Wrong metering with Neutral Denstiy Filter, compensation required?

[grantyale]

On all current EOS as far as I know. Metering is biased towards active AF points. Flash metering has been decoupled from AF points in ETTL2 but ambient AE is still linked.

 

[emlee]

So that takes out the possibility of stray light from VF affecting metering.
What was the metering mode and which AF point(s) was used?

metering mode is Evaluative. AF points... can't remember, didn't think it would matter.

what is your purposes of using ND filter to shoot? what effect you want to archive?

motion blur even in strong lighting. if you are asking on the context of the 2 pictures, then please dont. these 2 pictures are purely for testing the filter so that i can understand it's behavior.

Interestingly, the metering of the camera reflects the nominal difference (factor 8) the filter should make.

How reproducible is the effect on the pictures when you take several pictures in a row? Does it also happen when you open the aperture all the way? I've seen lenses where a bad aperture mechanism resulted in random exposure variations...

Edit: If the EXIF tags are to be believed, the first picture was taken at 1/125s, not 1/100s, which would make only 2.7 stops difference. Also, as grantyale points out in the post below, make sure you don't use evaluative metering - it is pretty much a random number generator.

To determine the effect of the filter, you could try a series of different exposures in manual mode. That should make it clearer what part of your troubles can be attributed to the filter, and what part to the camera/lens.

this is a series of 8 shots on different modes. all produced the same "error" in the brightness. i will try with different metering modes and with wide aperture as suggested. thks.

hrm, the condition looks the same to me.

well if this is the case, then be grateful for immediate feedback, i guess.. no camera is perfect.

i guess... lucky i found out now as opposed to actual shoot. i suspect the "error" is consistent hence i just need to remember to compensate for it when shooting. will continue to experiment.

If the camera was set in AF-point auto selection, probably the slight difference in composition has caused the camera to pick up different things and thus affecting AE.

Anyway... does this happen consistently or just for these two photos?

will try to change the AF point too. thks for suggesting.

 

[catchlights]

..........................
motion blur even in strong lighting. if you are asking on the context of the 2 pictures, then please dont. these 2 pictures are purely for testing the filter so that i can understand it's behavior.
......................................

you want motion blur in strong lighting,

blurriness has to depend on subject movement, 3 stops is not enough for a bright lighting condition

 

[gooseberry]

Considering that you used evaluative metering for such a scene, the first image (with ND filter) seems to be exposed appropriately (the sky is well exposed), however your second image (without filter) seems to be exposed a little too "hot" (ie. overexposed re: the sky and clouds). Your camera meter measured a Light value of 11.6 for the first image and a Light value of 14.3 for the second - 2.7 stops which seems in line with the ND8 filter you used.

The "hotter" exposure for the second image (using a faster shutter speed) is leading me to speculate that your shutter might be performing incorrectly - for the faster shutter speed of 1/800s set by the camera, the actual shutter timing might be slower than it should be, thus giving you a slightly overexposed image.

You might want to check your shutter mechanism timings by doing some tests at various different shutter speeds, especially for the faster shutter speeds above 1/250s.

Or as LittleWolf suspects, it could a bad aperture mechanism in your lens - not closing down fully to f/5 in the second image.

 

[emlee]

Considering that you used evaluative metering for such a scene, the first image (with ND filter) seems to be exposed appropriately (the sky is well exposed), however your second image (without filter) seems to be exposed a little too "hot" (ie. overexposed re: the sky and clouds). Your camera meter measured a Light value of 11.6 for the first image and a Light value of 14.3 for the second - 2.7 stops which seems in line with the ND8 filter you used.

The "hotter" exposure for the second image (using a faster shutter speed) is leading me to speculate that your shutter might be performing incorrectly - for the faster shutter speed of 1/800s set by the camera, the actual shutter timing might be slower than it should be, thus giving you a slightly overexposed image.

You might want to check your shutter mechanism timings by doing some tests at various different shutter speeds, especially for the faster shutter speeds above 1/250s.

Or as LittleWolf suspects, it could a bad aperture mechanism in your lens - not closing down fully to f/5 in the second image.

wow, i just got my camera for about 1 month... dont scare me leh...
anyway, how to test the actual shutter speed at 1/250 sec? send it to Canon Service maybe... else i just have to open wide aperture and test if the exposure changes at different speeds.
thks for highlighting.

 

[emlee]

thank you all for your comments. i did some more tests. to share:
- opened aperture to the fullest - same problem
- used manual focus - same problem
- changed to spot metering - same problem
- tested my shutter speeds from 1/100 to 1/8000 - appears to be ok (reducing brightness)

1 thing is consistent though - the problem. I have resigned to the fate that using this filter will require exposure compensation.
thank you all.

 

[catchlights]

actually, I don't understand what's your problem,

why don't you do the test with and without the filter by shoot a flat wall, this will eliminate the inconsistency results of difference metering mode on certain scene.

if the filter is state 8x, it cuts down three stops, that is for sure.

mostly photographers will compose and meter the scene without filter, select the speed or aperture he want to use, than add three stops to the speed or aperture which is less critical when with the filter attached, of course this is using manual mode.

 

[LittleWolf]

if the filter is state 8x, it cuts down three stops, that is for sure.

It is not uncommon that claimed product specifications do not match real product characteristics - from el cheapo products to high-end laboratory stuff. From my experience, I wouldn't trust anything blindly.

I found some spectral transmission curves online for Schott and Hoya ND filter glass types, and over the visible spectral range alone the transmission varies by almost a whole f-stop. A whole f-stop if you take the extended IR sensitivity of most sensors into account.