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Trying to reduce the size of jpgs by reducing resolution - doesn't work

krs


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All right - I need an image expert here.

I have jpg image sent to me by a friend that started out as an 8.8MB jpg.
Large file size at 600dpi

I reduced the image size to 1200pixels across which brought the file size down to 1.8MB, still at 600dpi
Since this image is only for the web, I would like the size to be below 1 MB, somewhere around 600 - 800K, but when I change the resolution in preview to 150 dpi first and then to 90 dpi, the size still stays at 1.8 MB
I checked each one of these images, and Preview reports the image as 600, 150 and 90 dpi per what is shown in the image name, but they are still all at 1.8 MB
What magic do I need to reduce the image to below 1 MB?
Does Preview report the wrong dpi, ie the dpi's never changed? or what is going on here?

Screen Shot 2019-07-14 at 2.56.44 PM.png
 

IWT


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I wonder if this would work:

Open the image in Preview. Then File > Export. At the bottom, you will see Format (jpeg) and underneath "Quality" with a slider. You can use that to reduce the File Size to suit your needs. Slide to the left till it shows 1MB or less.

Saves messing about with pixels and dpi.

Ian
 
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You are only changing the one aspect, maybe try changing the dimension size, or at least one other aspect, as well?
 

pigoo3

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One thing you can try is:

- Duplicate the file.
- With this duplicate file choose "Save".
- Then choose "JPEG" as the file format.
- Then you should see a slider for "Quality".
- Choose the quality level you would like with the slider (Least to Best).

As you slide the slider...you will see the file sizes. If choosing a lower quality level doesn't get you to a file size you like...then in addition reduce the image dimensions.

HTH,

- Nick
 

krs


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Thanks for the suggestions
@ Ian - Interesting idea but left me even more confused trying to understand what’s happening.
Will explain below
@Bob - I already changed the size from 2493 pixels wide and 8.3 MB to 1200 pixels wide at 1.8 MB.
1200 pixels wide is the standard I need to use, DPI (or quality) is the only other thing I can adjust, for the net even 90 DPI would be fine
@Nick - With those steps I end up at the same point and with the same issue as with Ian’s suggestion to use “Export”

So the strange things that happen with Ian’s or Nick’s suggestion - Sorry the images are a bit small, I had to reduce them to 800 pixels wide for the forum, I hope you can read the information.
So…
First thing that happens using “export” or when I get to the same screen using Nick’s method, is that the image size reported previously as 1.8 MB is now reported as 1.3 MB.
Just to be clear - that is without doing anything to the image parameters at all!

I then moved the quality slider to the left to 854K, so far so good
Saved it at 854K, size as the saved image shows aty that (855K actually), but when I open that image (that shows 855K) in Preview and check size - the size is indicated as 1.2 MB (third image)

And when I close that and then go do the same thing again, open that image and check size (again without making any changes to the image, it is no reported as 1.2 MB (was 1.2 MB)

Bottom line the way I see it - Preview software in that area iis totally messed up - one can;t depend on any of the sizes (in MB and KB) that are reported.

I think I will have to try another image program I have.

Screen Shot 2017-08-02 at 09.53.30.jpg

Screen Shot 2019-07-17 at 14.11.24.png

Screen Shot 2017-08-02 at 11.07.57.jpg
 
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All right - I need an image expert here.

I have jpg image sent to me by a friend that started out as an 8.8MB jpg.
Large file size at 600dpi
.................
Does Preview report the wrong dpi, ie the dpi's never changed? or what is going on here?
Hi Krs - you've already received plenty of advice which may have solved your issue? But simply changing pixel resolution (i.e. dpi or ppi) will not effect the image size but its dimensions if display and/or printed - as a baseline, 72 ppi is the standard resolution for web presentation (although higher resolutions still work fine) and 300 dpi for printing.

Now I'm assuming that you are using Preview to make these adjustments? Below in the first pic is an image of the Tennessee mountains from my camera's SD, note the large pixel sizes and the 180 resolution (this would print out as a huge image, but I would up the resolution to 300 if making a print, size would decrease accordingly). To reduce the image size, pixels have to be removed - in the second pic, note the altered pixel dimensions and the reduction in size to 285 KB; the third pic is the actual adjusted image which still looks fine. Hope this helps - Dave
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krs


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Thanks for the comments, Dave

One of the issues I have trouble getting my head around is that Preview reports dramatically different sizes for exactly the same image, 1.8 MB vs 1.3 MB, depending where one checks.
That makes absolute no sense to me.

You said:
To reduce the image size, pixels have to be removed
Yes, absolutely, agree 100%...but I thought reducing the resolution would do that - obviously not
 
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Thanks for the comments, Dave

One of the issues I have trouble getting my head around is that Preview reports dramatically different sizes for exactly the same image, 1.8 MB vs 1.3 MB, depending where one checks.
That makes absolute no sense to me.
Well, in my first pic, there are 2 numbers given in size, i.e. 3.2 MB & 4.8 MB - since these are lossy JPEGs, maybe the size on my camera's SD vs. the size Preview might make an adjustment is a possible explanation - this could be tested by using a lossless format, such as TIFF - have not given it a try?

Thanks for the comments, Dave

You said: Yes, absolutely, agree 100%...but I thought reducing the resolution would do that - obviously not
The resolution (i.e. the interchangeable terms, dpi or ppi) if changed w/o reducing the number of pixels will only expand or compact those pixels into different dimensions whether on screen or on paper; the image size will remain the same. So, pixels must be removed to reduce the 'file size', which you already knew. Dave
 

krs


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Well, in my first pic, there are 2 numbers given in size, i.e. 3.2 MB & 4.8 MB - since these are lossy JPEGs, maybe the size on my camera's SD vs. the size Preview might make an adjustment is a possible explanation - this could be tested by using a lossless format, such as TIFF - have not given it a try?
When one selects Preview>Tools>Adjust Size and gets two different sizes with one in brackets, like your first image - I can understand that.

What puzzles me is the first image for instance in my post #5, the image is reported as 1.8 MB in one place and 1.3 MB in another place.
 
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