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Preferred technique for downsizing a photo?
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Oct 12, 2021 10:56:50   #
TomHackett Loc: Kingston, New York
 
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)

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Oct 12, 2021 11:02:26   #
CHG_CANON Loc: the Windy City
 
This post was written to address this question: Recommended resizing parameters for digital images

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Oct 12, 2021 11:26:20   #
BebuLamar
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)
I have a CR2 file (Canon raw format) that is 2736x... (show quote)


I would try to see how far you have to go down on the quality to get 5MB. I hope not too much.

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Oct 12, 2021 11:40:33   #
BigDaddy Loc: Pittsburgh, PA
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)
I have a CR2 file (Canon raw format) that is 2736x... (show quote)

Most editors let you resize your photo to whatever you want, and show the before and after file size as well. I often use the free browser/editor FastStone to do this. You can punch in any number[s] you want and it will give you the new file size. For example, I just loaded a 6000x4000 9mb picture and resized it to 3000x2000 and it came up with 6mb file size. Changing to 2700x1800 gives 4.86MB.

This will not degrade your picture at all, or not to any noticable degree, but will make the picture smaller, but big enough for even 4k displays.

FastStone is super easy to use for this. Just open your pic in FastStone, Press Ctrl+R and punch in the numbers while maintaining the aspect ratio. It will show the original and new file size for you.

Changing the compression ratio will also change the file size, but will start to degrade quality pretty quick. For printing or a contest, I I'd leave the compression ratio low, at least 8 or 10.

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Oct 12, 2021 12:10:26   #
jeep_daddy Loc: Prescott AZ
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)
I have a CR2 file (Canon raw format) that is 2736x... (show quote)


If you are using Photoshop you can save the image as a jpg by holding down Ctrl/Shift/Alt then Enter - and it will open a dialog for "save for web" which saves it as a jpg and allows you to make changes in quality. As you can see in the image I've uploaded and where I've circled in red that you can simply change the quality as you see what size it will be saved as. I was able to take a 13mb image and reduce it down to 4.99mb by simply watching the lower left data as I reduce the quality slider. It doesn't really make a noticeable difference. It simply increases the compression used on the jpg file.

If using Lightroom it's even easier. You can tell it to export as a 5mb file and it will.



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Oct 12, 2021 14:26:41   #
DirtFarmer Loc: Way too close to New York City
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large...


A few months ago I studied what happened when jpgs were rewritten multiple times. I understand that you are coming directly from the raw file rather than editing jpgs, but the study also looked at the changes due to varying the "quality factor" during the initial writes to jpg. The study may be of some interest to you for a couple reasons: (1) the study of the "quality factor"; and (2) the description of a "blink test" which allows you to pixel peep two (or more) images by viewing them alternately so you can see if the differences are significant. Rather than depend on visual comparisons, I use a numeric approach to find the RMS change in the image pixel from the image file compared to the original image file.

The study was too large to post in UHH so there is a link to the study (14 pages) here

The "quality factor" is different for different editors. For Photoshop it runs from 0 to 12. For Lightroom, IrfanView, FastStone, and ImageMagick it runs from 1 to 100. (It appears to me that a Photoshop quality of 0 is roughly equivalent to a quality of 18 or thereabouts in the other editors).

Personally I found that 80 was a pretty good quality level for most photos and I really couldn't see much difference visually between a quality of 80 and 50, so the lower "quality factor" may be useful in reducing your file size. YMMV. I would recommend using the blink test to look at the image in high and medium quality to see if there are any significant differences. One advantage of doing the blink test using Photoshop is that you can expand the image to pixel peep any small area you want.




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Oct 12, 2021 14:37:17   #
TomHackett Loc: Kingston, New York
 
Fascinating work. Thanks.

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Oct 12, 2021 14:40:30   #
TomHackett Loc: Kingston, New York
 
I also have found Save for Web handy. But my question is whether you get more degradation by lowering "Quality" or lowering the Image Size, assuming the end result of either is the same file size.

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Oct 12, 2021 14:55:51   #
TomHackett Loc: Kingston, New York
 
2700x1800 will indeed give you 4.86 megapixels. But that's not necessarily the file size. Photoshop, for instance, tells me that a 2700x1800 pixel image has an "image size" of 27.8M. I guess there are other things (like metadata) in the file that take up this extra space. When I save it as a JPEG with the highest "quality," (12 in Photoshop), the resulting file is 4.2MB (or so says Finder or Windows Explorer). So, we're working with three different numbers here, and I don't know if there's a neat formula to connect them. It may depend on the amount of metadata, and it certainly depends on the compression algorithm and the nature of the image.

Interestingly, if I create a Photoshop image of 2700x1800 pure white, PS says the Image Size is 13.9 MB. If I save it as a JPEG with quality of 12, I get a resulting file of 336 KB.

So, I'm interested in whether resizing the image (in PS, for instance) or compressing it by saving it as a JPEG (with quality of x) results in more degradation.

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Oct 12, 2021 15:10:15   #
DirtFarmer Loc: Way too close to New York City
 
2700 x 1800 x 3 = 14.58 MB (3 is the number of colors)

But a jpg compresses the image, largely on the basis of color. So since all the colors are the same in an all-white image, it's not surprising that it could be compressed so much.

Personally, I would lower the quality and look at the result with the blink test. If you lower the image size you lower the resolution.

I'm assuming you are entering a digital file rather than a print.

