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03-22-2017, 04:42 PM   #1
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What is Jpeg Quality ?

On Picture Exif there is a Line Item called JPEG Quality followed by a numerical identity. Anybody know What this is a measurement of and or what factors are used to determine it?


e.g. JPEG Quality 85
e.g. JPEG Quality 98


Last edited by honey bo bo; 10-25-2017 at 07:59 AM.
03-22-2017, 04:44 PM   #2
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You can select the JPEG quality when converting from DNG, that's all I know.
I don't really know what it is...
03-22-2017, 04:52 PM   #3
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It affects the 'strength' of the compression algorithm. jpg compresses files by reducing the detail but in a way which is not obvious to the eye. The higher the quality, the more detail is retained but the less the file is compressed. Drop the quality to 50% and you will get a small file but with noticeable artefacts introduced by the compression process. jpg is a 'lossy' compression process because it irreversibly reduces detail. Other compression methods do not remove detail but may not be as effective as jpg. File size is still an issue for web photos but much less of an issue in other situations due to memory and storage being much cheaper.
03-22-2017, 05:01 PM   #4
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QuoteOriginally posted by honey bo bo Quote
Anybody know What this is a measurement of
Relates to how much compression is applied. The lower the number the poorer the quality of the resulting image but also the smaller the resulting file. So 100% is theoretically no compression, 80% is usually fine for most purposes except printing and 60% is OK for quick emails. Never used anything lower than that. There is a significant size difference so if you have a need to email files it is worth the time to check several compression strengths to see ho low you can go and still get acceptable image quality for your purpose.

03-22-2017, 05:58 PM   #5
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QuoteOriginally posted by jatrax Quote
Relates to how much compression is applied. The lower the number the poorer the quality of the resulting image but also the smaller the resulting file. So 100% is theoretically no compression, 80% is usually fine for most purposes except printing and 60% is OK for quick emails. Never used anything lower than that. There is a significant size difference so if you have a need to email files it is worth the time to check several compression strengths to see ho low you can go and still get acceptable image quality for your purpose.

I watch the Dimensions when I transfer Photos For this Site and Email etc. using Picasa to export e.g. Dimension 1024 X 681 for this site; larger for Flickr and smaller for Emails. I notice in the Picasa Exif on the side they have JPEG 99 (422) . Is the 422 relating to Width when you start losing Quality or is it related to something else? Also I notice the dimension is even less when I look at the Properties(right click) on the Photos Incl. the Ones I posted Woodpecker 309 X 400 JPEG 85, Hummer 437 X360 JPEG 98. Reduced even more by this site or Google or Who?
03-22-2017, 06:24 PM   #6
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The appropriate compression you use when exporting a picture is heavily dependent on what the picture is of. A high compression (lower #, such as 30 in photoshop vs 70+) will murder the gradiance of the sky/water or large area of smooth color as it will attempt to compress similar pixels as a single color mixed with dithering.

Where as images that don't have smooth tones will appear better with lower quality. Fine details however might get soft or lost with a higher compression.

The problem is, you don't want to do 100,as that would make the image huge (in file size affecting bandwidth) , you might as well use 24bit PNG in that case which is a lossless compression (jpeg is lossy, detail is discarded for sake of compression).

Rather you find your compromise, I tend to save my 1080p sized files as jpeg quality between 60 and 80, going with 80 usually when I have lots of smooth tones that could end up banded if I used a lower quality (ie higher compress, smaller file size)

---------- Post added 03-22-2017 at 09:26 PM ----------

Btw 422 might be file size if it had a letter next to it, such as 422 kilobytes (nearly half a megabytes) if the image is only 1024x768, half a meg is pretty hefty/excessive.
03-22-2017, 06:50 PM   #7
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"422" relates to color compression. In JPEG the image data is first converted from RGB to YCbCr. From there it then reduces the chroma resolution according to the user's choice in a process known as chroma subsampling: 4:4:4 means that it has the same resolution as the luminance channel and therefore uncompressed, while 4:2:2 indicates that chroma has half the resolution of luminance. This takes into account the fact that humans perceive chroma changes less well than luminance changes.

03-22-2017, 06:56 PM   #8
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One of the things I love about Lemke Software's GraphicConverter is the JPEG/JFIF options screen in the save dialog...

I set the quality ridiculously low to show the JPEG artifacts in the preview...

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03-22-2017, 07:41 PM   #9
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QuoteOriginally posted by honey bo bo Quote
I notice the dimension is even less when I look at the Properties(right click) on the Photos Incl. the Ones I posted Woodpecker 309 X 400 JPEG 85, Hummer 437 X360 JPEG 98. Reduced even more by this site or Google or Who?
I can't see where you got these dimensions from, but there is a slight difference between the EXIF information and the actual dimensions it is displayed at on this web page (on my 12.5" notebook's 1366 x 768 screen, anyway). It appears that the website templates resize the images to fit the device they are being viewed on so that it isn't necessary to scroll to see the entire image. I'm viewing it on Chrome, so possibly Firefox or Safari or IE handle this resizing differently, but generally speaking web site templates try to give the same look in different browsers, so I doubt if the resizing is browser dependent.

