Originally posted by Motorman I am no "expert" on lenses but I have 2 -1:1 macro prime lenses,a 2.5 and a 2.8,and have shot these both at their largest apertures at 1:1 and at infinity,2.5 and 2.8 respectively, and have not noticed a f stop change from what what was set.
no you won't.
the change in F stop results in a change in exposure not a change in any setting on the lens.
Quote: I have experienced exposure differences but nothing I could not compensate for while shooting.
that is usually managed by the TTL metering, but gets difficult if you are doing manual or "auto" flash, because the true aperture is not what is set on the lens
Quote: As for some kind of magic I am unaware of 2.8 going to 4 while still at 2.8 well I do understand the effective loss of light with extension but 2.8 is 2.8 is it not?Am I missing something here?
yes, you are missing something. F2.8 on any prime lens sets a specific diameter of the aperture, which relates to the mathematical calculation of F Stop, specifically
F-Stop = focal length / diameter
But note, focal length is only the length at infinity focus.
In reality, you need to consider not the focal length in an F-Stop calculation, but the equivelent lens to focusing plane distance.
Quote: Kinda reminds me of the argument that a 100mm lens is a 100mm on FF and 150mm on a cropped sensor,the lens is still a 100mm focal length.
no not the same thing cropping the middle out of an image (FF vs ASP-C) does not change the exposure of the cropped section or any other characteristic, but close focusing does cause a change in magnification ratio of the lens, making the image bigger on your sensor, making an image bigger means projecting the light entering the front of the lens over a bigger surface, and since the light entering the front is constant, spreading it out over a bigger area means that the light intensity hitting any one area is getting less and less as the image gets bigger and bigger
Quote: What I have noticed is that the DOF changes with distance and if there is a light change my cameras seem to handle it well shooting in A mode with minor EC adjustments.
yes the camera metering should sort this out
Quote: Please note I was not trying to be argumentative here and was just trying to give an answer to the OP and my experience.So I would like to apologize if I clouded the water with my reply it was not intended to do so........
no problem, and no offence taken.
Discussions like this are healthy