Close - up photography using extension tube.<br><p><b>1</b> - Camera lens.<br><b>2</b> - Extension tube.<br><b>3</b> - Film or sensor.<br><b>y</b> - Object height.<br><b>y'</b>- Image height.<br><b>l</b> - Width of extension tube.<br><b>H</b> - First principal plane.<br><b>H'</b>- Second principal plane.<br><b>e</b> - Distance between principal planes.<br><b>s</b> - Object distance.<br><b>s'</b>- Image distance.<br><b>ß</b> - Magnification.<br><b>f'ob.</b> - Camera lens focal lenght<br><b>Δ</b> = Object - image distance.<br><br><b>Example:<br></b> <b>f'ob.</b> = 50 mm ; 
<b>y</b> = 33 mm ; 
<b>y'</b>= 24 mm ; 
<b>ß</b> = ? ; 
<b>l</b> = ?  <br><b>ß</b> = y' / y = 24 / 33 = 0.727<br></p>
<b>l</b> = ß . f'ob. = 0.727 . 50 = 36.35mm
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