Who knows what is going on between 12 and 11..?

[Eigil Skovgaard]

My workflow normally ends up in a 8 bit JPG, which uncropped and uncompressed is about 70 Mb. Saving at quality 12 can produce files up to 30 Mb. A 30 Mb file is not very portable, so I reduced the quality setting 1 step (to 11) and was surprised that an 8% reduction down the scale resulted in a 43% smaller file - 16.9 Mb! (CS5)
Now my question is: Which pixels were lost - did the algorithme average contigous areas only, making my image less modulated, or was information along the edges averaged too, making my image less detailed. And most important - can you think of a situation where the loss will be of any importance in practice. A careful examination at 100% don't show any differences, so what have I lost.
Who knows what is going on between 12 and 11?
Br, Eigil

[carregwen]

I'm not sure I understand what is going on here. If I take a RAW file from a Canon 50D (15MP) and create a JPEG it comes out between 9MB and 12MB. How do you manage to get 30MB? BTW, a JPEG is only 8bit, it can't be 16 bit. You would need a TIFF or PSD for 16bit.

Sorry! Welcome to CiC! Forgetting my middle-class manners there.

[Colin Southern]

Hi Eigil,

What are the pixel dimensions of your image?

[Dave Humphries]

Hi Eigil,

Welcome to the CiC forums from me, great to have you join with a tricky question!

Personally, I did some 100% tests and didn't see any benefit in going further than 9 (of 12) for my photography.
i.e. Saving at 10, 11 and 12 only appeared to make the filesize bigger, not improve the quality (compared to a sample picture at 9).

The answer to your question is not straight forward, as it is my understanding that the jpg algorithm cuts the picture into 8 x 8 pixel blocks and works with those. I don't know the precise technical workings I'm afraid. I'm sure there's a 'spec' online somewhere, but I probably wouldn't understand all of it myself.

However, it is also important to know what use you intend to put these jpgs to?

a) Certainly re-opening to further edit, jpg is a bad idea, mainly because they are 8 bit and compressed. 16 bit Tif, or PSD, would be better.
b) If for print; via online service, or putting on a CD/DVD to get to a professional printer, then there maybe some benefit in having a higher quality setting - I have no experience. I suspect this is what you mean by "portable".
c) But if 'only' for web display, where you'll undoubtedly (indeed, hopefully) have down sized from 4-5,000 pixels to 700-1400 pixels width (for a typical landscape orientation image), plus post-downsize sharpen, I'd say anywhere between 7-9 will be sufficient.

I'll stop there pending knowing the answer to the "what use you intend to put these jpgs to?" question.

To be continued ...

[Dave Humphries]

He, he, all three mods pounce

[carregwen]

He, he, all three mods pounce

Crouching tigers? Or soft pussy-cats?

[Donald]

He, he, all three mods pounce

The rest of us are scared to intervene!

[arith]

I always save to 100% jpeg and put up on my site for sort of safe keeping; I say sort of because they have lost a few. Raw would be better but I don't know anywhere safe enough that takes RAW and anyway the finished thing is what is wanted.

Non of my files get bigger than 12MB on a 15 MP camera and so you must have a very big camera.

[Donald]

I don't want to divert attention away from the question being asked by Eigil, but find these discussions interesting would wish add to it. The question Eigil is posing is of a more technical nature. Mine is very simplistic but follows the same theme.

Rob - you saw & dealt with the jpg I sent you which you kindly printed for me. You are aware of the size of that which, from memory (not at home) was something just over 6mb. I start getting worried when I see people talk about 20, 30 or even just 9 -12 Mb jpg's. Am I doing something wrong? Should my jpg's be bigger? I still don't fully understand sizing (which is one of the things that's been frightenning me away from printing.

[Dave Humphries]

I like to keep things simple; I can't cope with more.

I have a simple rule of thumb for this,as most other things in life.

Just deal with the image, work in pixels, not physical dimensions, save an unsharpened fullsize jpg after all processing (except the final sharpen).

When it comes to print, don't downsize as you would for web use, but sharpen a little more agressively than normal, save as a separate file at 9 (75%) or more quality.

Only now consider what size you want to print. Take the longest dimension in pixels and divide by say 200, or by half and then 100, to make it easy in the head. That is the size in inches for a decent resolution print.

If you had in mind somewhat bigger than this, how much bigger?
If more than twice, your image probably won't stand it, unless being viewed from a distance.

Don't worry about filesizes, if you follow the above it'll be sufficient and if the result looks OK, why worry?

HTH,

[Shadowman]

I don't want to divert attention away from the question being asked by Eigil, but find these discussions interesting would wish add to it. The question Eigil is posing is of a more technical nature. Mine is very simplistic but follows the same theme.

