# Relay lenses and optical formulas

Started 2 months ago | Discussions thread
Re: Relay lenses and optical formulas

First you need to discover the effective focal length, taking into account all the stuff you attached. Here are two methods; both involve knowing the magnification of the resulting image. Not as easy as you might think. The viewfinder view only displays about 95% of the actual view because of masking and a magnification of 0.82X. You can, however, take a picture of a metric ruler. You can use this image to obtain the actual image size and the resulting magnification.

1. This method depends on you achieving a magnification of 1 (i.e. life-size) often called unity or 1:1. Set up a target; best is a ruler with a millimeter scale. Adjust the distance, camera-to- ruler to achieve life-size (magnification 1). Accurately measure the distance from the ruler to the image plane. This camera features an image plane measuring point. This is the symbol adjacent to the LCD data display atop the camera body. Assuming you have precisely obtained magnification 1 and an accurate measure of object-to- image distance, divide this distance by 4. This will be the effective focal length.

2. This method depends on you achieving a magnification of 10 to 1 reduction. You position the distance camera-to- object to obtain a reduction 10 to 1. That will be a 100mm span on the ruler adjusted to occupy 10mm on the image. Having achieved this magnification, measure the distance from the object to a definite point on the lens barrel. Now repeat this procedure. This time we want a 20 to 1 reduction. A 400mm span occupies the same 10mm image span. Again measure this revised distance, ruler-to- the same reference point, on the lens barrel. Now we calculate focal length. The effective focal length is 10 times the difference between these two measurements. The formula behind this method ---If the distance between two camera positions is D, and difference between the two magnifications is M, then the focal length is D÷M.

Alan Marcus (alanmaxinemarcus@att.net)

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