reverse macro/coupling questions

[ashyxt]

hi everyone, I'm looking to couple 55-250 IS with tamron 17-50 reversed, but I'm wondering:

- how does changing the 55-250 IS's aperture affect DOF? compared to.. like... when only reversing single lens? I mean... how does a wide-opened tamron 17-50 in front affect the DOF?

- how many stop of light would I be losing? given that the 55-250 IS is @ 55mm f/4 & the tamron is @ 17mm f/2.8

- has anyone here done this? there's not supposed to be any vignette right? the 55-250 IS is 58mm thread based, & even @ 55mm, I think the field of view only covers 50mm of the diameter or so... while the tamron is 67mm thread based

- regarding single lens reverse macro, shorter focal length = greater magnification? I have another lens in mind, it's an old sigma super-wide II 24mm f/2.8, but then... when reversing macro, the focus range is very very limited right? so I hope to have little flexibility in magnification ratio... this lens is 52mm thread based by the way


thanks in advance for any guidance :) :) :)

[LordV]

Just regard the reversed lens on the front as a fancy add-on diopter lens. The DOF is determined by the main lens aperture. You should leave the aperture of the reversed lens wide open to help prevent vignetting.

With a single lens reversed onto the camera body, the magnification is inversely related to the focal length as you suggest. I have only played with a reversed kit lens but found the limit was around 28mm which gave 3:1 magnification, smaller than that and the focus distance was impossibly short.

Brian V.