A practical guide to calculating exposure and handling color cast

Long-exposure photography unlocks a world of creative possibilities—silky waterfalls, dreamy clouds, streaking traffic lights, and serene seascapes. Neutral Density (ND) filters make these effects possible in daylight by reducing the amount of light entering the lens, allowing for much longer shutter speeds without overexposure.
But to use them well, you need two key skills: accurately calculating exposure and managing color cast.

This guide walks you through both, using proven field techniques that keep the process simple and repeatable.

1. Why ND Filters Matter in Long Exposures

ND filters act like sunglasses for your lens. Their purpose:

• Reduce light by a known number of stops.

• Allow slow shutter speeds even in bright conditions.

• Enable creative effects otherwise impossible in daylight.

Common ND strengths (in stops) include:

• 3-stop (ND8) – Mild blur for soft water in bright shade.

• 6-stop (ND64) – Strong blur for waterfalls and moving crowds.

• 10-stop (ND1000) – Extreme smoothing, ideal for oceans and clouds.

• 15-stop (ND32000) – Super-long exposures (minutes).

Understanding these strengths is the foundation for correct exposure calculations.

2. Step-by-Step: How to Calculate Correct Exposure with ND Filters

Step 1 — Meter Without the Filter

Compose, focus, then set your base exposure without the ND attached.
Example: Your camera meters:

• 1/60s, f/11, ISO 100

Lock in aperture and ISO. Only shutter speed will change after adding the filter.

Step 2 — Apply the ND Filter’s Stop Value

Each full stop doubles the required exposure time.
Use a simple rule of thumb:

New shutter speed = Base shutter speed × 2^(ND stops)

Let’s work the example:

Using a 10-stop filter

• Multiply 1/60s by 2¹⁰ (which is 1024)

• New shutter ≈ 17 seconds

A quick reference example using the same base exposure:

Step 3 — Use a Calculation Shortcut (Simplified Method)

Instead of math, you can count stops directly:

Starting at 1/60s → each stop:

• 1 stop: 1/30

• 2 stops: 1/15

• 3 stops: 1/8

• 4 stops: 1/4

• 5 stops: 1/2

• 6 stops: 1"

• 7 stops: 2"

• 8 stops: 4"

• 9 stops: 8"

• 10 stops: 16" → round to 15–20 seconds

This method is how most landscape photographers work in the field.

Step 4 — Compensate for Extremely Long Exposures

For exposures longer than 30 seconds, you’ll enter Bulb Mode.
Two considerations arise:

1. Reciprocity Failure (mostly film, some ND resins)

   • Not an issue for digital sensors.

   • Can matter for some very strong resin filters.

2. Sensor Heating

   • Expect increased noise in multi-minute exposures.

   • Enable long-exposure NR if time allows.

3. Practical Tips to Improve Accuracy

Use a dedicated long-exposure calculator

Mobile apps (e.g., PhotoPills, NDTimer) prevent errors and include compensation for stacked filters. I developed one of my own for Android phones, which you are free to download (see link below).

Always disable image stabilization on a tripod

IBIS or lens IS can introduce blur in a locked-down setup.

Shield your viewfinder

Required for DSLRs using long exposures.
Stray light entering the eyepiece can affect metering and the image.

Refocus before attaching the filter

ND filters—especially 10-stop or higher—are too dark for the camera to autofocus.

4. Understanding and Fixing Color Cast in ND Filters

Even “neutral” density filters may introduce a color shift, especially:

• Traditional resin filters

• Very strong NDs (10–15 stops)

• Stacked filters of different brands

Common casts:

• Magenta/purple

• Blue

• Warm/brown

• Green tint (cheaper filters)

Why Color Cast Happens

• Uneven spectral transmission

• Infrared/visible light imbalance in very dense filters

• Coating differences between brands

5. Techniques to Minimize or Remove Color Cast

Method 1 — Set a Custom White Balance

Before shooting:

1. Place the ND filter on the lens.

2. Photograph a neutral gray card in the same light.

3. Create custom WB from that capture.

Best for controlled environments or repeated use of the same filter.

Method 2 — Correct in RAW Processing

Shooting RAW is essential.
In Lightroom, Camera RAW or Capture One:

• Adjust Temp and Tint sliders.

• Use the White Balance eyedropper on a neutral object.

• Apply a preset for consistency across a series.

Method 3 — Use High-Quality ND Filters

Premium glass filters often have near-zero color shift:

• Breakthrough Photography “Dark Circular Polarizer + ND”

• NiSi Nano IR series

• Hoya PROND

• Lee ProGlass

• Freewell Pro

Higher-quality coatings reduce IR contamination, the usual cause of magenta cast in 10-stop filters.

Method 4 — Avoid Mixing Different Brands

Stacking filters with mismatched coatings almost guarantees uneven color.

Method 5 — Apply IR-cut or UV/IR filters for extreme exposures

Useful when using 15-stop filters or stacking large densities. Helps reduce magenta tint.

6. A Reliable Workflow for Perfect Long Exposures

Here’s a simple, repeatable field workflow:

1. Compose & focus without the filter

2. Meter and note the base shutter speed

3. Calculate long exposure based on ND strength

4. Attach ND filter and switch to manual focus

5. Enter calculated shutter speed / Bulb mode

6. Take a test shot

7. Fine-tune: adjust time ±10–20% if needed

8. Check for color cast and correct in post

9. Repeat for optimal cloud/water movement

This process keeps exposure predictable and creative decisions in your control.

Final Thoughts

Mastering long-exposure photography with ND filters is about consistency, not guesswork. When you understand how to calculate exposure and how to deal with color cast, ND filters become powerful creative tools that produce dramatic, ethereal images. With a solid workflow and a little practice, you can transform ordinary scenes into stunning long-exposure photographs that stand out with clarity, atmosphere, and motion.