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Alright La7200 again... on the Blackmagic 2.5k

Matthew Marshall

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Trying to understand / fix the LA7200 to make it clearer, less CA, sharper and more usable. Building a lens from other lenses to attempt to do this.


In any case, im trying to understand what is causing the issues in general using the LA7200 on DSLR or Blackmagic cinema camera.

Is it because of the sensor cropping then? The LA7200 was designed for a 4:3 sensor crop therefore it will not work correctly on the BMCC, or larger sensors?


Or is it simply a design flaw overall of the LA7200 , poor lens design / poor coating


If it is a crop sensor issue, what is happening on the LA7200 to cause issues because the sensors are larger crop.


Is the back lens element simply too small, where the image is being projected against the taking lens too small therefore when desqueezed you see parts of the image you really should not? i.e. the taking lens and the sensor is capturing too much of the la7200?

By adding a diopter behind it does that then, make it larger i.e. projects less of what is seen from the front of the la7200 to the sensor?

Or is it more complex than this.

I have tried using a diopter behind the LA7200 i think its a Tokina 0.5 Not sure. I mean it says No 0.05 so I dont know at this point. In any case it does not seem to help.

Just trying to understand, thanks anyone that can help.

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If possibly and if anyone has the lenses. Would someone be able to do a comparison between the Canon 16-35 2.8L and the Tokina 11-16 on the LA7200? Id be very interested in that. It appears though it may be because its was 35mm just at the focal length with it starts CA Plus i was using it at pretty fast speeds since it was dark out.

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I just cant help but think that all the Panasonic LA7200 needs is something like a focus module so it can be adjusted for different sensors and lenses. Something that replaces the existing back, add two more lens elements behind the existing back element. Adjusts them all to make it fit right. Plus a seperate ring for close and far focus. I dont know. something like that.

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the easiest way to lower ca is to use a smaller sensor or close aperture down a bit.  


I think the way you could modify the LA7200 for improved overall image performance is this method...


1.firstly attach your favourite/best performing taking lens to the la7200 and mark out where you need to set the lens in order to obtain infinity through the la7200.


2.separate the front and rear optics of the LA7200


3.place a SLR magic +0.33 diopter on the front. (or similar achromatic diopter with relatively low strength.  dont go stronger than +1) - it needs to be achromatic.


4. with the taking lens set to how you had it to get infinity, now change the distance between the two LA7200 optical elements in the same way he's doing on the video above.  Adjust it until the image is sharp at infinity.


5.  mark out the exact distance you need the two LA7200 elements to be spaced in order for it to be sharp at infinity.


6. make a frame to hold the optics in their new position.





what you will have done with this mod is changed the 2 element original design which is Afocal (it has no focal length), added a diopter which changes the total focal length of the la7200 diopter to a 1000mm focal length (for a +1 diopter) or a 3300mm focal length (for the slr magic +0.33) and changes the lens into a 'quasi' 4 element design, obtaining the correction properties from the achromatic diopter.  reducing the spacing between the two original LA7200 optical elements will counteract the magnification effects effects of the diopter and will bring the focal length back to 'Afocal'.  when infinity is achieved it means the lens is back to its afocal state, but with the achromatic diopter helping it out.


The stronger the diopter, the more you will need to change the distance between the two original la7200 elements so the +0.33x diopter will be a great purchase.  the included +1.3x diopter will also be helpful in getting closer and sharpening a little more I imagine:)


All theory, but i should work and will be a fun project if you can afford to take the risk!

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