Metering using grads/coloured filters/ND filters.

[SealBeard]

Good evening everyone, hopefully I have started this thread in the correct place.

I'm trying to get my head around metering a scene, when using such things as grads/ND/filters etc that effect the amount of light entering the camera. I think I know what I should do, by my brains not quite got a hold of the maths involved when trying to compensate for the extra stops lost due to the filters ND's etc.

So theoretical example coming up...

Nice bright sunny day, looking out over rugged hills, nice high clouds etc etc, now resisting the urge to set up the deckchair and getting my paper out, I'd like to instead to a long exposure of the scene. Now baring in mind I'm using 120 b&w film, this is what in my head I should be doing...

*meter foreground
*meter the sky
*choose an appropriate Graduated ND filter to balance out the foreground/sky and...
*the resulting exposure should be say... - ISO:50 (Ilford PanF50) Aperture:f/11 shutter speed:1/125 (theoretically - taken from camera metering)
*now I want to add two more filters, a red filter (2 stops of light loss), for darkening of the sky, and a 3 stop ND filter, for a longer exposure. So in total around 5 stops of light lost.

My question is... how do I go about working out how many seconds that exposure will be on bulb... I could figure it out up to a certain point, but once it starts moving into seconds, my brain stops:bonk:

Could anyone offer an explanation so I could understand what I should be doing better?

Many thanks,

Rob.

 

[HoppyUK]

Five stops down from 1/125sec is 1/4sec. You need much more ND to get into full seconds in bright sun.

Since you have a 5D2, which can compete on quality with medium format film, and you can do the red filter effects either in-camera or in PP, then a 10-stop ND filter plus grad would seem like a good way forward. One less filter to mess things up, more control, and you can see how the exposure is by chimping as you go. What you're trying to do is pretty hard to calculate really accurately anyway, let alone accounting for the film's reciprocity failure.

Edit: reciprocity failure isn't a problem down to one or two seconds, but after three or four you might need to add a stop or so, and after that it gets exponentially worse. Over ten seconds you're in the lap of the Gods, and with colour film you get variable colour shifts coming in. Reciprocity compensation data is available for most films.

It's the straight line response of digital to exposure, and the ability to review exposure immediately, that really makes these dark ND filters practical in a way that they were not with film.

 

[Chappers]

I'd agree with Hoppy, you can use the 5D to get a good approximation of exposure. Reciprocity over the range you are talking about for B&W film is going to be insigbificant.

However if you want to do the maths, try cutting it down into steps.

Primary exposure 1/125
2stops down ( Red filter ) 1/30 sec

3 stops down from 1/30 ( 3 stop ND ) 1/4. sec

however if we substitute a 10 stop ND for the 3 stop

it goes like this

1 stop= 1/15
2 stop= 1/8
3 stop= 1/4
4 stops= 1/2
5 stops= 1 sec
6 stops = 2 secs
7 stops = 4 secs
8 stops = 8 secs
9 stops = 16 secs
10 stops = 32 secs

Not that difficult, although we may start to see some slight reciprocity failure coming in at exposures longer than say 1/2 to 1 sec. Check out the data sheet from Ilford for exact data, it looks like an exposure of 32 seconds need to be increase considerably to about 2 1/2 mins