Using Filters

[GallantFoto]

I use a Nikon D7100 and recently purchased different neutral density filters along with a circular neutral density filter. The problem I have is my photos are always too dark. I realize that is the purpose of the filter is to reduce light coming into the camera. To get the correct brightness do I increase the ISO or slow down the shutter speed.

Regards,
JP

 

[astroNikon]

You can increase your ISO to keep the same Shutter speed for the type of shot you are doing.
You can also increase the aperture (if you want to change the DOF).

Of course, if you are using a ND to slow down shutter speed then you would slow down shutter speed to increase the total amount of light gathered. Such as taking photos of waterfalls where you want the water to be all silky white.

You didn't mention which NDs you have. Do you have a 1x (1 stop), 2x, 6x, 10x ?
The manufacturers have various ways of identifying them ... 1x or 0.3 or ND2
Neutral density filter - Wikipedia, the free encyclopedia

The key is with a ND filter you are affecting how much light is coming to the sensor.
Thus you have to compensate by ISO, Shutter, or Aperture dependent upon the type of photo you are taking.

 

[GallantFoto]

Thanks for that information. It's what I thought but wasn't sure. I currently have a 2 and 3 stop neutral density filter and hoping to get a ten stop filter soon.

 

[astroNikon]

Dependent upon what you are doing you also may want to look into Graduated Neutral density filters.
Graduated neutral density filter - Wikipedia, the free encyclopedia

There's good threads on here about their use and equipment.

 

[AlanKlein]

Why do you want to use ND filters? They serve no purpose if all you're going to do is increase the ISO as that will just add noise to the shot. Many people use these filters to slow down the shutter speed so water movement is blurred. You leave the ISO the same but just slow the shutter to compensate for the darkening of the filter.



The other thing I don't understand is why your photos are dark with the filter. The camera exposure system looks through the lens and the ND filter so it will set the exposure automatically for the correct openings. If you're shooting in manual, the exposure system will show that the setting is off. So why are you getting dark exposures?

 

[GallantFoto]

Thanks Alan, I guess it was a 2 part question I have. You answered my first one - Many people use these filters to slow down the shutter speed so water movement is blurred. You leave the ISO the same but just slow the shutter to compensate for the darkening of the filter.

The second part of my question was that the photo is too dark when I try to use a circular polarizing filter. My mistake I put neutral density filter previously. Do I also lower the shutter speed for this as well?

Thanks
JP

 

[ronlane]

Thanks Alan, I guess it was a 2 part question I have. You answered my first one - Many people use these filters to slow down the shutter speed so water movement is blurred. You leave the ISO the same but just slow the shutter to compensate for the darkening of the filter.

The second part of my question was that the photo is too dark when I try to use a circular polarizing filter. My mistake I put neutral density filter previously. Do I also lower the shutter speed for this as well?

Thanks
JP

I would say yes, you would because that is additional stop(s) of light that aren't getting to the sensor. To get correct exposure, you have to compensate at either the shutter speed, ISO or aperture. At some point there, I would figure that you may need a tripod to steady the camera.

 

[TCampbell]

The point of a "neutral density" filter is to change your shooting circumstances to allow you to use a different range of exposure settings than would be possible without it.

Examples:

Suppose it's a bright sunny day (Sunny 16 Exposure where even at ISO 100 and f/16 you need a shutter speed of 1/100th to correctly expose the shot) and you'd like to shoot at a slower speed. This would be useful if you needed to show some deliberately motion blur (waterfall shots are classic for this). By cutting the light (using the ND filter) you can decrease the shutter speed.

Or suppose you want to use a lower focal ratio to decrease the depth of field. You could always just increase the shutter speed and decrease the focal ratio... but if you also wanted to use flash (fill light) then you'd be constrained to the max flash sync speed. That puts an upper boundary on the shutter speed. An ND filter would allow you to decrease the focal ratio without exceeding a flash sync speed and also without needing to use "high speed sync".

