Question 1

How is the field of view (FOV) calculated?

Accepted Answer

FOV is calculated as (number of sensor pixels $	imes$ physical pixel size) / objective magnification. For example, a 512×512 sensor with 6.5 µm pixels behind a 16× objective gives a 208 µm $	imes$ 208 µm field of view at the sample plane.

Question 2

What is the Rayleigh resolution limit?

Accepted Answer

The Rayleigh criterion defines the minimum resolvable distance between two point sources: d = 0.61λ/NA, where $\lambda$ is the excitation wavelength and NA is the numerical aperture. At 920 nm with NA 0.8, the Rayleigh limit is approximately 0.70 µm. Pixel size should be at most 2× this value (Nyquist sampling) to fully capture the optical resolution.

Question 3

How do I choose between GCaMP indicators?

Accepted Answer

Choose based on your experimental needs. GCaMP8f has the fastest kinetics (10 ms rise, 80 ms decay) and highest brightness, ideal for fast spiking and dense populations. GCaMP6s has slow kinetics but large signals, suited for detecting sparse activity. GCaMP7f and jGCaMP8m offer intermediate trade-offs. Faster indicators require higher frame rates (Nyquist criterion), which may limit FOV on some systems.

Question 4

What does the Nyquist frame rate mean?

Accepted Answer

The Nyquist frame rate is the minimum acquisition rate needed to faithfully capture the calcium transient rise time: f_Nyquist = 1 / (2 $	imes$ t_rise). For GCaMP8f with a 10 ms rise time, this is 50 Hz. Acquiring below this rate risks aliasing fast transients and underestimating spike counts.

Question 5

How are estimated neuron counts calculated?

Accepted Answer

The calculator multiplies the FOV area (in $	ext{mm}^{2}$) by the regional neuron density (90,000/$	ext{mm}^{2}$ for cortex, 300,000/$	ext{mm}^{2}$ for hippocampus) and a modality-dependent visibility fraction (5% for widefield, 15% for two-photon, 8% for miniscope). These are order-of-magnitude estimates that depend on labeling density, expression level, and optical sectioning.

Question 6

Which objectives work best for two-photon calcium imaging?

Accepted Answer

Water-immersion objectives with high NA (0.8–1.1) and long working distance (2–3 mm) are standard. The Nikon 16x/0.8W offers the best balance of FOV and resolution for cortical imaging. The Nikon 25x/1.1W provides higher resolution at the cost of smaller FOV. For deep imaging, the Olympus 20x/1.0W is widely used.

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