Long exposure of urban buildings to eliminate the flow of people and traffic: ND1000’s extremely slow shutter speed creates an ethereal texture in the city

1. The fatal pain point of landmark photography during the day: the bustling tourist flow

When you stand in front of a magnificent classical cathedral or a modern minimalist art museum and want to take a picture of a pure, solemn, monumental, eternal and ethereal architectural blockbuster:

• The picture is densely packed with hundreds of noisy tourists holding parasols and wearing colorful fluorescent jackets, completely blocking the bottom of the building, and there is no artistic beauty at all.
• Even if you rush to shoot at 5 o'clock in the morning, there will still be cleaners and early vehicles in the shot.
• Physical long exposure magic: Through extreme light reduction methods, the shutter speed of 1/500 second during the day is forcibly extended to 3 minutes to 5 minutes (180 seconds ~ 300 seconds)!

2. Mathematical principle of "physical ablation" of human flow on photosensitive media

Why does a long exposure make pedestrians "evaporate and become invisible" out of thin air?

1. Physics of integral accumulation time ratio:

• A single pixel position on a sensor (or film) at which a solid, stationary building pillar continuously pours reflected light towards that pixel for the entire duration of a total exposure of up to 300 seconds;
• A tourist in red walking quickly walked in front of this stone pillar in only 2 seconds!
• The exposure energy left by the tourist in red on this pixel only accounts for the total exposure:
  $$\frac{2 \text{ seconds}}{300 \text{ seconds}} \approx 0.6%$$
• In the entire photo, this 0.6% weak energy cannot even break through the noise floor! The silhouettes of pedestrians were completely washed away, diluted and swallowed up by the constant light of 99.4% of the building that continued to enter, as if humans had never existed here.

3. ND1000 dimmer filter (10-Stop Neutral Density) and filter calculation

1. Physical parameters of ND1000:

• Has super strong optical light reduction capability up to 10 stops of aperture ($2^{10} = 1024$ times);
• The appearance of the lens is as dark as ink, and the light transmittance is only a pitiful 0.1%!

2. Accurate conversion table of exposure time:

• Without installing the filter, the measured normal exposure time is: shutter 1/250s, aperture f/8, ISO 100.
• After screwing on the ND1000 filter:
  $$t_{\text{actual}} = \frac{1}{250 \text{s}} \times 1024 \approx \mathbf{4 \text{ seconds}}$$
• 4 seconds is still too short (tourists standing still leave ghosts of translucent ghosts).
• Stop down to f/13 and ISO down to extended ISO 50:
• Base exposure time reduced to 1/30s;
• After superimposing ND1000, the shutter is extended to: $1/30 \text{s} \times 1024 \approx \mathbf{34 \text{ seconds}}$!
• Superimposed ND64 + ND1000 (a total of 16 stops of light reduction): The exposure time reaches 3 minutes to 5 minutes, reaching the ultimate level of erasure.

4. Actual operational standards and complete elimination of remaining “ghosts”

1. Three major pre-operations before installing the filter (the order is strictly prohibited!):

• Step 1: Before installing the black filter, complete the precise composition through bright field of view;
• Step 2: Accurately focus on the building door, then immediately switch the focus mode from AF to MF to lock the manual focus! (After screwing on the pitch-black ND1000, the viewfinder cannot be seen, and the camera’s autofocus will be blinded);
• Step 3: Use black tape to seal the eyepiece viewfinder hood of the SLR camera! (Prevent strong sunlight from leaking into the reflector compartment of the fuselage from the rear viewfinder and causing mysterious stray light streaks!).

2. Elimination of Occasional Visitors: Double Photoshop Statistical Median Stacking:

• If a tourist stops tying his shoelaces on the steps for 30 seconds, a faint translucent gray shadow will be left on the screen;
• Multiple combined ultimate skills:
• Take two separate 3-minute long exposure photos A and B, 2 minutes apart;
• Import into Photoshop: select two layers and execute Layer $\to$ Smart Object $\to$ Convert to Smart Object;
• Execute: Layer $\to$ Smart Object $\to$ Stack Mode $\to$ Median!
• The algorithm automatically compares each group of pixels and automatically eliminates abnormal motion data that appears at different time points;
• Moment: All the remaining translucent ghost images in the picture were completely erased, and the square appeared as ethereal, pure, majestic and sublime as the first day after the disappearance of human beings.

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