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How to select the "quality" and "pixel dimensions" when saving JPGs


Tom Mann

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The question of setting the quality and size of JPGs often comes up, and arose again a few minutes ago in a thread on underwater photography.

Here is how I handle this issue:

There are two parameters involved: "quality" and pixel dimensions. To save typing or saying "pixel dimensions" over and over, most people use shorthand and refer to this number as either "size" or "resolution". These terms should never be confused with the physical dimensions of a print (expressed in inches or cm), or with "pixels per inch" (sometimes also called "resolution"). Fortunately, selecting appropriate values for "quality" and "resolution" can be done quite independently of each other.

"Quality":

(a) If you think you might ever edit the JPG that you are about to save, set the quality to the absolute highest possible value. Depending on the software, this might be called 100%, "12", or sometimes, "10". This is a simple, almost completely mindless decision.

(b) If you are sending the file to someone who either isn't very critical, or who you know will only be looking at the image on a low resolution monitor or a smart phone, you can save space and easily go down to quality factors in the 6-8 range (on a scale of 12) with hardly anyone (in this category) ever noticing.

(c) If you are going to post the image on either your own website or on a website such as this one, where it is likely to be subjected to considerable scrutiny, but it's not ever going to be edited further, go for a quality factor of 9 or 10 (on a scale of 12).

(d) If you are having the file printed at almost any size, q = 9 or 10 (on a scale of 12) is usually fine, so long as the pixel dimensions are appropriate (see below).

"Pixel dimensions" (a.k.a., "resolution):

The end use determines how you should set this value.

(e) If you are going to make a large fine art print from the file, don't EVER intentionally reduce the resolution (the pixel dimensions) of the image. In other words, don't EVER "down-rez" your file. To get your image to fit a particular aspect ratio, or if you need to straighten the image, you might have to crop off a bit of the image, but don't reduce the pixel dimensions any more than absolutely necessary than to get the shape right.

(f) If you are going to post the image on your own web site (ie, one for which you have designed the page), you reduce the pixel dimensions to fit the spot you have set aside on your web page.

(g) If you are going to post the image on someone else's website (say, PhotoshopGurus.com), you have a couple of choices: you can either down-rez the image yourself to fit the rules of the website, or you can let the website software down-rez it automatically for you. If you are lazy, do the latter. If you want the best looking image, do it yourself. Often there are huge differences in quality between using the automatic software and sizing it appropriately yourself. Typically having the longer dimension be around 650 pixels is a reasonable compromise.

(h) If you are going to print the file at a small to medium size, set the ppi (pixels per inch) to something between 150 and 300 ppi, turn on "resampling", and tell it the physical print size, and let PS do the computation of an appropriate number of pixels.

(i) If you are going to make a HUGE print (ie, poster size to billboard size), do the same as (e), and if you have a good printing company, they will do a better job of up-rez'ing your image than you can possibly do.

(j) If you are printing something that will be produced using a half-tone process (eg, a large run of books, a newspaper, etc.) again follow (e). For the past 5 or so years, any good printer will do a better job at adjusting your image to fit their linescreen and other half-tone parameters than you will ever be able to do.

(k) If you are sending your image to a cell phone, follow (g).

(L) If you have serious concerns about people stealing your image from the web, either down-rez it to about 300 pixels (longest dimension) - the size of a big thumbnail, or watermark a larger version. At thumbnail dimensions, people will be able to get a reasonable impression of your image but they won't be able to use it to print anything larger than a couple of postage stamps. ;-)

HTH,

Tom M
 

ALB68

Dear Departed Guru and PSG Staff Member
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JPGs certainly serve a useful purpose these days, but "back in the day" I learned that jpgs were referred to as "lossy" and should be avoided except for e-mail and online. Subsequently, I developed a habit of saving work I intended to print in BMP (back then) or TIF that did not lose quality after multiple saves. I think I am correct in saying that jpgs suffer data loss when they are compressed and the more times they are compressed the more you lose. I use Tiffs and native PSDs for printed and engraving output. Lately I have been saving as PNGs for online as I believe they are less lossy than jpgs plus maintain transparency,,correct? Jpgs still work fine though and use them a lot.
Thanks for posting this info, it is huge source of confusion to a lot of people.
 

Tom Mann

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Hi Larry - Don't forget that there was a big discussion about the differences between the various JPG quality settings and a completely uncompressed file about a month ago:

http://www.photoshopgurus.com/forum/general-photoshop-board/44116-save-jpeg-quality-10-vs-12-a.html

The conclusion was that it was impossible to see ANY difference between q=12 and uncompressed, a tiny difference between q=10 and uncompressed, and substantial differences showing up at q=8 and below.

Best regards,

Tom
 

Tom Mann

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Hey, Larry - You're right. Even though, strictly speaking, the topic is only about JPGs, I probably should add some comments about other file formats, such as those you mentioned, for archival storage. As far as I'm concerned, these days, the main drawback to JPGs for archival storage is bit rot. If there is a single bit flip, uncompressed formats such as BMPs and uncompressed TIFFs show only one pixel being changed, whereas with JPGs all the data that follow the error is corrupted. I'll add something later tonight.

T
 

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