What's new
By:
Photoshop Gurus Forum

Welcome to Photoshop Gurus forum. Register a free account today to become a member! It's completely free. Once signed in, you'll enjoy an ad-free experience and be able to participate on this site by adding your own topics and posts, as well as connect with other members through your own private inbox!

Loss of quality ( JPEG )


H

hopper8

Member
Messages
10
Likes
0
When I download my pics into photoshop the file sizes are between 55KB to about 1 MB ( JPEG ) but when I alter them using curves, levels etc.. the image quality is noticeably poorer and unacceptable for print. does anyone know of a way round this problem ?

I have just found this site and this is my first post !!!
 
Not sure what you mean by *download*, but if you`re scanning them, save as a .tiff, and go from there. If you`re downloading from the web, the images are undoubtedly already compressed as far as they can be, and Curves and levels may just accentuate the JPEG compression.
JPEG, by definition, is a lossy format. Every save loses some info.
Hope that helps.
 
Compare it to the following: a pixel-based image is an image that is chopped up into little pieces so that a puter can remember of each one the place and the colour.
This means that, if, for example, the image is 100pixels long and wide, there will be 100x100 pixels, or 10.000.
Each pixel takes a byte per colour, so, for the three monitor or light colours Red, Green and Blue, this image will take 30.000byte or appr. 30 kB.

our example is a very small image, and most are much larger. They are even so large that it would take hours to download them from the internet. that is why there are compression methods. One of these is jpg, and it is mainly used for photographs.

What jpg does, is group pixels together. All neighbouring pixels that resemble each other are remembered as a group. How far this resembling goes, is something you as creator can decide with the quality setting. But even at best quality, jpg compresses and data is lost: indeed, pixels that were not identical but close in resemblance get now a mean value, so, when enlarging, you start seeing squares and other artifacts.

Yet: when you open that jpg in for example Photoshop, this application sees a bitmap image of X pixels wide and Y pixels long, and that is why you get once again a full-size fat image, yet, with all the pixels that in the original were approximately the same, changed into a mean value.

When you now jpg once again, Photoshop will start from what it has, and reduce variety even more. That's why jpg is always lossy and even the more so when you manipulate it again and save as jpg.

These lost data are lost forever. There is no way that any computer app can restore what isn't there as it only sees ones and zeros.

Now for print: to get an acceptable print, you need enough small dots into each inch, and a minimum is some 150. To get really good printing, 200 up to 300 (and even more) is necessary. That is why you have to reduce the size of your 72 ppi image by setting the resolution in the Image>ImageSize menu to 150 or 300 without resampling (uncheck the box).
 
Welcome to the forum hopper8! :) I'm confident that you'll learn loads here and have a lot of fun while doing so! :D

As always, marvelous, detailed (easy to understand) explanation Erik! :}
 
thanks for your explanation, should I then save the files as tiff or psd instead ? my digital camera produces jpeg !
on the same subject of quality one thing I have noticed recently is that when I select the crop tool no matter what file size I have it always shows in the resolution box 72ppi ,when I look at older pics they show about 150ppi is this some setting I have changed by accident ? or as you have explained when I am printing do I have to change the image size to 150ppi for every picture
 
should I then save the files as tiff or psd instead ? my digital camera produces jpeg !

The moment you camera takes pics as jpg and does not have any raw setting, the printing-size of your images will be limited as the quality loss will be visible when you try to make the image larger than what you camera delivers.
Saving as PSD or TIFF in phtoshop will halt the deterioration, but cannot restore what is lost. Yet, most people won't "see" this that much. It is only a problem for photographers who are member of a local club, or who "have the eye" to see it.

on the same subject of quality one thing I have noticed recently is that when I select the crop tool no matter what file size I have it always shows in the resolution box 72ppi ,when I look at older pics they show about 150ppi is this some setting I have changed by accident ? or as you have explained when I am printing do I have to change the image size to 150ppi for every picture

Probably your camera is set to 72 dpi, being the standard screen-resolution on Apple puters (dating from the time of matrix dot printers Apple made and sold...). It can also be a setting in PS for import of images from digital camera's. As I personally still prefer my 35mm slides (which are, as tests have shown, still better in quality when scanned on a good middle-class scanner like the Epson 2450 or higher than the results of a pro 6Megapixel camera), I have no real experience with this.

I cannot open that WIA import as I don't have a device for it...

You can easily set to 150 dpi for print in Edit>preferences Units and rulers, print resolution.

As for printing: If you camera produces images of 3MB, then the standard should be 1MB per colour. If the size is 3 to 2, then you will have some 1200x800 pixels. For good quality printing this gives you images of 4x less than 3 inch. For acceptable printing quality at 150 dpi, you get 8x5,3 inch.
Larger prints= lower resolution=less quality.
 
thanks for your detailed reply does this mean that any JPEG I print I will need to change the resolution to 150 or more ? as the resolution is only showing 72ppi because of the camera
 
Yes, hopper8. ;)

Bring up the "image size" dialogue box (image>image size) and fiddle with the resolution settings under document. Be sure to uncheck the resample box!!! This will give you, per my example image below, an understanding of the relationship between your camera's pixels (72 ppi) and what is required for print. Try changing the resolution of the document size (high quality printing) and you will see the print size! [excited]

Also, what Erik is saying is that most home printers produce acceptable to good results at 150 ppi. Something for you to experiment with on your own printer. :)

To better understand the relationship between pixel size and print size, I will also refer you to this article http://craiggoldwyn.com/links/resolution.html

The other thing that you can do is use PS's "crop tool" presets or put in your own values and let PS do the resizing for you. Click on the crop tool and take a look at what's available! ;) Click on the little arrow on the fly-out menu to bring up the drop-down menu so that you can see what's available.

Hope this helps! :)
 
once again thanks for taking the time to explain I think the problem was that I used another image editing package to import my pics from the camera but used photoshop to manipulate them and they used to show as 150 dpi so because I import straight to photoshop they show as 72 dpi but I now know how to increase the quality.
I only came across this site last month and think it is fantastic not only the site but in the main the people that contribute to it . I have learnt more in this short time (using the tutorials ) links etc...than I have in the last 6 months from magazines.

thanks again :righton:
 
B7 :righton: hopper8! Just so long as you're "sorted out", we're happy! ;)

And, thanks for acknowledgement GfX@rt! :) I'm glad that you found the link to be valuable!
 
Back
Top