Filters in Photography

A brief discussion about the most basic optical filters in conventional (film) and digital photography.

There's a lot of misconceptions about their effects and finality of use by beginners and even more advanced users.

The goal here is to explain what are the UV blocking, contrast filters for black and white and color correction filters.

Other filters, like polarizers, gradients and other effect filters will not be covered here.

UV FILTER

Let's start by the simplest filter ever, the UV. It looks like a flat glass disk without any obvious feature, but as its name suggests it blocks, or at least attenuates, the amount of ultraviolet light that reaches the film or sensor.

Film is sensitive to UV light and the excess of it can cause a bluish cast and haze on the photo. But why this ?

Let's talk about ultraviolet light. It's the light spectrum with wavelengths between 400nm and 100nm, being 400nm the very edge of the visible spectrum and 100nm a very penetrating and dangerous form of electromagnetic radiation. The Earth's blocks the shorter wavelengths more than the longer ones. 

To keep it simple, please accept that the 100-300nm wavelengths of the sunlight will be totally blocked from space to the altitude of about 7km. Some of the 315nm may reach the lower altitudes (1500-2500m) and in some places even the surface, Longer ones will reach it, depending on the latitude, season, solar activity, pollution, and other factors.

But what causes the bluish cast ? One of the causes is the very small dust particles in suspension. Small particles are able to reflect small wavelengths very efficiently, and thus, why they reflect more blue-violet than red light. That reflection makes the particles itself visible. Blocking the shorter light wavelengths will make the particles less visible, resulting in a clearer image.

Note that the particles also absorb light from what's behind them, and this absorption will not be reduced or influenced by the UV filter.  

Films are sensitive to some degree to UV light and this is also an important issue. Digital cameras are less prone to the problem but in a minor degree, they still are.

Camera sensors usually have a filter to block UV and IR (infrared) light and let pass the visible spectrum without being attenuated. This filter is technically a combination of a high pass and a low pass filters, resulting in a bandpass one.

But the filter cut-off limits aren't like a wall. It's a curve with a certain inclination.

Schneider B+W 010 UV Haze transmission curve
(C) Schneider Kreuznach 

The graphic above is the transmission curve of a typical good quality UV filter. The darker curve is for a multicoated filter. Note the higher transmission because of less loss of light due to reflections.

The visible light window is, for practical information, between 400 and 700 nanometers. At about 800nm note that the filter also starts to block infrared light.

This specific filter blocks 100% of the UV shorter than 350 nm but just 60% at 375 nm. This means that some UV will pass, depending on the wavelength. It's a more sophisticated filter if compared with the standard UV filter.

There are other types of UV filters with slightly different curves:

UV filter family curves
(C) Hoya

But there are lots of makers and filters belonging to the UV family and quality varies a LOT depending on this. Some filters are flawless and others will degrade the image quality.

Keep in mind, for any filter type:

- Good filters aren't cheap but they don't degrade the image sharpness.

- A good antireflection coating is a must. Ignoring this may result in undesired reflections and flare.

- Good UV filters have almost no light absorption.

- There are lots of counterfeit filters on the market., especially fake Hoyas. Be careful with eBay, Amazon and similar sources. If possible always buy from a trustable seller or store.

SKYLIGHT FILTER

First of all, Skylight and UV filters are NOT the same thing. The main difference is the pinkish tint on the Skylight.

The Skylight filter blocks UV light but also attenuate a little the purple-blue part of the spectrum, resulting in a slightly warmer image.

Skylight 1B filter curve
(C) Hoya

Both filters are extremely useful for film photography, especially for mountain/snow/beach areas.

There are different types of filters belonging to this family:

- SKY 1A

- SKY 1B

- Haze 

The difference between them is basically the intensity of the warming they add. Those filters are maybe the most widely used in film photography and have less impact on digital due to the possibility of post-processing.

On digital cameras with auto white balance set, the warm effect may be not present in the final image.

PROTECTION FILTERS

Those are supposed to be neutral filters, with no attenuation of the light spectrum and no color cast, at least on theory.

Again, a good coating is always a good idea and of course, good filters come for a price.

They are excellent for lens protection against impacts and they saved me more than once. I remember some years ago, I was taking pictures at a mountain place and a small pebble came from somewhere and hit exactly the center of my Leica Apo Telyt and cracked a Hoya HMC Skylight filter. I was using a metal lens hood and it didn't make any difference. If the lens was hit, I'm pretty sure that it would be damaged.

It's also easier to clean a filter than a lens and if by some bad luck you scratched it, just buy another one.

In my view, good quality protection (or UV) filters are a must-have. 

FILTERS FOR B&W PHOTOGRAPHY

Black and White FILM photography can be tricky if we're talking about contrast and tones.

It's easy to imagine a scene in plain colors, but not so easy to have a precise idea about the final look on black and white film.

B&W photography is all about contrast and tones and to make things worse, there are color tones that will be rendered in exactly the same way on film, even being quite different colors.


There are four main filter types for B&W photography:

- Yellow
- Orange
- Red
- Green

There are other ones, like blue, lime, but let's talk about the four main ones for now.

Light Spectrum

    
Take a look at the spectrum image above and let's talk about the workhorse of the b&w filters, the yellow one.

Transmission curves for filters (C) Hoya

Y2 = Yellow

YA3 = Orange

R1 = Red

X1 = Green

X0 = Lime

Filters like the yellow, orange and red are lowpass filters. They attenuate higher frequencies and let pass the lower ones with sight or no attenuation.

