Macro focussing rails

Started 4 months ago | Questions thread
mawyatt2002 Contributing Member • Posts: 502
Re: Macro focussing rails

The Wemacro is fine and the best value in a automated focus rail for general purpose use. I have one and used it often.

Here's an example of how DoF plays: With an Effective Aperture (EA= Aperture(1+M) ) of 24 (Aperture =4), the DoF is ~51um (DoF = Lambda/NA^2, Numerical Aperture =M/(2EA) with a resolution of ~2.64um (R= Lambda/2NA), all valid for small Numerical Aperture. Remember most cameras (except Nikon) report only Lens Aperture not Effective Aperture, so a F4 at 5X is actually F24

So you want your step size to be smaller than the DoF otherwise focus banding will occur, in the example a good choice would be ~30um.

Note that resolution is inversely proportional to NA, but DoF is inversely proportional to NA^2. So a 2 times increase in aperture (larger NA) causes 2X improvement in resolution but a 4X reduction in DoF. You want large NA for high resolution but eventually the wavelength of light becomes a barrier (Lambda is 0.55um for green), but the DoF quickly diminishes with larger NA and this is where small focus step sizes come into play. Tradeoffs everywhere you turn!!


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