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11-13-2011, 11:05 AM   #1
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Exposure With Neutral Density Filter

I am using a neutral density (10 stops) with my k-x. When not using live mode, the images are severely underexposed (the camera chooses .3 sec exposure).

Exposing in Live mode, the image is normally exposed (about 30 seconds).

Can anyone explain why this happens?

11-13-2011, 11:18 AM   #2
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If you're using a tripod, maybe the light meter is confused by the light coming in through the viewfinder? You can try blocking the viewfinder to see if that helps account for some of the difference you're seeing.
11-13-2011, 12:09 PM   #3
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with such filter, the exposure cell sees barely nothing, so it can not calculate properly the correct expo settings.

Anyway, you need a tripod first.
Take a pic without teh filter (check the focus and the xpo settings).
Set the focus on manual (readjuste if necessary).
Put the filter on the camera
Set the iso and/or the diaphragm and/or the time in order to expose 10 times more in comparison to your first pic.
Then you should have correct exposure.
Sometimes, some more finetuning will be necessary (add or remove few Ev).
11-13-2011, 06:26 PM   #4
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Both explanations above are probably contributing to what you are seeing.

Remember, the exposure calculation has to be made before the mirror lifts. (You can't read out the image from the CCD and then adjust the exposure.... too late then.) Exposure metering is done, with the mirror down, by sending part of the light coming through the lens to the metering system instead of to the viewfinder. Unfortunately, that light path isn't one-way, so a portion of light coming in through the viewfinder can find its way onto the metering grid. Usually your face blocks any direct light entering into the viewfinder, but you are using a tripod, so the viewfinder is exposed to ambient lighting conditions.

Now you go and put a filter on the lens. A 10 ND filter cuts *a lot* of light. In ordinary conditions, the amount of light leaking from the viewfinder is insignificant relative to that coming through the lens. The filter altered that relationship by 10x - in effect, you made the viewfinder leakage 10 times greater.

so you wind up with two problems:
1) light from the viewfinder corrupts the exposure calculation
2) there is so little light anyway that the camera has a hard time calculating exposure - you may be getting near the low limit of the metering system.

Why does it work in Live View? In Live View, the mirror is up. Because the mirror is up, the regular exposure metering cannot be used once live view starts, and light cannot pass from the viewfinder onto the CCD - all the light comes through the lens only. The camera can moniitor the brightness of the images coming in and use that information to calculate exposure. Assuming you are using Contrast Detect focusing (or manual focus) the camera can get everything it needs from the Live View image.

I am not sure what would happen if you used Phase Detect focusing with Live View. In that case the mirror has to flip down for focusing. I do not know whether the camera takes exposure information from the Live View image just before you release the shutter, or from the usual exposure system during the focusing phase. Probably (in order to allow focusing on one spot and then re-composing the image) it gets exposure from Live View, but I don't know.

11-14-2011, 02:52 AM   #5
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QuoteOriginally posted by jreynoldsME Quote
I am using a neutral density (10 stops) with my k-x. When not using live mode, the images are severely underexposed (the camera chooses .3 sec exposure).

Exposing in Live mode, the image is normally exposed (about 30 seconds).

Can anyone explain why this happens?
no can't explain but from the lee filter web site,

Calculating the correct exposure for a long exposure filter is very simple. Take a meter reading as normal WITHOUT the filter in place, and decide on the aperture required for the shot. If using a 10 stop long exposure filter you need to add 10 stops of exposure to the shutter speed that matches your chosen aperture (double the exposure length 10 times). For example a metered exposure of a 30th of a second at f11 would need to be increased by 10 stops making a new exposure time of 32 seconds

from 10stop - long exposure photographs by mark crocker

Don't expect the camera to meter correctly when the 10-stop filter is on the lens. You will sometimes get a meter reading that is below 30secs, but I have found that this will significantly underexpose. You will need to meter first without the filter in place (using Aperture priority). Once you have the meter reading, calculate the exposure time using the table below (basically the metered time multiplied by 1000) and then switch the camera to Build (B). If you own an Apple iPhone or iTouch, then you can download a great little application called NDCalc. This will do the calculations for you and provide you with a count-down timer. Taking into account reciprocity failure (for long shutter speeds), I will often double the time.

1/1000 1secs 1/60 16secs 1/4 4mins
1/500 2secs 1/30 32secs 1/2 8mins
1/250 4secs 1/15 64secs 1 16mins
1/125 8secs 1/8 2mins 2 32min

To increase the shutter speed, remember to use a small aperture. Typically I would use f/16. On a cloudy, but bright day, this should give an exposure time of around 2 minutes (ISO 50).
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