The idea here is not to make the image "darker" -- you want the image to have the same amount of light as it would have without the filter. The idea is that you can get that "same amount of light" in the exposure while either slowing the shutter or opening the aperture by the same number of stops that the ND filter reduces.

The filters are often indicated in density values where each "0.1" density value is equal to 1/3rd of a stop. So a "0.3" would be equal to 1 full stop. A 0.6 would be 2 stops. A 0.9 would be 3 stops. A 3.0 would be 10 full stops.


You mentioned "circular neutral density" filter and I'm thinking you mean to say "circular polarizer". A circular polarizer's purpose is to cut reflections. Any time light reflects off a shiny surface, it gets polarized. The "polarizing" filter allows light to selectively either pass or get blocked depending on whether the angle of polarization on the light matches the angle tuned by the polarizing filter. You rotate the filter while looking through the camera and you'll notice that the color a blue sky will get deeper blue. Grasses and foliage will appear to get "greener". What's really happening is the surface of the leaves is a bit waxy and that causes a bit of a glare - it's technically a reflection. The polarizer reduces or blocks that reflection so you can see what's "behind the reflection" (which, in the case of green foliage, would be more "green".)

But you can use a polarizing filter anywhere you want to cut reflections. Imaging you want to take a photo of someone inside a coffee house... while you are outside the coffee house shooting through the window. The window would normally produce the reflections of the street behind you. But the polarizer will allow you to cut the reflections to see through the glass. You can use some judgement here as sometimes eliminating 100% of the reflection doesn't look entirely realistic... so you can tune the filter to block most of the reflection but leave in just enough hints of it that it's clear to a viewer that it's "glass" but no so much that it's a distraction.

Polarizing works best when the original light source is orthogonal to the camera (coming from the side, etc.) If the is originating form a source nearly straight ahead, or reflecting from a source nearly directly behind, then it will have less of an effect (in fact it may so little that you don't notice any effect.) Also very wide angle lenses have problems with polarizers due to their extreme wide angles of view. The amount of polarizing depends on the angle of the light as it enters. Normal and Telephoto lenses have narrow enough angles of view that you get a uniform effect across the frame. But very wide lenses get "banding" issues (a dark band which fades to weaker levels of polarizing).

There are "circular" polarizers and "linear" polarizers (a linear polarizer is also sometimes called a top polarizer). The polarization itself is identical EXCEPT cameras that use phase-detect auto-focus and many metering systems are fooled by polarized light. To stop that from happening (fooling your AF system and metering system) a 2nd layer is added to the filter -- it's a quarter-wave plate that alters the polarity of the light AFTER it's already passed through the front polarization layer. This image is the same, but it doesn't confuse the AF and metering system.

A polarizer has the side-effect of blocking some light. How much light they block will depend on how they're tuned and the polarity of the inbound light... but it often works out to be somewhere near 2 stops (usually more than 1 stop).

There is also something called a "variable ND" filter. It's basically a linear polarizer stacked in front of a circular polarizer. When you have two polarizers in a row, you can rotate them to decide how much light you want to cut. It's not as good as just using a simple neutral density because you can get some bizarre effects on stacked polarizers -- people talk about a dark area roughly "x" shaped in the image. This would especially be an issue when using wide-angle lenses because amount of polarizing depends on the angle of the light and a very wide angle lens can have such a broad range of angles that you can't get uniform polarization.

 

[petrochemist]

A very full explanation of ND & polarizing filters, Tim. Though it's not just for flash that faster shutter speeds hit a limit. Shooting in sunshine at (f2 or wider) will often require speeds faster than many cameras can manage even at base ISO.

One extra point, if the OP's images are too dark, it's probably down to exposure compensation, rather than the filter. On most cameras this is set up to be very easy to adjust. Which can mean the control gets knocked by mistake. (I know I've done it many times on multiple cameras and lost the shot while resetting the control)