# Questions about ND filter and CPL filter



## shingfan (Jan 22, 2007)

I'm still a newbie...please correct me if my understanding is incorrect...i like constructive comments  

FIRST question is about ND filter...i heard that ND filter does 2 things

1a) reduce exposure, thus allow longer exposure time
1b) reduce DOF?

regard 1a)...the way that ND filter works....is the result the same as using a smaller aperature...does it have affect on the color...or just simply allow longer exposure time?.....if its function is allow longer exposure time....would it be the same as using negaive EC?....would negative EC and ND filter yield the same result?

regard 1b)...how does it change DOF? I'm just interested

SECOND question is about CPL filter....i heard that CPL filter reduce glare

say i'm taking indoor photo with a flash.....normally if i flash on an object directly...reflection is normally produced...creating overexposed spot...or whatever the technical term is....is CPL lense able to remove the reflection on the object....i hope you guys understand my question.....i understand that if i can find a ceiling and bounce the light off the ceiling....i wouldnt have the problem...but in some situation......bouncing is not an option and i woudl like to know how to avoid those hot spots....thanks


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## Big Mike (Jan 22, 2007)

> FIRST question is about ND filter...i heard that ND filter does 2 things





> 1a) reduce exposure, thus allow longer exposure time


Yes, that's correct.


> 1b) reduce DOF?


It will only reduce the DOF if you open up the aperture to compensate for the lower amount of light getting though the lens.

Polarizers reduce glare but I think it only works on things lit by sunlight.  I think it has something to do with the way that the sun's rays are scattered when they hit the atmosphere.  I don't think that a polarizer will help with reflections from your flash.


> i understand that if i can find a ceiling and bounce the light off the ceiling....i wouldnt have the problem...but in some situation......bouncing is not an option and i woudl like to know how to avoid those hot spots....thanks


The idea is to soften the light...which can be accomplished by enlarging the source of the light.  Bouncing is good because now the source of the light hitting the subject is effectively a large portion of the ceiling.  You can also use walls to reflect the light...which give you nice side lighting.  You can even turn the flash around a bounce it off the wall/ceiling behind you.

You can get a diffuser, like a softbox that will fit over your flash...but that doesn't really enlarge the light source all that much.

Another option is to move the flash as far away from the camera/lens as possible.  This will reduce glare and red-eye.  If you put the flash up higher, it will throw the shadows down, which really helps when people are against a wall or something.  This is where a flash bracket comes in handy.


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## shingfan (Jan 22, 2007)

thanks mike.

as always...i'm greedy and like to get to the root...lol 

1a)  if its function is allow longer exposure time....would it be the same as using negaive EC?....would negative EC and ND filter yield the same result?


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## Big Mike (Jan 22, 2007)

> 1a) if its function is allow longer exposure time....would it be the same as using negaive EC?....would negative EC and ND filter yield the same result?


No.

If you use a longer shutter speed, without changing the aperture or ISO...then the images will be darker.

If you put on an ND filter, without changing the shutter, aperture or ISO...then the images will be darker.  However, if you put the filter on the lens...the camera's meter will compensate for the filter.

An ND filter is useful for getting a longer shutter speed...when you can't or don't want to also change the aperture.

Here is an example of when people would use an ND filter.  Let's say I'm shooting a waterfall and I want a long shutter speed to make the water look blurry.  First thing I will do is make sure I have a tripod...then I will set a very small aperture (F22 or smaller) and set a low ISO, 100.  However, if it's somewhat bright outside...the meter might tell me that with the smallest aperture...the shutter speed for proper exposure is 1/2 (half a second).  That's not long enough for me...but I can't make the aperture any smaller and if I make the shutter speed any longer...it will just overexpose the image.  Here is where the ND filter comes in...you put it on the lens...and it block some light...so that at the same small aperture, I will need a longer shutter speed.  A two stop filter will let me use a 2 second shutter speed, without over exposing.  

Does that help?

As you can see, there are not many times that you might use an ND filter...but when you do need it...it's not something that can easily be replicated with Photoshop.


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## shingfan (Jan 22, 2007)

Big Mike said:


> No.
> 
> If you use a longer shutter speed, without changing the aperture or ISO...then the images will be darker.
> 
> ...


 
yes....the images will be darker in this sense with a negative EC...however...since i know that the imagine will be darker by so much...i can switch to M mode....and increase the exposure time to compensate for that......can i?...or the quality will be different?

