A filter is a piece of glass or resin placed in front of the lens to change the light before it reaches the sensor. In the days of film, photographers carried many colored filters. Today, editing software can copy many of those effects. But some filters still do things software cannot, and serious landscape, architecture and video work often depends on them. This lesson gives you the big picture: what filters are for, the main families, and how filters attach to lenses. The following lessons cover each family in detail.
What is a photography filter?
A photography filter is a transparent piece of glass or resin placed in front of the lens (or sometimes inside or behind it) that changes the light entering the camera. Some filters reduce the amount of light, some block certain reflections, some soften the image, and some add color or simply protect the lens.
Why use filters in the digital age?
Editing software can change color, brightness and contrast easily. So the key question for any filter is: does it change something the sensor records that cannot be recreated afterwards?
| Filter effect | Can editing copy it? | Why |
|---|---|---|
| Removing reflections from water or glass | No | The reflected light was recorded; the detail behind it is hidden |
| Allowing a long exposure in daylight | No | Motion blur from a long shutter speed must be captured at the time |
| Holding back a bright sky in one exposure | Partly | Editing can darken it, but clipped highlights cannot be recovered |
| Adding a warm color cast | Yes, easily | Color can be adjusted in editing, especially with RAW |
| Soft glow around highlights | Partly | Software can imitate it, but optical diffusion looks different |
This is why the most important filters today are the ones that change light in ways editing cannot: polarizers and neutral-density filters above all.
The major filter families
| Family | What it does | Lesson |
|---|---|---|
| Protection / clear (UV) | Protects the front of the lens; has little visible effect on digital images | protection and clear filters |
| Polarizing (CPL) | Reduces reflections and glare; can deepen blue skies | polarizing filters |
| Neutral density (ND) | Darkens the whole image evenly, allowing slower shutter speeds or wider apertures | neutral-density filters |
| Graduated ND and variable ND | Darken part of the image (usually the sky), or offer adjustable darkening | graduated and variable ND filters |
| Diffusion and creative | Soften, add glow, stars or special effects | diffusion and creative filters |
Remember:Colored filters, once essential for black-and-white film (Ansel Adams used a red filter to darken skies), are rarely needed for digital color photography because color is adjusted in editing. Some photographers still use them for specific looks.
Screw-in and square filter systems
Filters attach to lenses in two main ways:
| Screw-in (circular) filters | Square / rectangular filter systems | |
|---|---|---|
| How they attach | Screw directly into the thread on the front of the lens | Slide into a holder that clips onto an adapter ring on the lens |
| Best for | Protection, polarizers, solid ND and variable ND filters | Graduated ND filters you need to slide up and down, and stacking several filters |
| Using with several lenses | One filter per thread size, or step-up rings | One set of filters; buy an adapter ring for each lens |
| Size and cost | Small, simple and usually cheaper | Bulkier and usually more expensive |
Every lens with a front thread has a filter size printed on it or on its cap, such as ⌀67mm, as you learned in how camera lenses work. A step-up ring lets a larger filter fit a smaller lens, so many photographers buy filters for their largest lens and use rings for the others.
Warning:Some ultra-wide and fisheye lenses have a curved front element and no filter thread. They need special large holders, or small filters that fit at the back of the lens. Check before buying.
Filter quality matters
Everything the camera records passes through the filter, so a poor filter can undo the quality of a good lens. Cheap filters may reduce sharpness, add flare or create unwanted color casts. Look for good-quality glass and multi-coating, which reduces reflections. Thin ("slim") frames reduce dark corners on wide lenses, and stacking several filters increases the risk of vignetting and flare, as you saw in optical limitations.
Common mistakes
- Buying filters that only do what editing can do better: start with a polarizer and an ND if you need them.
- Buying a filter for each lens size: buy for the largest thread and use step-up rings.
- Stacking many filters on a wide lens: dark corners and flare.
- Saving money on cheap glass: a poor filter lowers the quality of every photo.
How professionals use filters
Landscape photographers often carry a polarizer, a few ND filters and graduated NDs in a square system. Architecture photographers use polarizers to control reflections in glass. Filmmakers rely heavily on ND filters, because video needs particular shutter speeds even in bright light, and many use diffusion filters for a softer look. Most professionals buy a small number of high-quality filters rather than many cheap ones.
Practical examples
A lake with reflections
- Situation
- Glare on the water hides the stones beneath the surface.
- What to do
- You use a polarizing filter and rotate it until the glare disappears.
- Why it works
- Reflections hide detail that editing cannot recover; only a polarizer can remove them at capture.
- Result
- The stones under the water become clearly visible.
Three lenses, one set of filters
- Situation
- Your lenses have 58mm, 67mm and 77mm filter threads.
- What to do
- You buy 77mm filters and step-up rings for the 58mm and 67mm lenses.
- Why it works
- Step-up rings let the larger filters fit the smaller lenses.
- Result
- One set of filters works on all three lenses.
Key points
- A filter changes the light before it reaches the sensor.
- Today's most important filters do what editing cannot: remove reflections (polarizer) and allow long exposures (ND).
- Main families: protection, polarizing, ND, graduated and variable ND, diffusion and creative.
- Screw-in filters suit polarizers and solid NDs; square systems suit graduated filters and stacking.
- Buy good-quality filters for your largest lens thread and use step-up rings.
Frequently asked questions
Do I need filters for digital photography?
What is the most useful filter?
Should I buy screw-in or square filters?
How do I know my filter size?
Conclusion
Filters still matter when they change the light in ways editing cannot. Learn the main families, choose the right attachment system, and invest in quality rather than quantity. Next, you will look at the simplest filters: protection and clear filters.
Finished reading? Track your progress through Lenses and filters.