Exposure settings with ND Filters

I have a K5 and I have recently been experimenting with using a Hitech 0.9 ND filter on a Pentax 17-70 lens.

Without the filter and using Manual exposure set to 0, the results are as expected and the exposure is correct.

If I then use the 0.9 ND, I re-set the Manual exposure to 0. However, the result is very under-exposed and (using the image below as an example) I had to then increase the exposure by 2 to 3 stops in Lightroom. Obviously I could have instead added exposure compensation at the the time of shooting.

I expected the K5 to still work out the correct exposure (ie at 0) when using the 0.9. I realise that using something like a Big Stopper you have to manualy over-compensate because it is so dense but I thought a 3-stop filter was still clear enough for exposure to be calculated automatically in-camera.

Hope this makes sense. Can someone please confirm or correct me ?




Many thanks
Nigel

Have you pressed the green button after fitting the filter? This will reset the meter, and yes, in theory the brightness should work out being the same as without the filter. It sounds like the EV is remaining at the previous unfiltered settings ... Can you note the exposure settings? If they are not changing when you put on the filter, then the image will darken. But the green button should sort it .. The settings should change to allow in more light and it should equalise.

Also, if you're using live view, this is not a reliable means to check the brightness of your images ... You'd have to check the actual settings and take test shots to be sure.

You do have to be aware that light can leak through the eyepiece and affect the exposure. (The light meter is in the pentaprism housing IIRC)

With no ND the light through the lens and through the eyepiece are going to be close enough.
With an ND, any light through the eyepiece will make the camera think the scene is brighter than it is (and will underexpose as you found out!).

Can't remember if there's an eyecap included with the K5... probably (and most wonder what it's for! :lol

Matt

Thanks guys for your comments and advice.

Nigel - no, I didn't use the Green button. Instead I reset it to zero manually, using the front wheel. I guess it has the same effect but the Green button is quicker so thanks for the heads-up.

Matt - so that will be the little plastic thing that I've just found unused at the bottom of the box then
Shame you have to remove the rubber eyecup to use it but I'll certainly give it a try next time - sounds like a good theory to me !

Regards
Nigel

Hi Nigel, the green button carries out a different function to adjusting the exposure adjustment indicator .. The zero indication simply means no adjustment away from the metering systems default reading and exposure value. Moving the indicator is applying user adjustment away from this metered value. So if the camera has not taken a new reading after changing the filter, then you are just returning the exposure value to what it was before.

The green button ensures that a new reading is taken, and so should produce the equivalent EV brightness, although with settings that are allowing more light in than before, compensating for the filter. Hope this and Matt's suggestions will help... Let us know how you get on.

You really do need the eye piece cover for long exposure work.

I use an iPhone app to tell me the settings I need, based on a correctly exposed test shot.
Shooting in full Manual (like real manual with a fixed ISO) so nothing can vary during shots.