# Flat-field Correction



## DMartin (May 29, 2019)

Being curious I thought to see what the new "flat-field correction" does for me.  As it turns out, maybe nothing.  It's always (as far as I can see) greyed-out.   Anyone know why?


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## Johan Elzenga (May 29, 2019)

Have you read the instructions on how to create a flat field corrected DNG? You need to select two images: one normal image taken with the lens, and one image where you covered the front lens with a diffusor.


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## PhilBurton (May 30, 2019)

Johan Elzenga said:


> Have you read the instructions on how to create a flat field corrected DNG? You need to select two images: one normal image taken with the lens, and one image where you covered the front lens with a diffusor.



What can we use for the diffuser?

Phil Burton


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## Johan Elzenga (May 30, 2019)

You need a good quality diffuser, otherwise your flat field corrected image will be corrected for a bad diffusor rather than its lens. They are for sale at (some) photo stores. ExpoDisc 2.0ExpoDisc 2.0 82mm White Balance Filter


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## clee01l (May 30, 2019)

Johan Elzenga said:


> You need a good quality diffuser, otherwise your flat field corrected image will be corrected for a bad diffusor rather than its lens. They are for sale at (some) photo stores. ExpoDisc 2.0ExpoDisc 2.0 82mm White Balance Filter


What is the difference between using this filter and shooting an 18% gray card?   Can you do flat field corrections using an 18% gray card instead?


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## Johan Elzenga (May 30, 2019)

clee01l said:


> What is the difference between using this filter and shooting an 18% gray card?   Can you do flat field corrections using an 18% gray card instead?


I guess you can, but you would have to make sure that you use a perfect illumination source. Even the slightest unevenness in the illumination will cause a problem.


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## clee01l (May 30, 2019)

The 18% gray card is used for determining WB.  The earlier Link is for a WB filter.   A Diffusion Filter seems to be a different animal:  Soft Focus. Using Diffusion Filters: A Comparison of LEE Soft Filters 1 to 5
B00466ZQUI
This can also be improvised by smearing petroleum jelly on a UV filter or shooting through a nylon stocking.


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## Johan Elzenga (May 30, 2019)

The idea is that you need to make a photo of a completely uniform surface, so that any light falloff you see is caused by the lens or the sensor, not by the light source or the surface itself. The LEE diffusion filters don’t do that, unless you would stack a few dozen on to each other. The ExpoDisc does create a uniformly lit image.


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