Author Topic: How to reduce file size  (Read 278 times)

Offline RobinBClay

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How to reduce file size
« on: Wednesday 17 February 21 18:27 GMT (UK) »
Advice, please.

I have just received some "digital images" of ancient photographs - but they are .JPGs  of over 2.5 Mb. each.
Alas !  They were taken using an I-phone, so only 72 d.p.i. "

And Yes, it irritates beyond measure that the sender hasn't got a scanner - but I have to be grateful that she does send these images.

However . . the image size is enormous, e.g. 34 x 45 INCHES.

File type: JPEG

Image :-
Dimensions: 2448 x 3264 Pixels    34.000x45.333 Inches
Pixels Per Inch: 72.000
Pixel depth/colors: RGB -8 bits/channel

Memory used
           On disk    In RAM
Image: 2590Kb   23410Kb

The Rules here specify a maximum of 500 Kb.
My graphics program - PaintShopPro - enables me to reduce the size quite easily,  either "By Pixels", "By Percentage" or "By Print Size"
Or I can reduce the quality by increasing the .JPG compression.

Yes, I have read https://www.rootschat.com/forum/index.php/topic,372537.0.html
but I'm still lost !
That was written 11 years ago, before mobile phones became quite so ubiquitous.   I can't "reply" to that, hence the new Topic. 

Which method is "best" for this purpose ?

Offline Kerrill

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Re: How to reduce file size
« Reply #1 on: Wednesday 17 February 21 21:17 GMT (UK) »
If you have photoshop just do as you said and reduce the size by pixels and save as JPG  simples.
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Offline RobinBClay

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Re: How to reduce file size
« Reply #2 on: Wednesday 17 February 21 22:11 GMT (UK) »
Thank you for your response.  I'll give it a go.

I don't understand what the process is when one "reduces the size by pixels".  Does that reduce the NUMBER of pixels ?  By combining adjacent ones ?  Or the size of each ?  Surely - whatever - it reduces the quality ?


Offline chlo

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Re: How to reduce file size
« Reply #3 on: Wednesday 17 February 21 22:13 GMT (UK) »
I think reducing by pixels, percentage or by Print Size will yield the same result in quality.
what I do is host my images on another site then embed them here using the [imgIMAGE URL HERE[/img ]http://line,
that way I have full control over the size.
the size on here is restricted for bandwidth reduction and because there's issues with the width clipping the images creating a horizontal scroll bar (because it's not responsive).
the fastest way is paint.
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Offline Treetotal

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Re: How to reduce file size
« Reply #4 on: Wednesday 17 February 21 22:15 GMT (UK) »
Please don't reduce the pixels, i.e. Resolution  as you will be throwing away data, thereby reducing the quality of the image as the resolution is already low at 72 dpi, (dots per inch) reduce the physical size, Prue's advice at the top of the board shows you the result of that in pictures.
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Offline chlo

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Re: How to reduce file size
« Reply #5 on: Wednesday 17 February 21 22:21 GMT (UK) »
As I understand it reducing image size resamples the dpi, so the only way to get a pic on here for others to edit it to it's full quality is host it off site.
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Offline Gadget

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Re: How to reduce file size
« Reply #6 on: Wednesday 17 February 21 22:22 GMT (UK) »

Many have used Dropbox
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Offline mrcakey

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Re: How to reduce file size
« Reply #7 on: Thursday 18 February 21 10:18 GMT (UK) »
Please don't reduce the pixels, i.e. Resolution  as you will be throwing away data, thereby reducing the quality of the image as the resolution is already low at 72 dpi, (dots per inch) reduce the physical size, Prue's advice at the top of the board shows you the result of that in pictures.
Carol

Agreed wholehearted with this!

What is your drive to reduce the file size? 2.5MB is not huge by any means. And DPI is a false construct - it's only relevant for printing. On the other hand, as someone else has said, if you were to to print the image, 72dpi is very low.

The other thing is that JPEG is what's called a "lossy" format. It uses algorithms to compress the information in the photograph so that instead of having information about every single pixel in the picture, it does tricks like "next 10 pixels are identical" or "keep the green levels the same for the next 20 pixels". Depending on your software, you could be losing information in the photo every time you save in JPEG format.
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Offline chlo

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Re: How to reduce file size
« Reply #8 on: Thursday 18 February 21 14:22 GMT (UK) »

The other thing is that JPEG is what's called a "lossy" format. It uses algorithms to compress the information in the photograph so that instead of having information about every single pixel in the picture, it does tricks like "next 10 pixels are identical" or "keep the green levels the same for the next 20 pixels".
This is whats known as resampling, I don't know what the obsession with DPI/PPI is on the board when DPI/PPI only effects an image if it's printed (mrcakey is 100% correct), you can raise the DPI/PPI to 800 in the editor of an image thats only 200 DPI/PPI  to edit it then lower it back down to 150 and it will be clear enough to be printed on a billboard in the street, DPI doesn't effect quality whatsoever, only editing.
DPI doesn't equal resolution, if editing for a print TIFF is the best format but if showcasing online just use PNG or JPEG.
learn more here on DPI/ppi.
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