How important is it...........

[Stephen]

Originally Posted by Don Hoey

This got me thinking. I seem to recall Hamma or Kaiser making a hotshoe mounted spirit level. A quick check on Wh/E site shows that someone still does. They have a 2 axis level so I would guess they are quite widely available.

This is no good though unless the camera is tripod mounted.

How to correct in Photoshop will be of interest to those who use fluid heads, as these are limited to 2 axis movement, and getting level horizons is more problematical.

I would say that if you have PS or any other decent editing prog, knowing how to correct sloping horizons, converging verticals etc is an essential must know skill, and one of the more basic skills needed by the digital photographer.

But this begs a question. I am sure that in my earlier readings in Jpeg v RAW that rotating jpegs in 90 degree increments is lossless but not so for any in between rotation. Cannot find the link to that info now.

I'm not sure this is the case in PS, however if you find this of concern you could always convert to TIFF first.

[Tannin]

Originally Posted by Don Hoey

I am sure that in my earlier readings in Jpeg v RAW that rotating jpegs in 90 degree increments is lossless but not so for any in between rotation.

As I understand the mathematics of it, Don, rotating any format by any amount other than 90 or 180 degrees cannot be lossless — doesn't matter if we are considering JPG, RAW, TIFF, PNG, PSD, or even BMP. This is because all formats relevant to our discussion store images as a rectangular collection of dots of particular colours.

If you rotate by 90 degrees, all the software has to do is move the dots around to new positions, keeping every dot exactly the same as it was before, just putting it in a new position.

But if you rotate by any other angle, the software has to interpolate - i.e., guess at - the colour to assign to each of the new dots. The "new dot" doesn't line up exactly with any of the "old dots" so the software takes a weighted average of the old dots that surround the position of the new dot. Obviously, the result involves some loss of quality.

There is no difference between doing this with a JPG or TIFF or any other raster graphics format.

Note, however, that the other differences between different file formats still apply: i.e., BMPs remain accurate across generations but huge and clumsy, JPGs drop quality with successive generations (if you are silly enough to use JPG for multiple iterations instead of converting to BMP or TIFF, manipulating, and converting back), and so on.