Why are my long-exposure photos (with ND filter) overexposed?

Question

I am using a Nikon D90 and this is my first time trying to take long exposure photos.

I have set the camera to Manual mode, exposure compensation to -5, Bulb mode, f/22 aperture, ISO 100, 6-stop ND filter, on a tripod, and using a wireless remote. When I shoot with these settings in broad daylight I can see nothing but total blank white image.

I tried the same setup in other modes; I am able to see the image, but I don't see those lovely dreamy clouds like we normally do in long exposure.

Where am I going wrong?

Answer 1

The short answer is that your ND filter is not strong enough.

As Alan Marcus's answer points out, according to the Sunny 16 rule, in bright daylight at ƒ/16, you will get a nominally-correct exposure when your shutter speed is the inverse of your ISO (i.e., at ISO 100, you should set the shutter to 1/100 s).

Setting the aperture at ƒ/22 buys you an additional stop on your shutter speed; The 6-stop ND filter buys an additional 6 stops (of course). 7 stops is equivalent to multiplying by 2⁷, or 128. So a correctly-exposed full daylight scene at ƒ/22, ISO 100, 6-stop ND filter is about 1.25 seconds.

I'm sure you've noticed that you don't get those dreamy clouds with a mere 1.25 second exposure. Depending on the clouds' altitude and wind speed, you might need at least 30 seconds, perhaps more. 30 seconds is about 4.5 additional stops beyond what your 6-stop ND can give you. Therefore, on a bright sunny day, I recommend at least a 10-stop ND filter.

Now, because you already have a 6-stop filter, it will probably be cheaper to acquire a 4-stop filter (and perhaps a 2- or 3-stop filter as well), than to get a 10-stop ND.

(I'm assuming that stacking 2 filters on your lens won't lead to strong vignetting. If that's the case, you'll need to go to a single 10-stop)

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Regarding your dialed-in Exposure Compensation:

In full manual mode, setting the exposure compensation does nothing to actually adjust your exposure. The camera can only control 3 parameters: ISO, aperture, and shutter speed. In full manual mode, if you have set ISO 100, ƒ/22, and are controlling shutter speed in Bulb mode, there's nothing more the camera can do for your exposure.

By setting exposure compensation, what you're actually doing is just adjusting the "0 point" of the camera's exposure meter. Even in Manual, the camera's exposure meter is still working, trying to tell you if something is under- or over-exposed. By setting EC to -5 EV, you're basically telling the camera adjust its reported "correct" exposure metering by 5 EV. So if the camera normally thinks a scene is correct at certain settings, instead it reports back "Well, you told me that I am making it too bright by 5 stops, so... User: darken the scene by 5 stops."

In any of the other modes (P, S, or A), instead of telling you darken the scene by 5 stops, the camera will do it itself.

And by the way, dialing in Exposure Compensation in Bulb mode is nonsensical. How can the camera's exposure meter even guess at exposure when it has no idea how long the shutter will be open? (Thanks to Michael Clark for pointing this out).

Answer 2

The approximate manual camera setting is likely achieved using the tried and true “sunny 16 rule”. Shutter speed is 1/ISO with the aperture set to f/16. Since you have chosen ISO 100, the exposure, according to this rule of thumb is 1/100 (likely you don’t have the 1/100 so we set the shutter @ 1/125 of a second @ f/16. You have chosen to set the aperture at f/22, that’s one f/stop less light so we compensate by setting the exposure time at 1/60 of a second.

OK, now we mount at + 6 stop ND. Now we need to compensate 6 f/stops worth by slowing the shutter.

1 f/stops worth = 1/30 --- 2 f/stops worth = 1/ 15 – 3 f/stops worth = 1/8 – 4 f/stops worth = 1/4 and 5 f/stops worth = 1/2 -- 6 f/stops worth = 1 second.

That the story.

Mount the 6 stop ND and you should set the camera, 100 ISO @ f/22 with the shutter set manually to 1 second.

Answer 3

It is clear from you question that you need some help understanding exposure. I suggest you start by looking at the Exposure Triangle.

Both standard M mode and Bulb are manual exposure modes, meaning that you are in control of the exposure. Metering by the camera becomes irrelevant and therefore Exposure-Compensation means nothing. Ignore it completely while exposing manually. While it has no impact on exposure, you may want to set it to 0 just so that the camera shows you the difference between the metered exposure and the one you set.

With Bulb mode, you are completely on your own since the camera does not know how long you will hold the shutter open. It cannot compare anything to the set exposure, so forget EC and forget what the meter shows. Most cameras operate Bulb in one or two modes, Press-and-Hold which you hold the shutter-release until the exposure is done or Press-Twice, once to open and once to close the shutter.

Now, what is usually done is to test the exposure in A and see what shutter-speed the camera gives. Then you switch to Manual mode and dial shutter-speed that the camera metered as a starting point for Manual exposure. To add the ND filter, select Bulb and perform the exposure counting with the set shutter-speed multiplied by the strength of the ND filter. Remove a few seconds of darker exposures, add some for brighter. Remember, exposures are quadratic, so if you want 1 EV less, then you divide the exposure time by two.

Answer 4

It's very hard for a camera to calculate the right exposure in low light conditions. Adding an ND filter will create a low light condition and you camera might over expose.

The best way to get the right exposure is to use manual settings. First get the right exposure without the filter, then put on the filter and lower the shutter speed accordingly.

With a 6 stop ND filter, you would lower the shutter speed by 2^6 = 64 times roughly. An easy way to calculate the correct shutter speed is to use a mobile app like this (ND filter timer).

When using bulb mode, remember that the first click will start your exposure and the second one will end it. The time in between the two clicks is the "shutter speed".