Rob - you saw & dealt with the jpg I sent you which you kindly printed for me. You are aware of the size of that which, from memory (not at home) was something just over 6mb. I start getting worried when I see people talk about 20, 30 or even just 9 -12 Mb jpg's. Am I doing something wrong? Should my jpg's be bigger? I still don't fully understand sizing (which is one of the things that's been frightenning me away from printing.

I was following this thread and thought I was doing something wrong. I usually set my camera to maximum resolution which gives me the largest file size possible, I have a 10MP camera so I will get anywhere from 6-10MP jpeg file size. I think what Eigil is talking about is the uncompressed file size shown in the photo's metadata. If I look at the meta data for either the 6 or 10MP jpeg file they will both state that the uncompressed size is 28.7MB.

[Meremail]

Hi interesting and ironic discussion. I have just registered for the forum, and the irony is that I was reading the website articles on sensor size etc as I am wanting to write a succinct story on Flickr image posting with respect to compression and size as many people have a poor understanding.
So I ended up here on the forum and the first post I read is on the subject, almost.

When I started using Flickr I would compress down to get a practical upload file size, as I noticed that Flickr post-processes and it doesn't seem to impact on the biewing experience. However, I started to see a problem when the image has a smooth light level or tonal gradient, as posterisation was introduced.
So I changed to reducing the edited file size to 1000 pixels wide which allowed me to use a lower compression rate and I still target about 250K upload size and posterisation seems to be gone now. I still don't understand why people upload 2M files to a web page. I could probably resize lower, so was looking up further detail.

I had not done any tests comparing quality to compression rate as raised here in the context of visible differences, so it will be interesting to hear more. I am thinking that the results will be also determined by the content of the image too, and perhaps different quality results may be seen with different subject matter.

So if I have diverted from the original subject.

Jim

[Shadowman]

Hi interesting and ironic discussion. I have just registered for the forum, and the irony is that I was reading the website articles on sensor size etc as I am wanting to write a succinct story on Flickr image posting with respect to compression and size as many people have a poor understanding.
So I ended up here on the forum and the first post I read is on the subject, almost.

When I started using Flickr I would compress down to get a practical upload file size, as I noticed that Flickr post-processes and it doesn't seem to impact on the biewing experience. However, I started to see a problem when the image has a smooth light level or tonal gradient, as posterisation was introduced.
So I changed to reducing the edited file size to 1000 pixels wide which allowed me to use a lower compression rate and I still target about 250K upload size and posterisation seems to be gone now. I still don't understand why people upload 2M files to a web page. I could probably resize lower, so was looking up further detail.

I had not done any tests comparing quality to compression rate as raised here in the context of visible differences, so it will be interesting to hear more. I am thinking that the results will be also determined by the content of the image too, and perhaps different quality results may be seen with different subject matter.

So if I have diverted from the original subject.

Jim

Hi Jim,

Welcome to the group. I guess you noticed that 1000x... is the maximum allowed on this site and in answer to your question "why people upload 2M files...". I think it's because when we are sold technology (printers/scanners) we are told that more is better and it is for this type of technology. But one thing you will notice, especially with scanners, is that our desired output (web publishing, fine printing) comes into question when our choice of resolution is selected. Whenever I try to scan above 300MP I get the message that the output may not differ depending on how the image will be viewed. So when we decide to post an image that we have scanned or photograph at a higher resolution we believe we are losing quality when we downsize.

[Dave Humphries]

~ I think what Eigil is talking about is the uncompressed file size shown in the photo's metadata. If I look at the meta data for either the 6 or 10MP jpeg file they will both state that the uncompressed size is 28.7MB.

Hi John,

You may well have spotted something here we missed.

As per my post above, I just ignore those figures (I don't see the practical relevance) and when saving files, I never look to see what size they are. OK I just looked, my recent band shots are between 200k and 400k for (on average) about 1,300 x 1050 pixels, so Jim's point (Welcome to CiC, Jim) about 2M jpgs gets my support.

I deliberately avoid any site or storage method that makes me have to conform to a 'maximum' filesize as inevitably the finished image exceeds it by a few kilobytes, then you have to phaff around reducing the quality and/or the dimensions to comply. I just can't be bothered and find another way, usually elsewhere.

Cheers,

[Dave Humphries]

I guess you noticed that 1000x... is the maximum allowed on this site

Hi John,

I don't believe we have a hard maximum dimension, sure we advise people not to post images larger than about 1600px wide or 800px tall, but that's mainly to avoid viewing problems and excessive thread loading times.

It is also true that all images get resized to 700px on the longest side in the posts, but clicking the image will usually show it bigger in a Lytebox, how much bigger depends on lots of things; the viewers screen dimensions, whether the browser is in (F11) fullscreen or not. But you can always right click the image and open it in another browser tab or window to get the image full size (that may require an additional mouse click on the image in the new tab/window, if large).

