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06-30-2009, 09:13 PM   #1
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Image Dimensions Out of Camera

I don't shoot Raw often so maybe this makes sense with more experience, but I need some help to understand this...

I happened to shoot some night shots using the Raw+JPEG setting.

When I opened one of the raw file in Photoshop the initial image dimensions were:

12.933 inches by 19.467 inches at 240 pp

opening the out of camera JPEG for the same image, the initial dimensions were:

43.111 inches by 64.889 inches at 72pp


I understand that the RAW file captures more info per pixel so the higher pp values seem intuitively sensible. It also makes sense that the Height*Width ratios are essentially the same.

But what I can't get my head around is why would the JPEG width and height be more than those of the raw file?

Thanks in advance for the help with this.

Re-tom

06-30-2009, 09:35 PM   #2
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Cameras don't take pictures in "inches" or in "pixels per inch". they take images in *pixels*, period. Any other being reported to you is a fantasy invented by the program you are using to display the numbers - it's making some sort of guess as to how many pixels yopu might by trying to squeeze into an inch, and then telling you how big the image would be *if* you in fact did squeeze that numbers of pixels into an inch.

The K20D takes pictures at 3104 pixels by 4672 pixels, period. The camera doesn't camera how many of those pixels you squeeze into an inch when viewing or printing the images.

If you try to squeeze 240 of those pixels into an inch, you'll end up with 12.933 by 19.467 inches (3104 / 240 = 12.933, and 4672 / 240 = 19.467). On the other hand, if you happen to try to squeeze 72 pixels into an inch, it works out to 43.111 by 64.889 inches. Again, the camera couldn't care less how many pixels you decide to squeeze into an inch when viewing or printing. The fact that Photoshop for whatever silly reasons assumes you might want to squeeze 240 pixels into an inch for a RAW file but only 72 for a JPEG file is irrelevant - the picture is 3104x4672 pixels, period. How many of those pixels you squeeze into an inch is between you and Photoshop; the camera doesn't know or care about that. Nor does it matter if you start with JPEG or RAW - you are welcome to print or view a JPEG file at 72ppi, 240ppi, 3dpi, or 30000dpi.
06-30-2009, 09:39 PM   #3
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Time to try out my new calculator.

240/72 = 3.333333

3.333333 * 12.933 = 43.111

It's the display settings in Photoshop that is confusing the issue.
06-30-2009, 11:40 PM   #4
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Or, another way of calculating it:

RAW:
12.933 inches by 19.467 inches at 240 pp = 3103.92 pixels by 4672.08 pixels
(12.933 x 240 and 19.467 x 240)

JPEG:
43.111 inches by 64.889 inches at 72pp = 3103.992 pixels by 4672.008 pixels.
(43.111 x 72 and 64.889 x 72)

Taking into account the differences due to decimal places, the dimensions of both shots are therefore identical.

QuoteOriginally posted by tpeace Quote
I understand that the RAW file captures more info per pixel
That is incorrect. The RAW file contains the information as captured direct from the camera and stored as a 12-bit RAW file. The JPEG is the same 12-bit RAW file that has then processed by the camera using whatever settings were in the camera, to produce the final 8-bit JPEG file.

So yes the RAW file has more detail that the JPEG file, but both RAW and JPEG both start out as a RAW file anyway. Its just that the camera does all the work to spit out the JPEG.

07-01-2009, 09:32 AM   #5
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Same number of pixels either way.

240ppi would be considered an optimum *print* resolution but you could still take this number up or down depending whether you want a smaller or larger print size...without changing the number of actual pixels. 240ppi works out nicely for Epson printers that have a native resolution of 720ppi (720/3=240) and is about the lowest resolution that will still guarantee a high quality or "photo quality" print. Below about 200ppi (180ppi would be the next easily divisible resolution for an Epson printer) and you may start to see resolution artifacts upon close inspection of the print. At 120ppi and below, you're definitely into the lower "poster quality" type resolutions.

72ppi is a resolution more appropriate for viewing on a screen or display since that is the typical "native" resolution of a computer display (many are closer to 96 ppi though).

The resolution coming from a raw file is completely up to the raw converter that's used. ACR/Photoshop simply assumes 240ppi (I noticed in PS CS4 this can be changed to a different default resolution). The Pentax "native" raw conversion software may assume a completely different resolution when exporting raw compared to ACR/PS. I can remember that the raw converter for my Sigma SD9 (Sigma Photo Pro software) always defaulted to 180ppi giving a bit of a false impression as to the maximum printable size of the image.

Remember, it's the total pixel count that really matters, not the final resolution in ppi. PPI is simply a way to put a "number" to the total pixel count to get an idea of the final size that's appropriate to it's intended purpose. Also, there's no free lunch when it comes to interpolating more pixels in something like Photoshop. There's really never more detail in a digital capture than what was in the original so upsampling isn't really going to get you much.

Regards,
Terry Wyse
07-01-2009, 01:18 PM   #6
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Thanks everyone, appreciate the education.

Seeing the formulas that Pop posted really helps clear it up for me. I knew the numbers had to reconcile somehow, but I couldn't figure out to put them together. Obvious now that I see it.

re-tom
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