Originally posted by Bob 256 You didn't mention the exposure you're using. If it's a short shutter speed that's forcing the lens near wide open, that would probably explain some of the "out-of-focus" portions you're seeing. You want to use a small (though not too small aperture) to gain sharpness in shooting slides. If vibration is well controlled, longer exposures are fine and you can get the aperture down to f8 - f11 which should improve sharpness, especially toward the edges.
You could try some focus bracketing to see if somehow you're missing the sweet spot for best focus. If you're going to be doing a lot of this, it might be best to see if you can lay your hands on an enlarger lens (like an El-Nikkor) which you can adapt to fit your bellows. Usually, a lens like the El-Nikkor will beat most other lenses for sharpness in this application. Again, you want to be stopped down to gain on sharpness and improve depth of field so center and edges are both in focus if there's any warping of the slide.
Keep in mind that since you're shooting 1:1, the indicated f-stop on the lens is not the effective f-stop. A 55mm lens will be working at an effective focal length of about 110mm for 1:1 and that doubles the f-stop from what is indicated by the lens (for a full-frame camera - the effective focal length will be less than a factor of 2 for crop frame shots since you won't be working at 1:1).
There is a link for EXIF data just above the posted image - indicates ss = 1/10th second, but does not include f-stop.
If the image looks sharp through the viewfinder, then I can think of only a few explanations:
1. motion blur
2. focus calibration is not accurate for this combination
3. dirty/damaged sensor
I assume the camera works normally without the bellows (?) which would rule out #3
To check #2 you could set up a scene with more front-to-back depth - lay a hair comb flat and shoot it lengthwise to see if there is anything in focus, even if the focus is not where you would expect it.
I can't think of anyway to prove that the problem is due to motion blur, but the solutions for preventing it have already been mentioned - faster shutter speed, solid setup (tripod, slide holder, etc), mirror lockup, use time delay or shutter release, etc. Also, be sure shake reduction is turned off for tripod shots.
Unlikely, but possible: maybe some kind of "special effects" filters have been accidently applied either in camera or when imported into Photoshop? Rule out by checking a RAW exposure.
As for the lens, I have read a dozen posts on several forums where people have used cheap zoom lenses to photograph slides, and the results were far better than this. Whatever the problem, I'm pretty sure it is not due to the lens, alone. Also, if the problem was a combination of film curvature and too shallow depth of field, that would be seen through the viewfinder, right?
---------- Post added 01-02-19 at 09:15 PM ----------
Final thought. Are you focusing with aperture wide open, then manually rotating the aperture to the correct f-stop before taking the exposure? Or does your rig automatically stop down at the time of exposure? Or are you trying to focus with the lens stopped down?