10 stop ND filter? What happened?

[Ron Marshall]

HP5 has pretty bad reciprocity failure. I have used a ten stop with TMY for an ocean scene, which gave me an exposure of 1 min, which then became two because of reciprocity failure.

[Eric Leppanen]

This doesn't make sense to me either. Assuming Hugo shot at ISO 250 (just to make calculating the Sunny 16 rule easier):

Exposure without ND:
f/16 = 1/250 sec (Sunny 16 rule)
f/22 = 1/125 sec
f/32 = 1/60 sec
f/45 = 1/30 sec

Exposure with ND:
1 stop ND = 1/15 sec
2 stop ND = 1/8 sec
3 stop ND = 1/4 sec
4 stop ND = 1/2 sec
5 stop ND = 1 sec
6 stop ND = 2 sec
7 stop ND = 4 sec
8 stop ND = 8 sec
9 stop ND = 15 sec
10 stop ND = 30 sec

Per the HP5+ reciprocity chart, a 30 second measured exposure should be given roughly 155 seconds of actual exposure. So by exposing for 120 seconds, Hugo underexposed by maybe half a stop or so, certainly not enough of a difference to cause a blank sheet of film.

Hugo, did you remove the dark slide when making your exposures?

How old is the ten stop filter? Could the dyes have become unstable, causing the filter to block up? I've never heard of such a thing, but with such a large correction who knows?

How about shooting through the filter with a digital camera to estimate the actual density of the filter?

[Hugo Zhang]

Hi Everybody,

I think I severely underexposed those negatives. I came home after lunch today and set up my WP camera inside my house and pointed my lens outside the window and focus the tree bathed in the sun. I took a meter reading of the dark green tree leave and the window frame inside. Both gave me the same reading: f/22 at 1/15 second and I placed at zone v. Then I added 10 stops. At f/32, the exposure time should be 2 minutes. My WP film is TMY 400 and I rated it at 200. According to my sticker on my meter which shows Tri-X Reciprocity, 2 minutes will need 22 minutes. I guess TMY400 should not be too far. So I exposed one sheet at f/32 for 20 minute and souped it in D-76 and the negative looked very nice.

The lesson is: don't take meter reading through the 10 stopper ND filter.

BTW, the filter is brand new B&W filter screwed into the rear element of the 150mm lens.

Thanks,
Hugo

[Bob Salomon]

Sounds like your metering through the filter was not a good idea. I don't think many meters perform well at low light levels, and a small light leak could throw off the results.

I'd meter the scene then apply the ten stops of compensation. THEN figure out reciprocity.

Neither was putting any piece of glass behind the lens. Put it on front where it was made to go.

[Hugo Zhang]

Neither was putting any piece of glass behind the lens. Put it on front where it was made to go.

Even an UV filter on the front of my 150mm ssxl lens will make it unable to cover my 7x17 plate. A 67mm filter work perfectly behind the lens, at least according to what I have heard and my very limited experience.

[Peter K]

Many light meters use silicon elements that are IR sensitive and most likely, even a 10-stop ND filter will pass IR. I know ND Lee lighting gels are completely transparent to IR, because I use them as IR filters for IR photography.

Most light meters today are equipped with "silicon blue cell" silicon elements with extended blue sensitivity and an IR-blocking filter. BTW similar with CCD-cells used often in digital cameras.

As I know B+W uses Schott "NG" filter glass for ND filters. This filter glass blocks all rays nearly even from 400 nm up to at least 1000 nm.

Also Wratten filter 96 transmitts IR, to block it one has to add a filter like B+W 489 or Helioplan 103.

Peter

[Peter K]

A 67mm filter work perfectly behind the lens, at least according to what I have heard and my very limited experience.

Any (filter-)glass between lens and film will overcorrect the spherical aberration of the taking lens.

The influence on the image quality if taken with long focal lengths resp. small angle of view isn't such a problem. But with UWA-lenses the result will be uneven sharpness.

Of course only visible if other causes like vibrations, uneven film flatness, and focus errors etc. are ruled out.

Peter