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When I take a panorama with a 80mm lens at F/2.0 on a fullframe body such that the resulting image has a equivalent field of view of a 50mm lens on the same body, what would then be the equivalent aperture I would need to use on the 50mm lens to get the same DOF as in my panorama?

Im guessing F/1.25. There I calculated the factor 1.6 = 80mm/50mm and divided 2.0 by it. If this is the right way to calculate the equivalent aperture, what is the physical reason behind that?

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  • "When I take a panorama with a 80mm lens at F/2.0 on a fullframe body such that the resulting image has a equivalent field of view of a 50mm lens on the same body," This does not make sense to me, an 80mm lens does not have an "equivalent field of view" on a full size sensor camera, it has a field of view of 80mm. Perhaps you have stated it backwards. Also a 80mm would not give a panoramic view. Clarification is needed. F2.0 with a 80mm lens is going to give you a shallow depth of field, is that your goal ?? – Alaska Man May 12 '20 at 17:09
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    @AlaskaMan just a gues, but I think he means that he wants to take two or more photos whith an 80mm lens and stirch them toghether to get a FOV equel to 50mm – Jonas May 12 '20 at 17:24
  • @Jonas thats true. That is what I mean. – Arjihad May 12 '20 at 19:38
  • @Arjihad Your question is based on a bit of a misconception regarding what Depth of Field (DoF) is and what it is not. There's no real thing as "the" depth of field of any image until the display size and viewing distance is factored into the equation. For more, please see Is camera lens focus an exact point or a range? – Michael C May 16 '20 at 5:33
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Depth of field changes with the resolution. If we disregard resolution and state that we want things blurred in the same manner assuming that resolution is infinite (and consequently depth of field is mathematically speaking 0 and not useful as a measure), the principal deciding criterion once we ascertain the same focusing distance is how large of an aperture we are looking out of.

80mm/2.0 is the same as 50mm/1.25 as you correctly determined. That makes for an "eye" (it's actually called "entrance pupil") of 40mm and means that background blur (at infinite distance) will have the same diameter that a disk of 40mm would have at the focusing distance.

With regard to foreground blur (at quite closer distance than the focusing distance), the blur disk is about 40mm in the world (it actually shrinks to zero as you get to the focusing distance and then grows again). So that kind of setup will be good at making a fence significantly closer than the focusing distance and with holes/pattern smaller than 40mm disappear on the photograph.

With regard to actual "depth of field", it all depends on what kind of blur circle you are prepared to call "in-focus" and that depends on viewing scale and/or sensor resolution and/or optical resolution. But if you are not looking at the detail level but at the big picture, the blur radius thing is what will be apparent and scale-independent and will define the equivalence of pictures/views.

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