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Front Filter Corrects Corner Smearing

thrice

thrice

Active member
I thought I should repost this here as well since it really is such a revelation for those who enjoy using rangefinder and legacy wide angles on A7* bodies.

The original solution was theorised by HaruhikoT on Fred Miranda forums here: Front-End Filter Improves Corner Smearing - FM Forums :clap:

I have done none of the major R&D myself. What I have done is calculate a thinner filter and have it fabricated with AR Multicoating on both sides. This fits into a 49mm filter ring and works a treat with my 35mm ZM.
Next step is to fit a very thin o-ring inside the filter threads on the lens itself and screw the correction filter onto the front with a 49mm retaining ring.

With filter on left, without filter on right.

Centre.JPG
f/1.4
35_1.4Centre.JPG
f/2.0
35_2.0Centre.JPG
f/2.8
35_2.8Centre.JPG
f/4.0
35_4.0Centre.JPG


AlmostCorner.JPG
f/1.4
35_1.4Corner.JPG
f/2.0
35_2.0Corner.JPG
f/2.8
35_2.8Corner.JPG
 
D

daf

Member
impressed by the corner improved performance ... i'm gonna try this on few of my leica lenses.
Do you know if this have any impat on colorcast?
 
thrice

thrice

Active member
Makten said:
Indeed, but the center obviously gets a bit worse. :lecture: :p
Click to expand...
It has to be refocused because the filter is planoconvex. As I said this is likely due to refocusing error on my part, you can see more purple fringing on the filtered image in the centre.. that to me indicates the plane of focus is different. I will do more controlled tests if I can be bothered.

Even if it is not focus error, "a bit" is an exaggeration.
 
thrice

thrice

Active member
In a 100% crop the difference is less than the camera's own smearing algorithm at 4.0s.

Anyway, until you do your own testing you'll have to live with me saying it is focus error.

“A bit” can mean very little or quite a lot. An English peculiarity.
 
Makten

Makten

Well-known member
thrice said:
In a 100% crop the difference is less than the camera's own smearing algorithm at 4.0s.
Click to expand...
I read your thread about that at FM forums, and I think it's funny that you seem to think the "smearing algorithm" is a problem, but not this. To my eyes the difference with the filter is worse than the long exposure smearing. :eek:

Anyway, until you do your own testing you'll have to live with me saying it is focus error.
Click to expand...
Of course. But you didn't say anything about refocusing in the first post, so how could I have known about it?

“A bit” can mean very little or quite a lot. An English peculiarity.
Click to expand...
I'm not a native english speaker (I thought that was obvious), so... Anyway, I meant that the difference is small but noticeable. :thumbup:
 
thrice

thrice

Active member
No harm no foul.

I'm really reluctant to remove the pcx glass from the front of my lens. Took me a while to get both the lens and the pcx totally dust free (under bright sunlight) before attaching them.

Will you accept a shot more carefully focused from the exact same spot? It's just outside my door.
 
Makten

Makten

Well-known member
thrice said:
No harm no foul.

I'm really reluctant to remove the pcx glass from the front of my lens. Took me a while to get both the lens and the pcx totally dust free (under bright sunlight) before attaching them.

Will you accept a shot more carefully focused from the exact same spot? It's just outside my door.
Click to expand...
No efforts for my sake! I'm just curious and a lens geek, and I couldn't resist commenting on the obvious (?) difference. :grin:

I'm not gonna buy a ZM 35/1.4 anyway (I think), since the bokeh at mid distances (~3 meters) is not to my liking. I love the solution with a front filter to get rid of field curvature, but if it also affects center sharpness/clarity/mojo, I think that's an important aspect. Especially if that holds for other lenses too. :eek:

Edit: And yes, I think you're right about the focusing. The filter will change the focal length, and thus need refocusing when attached. So that's likely the cause of the difference.
 
thrice

thrice

Active member
Makten said:
Edit: And yes, I think you're right about the focusing. The filter will change the focal length, and thus need refocusing when attached. So that's likely the cause of the difference.
Click to expand...
The filter drastically changes the focal distance, even with the hawks helicoid I had to remove the thickest brass shim under the mount (3mm slot screwdriver with a sharp tip necessary). If i didn't adjust the lens, the infinity hard stop would be 5m.

Without the filter I now focus WAY past infinity. With filter, without shim and after tweaking my hawks infinity is at the hard stop.
 
thrice

thrice

Active member
Sorry very different light today.
I might re-shoot it AGAIN when it's drab and overcast again (so, probably tomorrow in Melbourne).

Comparing without filter on the left, to with filter on the right, ignoring differences in lighting the resolution looks the same on centre to me.

Thoughts?

351.4Redone.jpg

And the corners from both images - with CA correction.

351.4CRedone.jpg
 
V

Vivek

Guest
As long as you capture an image under 3.2s, this would be useful. :D
 
Makten

Makten

Well-known member
thrice said:
Thoughts?
Click to expand...
It's a lot harder to tell the difference this time, but I think I can see just a little more resolution in the left image. Look at the cracks to the right in the brick wall.
Could just be different lighting though.

Anyway, the difference is so small that the filter is obviously worth it when seeing the huge benefits in corner performance. :eek:
 
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