today i tried to capture an image of a very small item, about 1cm long. So i grabbed the ZE100 and added my 3 Kenko extension tubes (12,20 and 36mm) to get a greater magnification (with Canon 5D2). The resulting image was not very sharp or contrasty and there was severe vignetting in the conrners (see image below).
DoF was very thin so i set the ZE100 to itsīminimum focus distance and moved the camera so that the closest part of the item was in focus. I took a photo and turned the focus ring back a bit, took a photo, turned the focus ring a bit, took photo and so on.... Looking at the images on the computer none but the first have anything in focus! WTF? I noticed that the vignetting was reduced by turning the focus ring back. What is going on here?
I guess that the focal length changes when focus is pulled but what about the vignetting? Is getting a marco rail the only chance for DoF stacking with the ZE100?
The ZE 100/2 has floating elements, so the focal length probably varies with focus distance. And when you put it on tubes, they might obstruct the light from the rear pupil of the lens. Or focus could actually be behind the front element, which would of course give vignetting and nothing in focus.
With the extension tubes and the lens own macro abilities, I think you are reaching around 1.18:1 ratio. I would reduce the extension a bit, since 50mm tube should give you 1:1. Also you should normally focus by moving the camera, or the subject (I put it on a sheet of plexiglas and slide it around to reach the best focus).
Makten wrote:
The ZE 100/2 has floating elements, so the focal length probably varies with focus distance. And when you put it on tubes, they might obstruct the light from the rear pupil of the lens. Or focus could actually be behind the front element, which would of course give vignetting and nothing in focus.
That sounds improbable to me, but maybe you're right. I have a Tokina 90/2.5 which also has a floating element but it works fine even on a fully extended bellows. The way to check would be to start with the full set of tubes and the lens focused at infinity and then see what happens to the focus distance when you turn the focus ring gradually towards the MFD.
By the way, I believe focal length always changes with the focus distance, except for lenses that don't have focus breathing (like cine lenses that are corrected for this). It's lenses with internal focusing (which this none of the Zeisses have) that have the strongest reduction in FL.
The 100MP has quite a large rear element. I wonder if the Kenko tubes were not designed for that, and obstruct in some way? Since macro shots with rings are just a crop of the full shot, I would not think that there was any other kind of possible vignetting.
I have not tried my 100MP with rings, so I don't know why the focus problem would occur, unless you twisted the focus ring too much between the original position and the second position. Sounds a bit unlikely to me.
The tubes are probably blocking the light. I've seen that before. Instead of stacking all three can you use only two? I had two sets at one time and found a two-tube combo that worked best and was more stable too. Of course the circle of coverage could be too small.
