Canon Hackers Development Kit Trial

[JohnRostron]

I recently installed CHDK on my Canon Powershot SC 260HS, mainly so that I could take raw files. After some experimentation, I settled on DNG output. This loads into Photoshop via ACR. I thought I would try it out on some vegetables I was preparing for Sunday lunch. I was expecting that the DNG file would show more dynamic range than the recorded Jpeg, which it did, although not as much as I thought it might.

What I was not expecting is that the DNG and Jpeg images seemed to differ in perspective. For each of the Turnip and Celeriac images below, I loaded a pair into the stack in Photoshop, ticking the align option, then cropping before exporting back. Comparing them there is a definite difference in perspective, as if each was photographed at a different distance. As I understand it, the Camera actually records a RAW file, then converts it to Jpeg, which means that the perspective should be identical. I did note that the DNG image was larger than the Jpeg one in its field of view, but not in the pixel width. As I understand my optics, although the DNG image is actually smaller in the original image, the perspective should be identical. Is this a real phenomenon, or is it a purely perceptual one?

#1 Turnip from DNG


#2 Turnip from Jpeg


#3 Celeriac from DNG


#4 Celeriac from Jpeg


John

[Dave Humphries]

Hi John,

I strongly suspect the jpg images undergo 'lens corrections', to account for barrel/pincushion distortion and the lack of that on the RAW images is what you are seeing as the difference.

When you put them into ACR, does that have a lens profile for your camera?
If so (it may not, as RAW isn't 'standard fare' for that camera), you could apply the 'same' correction in ACR and see if it looks similar.

Cheers, Dave

[chauncey]

the DNG and Jpeg images seemed to differ in perspective

I have no clue what you were meaning to say...
Did you take one image that was preserved as a jpeg and as a RAW, then, doing nothing else,
converted the RAW image to DNG?

[JohnRostron]

Dave, I have just looked at a couple of images taken at a moderate distance (my garden at around15 metres) and there is no 'perspective' effect visible. It could be that, as you suggest, the Powershot suffers from barrel distortion when close up, which is corrected in the Jpeg. The DNG does correctly specify the camera. This is not really a problem, but I was curious about it.

Chauncey, by a perspective effect, I meant that the two images looked as if they had been taken from a different viewpoint or distance. If Dave is correct, which seems likely, then the barrel-distortion correction is probably the explanation for the apparent change in perspective. As I suspected in my original post, the perspective effect is purely a perceptual one.

John

[Manfred M]

John - the camera's firmware, in the raw to jpeg conversion, commonly applies sharpening, contrast, saturation and distortion correction adjustments. This is in addition to setting the white balance. If you shoot raw, you have to do all of these operations during the raw import operation.

In general, a jpeg will look like it has more "punch" when compared to a raw file just opened in your editor.

[JohnRostron]

Thanks Manfred. Although I knew about this in theory, I have not explored RAW as much as I ought to have done and I had not put two and two together to make four. My limited exploration of RAW has not hitherto explored the lens aspects, probably because the lenses I use on my Sony DSLR have not given me any need to. (This is not to say that they have not needed correction, just that I have not spotted anything amiss.)

John

[Manfred M]

Thanks Manfred. Although I knew about this in theory, I have not explored RAW as much as I ought to have done and I had not put two and two together to make four. My limited exploration of RAW has not hitherto explored the lens aspects, probably because the lenses I use on my Sony DSLR have not given me any need to. (This is not to say that they have not needed correction, just that I have not spotted anything amiss.)

John

Panasonic for sure, I believe Olympus does so too and I have read that Sony does as well, embeds lens correction profile data in the raw data. With my Panasonic GX7, in Lightroom or Camera Raw, I cannot override the embedded lens profile (correction) data.

[Dave Humphries]

Panasonic for sure, I believe Olympus does so too and I have read that Sony does as well, embeds lens correction profile data in the raw data. With my Panasonic GX7, in Lightroom or Camera Raw, I cannot override the embedded lens profile (correction) data.

I am partly surprised Manfred, usually proprietary/commercial considerations take precedence and I might have expected this to only apply to those manufacturers already working 'with' Adobe by supporting native, in-camera DNG generation (e.g. Pentax and Ricoh).

That said, it is obviously in all camera/lens manufacturer's best interests to share the protocol used to transfer and apply this data, so may be that's why. I don't believe it is true of Nikon though - if I am lucky, it will determine the lens in use and the focal length (and focus distance?), and apply lens corrections according to data tables contained in the Camera RAW updates supplied to LR/ACR by Adobe themselves, not embedded in the RAW data from the camera. That's what I believed, but I may be mistaken.

In ACR I can manually choose a different lens profile (from the one named in EXIF) and/or uncheck the lens corrections to ignore it if I desire - but I have no experience with Panasonic RAW or the GX-7 in particular.

After-thought - I have a feeling that 4/3 lenses were designed to always need corrections applied, as - unlike other manufacturer's, who try to produce an optically perfect (within compromises) lens - the 4/3 spec. moved with technological progress and decided that a marriage of physical lens plus electronic compensation was the way forward. (to produce smaller, cheaper lenses for their customers)

[dje]

Another vote from me for "lens correction applied in camera on jpeg" !!

Dave