Panasonic GX7: matching the embedded JPEG crop

(Fulvio Senore) #1

I have discovered RT a month ago and I like it very much. It took some time but now I have been able to create very good images from my Panasonic GX7 RAWs.

I still have a problem with the lens profile support. Micro 4/3 raw files embed file correction data and (AFAIK) RT cannot read them. This is a pity because with ACR I can create an image that has exactly the same distortion correction and the same crop as the JPEG embedded in the RAW file. This is useful because the viewfinder gives me a 100% coverage. Micro 4/3 lenses usually have a very large level of distortion that is corrected in software.

RT uses lensfun and I have discovered that the distortion correction for my 15-45 lens is rather poor. I have photographed a test image and I have calibrated the lens, then I have edited my lensfun copy and now distortion correction is very good.

I still have a problem: RT shows a much larger image than the embedded JPEG: I must crop the width from 4600 pixels to about 4400 to have the same field of view. On the other side ACR shows the same crop as the JPEG with an image 4592 pixels wide.

I know that I can crop the image, but the needed crop probably changes from lens to lens, so this would require a manual crop each time. This is far form an optimal solution.

I would like to obtain exactly that same crop as the jpeg to have a 100 coverage in the viewfinder. Is there a way to solve this problem?
It would be nice to be able to do it at the lensfun level but I do not know if it is possible.

(Mica) #2

You could use a partial profile to detect the lens and apply a crop to your image.

(Alan Gibson) #3

Does it change between lenses? My Nikon D800 has a sensor with 7378x4924 pixels, and the JPEG is always 7360x4912+8+6. That is, 7360x4912 pixels starting at (8,6) in the raw image. This is with a variety of lenses.

I would expect a camera to always crop every JPEG to the same size. In theory, the offset might vary between lenses. But does it actually?

(Morgan Hardwood) #4

Send your improved calibration upstream to the Lensfun developers for everyone’s benefit.

(Fulvio Senore) #5

Thank you for your suggestion. I did not know about partial profiles and they look like a great solution to my problem.

(Fulvio Senore) #6

In my camera the size difference is much larger. Most Micro 4/3 lenses have huge distortion that is corrected in software: for this reason there is a big difference between the image recorded by the sensor and the JPEG created by the camera. For my lens the designers have decided that, after distortion correction, the image must be cropped by a visible amount.

(Fulvio Senore) #7

The lens is a zoom one so it needs to be calibrated for many focal lengths. At the moment I have calibrated only the shortest ones: as soon as I will calibrate the full zoom range I will surely send the calibration data to lensfun.