With an old manual lens and modern digital camera, sharp image on the matte focusing screen, blurred one on the sensor

Question

I tried to use an old Practicar lens 50mm/1.8 with Canon 450D, using an appriopriate adapter ring Practica B/Canon EF. Of course, this lens is completely manual in this configuration. Via the viewfinder, it seemed ok: the focus was perfect if the scene was far and the focusing ring set to infinity.

Yet, the photos took in the same situation are considerably blurred and it is not merely an illusion, e.g. small objects clearly visible via the physical viewfinder are almost completely blurred in a photo, bookeh disks are clearly visible in the latter.

I would think that the body is out of order, yet the factory zoom lens which came with this 450D, in manual focus mode, shows the same sharpness on the focusing screen and in the photographs taken.

I can not explain this. Do you have an answer to this riddle? Is there any fix?

Image made by the kit lens: auto mode, 1/10 sec, 1:4.5, ISO 800, crop 1:1

(very similar to what is seen via the physical viewfinder using any of the two lens)

Image made by the Practicar lens: auto mode, 1/50 sec, 1:1.8, ISO 500, crop 1:1

(very blurred, while in the physical viewfinder for example the fence bars were completely crisp when taking this photo; see that the defocus disk of the door handle is round, thus there was no shake)

The Practicar lens has been maximally open, i.e. 1.8 all the time, i.e. when looking through the viewfinder and when making photos. As said, it has also always been set to infinity. The photographed have been at a distance of about 50 m. The kit lens is EF-S 18-55mm 3.5-5.6 IS. The problem is not related to camera shake, as in any photo there are similar, clear defocus disks when using Practicar.

I also tried to focus at distances smaller than infinity with the Practicar. It made the viewfinder image lose its crispness and the sensor image even more blurred.

Answer 1

With the adapter it seems your lens is too far from the camera to focus at infinity. The registration distance of the EF mount lens is 44mm. The registration distance of the Praktica B mount is also 44mm. The thickness of your adapter pushes the lens too far away from the camera's imaging sensor.

The reason the viewfinder looks better is because the viewscreen/focusing screen is made in such a way that it "cuts off" the light rays coming from the edge of the lens. So even though the lens' aperture is at f/1.8, the light getting through the viewfinder are only the rays coming through the center of the lens equivalent to around f/2.8 or so.

Objects that are too far from the camera to be in focus will be less blurry due to the increased depth of field at f/2.8 or so as compared to f/1.8. This is even more the case when you used an even narrower aperture such as f/4.5 as the angles of light rays reaching the sensor are even narrower, which is what elongates depth of field when we stop down a lens.

For more about how the shape of the surface of the front side of the viewscreen doesn't give a true depth of field preview for very wide apertures, please see:

Why is the depth-of-field preview in the optical viewfinder of my Canon 500D inaccurate?

and

Should the depth of field in the viewfinder be the same as the final images with the aperture set to wide open?

Answer 2

This might be an issue with modern focusing screens vs fast lenses.

AF SLR cameras tend to use focusing screen materials/textures optimized for brightness and manufacturing, not for accurate representation of images. They might be partially transparent instead of fully diffusing, allowing your eyes to focus THROUGH the screen. They might use light-guide type structures that discard or reroute some of the more oblique light rays that make a wide-open image, showing you an f/4 or so image with the lens at f/1.8. They might even be installed with less accuracy than what is required for proper manual focus.

All in all, you could end up with severely misrepresented depth of field.

If the camera has a live view+magnifier feature, use that to focus precisely. Also know that manually focusing an f/1.8 lens perfectly is not as easy as it seems.