Hope that clarifies things,

[Shadowman]

Hi John,

I don't believe we have a hard maximum dimension, sure we advise people not to post images larger than about 1600px wide or 800px tall, but that's mainly to avoid viewing problems and excessive thread loading times.

It is also true that all images get resized to 700px on the longest side in the posts, but clicking the image will usually show it bigger in a Lytebox, how much bigger depends on lots of things; the viewers screen dimensions, whether the browser is in (F11) fullscreen or not. But you can always right click the image and open it in another browser tab or window to get the image full size (that may require an additional mouse click on the image in the new tab/window, if large).

Hope that clarifies things,

Hi Dave,

No issues with the format, I usually dance very close to the limits usually trying to target file size more than pxs. Although, I have noticed that some of my images does extend beyond the screen.

[timo2]

In Photoshop, there is no visible difference between 12, 11 and 10, the only difference is compression level and file size. Visible loss starts at 9.

[revi]

OK, my 2¢

First, about image sizes: OP doesn't give image dimensions, but later on 10MP files are mentioned. Uncompressed those should take 30 MB (3 bytes/pixel in 8-bit depth) to give a starting point.

Jpeg is a lossy algorithm, which (if I understood correctly) uses a variation of Fourier transform to calculate spatial frequencies in the image, and throwing away a number of the calculated coefficients. At lower quality settings, more of those coefficients are thrown away, resulting in a smaller file with less high frequency details ('fine details'). I don't know exactly how the algorithm determines what to keep, but my guess is that the values of the coefficients play a role (which explains why simple images with mostly soft colour gradients compress very well: high frequency coefficients are very small or zero). It's also the reason line-art and text suffer from jpeg compression: the sharp edges are high-frequency detail.

Taking all this, the final size of a jpeg file depends on the quality setting (but not in a linear way!) and the image content.

Some figures:
original image: 2476*2476 pixels (6.13 Mpix, so uncompressed size at least 18.5 MB), saved from Digikam as jpeg @ 100 quality took 5.1 MB, @ 90 quality, 1.6 MB, and @ 80 quality, 1.0 MB. Al these include the same metadata.
(Fwiw: I can't see any differences between the best and worst of these three settings...)
The same image @ 80 quality after a very strong blur to get rid of sharp edges: 225 kB (i.e. less than a quarter...)


imaged reduced to 700*700 @ 80 quality (as attached here): 82.7 kB

[Eigil Skovgaard]

Hi Eigil,

Welcome to the CiC forums from me, great to have you join with a tricky question!

Personally, I did some 100% tests and didn't see any benefit in going further than 9 (of 12) for my photography.
i.e. Saving at 10, 11 and 12 only appeared to make the filesize bigger, not improve the quality (compared to a sample picture at 9).

The answer to your question is not straight forward, as it is my understanding that the jpg algorithm cuts the picture into 8 x 8 pixel blocks and works with those. I don't know the precise technical workings I'm afraid. I'm sure there's a 'spec' online somewhere, but I probably wouldn't understand all of it myself.

However, it is also important to know what use you intend to put these jpgs to?

a) Certainly re-opening to further edit, jpg is a bad idea, mainly because they are 8 bit and compressed. 16 bit Tif, or PSD, would be better.
b) If for print; via online service, or putting on a CD/DVD to get to a professional printer, then there maybe some benefit in having a higher quality setting - I have no experience. I suspect this is what you mean by "portable".
c) But if 'only' for web display, where you'll undoubtedly (indeed, hopefully) have down sized from 4-5,000 pixels to 700-1400 pixels width (for a typical landscape orientation image), plus post-downsize sharpen, I'd say anywhere between 7-9 will be sufficient.

I'll stop there pending knowing the answer to the "what use you intend to put these jpgs to?" question.

To be continued ...

Thank you all - and sorry about this late respons. I have been waiting for an email to pop up.

Dave,
Your answer confirms my assumption, that reducing the quality setting from 12 to 11 would do not harm to the image quality in practice.
My workflow goes from ff raw managed in CO5, then output as tiff to be edited in Photoshop. I do all editing in 12 bit, save the tiff as a master and finally converts to jpeg for upload to different sites. When the tiff is flattened, the file size is 69,8 Mb (uncompressed) for an uncropped 6048 x 4032 image. If the pattern is of high frequency, the output jpg file can be as large as 30 Mb (using Q12).
I primarily fotograph for stock and try to keep a high technical standard.
Br, Eigil

[Eigil Skovgaard]

In Photoshop, there is no visible difference between 12, 11 and 10, the only difference is compression level and file size. Visible loss starts at 9.

Tim,
This is my point. "No visible difference.." not on a normal screen at least - then what is the overflow used for - the machines printing large posters.. or maybe because we don't want to throw good image information away..? There must be a logical reason for levels 10 to 12. Otherwise my new choise will be Q10. Is there an engineer from Adobe out there