Photoshop blending of sky and foreground

[davids245058]

Hi All, I have two exposures of the same image. One for the sky and one for the foreground. I created layers of each and a layer mask for the sky. I am painting the sky with a white brush and despite the images being identical when I get to a little part of the foreground the images become blurred. I'm sure that I didn't move my tripod so I'm at a loss as to why the images would be come blurred. Can anyone help?

[Leslie Moak Murray]

Can you post a screen shot?

[davids245058]

I was able to auto align the images but I'm still scratching my head as to why it was off.

[D Fosse]

You really need to show this with a screenshot.

Did something move in the scene between exposures? Trees? Clouds? Water?

Even the sturdiest tripod can easily move some pixels when you touch the camera, happens all the time. Maybe the camera fastening plate didn't fully snap into position.

[davescm]

If you think how little the camera has to move in order to move the image over by a pixel or so then some misalignment is not surprising.

Some possible causes:

Mirror vibration - did you lock up the mirror?

Manual vibration - on a tripod I usually use a delay of a couple of seconds so my finger is out of the way and any vibration gone before firing.

Exposure change - if the aperture is changed then the depth of focus changes with it.

Focus change - unlikely to be the cause here, but many still camera lenses change focal length slightly when moving from close to distant focus i.e they zoom slightly. Known as focus breathing, it is avoided in (very) expensive cinema lenses, but can catch out the unwary when focus stacking.

Subject movement. Very hard to avoid outdoors. Even the slightest breath of wind will move long blades of grass. leaves on trees etc.

Dave

[Norman Sanders]

Did you use an automatic camera setting where changing from sky to foreground point of focus tone level altered the f/stop & depth of field of the images?