What Focal Lengths do I take photos at?

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I’ve been working with PowerShell for sometime and I have recently been encouraged to share some of the work that I’ve been doing.  So I thought I’d start out with a fun one. 

I have a SLR camera and I’m considering buying a new Lens, but what I’d like to know before I go and purchase one is: What Focal Lengths do I take my Photos at?  More importantly what focal length do I take most of my photos at; so I can focus my money on a lens that I’m going to use a lot!  Here’s what I came up with:

What the script will do:

  • Find all the files in a directory structure with a given file extension
  • Load each image and gather up it’s focal length and the type of camera that took it
  • If the camera type matches what we are looking for, add the focal length to the running score
  • Print the frequency distribution on focal length to screen

Script

# setup default parameters if none were specified
param([string]$path = (get-location), # current path 
    [string]$fileType = ".jpg", # search for .jpg
    [string]$model = "Canon EOS 550D") # default camera type

# Clear the Screen
clear

##### Assemblies 
# load the .NET Assembly we will be using
Add-Type -AssemblyName System.Drawing

##### Constants 
$filter = "*" + $fileType # clean up the search filter
$Encode = new-object System.Text.ASCIIEncoding

# find all the image files we are interested in
$files = get-childitem -recurse $path -filter $filter 
# and how many we found
$totalFiles = $files.count

##### Varibles 
$image = $null
$imageHash = @{}
$i = 0
$focalLength = $null 

##### Main
# if some files were returned
if ($files -ne $null)
{
    foreach ($file in $files)
    {
        # load image by statically calling a method from .NET
        $image = [System.Drawing.Imaging.Metafile]::FromFile($file.FullName)
        
        # try to get the ExIf data (silently fail if the data can't be found)
        # http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/EXIF.html
        try
        {
            # get the Focal Length from the Metadata code 37386
            $focalLength = $image.GetPropertyItem(37386).Value[0]
            # get model data from the Metadata code 272
            $modelByte = $image.GetPropertyItem(272)
            # convert the model data to a String from a Byte Array
            $imageModel = $Encode.GetString($modelByte.Value)
            # unload image
            $image.Dispose()    
        }
        catch
        {
            #do nothing with the catch
        }
        
        # if the file contained both focalLength and A modelName
        if(($focalLength -ne $null) -and ($imageModel -eq $model))
        {
            if($imageHash.containsKey($focalLength))
            {
                # incriment count by 1 if focal length is already in hash table
                $count = $imageHash.Get_Item($focalLength)
                $count++
                $imageHash.Set_Item($focalLength,$count)
            }
            else
            {
                # Add focal length to Hash Table if it doesn't exist
                $imageHash.add($focalLength,1) 
            }
        }
        
        # Calculate the current percentage complete
        $i++
        $percentComplete = [math]::round((($i/$totalFiles) * 100), 0)
        
        # Update that lovely percentage bar...
        Write-Progress -Activity:"Loading Focal Lengths" -status "$i of $totalFiles Complete:" -PercentComplete $percentComplete 
    }
    # print results in ascending order of focal length
    $imageHash.GetEnumerator() | Sort-Object Name
}
else
{
    Write-Host "No files found"
}

Usage

Copy the script into a ps1 file and call it like so:

.\get-Focallengths –Path “c:\images” –fileType “.png” –model “Canon EOS 7D”

or drop the parameters you don’t need and let the script use it’s defaults e.g.

.\get-Focallengths –Path “c:\images”

Summary

I started out thinking this script was going to be trivial, but it’s turned into quite a good demonstration project of a lot of PowerShell’s abilities, specifically it’s ability to access the .NET framework.

THIS POSTING IS PROVIDED “AS IS” AND INFERS NO WARRANTIES OR RIGHTS, USE AT YOUR OWN RISK

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