ND filter


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Jun 29, 2003
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#1
Can I ask, what's the use of a ND filter? when I have a circular polariser. I know the ND filter is suppose to stop down the exposure so that u get silkier water etc. but won't the polariser do the same?
 

kensh09

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Feb 1, 2005
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#2
alexljy said:
Can I ask, what's the use of a ND filter? when I have a circular polariser. I know the ND filter is suppose to stop down the exposure so that u get silkier water etc. but won't the polariser do the same?
The silker water part u shld be using polariser filter instead.
The main purpose of using neutral density (i.e., ND) filters is to reduce the amount of light that can pass through the lens. As a result, if a shutter speed is kept the same, after adding a neutral density filter, a larger aperture must be used to obtain the same exposure. Similarly, if an aperture is kept the same, after adding a neutral density filter, a slower shutter speed must be used to obtain the same exposure.
 

tunster

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#3
... What actually a polariser for is to throw away stray lights. Which normally gives a better colour for sky and cut off reflections. However, since stray lights are thrown, the stop down effect is there. Just that it is lower value than ND. Normally 2-stops +-.

Of course you can achieve the silky effect with polarising filter, just take note that an ND can give a longer exposure which you might achieve a silkier effect. And sometime when the sun is too bright, the polarising filter won't help much.
 

Pro Image

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Oct 31, 2003
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#4
Please do not forget there is also Gradual ND filters as well which you can control the sky exposures and the land exposures.
 

student

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Jul 26, 2004
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#7
kensh09 said:
The silker water part u shld be using polariser filter instead.
I believe you are a little mistaken.

The purpose of the polariser is to "polarise" light, which in the case of water, to reduce reflection.

The "silky" or "milky" effect is a result of slow shutter speed. The slower the more "milky" the water. A ND filter reduces the light, and therefore allows one to use a slower speed.

Perhaps you have been confused because a polariser, used to its maximun effect, do reduce light by 2 stops.
 

reachme2003

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Oct 6, 2003
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#8
hi, i was asked this question recently, but because of time and place constraints, i did not explain it as well as student did. thanks. if you are reading this, pls note.

what is essential is to ask oneself the intent of using one or the other.
 

Jul 23, 2005
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#9
kensh09 said:
The silker water part u shld be using polariser filter instead.
The main purpose of using neutral density (i.e., ND) filters is to reduce the amount of light that can pass through the lens. As a result, if a shutter speed is kept the same, after adding a neutral density filter, a larger aperture must be used to obtain the same exposure. Similarly, if an aperture is kept the same, after adding a neutral density filter, a slower shutter speed must be used to obtain the same exposure.
Let's say I wanna shoot a waterfall with the 'silky water' effect. I want everything sharp so DOF is max, hence I can live with small aperture.

My question is, would ND filter vs use a long shutter time + tiny aperture produce the same effect in this case?
 

catchlights

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Sep 27, 2004
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#10
ConnorMcLeod said:
Let's say I wanna shoot a waterfall with the 'silky water' effect. I want everything sharp so DOF is max, hence I can live with small aperture.

My question is, would ND filter vs use a long shutter time + tiny aperture produce the same effect in this case?
The 'silky water' effect actually is a motion blur of the water, when ND filter is needed, it means you can't get a shutter speed long enough.

Lets say you need at least 15 sec to get the 'silky water' effect, if you can stop down the aperture to get shutter slow enough, ND filter is not needed. But if you set the minimum aperture and still getting only 2sec for shutter speed, than you need a ND filter to help you cut down 3 stops of light.
 

student

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Jul 26, 2004
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#11
ConnorMcLeod said:
Let's say I wanna shoot a waterfall with the 'silky water' effect. I want everything sharp so DOF is max, hence I can live with small aperture.

My question is, would ND filter vs use a long shutter time + tiny aperture produce the same effect in this case?
The issue is not ND versus long shutter speed.

The issue is "how long you want the shuuter speed to be, with regards to the effects you want".

Let us say I am using film, and I shoot at ASA 25 (I think no DSLR can shoot at that ASA) in the evening in a waterfall, and the light level is very low, I am quite sure that I will get very long shutter speed, especially when my aperture is, say, f22. Assuming the shutter speed is now 1 sec, I can get a nice motion blur from the flowing water, but still recognisable as flowing water. However if I want the water to be nothing more than a blur white sheet over the rocks, where the water does not even look like water, more like a white mist, then I may have to reduce the light even more, say from one second to one minute. So I use a ND to reduce the light by say 6 stops (assuming no reciprocity failure).

Hope I have not confused you!
 

Jul 23, 2005
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#12
Student & catchlights:

Thanks for clarifying the issue that ND is useful when a particular aperture is needed for the desired effect, and also on occasions which ND is needed with small apertures.
Cheers Mates!
 

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