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<!doctype html>
<html lang="en">
<head>
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<title>NPF/Shutter Time Calculator</title>
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<meta name="description"
content="This shutter time calculator has: The Plate Scale calculation, NPF Calculation, simplified NPF Calculation, the 500 rule and the 4 crop rule.">
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<div class="div-to-align">
<h1>NPF/Shutter Time Calculator</h1>
<p>
This NPF & shutter time calculator uses 5 different techniques of calculating your shutter time.
Most people are used to the default "500" rule, but this calculator also has: The Plate Scale calculation,
the NPF rule, the simplified NPF rule and the 4 crop rule.
</p>
<p>
Below the calculator you will find the explanation behind every way of calculating your shutter speed, so
you can pick the one that suits you the most.
</p>
<form onkeyup="calculateNPF()" id="NPF-calc" class="form-horizontal">
<fieldset>
<div class="form-group">
<label class="control-label" for="sensorSize"><b>Sensor Size</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Sensor size will determine the crop factor of your images.</p>
<ul>
<li><b>Medium Format</b>: Crop factor of 0.6x</li>
<li><b>Full Frame</b>: Crop factor of 1x</li>
<li><b>APC</b>: Crop factor of 1.5x</li>
<li><b>APC (Canon)</b>: Crop factor of 1.6x</li>
<li><b>3/4</b>: Crop factor of 2x</li>
<li><b>1 Inch</b>: Crop factor of 2.7x</li>
</ul>
<p>If you do not know your crop factor go to <a target="_blank"
href="https://www.digicamdb.com/">digicamdb.com</a> find your camera, and search for "crop
factor"</p>
</div>
<div>
<select onchange="calculateNPF()" id="sensorSize" name="sensorSize" class="form-control">
<option value="0.6">Medium Format</option>
<option value="1">Full Frame</option>
<option value="1.5">APC</option>
<option value="1.6">APC (Canon)</option>
<option value="2">3/4</option>
<option value="2.7">1 Inch</option>
</select>
</div>
</div>
<div class="form-group">
<label class="control-label" for="aperture"><b>Lens Aperture (F-ratio/F-stop)</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Aperture of the lens you are shooting with, this is usually found on the lens itself or the
interface, indicated with an F value (example F/1.8) </p>
</div>
<div>
<input id="aperture" name="aperture" type="text" placeholder="1.8" value="1.8"
class="form-control input-md">
</div>
</div>
<div class="form-group">
<label class="control-label" for="focalLength"><b>Focal Length (mm)</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Focal length of the lens, can be found on the lens/focus rinng, indicated in mm</p>
</div>
<div>
<input id="focalLength" name="focalLength" type="text" placeholder="50" value="50"
class="form-control input-md">
</div>
</div>
<div class="form-group">
<label class="control-label" for="pixelSize"><b>Pixel Size/Pitch (µm)</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Pixel size/pitch of an individual pixel on the camera sensor, you can find the pixel size/pitch
on <a target="_blank" href="https://www.digicamdb.com/">digicamdb.com</a>.</p>
</div>
<div>
<input id="pixelSize" name="pixelSize" type="text" placeholder="7" value="7"
class="form-control input-md">
</div>
</div>
<div class="form-group">
<label class="control-label" for="declination"><b>Declination (Δ°)</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Declination is the angular distance in degrees measured from the celestial equator along the
meridian through the star. If you are not sure what the declination is, set this to 0 or find your
target on <a target="_blank" href="https://stellarium-web.org/">stellarium-web.org</a></p>
</div>
<div>
<input id="declination" name="declination" type="text" placeholder="0" value="0"
class="form-control input-md">
</div>
</div>
<div class="form-group">
<label class="control-label" for="accuracy"><b>Accuracy*</b></label>
<button type="button" class="collapsible">?</button>
<div class="content">
<p>Accuracy will be used to calculate the NPF rule, you can select the following:</p>
</p>
<ul>
<li><b>Pin Point</b>: Stars will look like points (very short exposures)</li>
<li><b>Slight trailing</b>: Stars will have some slight trailing (longer exposures but less
accurate)</li>
<li><b>Major trailing</b>: Stars will have major trailing (very long exposures, very inaccurate,
not recommended)</li>
</ul>
<img src="img/NPF_Calculator_Accuracy.png" class="img-fluid" title="NPF Calculator Accuracy Example" alt="NPF Calculator Accuracy Example"></a>
</div>
<div>
<select onchange="calculateNPF()" id="accuracy" name="accuracy" class="form-control">
<option value="1">Pin Point</option>
<option value="2">Slight trailing</option>
<option value="3">Major trailing</option>
</select>
</div>
</div>
</fieldset>
</form>
<p>*Only used for the <b>NPF & 500/300/200 Rule Calculator</b></p>
<div id="err"></div>
<h2>Calculated Exposure times:</h2>
<div class="row w-100">
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2>Plate Scale</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="plateResult">0s</p>
</div>
</div>
</div>
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2>Plate Scale**</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="noEqplateResult">0s</p>
</div>
</div>
</div>
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2>NPF</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="result">0s</p>
</div>
</div>
</div>
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2>Simplified NPF</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="simplifiedResult">0s</p>
</div>
</div>
</div>
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2 id="numberRule">500 rule</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="numberResult">0s</p>
</div>
</div>
</div>
<div class="col-md-6">
<div class="card border mx-sm-1 p-3">
<div class="text text-center mt-3">
<h2>4 crop rule</h2>
</div>
<div class="text text-center mt-2">
<p class="resultTime" id="fourcropResult">0s</p>
</div>
</div>
</div>
</div>
<h2>Plate Scale Calculation</h2>
<code>Shutter Speed = (206265 * Pixel Size in mm / Focal Length) / 15</code>
<p>The more technical equations for keeping stars round. Stars on the celestial equator move 15 arc-seconds
per time second. So compute the angular size of a pixel to determine the star drift rate. The pixel pitch
is called the Plate Scale.
