Image reduction jaggies

[dwdallam]

I'm not seeing this problem with landscape or people photography, but I'm doing an automobile job, and when I re seize from, 5000px to 700, or even 2000, I'm getting really shitty jaggies on hard edges of the car. It''s not all of the edges either, but some of them. I've never noticed this problem before. Any ideas?

http://www.dwdallam.com/albums/main.php?g2_itemId=25109

for example.

As you can see here, no jaggies and same reduction type:

http://www.dwdallam.com/albums/main.php?g2_itemId=25112

Something must be different about the files.

[Graystar]

You need to apply a small amount of blur before downsizing.

http://www.cambridgeincolour.com/tutorials/image-resize-for-web.htm

[Bart_van_der_Wolf]

I'm not seeing this problem with landscape or people photography, but I'm doing an automobile job, and when I re seize from, 5000px to 700, or even 2000, I'm getting really shitty jaggies on hard edges of the car. It''s not all of the edges either, but some of them. I've never noticed this problem before. Any ideas?

It's due to the downsampling method you (Photoshop) used. When using a proper method, the risk of aliasing artifacts is hugely reduced or even virtually eliminated. The reason you see it is some images and not others, is due to the fact that it's  unpredictable (but it will bite you when it's the least welcome) due to image content, and it shows better at certain angles, not all angles. Best is to avoid surprises as much as possible.

Here is an overview of a number of methods, and the effect they can have on image content. You can test your method and see how it does:
http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/down_sample.htm

And here are some examples based on a regular image:
http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/example1.htm

The best you can do from within Photoshop, which obviously is not my preferred application for this, is applying some Gaussian blur in proportion to the downsampling factor.

Cheers,
Bart