camera lens. Each film I currently use responded slightly or significantly different to different filters. Also, prescribed filter factors were usually too high. For example, a Wratten 25 on HP5+ is 3 stops more but for me it's only 1 2/3 stops more. I do believe that TMY and Efke 25 had the oddest responses to certain filters...I'll have to dig up my data.
Thanks John.
Hey, where have you been hiding your Rollei??

[Alan Rabe]

There is one meter out there that is tailored to B&W and was designed for metering thru filters. It is a Zone VI modifed meter. There are pentax and soligors available used. I have been using the Soligor for 20 years and would not use anything else. Metering thru filters is the only way to get it right.

[aporodagon]

Tailored to which B+W film?

[Arne Croell]

Tailored to which B+W film?

Tri-X

[renes]

Anybody used Adox CHS 25 with orange filter in the field? What was a real filter factor?

[Stephen Benskin]

The meter's photo cell has a spectra bias. The color depends on what material the cell is made from. Attached is a graph comparison of meter sensitivity vs color temperature.

Filter factors are the adjustment in exposure required to make a neutral colored object have the same illuminance value at the film plane with the filter as it would without. Many modern meters have bias to red. It will see the red as brighter than it is. This could translate into a one stop difference when taking a reading through a red filter. The meter says open up two stops and the filter factor is three.

The reason why there filter factors exist is to eliminate the problem of the spectral bias of the meter.