Bottom line, you lose some control over how your images look when you post them online, for a variety of reasons. The positive side effect is that when some pixel peeper makes disparaging remarks about the photos you post, it's what he is viewing it on that is the problem, not your photographic skill.
03-22-2017, 07:54 PM   #10
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QuoteOriginally posted by honey bo bo Quote
I watch the Dimensions when I transfer Photos For this Site and Email etc. using Picasa to export e.g. Dimension 1024 X 681 for this site; larger for Flickr and smaller for Emails. I notice in the Picasa Exif on the side they have JPEG 99 (422) . Is the 422 relating to Width when you start losing Quality or is it related to something else? Also I notice the dimension is even less when I look at the Properties(right click) on the Photos Incl. the Ones I posted Woodpecker 309 X 400 JPEG 85, Hummer 437 X360 JPEG 98. Reduced even more by this site or Google or Who?
I see your woodpecker at 695x900 pixels, the hummingbird as 437x360. It's possible your browser is resizing the image? Or your settings on Pentax Forums have a max width of the image?

It's specifically referencing Lightroom, but I always like this link for a practical discussion on jpeg settings: Jeffrey Friedl's Blog » An Analysis of Lightroom JPEG Export Quality Settings It's worth pointing out that the quality levels in lightroom won't directly correspond to the quality levels in Picassa. It's still a good read.

If you're really curious on how jpeg works, JPEG - Wikipedia is worth a read. By no means necessary for any photographer, but interesting if you're math inclined.

QuoteOriginally posted by jatrax Quote
So 100% is theoretically no compression,
There is a lossless version of jpeg, but afaik no standard programs implement it. The "100" setting in any program will involve the lossy compression scheme. It will be mostly indistinguishable from the original though, at least through most human eyes and viewing devices.
03-22-2017, 08:13 PM   #11
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It's not a measure of anything. It's just an arbitrary unitless scale. Not every program uses the same 0-100 scale (e.g. some are 0-10), and 80 in one program does not mean the same thing as 80 in another.
03-22-2017, 08:39 PM   #12
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QuoteOriginally posted by RGlasel Quote
not your photographic skill.

I knew it ! I'm a victim of an International Webborn conspiracy. It appears that I have no Control over the size of my end product or the quality depending on the medium and the end receptor. So that in mind I'll put my Tin Foil Helmet back on and push send hoping that the recipient will understand or at least colour in the missing parts.
Thanks to all for your responses.
03-23-2017, 05:36 AM   #13
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QuoteOriginally posted by BrianR Quote
It's specifically referencing Lightroom, but I always like this link for a practical discussion on jpeg settings: Jeffrey Friedl's Blog » An Analysis of Lightroom JPEG Export Quality Settings It's worth pointing out that the quality levels in lightroom won't directly correspond to the quality levels in Picassa. It's still a good read.
It is indeed. As a heavy LR user, I really needed that.

Some cameras output crazy huge JPEG files that are effectively lossless. For example, they have settings like NORMAL, FINE, SUPERFINE, and if you set it to super then the files become enormous. And yet, the only possible way to see any difference between fine and super is by taking a shot that's all noise and then pixel-peeping it. Super can capture the noise accurately, pixel-for-pixel, but for any other kind of subject matter it's indistinguishable.

It's been rumored that some cameras are designed to produce huge JPEGs simply because people now associate bigger = better, so the superfine option would be a way to appease that belief, even though it has little-if-any practical value.

I'm a believer in JPEGmini. I've been habitually exporting images from Lightroom at 85% -- which, according to the article I just looked at, that's the second-highest level on LR's 12-step scale. Then I've been running them through JPEGmini. It cuts them down to sometimes half the size, while the visual change is very minimal. I've tried pixel-peeping the files, and sometimes it's just barely possible to pick out some difference at that level. (Maybe I should just export at 100%, if I'm going to use JPEGmini on them anyhow.)
03-23-2017, 09:41 AM   #14
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QuoteOriginally posted by aremmes Quote
"422" relates to color compression. In JPEG the image data is first converted from RGB to YCbCr. From there it then reduces the chroma resolution according to the user's choice in a process known as chroma subsampling: 4:4:4 means that it has the same resolution as the luminance channel and therefore uncompressed, while 4:2:2 indicates that chroma has half the resolution of luminance. This takes into account the fact that humans perceive chroma changes less well than luminance changes.
Not sure jpeg even has a 4:2:2 YUV option since that's a video thing.
03-23-2017, 09:52 AM   #15
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QuoteOriginally posted by kb244 Quote
Not sure jpeg even has a 4:2:2 YUV option since that's a video thing.
Look at boriscleto's screenshots above, they clearly show an option for subsampling. Also, from the Wikipedia page:

QuoteQuote:
The transformation into the Y′CBCR color model enables the next usual step, which is to reduce the spatial resolution of the Cb and Cr components (called "downsampling" or "chroma subsampling"). The ratios at which the downsampling is ordinarily done for JPEG images are 4:4:4 (no downsampling), 4:2:2 (reduction by a factor of 2 in the horizontal direction), or (most commonly) 4:2:0 (reduction by a factor of 2 in both the horizontal and vertical directions). For the rest of the compression process, Y', Cb and Cr are processed separately and in a very similar manner.
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