Originally posted by mroeder75 I had someone take some Autofocus photos, and similar Manual Focus photos today. The autofocus photos were in the 6.4 - 7.6 MB range. The manual focus photos were 10.2 - 12.4 MB range. I am using a Pentax K-5.
Why the difference in size? I used my f/1.7 autofocus Pentax 50mm lens, and an old Focal 28mm f/2.8 lens.
Besides the ISO effects mentioned by others, your lenses might be causing this:
1) A wider angle lens will often create a larger file both by encompassing more objects in the scene and by typically having greater depth of field (more objects in focus).
2) A sharper lens (or sharper aperture) will create a larger file.
Can you post a pair of the "similar pictures"?