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Oct 12, 2021 15:29:16   #
TomHackett Loc: Kingston, New York
 
Save for Web is a great option, as your image shows. Unfortunately, it does not support Adobe's newer algorithms (e.g. Preserve Details, Preserve Details 2.0). I suppose that's why it is labeled (Legacy). Then again, we're trying to downsample, and Preserve Details is really for upsampling.

It looks like Export As. . . might be the method of choice, although it does not appear to support Preserve Details 2.0.

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Oct 13, 2021 06:20:31   #
Jimmy T Loc: Virginia
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)
I have a CR2 file (Canon raw format) that is 2736x... (show quote)


CHG_CANON has already given you a great answer.
However, I use PhotoScape X http://x.photoscape.org/ (Free)
PhotoScape X (Under the "Batch" tab) will allow you to select any of about 12 different variables to "Save As" and will process many pics at a time.
Best wishes on your choice, and . . .
Smile,
JimmyT Sends

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Oct 13, 2021 06:43:31   #
Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)
 
TomHackett wrote:
I have a CR2 file (Canon raw format) that is 2736x3648 pixels (about 10 megapixels). I want to submit it to a juried show where the conditions are that it must be JPEG no larger than 5 mb. If I take the original file and convert it to JPEG, I have serveral options as regards to "quality." It can be 0 (low) to 12 (maximum). Using 12 gives e a file size on disk of 10.3 mb, which is too large.

I have a few options to meet the constraint. I can use one of the editing tools (Photoshop, Preview, Photos, etc.) to reduce the number of pixels in the image; I can increase the JPEG compression (by reducing the "quality") or I can use some combination of the two. In any case, it seems like a bit of trial and error to find the right combination to get as close to 5 mb as possible without going over.

What technique or combination would do the least "damage" to the image? (I'm not resaving JPEG files; I'm always starting from the original CR2 file.)
I have a CR2 file (Canon raw format) that is 2736x... (show quote)


Events like these usually specify the file size and the maximum number of pixels on the longest side - usually something like 2048 px on the long side and not more than 5 mb file size. The file size constraint is driven by the number of participants and the software used to review and the pixel dimension is driven by the viewing distance and/or print size. Shows that are just digitally projected usually do not allow images that are more than 2048 px on a side, because the viewing distance rarely requires more pixels for the perception of sharpness and fine detail.

So my guess is that you will likely contact the show officials to find out if there is pixel dimension limitation, then resize, and ultimately adjust the quality parameter to get you as close as possible to the 5 mb file size limit without exceeding it. You'll find that there is very little difference between an image at 95% and one at 70% quality, other than file size.

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Oct 13, 2021 08:14:27   #
SuperflyTNT Loc: Manassas VA
 
DirtFarmer wrote:
2700 x 1800 x 3 = 14.58 MB (3 is the number of colors)

But a jpg compresses the image, largely on the basis of color. So since all the colors are the same in an all-white image, it's not surprising that it could be compressed so much.

Personally, I would lower the quality and look at the result with the blink test. If you lower the image size you lower the resolution.

I'm assuming you are entering a digital file rather than a print.


Actually until the resolution is the same width in pixels as the display device you’re better off reducing image size. Any resolution greater than the display you won’t see so level of compression will have more of an effect on IQ. Refer back to Paul’s post on resizing images. Follow those instructions.

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Oct 13, 2021 09:14:23   #
BigDaddy Loc: Pittsburgh, PA
 
TomHackett wrote:
2700x1800 will indeed give you 4.86 megapixels. But that's not necessarily the file size. Photoshop, for instance, tells me that a 2700x1800 pixel image has an "image size" of 27.8M. I guess there are other things (like metadata) in the file that take up this extra space. When I save it as a JPEG with the highest "quality," (12 in Photoshop), the resulting file is 4.2MB (or so says Finder or Windows Explorer). So, we're working with three different numbers here, and I don't know if there's a neat formula to connect them. It may depend on the amount of metadata, and it certainly depends on the compression algorithm and the nature of the image.

Interestingly, if I create a Photoshop image of 2700x1800 pure white, PS says the Image Size is 13.9 MB. If I save it as a JPEG with quality of 12, I get a resulting file of 336 KB.

So, I'm interested in whether resizing the image (in PS, for instance) or compressing it by saving it as a JPEG (with quality of x) results in more degradation.
2700x1800 will indeed give you 4.86 megapixels. B... (show quote)

Your right, 4.86MP is not the file size after jpg compression, but before jpg compression, my mistake. But your image that is 2736x3648 is also 10 megs before compression. What is the file size of the jpg image that you wish to submit? The raw file size of 9 megs doesn't matter. If the image size if 2736x3648 then I'd bet the jpg file size way under the 5mb limit?

It's a good question if compression or downsizing the photo effects quality the most. JPG Compression has very little effect at low values. For screen display I've used the blink test to get down to 60% before noticeable artifacts, but for a contest, I wouldn't go below 80%. If the contest is judging via digital display, I'd want to know the display size and would size my photo to those dimensions, or that aspect ratio for sure.

The most important factor I think in downsizing is matching the image size ratio to the display size ratio. Most displays are 1920x1080 (1.77:1), but 4k displays are generally 3840 × 2160 (also 1.77:1) These dimensions should be way under your 5mb file sizes I'd think. If you keep your ratios within these limits, or whatever display size they use, you should have no noticeable deterioration, and compression at 80% or above should be fine.

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