magnification ratio and APS-C

<p>Hi!<br>

I tried to calculate a lot, but now I'm stuck.<br>

In theory, APS-C Nikon sensor is approximately 24*16 mm. So 1:1 macro would be something this large in life. That's OK.<br>

My 50mm 1.8 D lens is about 1:6 ratio max. - but to fill the frame with a 23mm subject, I only had to add 30mm extension. Is this 1:1?</p>

<p>How can I calculate the magnification ratio with the following: lens focal length, sensor size, subject size, lens max. magnification ratio and extension length?<br>

Thanks,<br />Peter</p>

<p>Magnification ratio is independent of the format size. A ratio of 1:1 on 35mm is 1:1 on 4x5 film too. The only thing that changes is the area covered in the image.</p>

<p>"Filling the frame" is something different. Divide the image size by the magnification an you have the subject size covered by that combination. Filling the frame on an APS-C camera will require less magnification than for a full-frame camera, by a factor of 1.5x or so.</p>

<p>The magnification ratio is approximately equal to the total extension (focal length plus extension) divided by the focal length. The focal length is a constant, based on the focus at infinity. The extension equals the distance added by the focusing helix plus any extension tubes. It is "approximate" because some lenses focus internally and change the focal length (hence the rear node position).</p>

<p>To achieve unity (life size) on the focal plane, of any format size, an object is placed four focal ahead of the focal plane. In other words if a 50mm lens is mounted, unity (1:1) will be achieved if the object is 200mm from the focal plane. When sharply focused the rear nodal, the principle point used to originate measurements as to focal length, will be 100mm from the object and 100mm from the focal plane.</p>

<p>You can dismount the lens; set it on a makeshift stand (I use books). Aim the lens at a printed page. On this page, scribe some lines of known length. Illuminate the page with strong light. Using a ground glass or tissue paper or plain white paper as the screen set the screen 4 times the focal length away from the object. On the screen, you will see the projected image of the printed page. Adjusts the position of the lens and screen to achieve unity using a ruler to measure the lengths of the lines you have scribed. Place a white adhesive label on the lens barrel and place a mark half way between object and screen. You have achieved unity and discovered the position of the rear nodal.</p>

<p>Conjugate Distances and Image Scale:<br>

F = focal length<br>

U = object - lens distance<br>

V = in lens - image distance<br>

F = ( VU) ÷ V + U)<br>

U = (FV) ÷ (U-F)<br>

V = (FU) ÷ (U-F)<br>

The linear ratio of the object and the image ® is:<br>

R = V ÷ U<br>

R = F ÷ (U-F)<br>

R = (V-F) ÷ F </p>

<p>I use this:</p>

<p><a href="http://www.eosdoc.com/manuals/?q=jlcalc">http://www.eosdoc.com/manuals/?q=jlcalc</a></p>