From 5D Mark II Team
Description of the problem:
When a Canon EF 100mm f/2.8 Macro USM lens is mounted on the camera and the user manually changes focus (rotates focus ring), the lens changes the iris/aperture “by itself”, even when the camera is in full manual mode.
It is not normal, and should not happen. The tests were made by several users with many copies on many camera bodies, so it’s not an isolated lens problem.
▪ This malfunction always occurs in this situation:
When the camera is set to M (Manual) mode and in Live View mode (Still+Movie – Movie Display), which is the correct mode for “full manual control” in movie mode.
Read more about the issue below.
http://5dmark2.wordpress.com/2010/04/05/malfunction-in-firmware-2-0-4/
thanks everyone that sent this in
cr
72 Comments
Well, not exactly. New macro lenses with internal focus will have a shorter focal length at close focus, but the old ones (and some short ones, like the EF 50mm 2.5) focus by moving all the lenses from the focal plane. These will keep their focal length from infinty to 1:1.
No. Not all macro lenses change focal length as a function of focus distance. Rather, it is the internal focus design (regardless of macro capability) that determines whether a lens changes focal length upon turning the focusing ring.
Furthermore, I am well aware of the formula N[eff] = N (1+M). I posted it in the FM thread. And it has NOTHING to do with changing focal length or internal focus lenses or even the size of the entrance pupil. None of these appear in the formula. What the effective aperture formula tells you is that the true light-gathering ability (assuming ideal, 100% transmission) is magnification-dependent. The relative f-number (at infinity) doesn’t change even if you are not focused at infinity. It doesn’t change even if your focal length changes. A lens at f/2.8 shooting at 1:1 magnification will yield an effective aperture of f/5.6, two stops less light-gathering ability.
For an internal focus lens at a fixed f-number, say f/4, changing focus will also change the entrance pupil diameter because N = f/D, where f is the focal length and D is the entrance pupil diameter. That is why the pupil appears to change size, because the focal length is changing and N is fixed. Again, this has no relationship to the effective f-number.
After the statement
“A lens at f/2.8 shooting at 1:1 magnification will yield an effective aperture of f/5.6, two stops less light-gathering ability,”
add the following:
“Even though the effective aperture is f/5.6, that doesn’t mean the aperture itself has stopped down to the relative f-number f/5.6. Effective f-number is a calculated value, not a physical ratio of focal length to entrance pupil diameter.”
All lenses change focal length when they are not focused on infinity, that is an optical fact. Indeed a focal length can only be quoted for any lens as the distance behind it that parallel light rays come to a focal point, rays can only be parallel when coming from infinity, if they are not coming from infinity then the focal point can’t be where it is when they are.
The effect of focal length shortening can be mitigated by lens extension, either internal, as on some expensive lenses, or as on many cheaper lenses, by extending the front, primary optic, away from the film/sensor plane. The film/sensor plane is not where the focal point is.
Some lenses are better than others at achieving this, the latest infamous example is the Nikon (sorry for swearing) 70-200 f2.8 VR MkII, at closest focus distance the 200mm becomes a 128mm! These figures are easily worked out by magnification ratios and subject distances.
Now apparent and effective apertures are not “adjusted” as focus distance changes, f8 at infinity is not f8 at any other distance, but the absolute size of the iris opening stays the same where ever you focus. The focus group changes the apparent size of the iris opening, but this is just the effect of looking at a hole through lenses that move, it looks like it changes but it doesn’t.
None of this has anything to do with the effects being reported about some lenses on the 5D MkII in some modes. The jittering of the iris clearly seen in the videos is not normal lens behavior. If it was, all lenses on all bodies would do it.
It is a bug.
In the FWIW category, I can confirm that this phenomenon is present in my 5DII/2.04 with a Tokina 100mm f2.8 macro lens. Definitely not internal focusing…