Usually it's caused by the image being displayed at a larger size than its resolution supports In other words, each pixel of the image using multiple pixels of the screen.
In Preview go to Preferences, both Images and PDF tabs, and select "1 image pixel equals 1 screen pixel" - that should fix it. Trade-off: Smaller images at lower resolution will appear in a small size on the screen, too. Command and "+" will let you zoom in (= enlarge the image), but the sharpness will suffer again.
Thank you for you response! However, this is not the case. The images are way bigger than the screen and are always downscaled in this case by Preview. I am a photographer and I work with very large images. Your are pointing at something though and it SOMETIMES happen but only when the images are downscaled (not viewed at 100%), like if the downscaling algorithm was adding a amount of blur sometimes.
One thing I just saw, when I move from one picture to the other very quickly with the arrow keys (when opening multiple file), I see the properly sharpened image for a fraction of a second and then some soft filter gets applied. It would be interesting to have a idea of what is happening behind the scene in Preview's algorithm...
Oh okay, it happens when downscaling...
Well, it appears that Preview tries to display the entire image and as large as possible, meaning in a window as large as your desktop allows. Depending on your screen resolution, it's very likely that the scaling then happens at an "odd" ratio, e.g. not 50% but 47.9% or something. How good the result then is, depends on how well the "scaling engine" works, in other words, what and how much processing happens to recalculate the image for the display at that scale. Preview seems to be optimized for speed while other apps are more exact - I've noticed that, too.
If a different app works better for your purposes, you should either select it per "Open With" on a case-by-case basis, or generally define this different app for opening the file type(s).
Well, I agree that the scalling algorithm will yield different results at different scaling ratios. But I have this issue with images of exactly the same size. There has to be something else. In a typical sharpening workflow, sharpening is done at all stages and when an image is scaled. Downscaling an image typically requires a new pass of sharpening. I have the feeling Preview does that but the results are inconsistent. You might be right that this is the consequence of a decision between performance and quality. But then, why is the Canon app (for example), delivered with the camera, performs as fast but gets better results?
It still nags me that I see a proper version for a fraction of a second before the image get blury. It seems like I would just need some control over the sharpening decision but there are absolutely no settings for image quality.
I know I can always open the image with another app and it is what I do now since I get these inconsistent results. It is just annoying that the default app, which works perfectly fine otherwise, does not work better. I wish there were quality options and I wouldn't mind waiting a few more milliseconds to have consistent results. Considering that apple computers are the platform of choice for photographers, shouldn't the Preview app do that as best as possible? Preview is best integrated to the os, it's a bugger that it does not do it's job better.
Thanks for your help
Considering that apple computers are the platform of choice for photographers, shouldn't the Preview app do that as best as possible?
Well, after all it's named "Preview", and not "Exact View" or even only "Aperture Lite" or so...
Just kidding, I know what you mean! But then, as you say, Photoshop and other apps made for more editing (even iPhoto) are doing a better job with scaling.
So, have you been to Preferences and checked your settings there as explained above? If set right, it should improve the display on your screen anyway, me thinks.
Ok, I may have not put that correctly. I realize Preview is meant to show a preview but we are talking basic stuff here (like showing a decent preview) and it is nice to have a quick double-click way of checking out an image without opening PS or other more sofisticated editor that take ages to load and are too fat to leave opened all the time. (I know you were joking... )
The preferences setting you suggested does not change a thing and it is normal as this just defines what 100% means between max screen size and print size. Nothing to do with my problem since I always look at images that are larger than my screen in terms of pixels (and the print size). Anyways the settings was at 1 for 1 already.
Seems I am the only person with this issue... Alright, I will try to find an alternative, thanks for you help!