Metering for Long Exposures

Saltburn Pier
Originally uploaded by thefatcat44

These long exposure images are becoming really popular amongst photographers and customers alike. This is seen by the massive increase in price suppliers are putting on the 10 stop filters. Warehouse Express six months ago were charging about £54 for B&W 10 stop ND filters (77mm). Recently they were up to £156!! How about that for ripping us off?

I have done a previous post on how to set up for these shots, which takes precision and patience but an issue that keeps arising is 'how do I meter for the long exposure?'

You have a couple of options. The easy brain dead way is to just guess it, and this becomes easier with experience. However, when you are talking about 2, 3 or 5 minute and longer exposures, if you are a little off, either over or under-exposing it can get very frustrating as you try various lengths of exposure in an effort to get the image you want.

If you really want to do it properly and consistentley the technique is, in fact, very similar to calculating any other exposure using grads. Here's how it's done.

1. Set the aperture you want - usually for these types of images somewhere between f11 and f22. F22 will give you a much longer exposure than f11 or f16. F16 to f22 will give you good depth of field and even in daylight with a 10 stop ND filter attached you will get a long exposure of 1 minute plus in most circumstances.

2. In Manual mode with the aperture set point the camera to fill your frame with the foreground (have the camera set to matrix metering). Centre the light meter guage by adjusting the shutter speed (do not alter the aperture to centre the guage). Note the shutter speed.

3. With the same aperture now point the camera at the sky (not at the sun or the brightest part of the sky, but an average brightness area) and again use the shutter speed dial to centre the light meter guage. Note the shutter speed.

4. If this shutter speed is 2 or more FULL stops brighter than the first reading you will need ND Grads - select the one or combination of grads you need to balance the exposure to your liking.

5. Focus the camera and frame the shot. Fit your ND Grad filter holder to the lens and compose the shot without the 10 stop filter attached - setting the ND grads to the right position for the horizon.

6. CAREFULLY remove the ND Grad filter holder without sliding the grads up or down and without moving the camera or changing the focus etc.

7. Set the camera to BULB mode and make sure the aperture is set to the setting you used for the metering in steps 2 and 3 above.

8. Fit your 10 stop ND filter - again without moving your camera or altering the focus etc.

9. Very carefully re-fit the ND Filter holder with the filters in place - this will ensure the ND Grads are set correctly to the horizon line - something it is very hard to do after you have fitted the 10 stop filter!

10. Now, remembering your shutter speed from step 2 above - you double the shutter speed for every stop of ND filter you added. So if you added a 10 stop filter and say the base exposure from step 2 was a quarter of a second this is how the calculation would go -
2 x 1/4 = 1/2 sec.
2 x 1/2 sec = 1 sec.
2 x 1 sec = 2 sec.
2 x 2 sec = 4 sec.
2 x 4 sec = 8 sec.
2 x 8 sec = 16 seconds.
2 x 16 secs =32 seconds.
2 x 16 seconds = 32 seconds.
2 x 32 seconds = 64 seconds.
2 x 64 seconds = 128 seconds (or just over 2 minutes).

So your exposure time would be just over 2 minutes. You can adjust this to over or under expose if you wish for artistic effect.

Just remember for every stop of ND filtration (you don't include the grads in the calculation) just double the exposure.

There you have it. I hope you have fun experimenting and if you want any help, drop me a line, or consider booking some one-to-one tuition time with me - you will be producing wonderful images in no time at all.