Why does a manual Praktica B 50mm f/1.8 look sharp in the viewfinder but blurry in the photo on a Canon 450D?
Asked 8/31/2019
1 views
2 answers
0
I'm adapting a Praktica B 50mm f/1.8 to a Canon 450D with a Praktica B-to-EF adapter. Through the optical viewfinder, distant subjects look sharp when the lens is set to infinity, but the captured image is clearly out of focus, with obvious blur circles. The Canon kit lens does not show this mismatch between viewfinder and final image.
The manual lens stays wide open at f/1.8. If I turn the focus ring away from infinity, the viewfinder image gets softer and the photo gets even blurrier. Why can the viewfinder appear sharp while the sensor image is not, and is there a fix?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
5
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
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
6y ago
0
Generated from our catalog & community — verify before relying on it.
Most likely the adapter is preventing correct infinity focus. Canon EF and Praktica B have essentially the same flange/register distance, so a simple mechanical adapter adds thickness and moves the lens too far from the sensor. That means the lens cannot actually focus at infinity on the camera, even if the ring says it can.
The optical viewfinder can still mislead you because DSLR focusing screens are optimized for brightness, not precise manual focus with fast lenses. They often show less depth-of-field blur than the sensor records, especially at f/1.8, so an image can look acceptably sharp in the finder but be clearly out of focus in the photo.
Fixes:
- Use live view with magnification, if available, to focus from the sensor plane.
- Stop the lens down to gain depth of field.
- Use a corrective optical adapter, if one exists, though it may affect image quality.
- Best solution: use a lens/camera combination with compatible register distances, or a different camera mount better suited to adapting that lens.
Recommended products
UniqueBot
AI6y ago
Your Answer
Related Questions
How can I get accurate focus for night long exposures when autofocus won’t lock?
Why doesn’t turning my Canon 18-135mm STM to infinity give sharp night shots?
How can I get larger bokeh circles behind a portrait subject with a 50mm f/1.8?
Why does my Canon 50mm f/1.8 STM front/back focus on an EOS 1200D through the viewfinder but not in Live View?
Is it normal for the viewfinder image to look blurry when using a split-prism focus aid?