A javascript calculator to compare full frame to crop sensor equivalents for focal length, aperture, and ISO.

crop2ff.html

Calculate full frame to cropped equivalent. First order approximations only. Designed as described by <a href="https://northrup.photo/gear-basics/camera-body-features/sensor-size-crop-factor/">Chelsea and Tony Northrup</a>.

<form name="cropCalculator" action="" method="get">
	<table>
    	<tr>
        	<td>Crop factor:</td>
            <td><input type="text" name="cropFactor" value="1.53"></td>
        </tr>
        <tr>
        	<td>Direction:</td>
            <td><input type="radio" name="direction" value="toCrop" checked="true" onchange="changeDirection(this);"> to crop</td>
            <td><input type="radio" name="direction" value="to35mm" onchange="changeDirection(this);"> to full frame</td>
        </tr>
        <tr><td><br></td></tr>
		<tr>
        	<th></th>
            <th>Min</th>
            <th>Max</th>
        </tr>
        <tr>
			<td><span id="cropOrFFLabelfocal">Crop</span> focal length:</td>
            <td><input type="text" name="inputFocalMin" value="20"></td>
            <td><input type="text" name="inputFocalMax" value="70"></td>
        </tr>
        <tr>
			<td><span id="cropOrFFLabelaperture">Crop</span> aperture:</td>
            <td><input type="text" name="inputApMin" value="1.8"></td>
            <td><input type="text" name="inputApMax" value="16"></td>
        </tr>
        <tr>
			<td><span id="cropOrFFLabeliso">Crop</span> ISO noise:</td>
            <td><input type="text" name="inputIsoMin" value="100"></td>
            <td><input type="text" name="inputIsoMax" value="1600"></td>
        </tr>
        <tr><td><i>is equivalent to</i></td></tr>
        <tr>
			<td>Crop Focal Length:</td>
            <td><span id="outputFocalMin"></span></td>
            <td><span id="outputFocalMax"></span></td>
        </tr>
        <tr>
			<td>Crop Aperture:</td>
            <td><span id="outputApMin"></span></td>
            <td><span id="outputApMax"></span></td>
        </tr>
        <tr>
			<td>Crop ISO noise:</td>
            <td><span id="outputIsoMin"></span></td>
            <td><span id="outputIsoMax"></span></td>
        </tr>
	</table>
	<br>
	<input type="button" value="Calculate" onClick="calculateCrop(this.form)">
</form> 

<script>
function changeDirection(radio) {
	var toCrop = radio.value == "toCrop";
    var doc = radio.ownerDocument;
    var typeString = "Full Frame";
    if (toCrop){
	    typeString = "Crop";
    } 
    var allSpans = doc.getElementsByTagName("span");
    
    for(ii=0; ii<allSpans.length; ii++) {
    	if (allSpans[ii].id.includes("cropOrFFLabel")) {
        	allSpans[ii].innerHTML = typeString;
        } 
    }
}

function roundFstop(x) {
    var localError = [1e10, 1e9];
    
    var ii = 0;
    while( (ii<15) && (localError[0]>localError[1]) ) {
    	localError[0] = localError[1];
    	localError[1] = Math.abs(x - Math.pow(1.414, ii));
        
        ii++;
    }
    
    var y = Math.round(Math.pow(1.414, ii-2)*10)*0.1;
    return y.toFixed(1);
}

function roundISO(x) {
	commonISOs = [100, 200, 400, 800, 1600, 3200, 6400];
    
    var ii=0;
    var err = [1e10, 1e9];
    while( (ii<commonISOs.length) && (err[0]>err[1]) ) {
    	err[0] = err[1];
    	err[1] = Math.abs(x - commonISOs(ii));
        
        ii++;
    }
}

function calculateCrop (form) {
    var cf = form.cropFactor.value;
    var toCrop = form.direction.value == "toCrop";
    
	var scalingFactor = cf;
    if(!toCrop){
    	scalingFactor = 1/cf;
    }
    var document = form.ownerDocument;
    
    document.getElementById("outputFocalMin").innerHTML = (form.elements.namedItem("inputFocalMin").value*scalingFactor).toFixed(0);
    document.getElementById("outputFocalMax").innerHTML = (form.elements.namedItem("inputFocalMax").value*scalingFactor).toFixed(0);
    
    document.getElementById("outputApMin").innerHTML = roundFstop(form.elements.namedItem("inputApMin").value*scalingFactor);
    document.getElementById("outputApMax").innerHTML = roundFstop(form.elements.namedItem("inputApMax").value*scalingFactor);
    
    document.getElementById("outputIsoMin").innerHTML = roundFstop(form.elements.namedItem("inputIsoMin").value*scalingFactor*scalingFactor);
    document.getElementById("outputIsoMax").innerHTML = roundFstop(form.elements.namedItem("inputIsoMax").value*scalingFactor*scalingFactor);
    
}

calculateCrop(cropCalculator);
</script>
</p>