The number in the f/x is actually obtained dividing the FL by the
diameter of the lens, with the actual diaphragm.
So, yes, in mm the 100mm @f/4 will have an aperture (in mm) of 25 mm,
and the 50 mm @ f/2 will have the same 25 mm.
That is regardless of format, it will be the same if you use the lens
with a tiny P&S sensor or a medium format sensor.
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Antonio
http://ferrer.smugmug.com/
Thank you very much Pedgydusz, this has helped greatly.
If someone could confirm this, all my troubles would be over:
If if I am standing with an Olypmus 5mm f/2 and a FF 100mm f/2 at an
equal distance from a subject, why wouldnt the DOF be the same?
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Daniel
Look at my posts in this thread. Precisely because the depth of field
of a 50 mm lens at f2 is the same regardless of the size "sensor"
behind it, whether the sensor is four thirds, a roll of 35 mm film in
an Olympus OM2 camera or 120 roll film (6x7) in a Mamiya 7 or for
that matter 4x5 sheet film in a large format camera. The image
circles of these lenses (all do in fact exist, though you would have
to substitute 47 mm or 55 mm in 4x5 in reality) will obviously need
to be differnet to cover these differing "sensors", but that would
open an entirely new can of worms. Their field of view is also
different it is a normal lens on 35 mm film but a megawide on 4x5.
Are you saying that because the 50mm lens is wider, its DOF is greater because its focused area takes up more space in the frame, and when it is cropped, the focused area is blown up (magnified due to cropping of the other area) and then the DOF is greater?
If that is right, then I can wrap it up (for myself) by saying that for a given focal distance, sensor size is quite (if indirectly) relevant DOF. Take before's example, where weare talking about a 100mm focal distance. Because there is a crop factor of 2 on the Olympus lenses, the focal length of the lens must be 100/2 and therefore bearing in mind Pedaguydusz's formula where f=fl/d it is harder to obtain larger apertures because the focal length is always shorter.
Now to return to the original question, if anyone still remembers, which was can a direct comparison to be made to transpose the aperture to FF or APS-C, because they are on different scales, and by this I mean that a true aperture (diameter) is the same but the f values are different. To demonstrate this, I will use the previous example. To get a 25mm diameter, one needs a 50/2 4/3 lens and a 100/4 FF lens, because D=FL/A so 50/2=25 and 100/4=25. Therefore using the formula, on DX format with its 1.5 crop factor, a FL of 66.6 and an aperture of 2.6 would be needed.
However, if one were to make a direct comparison between FF and 4/3, where one would see that to get the same diameter, one needs a FF f value that is double that of the 4/3 f value, so we can double it. I now wonder why one cannot do the same with DX. To get to FF sensor size you (roughly) double and to get the crop factor for DX sensor size you divide DX sensor area by 4/3 sensor area which is 372.09/224.9
=1.6. However, that number does not work in application. I will try the number on the previous equation. 50/2 Olympus lens becomes 100/4 (w/ crop factor of 2). Therefore, DX lens should be 66.6 (because 66.6*1.5=100) and should have an aperture of (2*1.6) 3.2, but it doesnt. Do any of you know why?
I suppose this also answers the question whichI posed in an earlier thread, asking why there are no Olympus lenses with an aperture larger than f/2, which I stopped following because it went ridiculously technical and off topic, and I couldnt follow it. The answer to that then is that it physically requires a larger diameter for any f stop. It all makes sense now (apart from above). Anyone?
--
Daniel
Olympus e-five10 + 14-42, 40-150
