question about downsized images

[raymo]

I don't fully understand the ins and outs of downsizing my images for
posting on IIS.
How is it that some peoples' images are so large that I have to lower my
screen mag in order to see the whole image, and yet when I take mine down to the 200 limit they end up about 100mm across? I assume that there is some way of lowering the quality of the image, rather than just making it smaller, but retaining the original quality, so that it remains large enough to view without squinting at it. Perhaps I am using the wrong software, and there is software that reduces the quality rather than the physical size of the image. What is the best software to use?
raymo

[multiweb]

Compression to 200KB will greatly depend on the colours in your pic as well as the number of features, neb, etc... For example a small galaxy shot with a lot of black background will compress better than let's say a milkyway shot with a lot of colors, stars, nebs. Also monochrome might be a little lighter than colour. Finally the software you use to save as a compressed JPEG will defiitely make a difference too. The way I usually do it is post highres on a separate URL then change size to 1280 or 1024 wide for IIS than tweak the slider to go under 200KB. If it looks bad then reduce further the width. Try to keep quality above ~50%. I also vaguely recall that yellows or greens can be a problem when compressing JPEGs (Troy can elaborate).

[RickS]

Most (just about all?) image processing software should give you a choice of quality settings when writing files in a lossy format, e.g. JPEG. What software are you using?

Cheers,
Rick.

[raymo]

Thanks for the replies.
I used to use PS, but discontinued renting it when I gave up creating new
images some months ago. I am now using Irfanview, which doesn't seem
to have a quality adjuster, only physical size. Oh Dear, what a dill, I've
just remembered the slider I used to use in PS, [Marc mentioning a slider
brought it back to me]. I do have an old version of PS on my other laptop, so I'll have to transfer my images to that one, unless there is other software
that I can download to make it easier for me. I apologise for wasting your time, and can only use my appalling memory, and advancing age as excuses.
raymo [I think that's my name]

[Atmos]

I think the answer to the question is that some people upload images that might be 2000x1500 pixels and HEAVILY compressed to get to the 200kb limit while others save as a 1200x800 and compress less heavily.

[RickS]

Raymo,

My copy of Irfanview (version 4.44) displays a dialog called JPG/GIF save options when you save a file. At the very top is a "save quality" slider. You can also tick a box and specify the file size instead.

Cheers,
Rick.

[raymo]

Thanks everyone, all should be good now.
raymo

[Regulus]

This site from Cambridge Uni is an excellent, easy to use, digital image learning site.
This explains the various resizing tools like: Bilinear, Bicubic, and interpolation that is used. It is certainly important to use the correct one when resizing for the web.
http://www.cambridgeincolour.com/tutorials/image-interpolation.htm

Trev

[raymo]

Thanks Trevor, I'll take a look at it.
It's all a bit embarrassing really, since it will shortly be the 64th
anniversary of my first astro image, but of course the film days were
very different. It's even more embarrassing because six months ago
I was happily downsizing with PS including reducing the quality of images.
I am actually getting frightened; my short and medium term is that bad.
raymo

[sil]

Downsizing was just a matter of getting the oven temperature correct? shrinkie dinks!