On 18.04.2011, at 16:46, Geoffrey Heard wrote:

>> 
>> Every time I open a jpg file in Finder with a double-click it is decompressed when it opens. When I close the file it is compressed again. 

> 
> When you double click and open the JPEG to look at it, it doesn't get compressed again when you close it, it stays the same.

So my assumption was wrong then and my fear unsubstantiated. 

I hardly make any image editing at all, the jpg photos I download from the Web are usually good enough for me, so what I have learned now from you is that as long as I don’t edit them their quality will remain the same regardless of how many times I open and close them and how frequently I watch them in a slideshow. 

I guess I should also abandon the habit of converting important jpg photos to PNG (which according to Wikipedia employs lossless data compression) because that does not make any sense in my case. Only if I want to *edit* them I should convert them to PNG or TIFF before I begin. If this is not correct then please correct me.

> 
> PNG format? That's a web format. It is pretty stripped out but superior to JPEG in retaining transparency (JPEG makes transparent areas white). I'm not sure how iPhoto handles PNG.

I thought PNG was good because it employs lossless data compression and the default format for screenshots in OS X is PNG, I think; the PNG files are also smaller than TIFF. 


> I would store important photos in TIFF format. It's big but it works. There is non-lossy compression available.

As long as I don’t *edit* my jpg photos they can be kept in the original jpg format in iPhoto for slightshows, isn’t it? 


Þorvarður