Why does this happen

[Alfisti]

Google it and all you need to know is there.
In practice, don't go past f/10 unless you REALLY need to for macro work. Most consumer lensesd are at their sharpest at around f/8. By f/11 there is drop off and by f/13 is a freakin mess. At f/19, well, you see what you got.

[walter]

you can see the effects of diffraction by going to Sigma Lens: Zooms - Sigma 18-125mm f/3.5-5.6 DC (Tested) - SLRgear.com!
Select the blur index plot and when the graph appears, set the focal length to 125 and move the aperture slider around. You can see how diffraction affects sharpness.

If you want to understand diffraction better, take a look here: Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

I can't tell if the problem is diffraction, focus error, motion blur or a combination of all three, but I'd try a larger aperture setting next time, probably f/5.6 or f/8. That should fix any diffraction and motion blur problems.

[wildman]

I do a lot of distance shots through very long (over 500mm) FL glass at
a distance.

In my opinion there is nothing unusual about your shot. You are shooting
at a distance over open water in the middle of the day the worst
conditions possible. The problem is not you or your equipment it's all
that air between you and your subject. It's just normal air turbulence.

After all that air between you and the subject is as much a part of your
optical system as your eyes or your camera lens.

You can't expect to point a camera at something 3000 feet away and
expect it to be as sharp as it would be at 30 feet. Especially on a hot
afternoon when you are standing more or less parallel to the water's
surface and just a few feet above it.

I have had many a day where anything much over 100 feet and I can't get
a sharp picture no matter what I do. It's unstable turbulent air.

[Fritzvds]

Thanks again for all the comments, it looks to me as if it could be a combination of things but with all the advice I will be more aware of these things in the future.

Thanks again for helping my development!!!!!

[sabarrett]

Originally Posted by Fritzvds

Thanks again for all the comments, it looks to me as if it could be a combination of things but with all the advice I will be more aware of these things in the future.

Thanks again for helping my development!!!!!

Man, I really learned lots from this thread. Thanks for the post. I would agree with your last post, probably a little of all of it (seems like that is usually the case).

I never thought of the air issue - a good thought. Not only the air, but probably some splashing and misting of the water between you and the subject too. Good things to think about.

[Marc Sabatella]

Originally Posted by Fritzvds

I was acctually standing on the shore not a boat (you would be suprised how close these animals come in)

My mistake. Still, like I said, you were kind of borderline - at that shutter speed and focal length, you have a right to expect more than 50% of your shots to be sharp, but not all. And the ones that won't be are the ones where you were kind of hurrying rather than taking your time to carefully brace yourself.

Throw in the other factors - the fact that the camera may well have chosen to focus on the horizon rather than whale, the diffraction from the way-too-high aperture, the quality of the air itself - and as you say, there were indeed quite a few factors contributing to this shot coming out the way it did.

The only thing I could think that would maybe effecting it is the blue sky and blue water making it hard for the autofocus

It's not the fact that they are blue. It's the fact that by far the "easiest" target for a AF system is a clear line between a dark object and a light object. And the nice, long, straight, non-moving, horizontal border between the dark water and light sky is just *begging* the AF system to focus there, not on the little dark non-descript shape that popped into view for a fraction of a second. There's basically no reason in the world to expect the AF system to have found the whale when the horizon made a much easier target.

[thomasjmpark]

whoops, sorry posted in wrong thread, lol, sorry

[Peter Zack]

Wildman beat me to some comments. Over water in the middle of the day can add air turbulence ans well that could play into the mix even on a cool day. Water is evaporating constantly and effecting the image close to the water surface.

The old film shooters rule of thumb is "shutter speed needs to equal the focal length". Since this camera has no SR, you need to be shooting at 1/250th or higher when hand held. Use a tripod and a higher shutter speed to cure most of the issue (the most important tool a wildlife pro has with him is a solid tripod).

Stopping down has been commented on and is correct. The lens should be between f8 to f11 for best results.