Take a look at the figure and let's talk about the transmission curve of a yellow (Y2) filter. The cut-off frequency of this filter is about 475nm. It will block all wavelengths below, let's say, 425nm. But the blue is at something like 470nm and will be just partially blocked. Looking at the graph I would guess that the blue light will be reduced by something like 10-15%.

This slight reduction of blue light can result in a slight contrast enhancement between a cloud and the blue sky. The sky will look a bit darker than the clouds.

The orange filter transmission curve "knee" is shifted to a longer wavelength, and will reduce dramatically violet, blue and some green.

And the red filter will cut from green to violet. 


A yellow filter makes colors with shorter wavelengths than yellow to appear darker than the longer wavelengths and all the longer wavelengths will have the same look as without the filter. The orange and red ones follow the same rule.


Green filters have different transmission curves. The Green (X1) is a band-pass filter, with the central point at something like 540 nanometers and it will attenuate any wavelengths in both directions but in a not so steep way.

Green filters make green objects lighter (or brighter if you prefer) than objects of other colors. They're also nice for portraits of people with white skin, giving a more natural tone.

The lime filter is something like a combination of a bandpass and a low pass filter and can be used as a general purpose filter for b&w photography if you plan to photograph people and vegetation.

The BLUE filter does the opposite of what the Red does. It will lighten the colors at the blue side of the spectrum and darken the ones on the red side. Blue filters are high-pass or band-pass filters, depending on the manufacturing process.

Important to mention is the fact all the above-mentioned filters block 100% of the UV light.

FACTORS

All filters blocks some light and this quantity is called FILTER FACTOR. It's expressed in how many times the light will be attenuated. It's simple, if you're using a red filter it will reduce the light to 1/8th or -3 stops. 

Remember to compensate the exposure if you're not using TTL metering or if the filter isn't covering the light sensor of your camera, for example if you're using a Yashica 35 Electro GTN or an Olympus 35 SP.

B&W Filter effect table

The skylight filter is like the Y1 filter, but for color photography.

Do NOT use physical Y/R/O/G filters on digital cameras with Bayer or X-Trans sensors, even if the camera is set to monochrome. This WILL cause interpolation errors during the demosaic process and may give very strange results. In this case, you need to use software-based filters.

But if you have a Sigma camera with the Foveon sensor or a Leica Monochrom and plan to shoot in b&w, you need the filters, like you were using b&w film. By the way, Foveon's pictures shot this way are amazing. 

Original image in color

Yellow filter, slight blue attenuation

Orange filter

Red filter

Green filter

Blue filter, severe yellow to red attenuation

Another set with more green.

Color, no filter

B&N no filter

Yellow

Orange

Red

Green

Blue

FILTERS FOR COLOR PHOTOGRAPHY


As a motto, you really want a Skylight filter attached to your lens for normal photography. But sometimes we need to use a color correction filter.


Color Warming filters

The #81 family cuts off the UV light and attenuates the bluish part of the spectrum in a way that contrary to the Y/O/R filters used in BV, would not totally mess up with the colors. The result will be a "warmer" light in the whole image, looking like the spectrum was shifted to the longest wavelengths. But since it's a subtractive process, this not actually happens.

(C) Hoya

There are three 81 filters in different intensities:

• 81A (1.2x factor, 3400K to 3200k, minus 200K)
• 81B (1.3x factor, 3500k to 3200K, minus 300K)
• 81C (1.4x factor, 3600K to 3200K, minus 400K)

A stronger effect can be obtained using a #85 filter.

(C) Hoya

• 85A (2.0x factor, 5500K to 3400K)
• 85B (1.6x factor, 5500K to 3200K)
• 85C (1.8x factor, 5500K to 3800K)

The #85 series can be used to balance tungsten light film to be used in daylight conditions.

Opposite to the warming filters are the cooling ones. There are two main groups:

The #82 family is the opposite analog of the #81 family (sort off). They cool down the color temperature by absorbing the longer wavelengths.
Like the previously mentioned warming filters, they come in three strengths 82A, 82B, and 82C

They look like pale blue filters. Below, the transmission curves of the #82 family. Note the opposition to the warming filters.

(C) Hoya

The #80 family is mostly used when someone needs to use a daylight balanced film under tungsten light. They also come like 80A, 80B, and 80C. They are deep blue filters.

(C) Hoya

• 82A (1.3x factor, 3000K to 3200K)
• 82B (1.5x factor, 2900K to 3200K)
• 82C (1.6x factor, 2800K to 3200K)
• 80A (4.0x factor, 3200K to 5500K)
• 80B (3.0x factor, 3400K to 5500K)
• 80C (2.0x factor, 3800K to 5500K)

There are many other special purpose filters, like neutral density, gradient, color enhancers and a myriad of special effect ones, like stars, diffusers, soft focus, diffraction, vignetting and more.

NEUTRAL DENSITY FILTERS

Also known as ND filters, they are made to reduce the light by a known factor. Usually, they are marked in a very easy way, just an ND followed by the factor, like ND-2, ND-4, ND-8, and so on.

An ND-2 will cut the light to a half (or 1 point), an ND-4 to a quarter (or 2 points), etc.

So, why would someone need less light? There are so many reasons like the following:

• A camera having a "slow" maximum shutter speed and using high ISO film, like a Konica Hexar Silver with an 800 ISO film in daylight because the maximum shutter speed for this camera is 1/250s
• To allow the use of large apertures in high light conditions
• To allow the use of slow shutter speeds in high light conditions, like to create the "water blur" effect.

ND factors can be very high. There are also variable ND filters, made by the combination of two polarizers. Personally, I don't like the variable ones because sometimes you don't want any polarizing effect at all.

ND filters are useful in both film and digital photography. There are hundreds, even thousands of tutorials about ND filters on the Internet.