P.S. i'm using a nikon D80 with exposure compensate on the camera


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## Big Mike (Jan 22, 2007)

> yes....the images will be darker in this sense with a negative EC...however...since i know that the imagine will be darker by so much...i can switch to M mode....and increase the exposure time to compensate for that......can i?...or the quality will be different?


Yes, you could do that.  Keep in mind that when you put on the filter...the camera's meter will change.  So if you are in manual, without the ND filter and the meter is on zero...then you put on the filter, the meter/needle will slide down the scale...and when you increase the shutter speed, the needle will just come back up to zero.  If you are in an auto mode, the camera will do this automatically.  

As for quality...you almost always degrade the image quality when you put more glass in front of the lens...and of course, a longer shutter speed will cause anything that moves to be blurred.


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## shingfan (Jan 22, 2007)

i think i know the answer to my qeustion....EC doesnt allow longer exposure time....it changes only the light meter on the camera to compensate incorrect exposure....lol .....to truly increase the exposure time....i guess a ND filter is still needed....


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## Big Mike (Jan 22, 2007)

I think you are getting it.

If you change the exposure (EC)...then the image will be brighter or darker.  If you increase only the shutter time, then you will get a brighter image (over exposed).
That's not the idea of an ND filter.  An ND filter blocks some of the light so that you can compensate for it with a longer shutter speed...to keep the proper exposure.

If all you wanted to do...is make the image darker, then you could shorten the shutter speed...no need for a filter.



> to truly increase the exposure time....i guess a ND filter is still needed....


Only if you are already at the limits of your aperture and ISO.  If you are shooting at F8 and 1/125...and you want a longer shutter speed...you can just change the aperture to F16 (two stops)...and then the shutter speed will have to be 1/30 to keep the same exposure.


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## rmh159 (Jan 22, 2007)

I have a statement / question along these lines.

Mike gave the example of using the ND filter for a waterfall shot but you can also use it to add motion blur into a well lit scene without overexposing.  

I've also seen pics where ND filters are stacked in order to produce such a long shutter speed that anything moving simply won't show up in the image.  Does anyone know how many stops are required to get that affect?  Since it's dictated largely by the amount of light, for the sake of simplicity let's assume we're going for the affect around noon on a clear, sunny day.


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## Big Mike (Jan 22, 2007)

> I've also seen pics where ND filters are stacked in order to produce such a long shutter speed that anything moving simply won't show up in the image. Does anyone know how many stops are required to get that affect? Since it's dictated largely by the amount of light, for the sake of simplicity let's assume we're going for the affect around noon on a clear, sunny day.


It depends on a couple of things.  The amount of movement by the subject and the camera.  If you are hand holding the camera with a shutter speed of say, one second...then you won't get very much sharpness at all.  If you want a moving subject to be blurred...it depends on how much they are moving and how long the shutter is.  To know just how much...you will have to experiment.

To get a long shutter speed you can close down the aperture and keep a low ISO...but that may not be enough.  So, as you mentioned, in bright light, you could use an ND filter.

Or, if you wanted to keep a wide aperture for a shallow DOF...but also wanted a longer shutter speed...then you could use an ND filter.


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## shingfan (Jan 22, 2007)

rmh159: i think the answer is how long would you need the shutter to be....you just keep stacking ND filter until you reach the desired shutter speed i think

and yes....ND filter can be applied to a lot of areas relating to increasing exposure time.....at times when you need longer exposure and run out of method (aperature already as small as it can get...ISO already as slow as it can get).....that is when ND filters kick in


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## rmh159 (Jan 22, 2007)

shingfan said:


> rmh159: i think the answer is how long would you need the shutter to be....you just keep stacking ND filter until you reach the desired shutter speed i think...


 
Yeah that's what I was getting at... what shutter speed would you need to remove moving objects?  For example if you're shooting a building in where people would be walking by... what shutter speed would remove them (for the most part).  I know some people might be standing still or whatever so I'm not expecting a definite answer but thought maybe someone could take a decent guess.


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## Big Mike (Jan 22, 2007)

> I know some people might be standing still or whatever so I'm not expecting a definite answer but thought maybe someone could take a decent guess


I would say that you would want at least a couple of minutes in that situation...but it completely depends on how long the people are standing in one spot and how much they contrast with their background.  If you shoot a building with just people walking by...then it may not take all that long to get a shot with no people visible...but if someone in a bright white shirt comes and stands there for a few seconds...that will really mess up the effect.


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