While playing with the 105mm macro I had noticed that, on occasion, it would not go below f3. What is going on here I thought??????
So I got it out today and it would appear that the closer I got to the object I wanted to photograph, the more the lens stopped down but the DOF was still looking all nice and shallow.
Had a look in the manual.......nope, nothing....
So google being my friend I came up with a really rather good reply as to what was going on with the 105mm.
Michael R. Freeman , Jan 13, 2010; 09:42 a.m.
Basically, when you focus closer, a lens moves away from the image plane and casts a larger circle of light. But since the amount of light coming through the lens is exactly the same, but spread over a wider circle, the light intensity (effective f/stop) within that circle is now dimmer.
Take a flashlight into a dark room and hold it about 6 inches from the wall. Now move the flashlight further away from the wall. The circle of light gets larger, but dimmer. That is essentially what happens with a macro lens at close distances. ALL lenses exhibit this behavior, but with macro lenses it is much more noticeable because the extension away from the image plane is much larger. We now have "smart" cameras and lenses that will report this decrease in effective aperture (the actual aperture opening remains at f/2.8) as the lens is focused closer.
Some third party macro lenses may not show this decrease in effective aperture in the viewfinder, and may appear to give you f/2.8 at minimum focus, but in reality the exact same thing is happening and the effective aperture decreases with increased magnification.
So there you go, blooming smart lenses!
Just the thick operator who will have to learn how to use it right.
So I got it out today and it would appear that the closer I got to the object I wanted to photograph, the more the lens stopped down but the DOF was still looking all nice and shallow.
Had a look in the manual.......nope, nothing....
So google being my friend I came up with a really rather good reply as to what was going on with the 105mm.
Michael R. Freeman , Jan 13, 2010; 09:42 a.m.
Basically, when you focus closer, a lens moves away from the image plane and casts a larger circle of light. But since the amount of light coming through the lens is exactly the same, but spread over a wider circle, the light intensity (effective f/stop) within that circle is now dimmer.
Take a flashlight into a dark room and hold it about 6 inches from the wall. Now move the flashlight further away from the wall. The circle of light gets larger, but dimmer. That is essentially what happens with a macro lens at close distances. ALL lenses exhibit this behavior, but with macro lenses it is much more noticeable because the extension away from the image plane is much larger. We now have "smart" cameras and lenses that will report this decrease in effective aperture (the actual aperture opening remains at f/2.8) as the lens is focused closer.
Some third party macro lenses may not show this decrease in effective aperture in the viewfinder, and may appear to give you f/2.8 at minimum focus, but in reality the exact same thing is happening and the effective aperture decreases with increased magnification.
So there you go, blooming smart lenses!
Just the thick operator who will have to learn how to use it right.
you know, I can't remember the amount of times I wondered why my Sigma 150mm macro did that, sometimes 3.2 is as low as it will go....thanks Ali 
