Focus stacking is like exposure bracketing. You take multiple images of the same scene. The only setting you change though is the focus. Sometimes we want to place the camera right close to something in the foreground. But when the lens is so close to an object the background cannot be rendered sharp enough. Even using a small aperture does not suffice. Focus stacking is the solution.
Focus on the closest thing to the lens and take an image. Then focus just a bit further into the image and take another image. Repeat the process a few times. But how many times should you repeat the process? Macro photography may require a large number of images as the depth of field can be razor-thin in this genre of photography. For landscape photography, the number of images needed to produce a pin-sharp image from back to front depends on the focal length of the lens in use. Wide-angle lenses naturally have more depth of field at the same aperture than a telephoto lens. Therefore, wide-angle lenses require less images than telephoto images. With my 16mm to 35mm lens, used at the short end, I find that three or four images are enough. Just remember that you will need more images with longer lenses.
Shoot in manual and in raw. We want to shoot in manual mode so that the exposure does not change between the images. We shoot in raw so that we can make the ISO uniform during post-processing. In post, we will want to ensure that chromatic aberrations are removed and that lens profiles are applied. Now just merge the images. There are many tutorials out on how to do this so I will not cover that here. Once merged I take the image back into Camera Raw to start working on it.
You can see that my lens is right up against these rocks. Yet, they are sharp, so is the background. I used three or four focus stacked images to produce this final image. It is the only way to make sure that everything in the image is in focus.
Watch out for moving objects, especially close to the lens. Blending images where movement took place is a nightmare. If there is moving such as water flowing, make sure, if possible, that all the movement is sharp in one shot to make blending easier. With a wide-angle lens movement in the scene beyond a certain distance is not a problem provided your shutter speed is fast enough to freeze the action. Focus stacking with wide-angle lenses really only concerns itself with the first third of the distance into the image. The rest can easily be captured sharply with one exposure if a small aperture is used.
Use a tripod. Merging images where the camera moved in between shots is a problem you don't want to face. I also take the images in manual focus mode. The less fiddling you need to do in between images to adjust the focus the better. Happy focus stacking. Get those images sharp front to back.
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