[Alex Zhang]

I have always wondered whether modifying the rear baffle on the HD DA 20-40 would improve the lens usability on K-1. Now that my 20-40 is out of warranty, I pulled the trigger today. I must say that I am very pleased to find that the improvements exceed my expectations.

By filing the opening on the rear baffle from circular to rectangular (as shown in the attached photo), all vignetting ranging from 23mm to 40mm are gone. Even the vignetting at close focus near 23mm are gone. Furthermore, it is even possible to get usable image at 21mm from F5.6 to F11. The corners are still a bit soft due to the DA design, but I am perfectly happy to get a lightweight HD 21-40mm F2.8-4 DC WR lens for K-1.

The disassembling is pretty easy. There are 3 screws (out of the 4 screws on the side/neck of the rear mount) holding the baffle. Unscrew them, then you should be able to pull the baffle off with a little force. You need to use a little force because the two electrodes(for auto focus) are plugged into two sockets underneath the baffle. You have to pull them out of sockets for the baffle to break free. Once the baffle is off, you can cut or file its opening to the rectangle shape. You might want to shoot the lens with the baffle off to see the improvements on vignetting before actually cutting. With the baffle and electrodes off, the lens is still usable as a manual focus DA lens on K-1.

Please be aware that this modification is non-reversible, so do it at your own risk. I am not sure whether it will affect the image quality of the les on APSC or the aftermarket value of the lens. My guess is that it shouldn't.

Also remember to remove the hood while doing testing on the K-1.

Have fun hacking,

Alex

Edit: I've added another set of sample images taken after the baffle was removed at post #26. This set is more complete and has sky included to better show the effect of vignetting. For comparison, see the samples before the modification at post #12.
One may also find more detailed pictures showing which screws need to come off for removing the rear baffle at post#25

Attached Images

 

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Last edited by Alex Zhang; 04-20-2021 at 08:12 PM.

[MarkJerling]

I have always wondered whether modifying the rear baffle on the HD DA 20-40 would improve the lens usability on K-1. Now that my 20-40 is out of warranty, I pulled the trigger today. I must say that I am very pleased to find that the improvements exceed my expectations.

By filing the opening on the rear baffle from circular to rectangular (as shown in the attached photo), all vignetting ranging from 23mm to 40mm are gone. Even the vignetting at close focus near 23mm are gone. Furthermore, it is even possible to get usable image at 21mm from F5.6 to F11. The corners are still a bit soft due to the DA design, but I am perfectly happy to get a lightweight HD 21-40mm F2.8-4 DC WR lens for K-1.

The disassembling is pretty easy. There are 3 screws (out of the 4 screws on the side/neck of the rear mount) holding the baffle. Unscrew them, then you should be able to pull the baffle off with a little force. You need to use a little force because the two electrodes(for auto focus) are plugged into two sockets underneath the baffle. You have to pull them out of sockets for the baffle to break free. Once the baffle is off, you can cut or file its opening to the rectangle shape. You might want to shoot the lens with the baffle off to see the improvements on vignetting before actually cutting. With the baffle and electrodes off, the lens is still usable as a manual focus DA lens on K-1.

Please be aware that this modification is non-reversible, so do it at your own risk. I am not sure whether it will affect the image quality of the les on APSC or the aftermarket value of the lens. My guess is that it shouldn't.

Also remember to remove the hood while doing testing on the K-1.

Have fun hacking,

Alex

Oh, cool! Very nice work!

[robbiec]

Is the part that you trimmed one unit? Like could we create a 3D print STL file, keep the original part and put a revised 3D print in its place?

[Alex Zhang]

The rear baffle is a two-parts unit. The electrodes can be removed from the baffle by unscrewing the two black screws. I am not familiar with 3-D printing. I guess it should be possible to print a similar baffle.

[Belnan]

When you said hacking I was a little worried. Please share some photos when you get a chance.

[Kevin B123]

Brave. Did you paint the edge of the cut aperture with black paint?.

It would be interesting to see some wall images to assess the improvement.

[Alex Zhang]

The rear baffle is made of black plastic, so no need for black paint. There really is very low risk for removing the baffle. Then you can try the lens without the baffle to see the improvements yourself. If you don't like it, just put the baffle back on. I am not very familiar with sending post with many pictures, and that's why I didn't upload any sample photos. I shot more than 100 photos at different focal lengths and apertures before drawing the conclusion. I can try to upload some of them maybe later.

[AfterPentax Mark II]

This looks to me to be a case of GBH!

[savoche]

I have always wondered whether modifying the rear baffle on the HD DA 20-40 would improve the lens usability on K-1. Now that my 20-40 is out of warranty, I pulled the trigger today. I must say that I am very pleased to find that the improvements exceed my expectations.

Nice! Now I'm wondering if a similar modification could work with the DA11-17...

[Kevin B123]

Nice! Now I'm wondering if a similar modification could work with the DA11-17...

Gonna need to start a Frankenlens thread now...

[11GTCS]

This is pretty awesome! I’m definitely in the group that wishes a similar restricted focal length limited zoom existed for full frame. Not sure I would do this to my 20-40, but it could be interesting.

[Alex Zhang]

When you said hacking I was a little worried. Please share some photos when you get a chance.

Ok, so here are some of the test shots I took. It appears to be a limit on how many photos can be uploaded per message, so I will upload first 9 images taken before the baffle was modified. They were taken at 23mm (F2.8, F5.6, F11), 30mm(F3.5, F5.6, F11) and 40mm(F4, F5.6, F11) in that order.

[Alex Zhang]

Ok, so here are some of the test shots I took. It appears to be a limit on how many photos can be uploaded per message, so I will upload first 9 images taken before the baffle was modified. They were taken at 23mm (F2.8, F5.6, F11), 30mm(F3.5, F5.6, F11) and 40mm(F4, F5.6, F11) in that order.

It turned out that the limit of upload images is per thread. So I can only upload 5 more pictures. Instead of the whole 9 images after the mod I intended to upload, I will only upload the last 5 of them, which were taken at 30mm(F5.6, F11) and 40mm(F4, F5.6, F11) . This is where the most improvements happen. As you can see there are huge improvements at 40mm and even 30mm at F11 (which is unusable before the mod) become free of vignetting.

Edit: I see that I only managed to upload 4 more images in this post, so the 40mm F11 sample after modification is missing from the set. I may try later today to upgrade my account to see whether I can upload more samples.

[wstruth]

Very cool. From my quick viewing it looks like a success.

[shyrsio]

This is an amazing intervention. I urge you to zoom at 100% and compare the results for each setting to see if there's any change in IQ from difraction.