</p>
<p>This calculation will result in stars having exactly 1 pixel smear.</p>
<p>** For stars not on the celestial equator, the exposure time will be divided by the cosine of the
declination to get the same drift amount in pixels:
</br><code>Shutter Speed = ((206265 * Pixel Size in mm / Focal Length) / 15) / cos(Declination)</code>
</p>
<p class="source">Clark, R. N. (2012, October 20). Nightscape Photography with Digital Cameras.
Clarkvision.com. Retrieved
August 12, 2022, from <a
href="https://clarkvision.com/articles/nightscapes/">https://clarkvision.com/articles/nightscapes/</a>
</p>
<h2>NPF Rule Calculator</h2>
<code>Shutter Speed = Accuracy * (16.856 * Lens Aperture + 0.0997 * Focal Length + 13.713 * Pixel Size) / (Focal Length * cos(Declination))</code>
<p>The NFP rule is a rule that makes it (fairly) easy to calculate the maximum exposure time to take a photo
of a starry sky without the stars trailing too much. This rule replaces the old “rule of 500” which gives
results that are too uncertain.</p>
<p class="source">Michaud, F. (2020, December 1). La Règle NPF – Société Astronomique du Havre. sahavre.fr.
Retrieved August 12, 2022, from <a
href="https://sahavre.fr/wp/regle-npf-rule/">https://sahavre.fr/wp/regle-npf-rule/</a></p>
<h2>Simplified NPF Rule Calculator</h2>
<code>Shutter Speed = (35 * Lens Aperture + 30 * Pixel Size) / Focal Length</code>
<p>A Simplified version of the original NPF rule that uses a rough formula which uses a declination of 60° instead of 0° and does not take into account the (small) effect of the seeing. </p>
<p class="source">Michaud, F. (2020, December 1). La Règle NPF – Société Astronomique du Havre. sahavre.fr.
Retrieved August 12, 2022, from <a
href="https://sahavre.fr/wp/regle-npf-rule/">https://sahavre.fr/wp/regle-npf-rule/</a></p>
<h2>The 500/300/200 Rule Calculation</h2>
<code>Shutter Speed = 500 / (Sensor Crop Factor * Focal Length) </code>
<p>The 500/300/200 rule is a measurement used to determine the maximum exposure time that can be used in a
photograph before star trails appear or before the stars become blurry. If you set the shutter speed for a
longer amount of time than what is permitted by this rule, the photographs you take will not have sharp
stars.</p>
<ul>
<li>200 Rule: Pin Point Accuracy</li>
<li>300 Rule: Slight Trailing</li>
<li>500 Rule: Major trailing</li>
</ul>
<h2>The 4 Crop Rule Calculation</h2>
<code>Shutter Speed = (4 - Sensor Crop Factor) * 100 / Focal Length</code>
<p>The simplest rule (except for the 500 rule) that is less precise then the NPF rule, but much more precise
then the 500 rule.</p>
<p class="source">Michaud, F. (2020, September 18). “4-crop” rule instead of rule of 500 for pin point
stars. Cloudy Nights. Retrieved August 12, 2022, from
<a
href="https://www.cloudynights.com/topic/729753-4-crop-rule-instead-of-rule-of-500-for-pin-point-stars/">https://www.cloudynights.com/topic/729753-4-crop-rule-instead-of-rule-of-500-for-pin-point-stars/</a>
</p>
</div>
</div>
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