Hi Folks,
I would like to ask for info in order to help diagnose a possible lens issue I am seeing while attempting focus stacking.
It seems that simply changing the focus point is also slightly changing the composition. It seems as if the focal length is also changing with the focus.
Not evident while shooting, but most evident while viewing side-by-side on the monitor.
Here are two photos taken in the front yard.
The photo with the close focus shows the street light bulb, the far focus does not. This is easy to see. Other dramatic differences are visible if one were to click between the two images, say in Photoshop.
I was very careful to not touch the zoom ring while performing the test.
Although I don’t think this is normal I thought I could ask and learn a bit more before contacting Canon service.
Canon 5d MK II
16-35 mm 2.8
Also, while I’m asking, it seems that the infinity (far focus) point changes. When the lens was new the infinity point lined up with the mark on the lens. Now the infinity point is way past the mark. I’m guessing temperature has something to do with it, but the difference seems substantial.
As I understand it, one needs to use software to stack the images. The software will auto align the images in order to overcome the apparent zoom affect caused from refocusing.
From the article
“STAGE 2: ALIGNING THE PHOTOS
Unlike extending the depth of field by varying the f-stop, varying the focusing distance causes each individual photo to become misaligned. This is because the effective focal length of a lens actually changes slightly when one alters the focusing distance (see tutorial on macro lenses & extension). As a result, the angle of view also changes — giving images the appearance of zooming in or out when focusing near or far, respectively:
Special software therefore needs to re-align (aka "register") the separate images. Necessary software packages might include TuFuse, Helicon Focus, CombineZM or some combination of Adobe Photoshop's Auto-Merge and Auto-Align features. Fortunately, all of these produce very similar results when aligning images. In Photoshop, you can use either "File > Automate > Photomerge..." or "Edit > Auto-Align Layers..." as shown:”
Still, seems funny that the street light in one of the photos is missing the lamp. Not sure if and how the software deals loss of information.
Well, that was easy. Just watched a youtube vid showing how to use CS-6 for focus stacking.
Really simple and fast, at least for the two photo stack in my photo.
The software seemed to realign the images, even though the two photos looked to be at slightly different focal lengths.
Posted by David Bostedo on January 7, 2015 at 10:10 PM
Glad you got it sorted Richard. As pointed out, lenses DO change focal length somewhat as they change focus. Some really long lenses (like 200mm+) can change by 30mm or more from the MFD to infinity!!
The way the software usually aligns the images is to look for points and edges in the image and align them. This usually stretches or squashes some of the images (or parts of them) so that you lose the edges because they don't all wind up covering the exact same scene. If you have an exact stacked composition you want, leaving some extra space around the scene can be necessary.
“Some really long lenses (like 200mm+) can change by 30mm or more from the MFD to infinity!!”
Wow, that’s a lot.
Yes, the photo I edited became a bit scrunched, this became evident when I attempted to blend the sky exposure with the focus stacked ground exposure.
I used auto align to make the sky exposure work with the ground exposure.
I wonder if all of this auto blending and aligning has a negative impact on the photo. As I understand it, the lens correction software in Photoshop can negatively impact the sharpness.
Normally my comps have some space for cropping. Very helpful in this case.
Discussion
5 repliesI would like to ask for info in order to help diagnose a possible lens issue I am seeing while attempting focus stacking.
It seems that simply changing the focus point is also slightly changing the composition. It seems as if the focal length is also changing with the focus.
Not evident while shooting, but most evident while viewing side-by-side on the monitor.
Here are two photos taken in the front yard.
The photo with the close focus shows the street light bulb, the far focus does not. This is easy to see. Other dramatic differences are visible if one were to click between the two images, say in Photoshop.
I was very careful to not touch the zoom ring while performing the test.
Although I don’t think this is normal I thought I could ask and learn a bit more before contacting Canon service.
Canon 5d MK II
16-35 mm 2.8
Also, while I’m asking, it seems that the infinity (far focus) point changes. When the lens was new the infinity point lined up with the mark on the lens. Now the infinity point is way past the mark. I’m guessing temperature has something to do with it, but the difference seems substantial.
I found this article on
focus stacking
I thought I would post for other rookies.
As I understand it, one needs to use software to stack the images. The software will auto align the images in order to overcome the apparent zoom affect caused from refocusing.
From the article
“STAGE 2: ALIGNING THE PHOTOS
Unlike extending the depth of field by varying the f-stop, varying the focusing distance causes each individual photo to become misaligned. This is because the effective focal length of a lens actually changes slightly when one alters the focusing distance (see tutorial on macro lenses & extension). As a result, the angle of view also changes — giving images the appearance of zooming in or out when focusing near or far, respectively:
Special software therefore needs to re-align (aka "register") the separate images. Necessary software packages might include TuFuse, Helicon Focus, CombineZM or some combination of Adobe Photoshop's Auto-Merge and Auto-Align features. Fortunately, all of these produce very similar results when aligning images. In Photoshop, you can use either "File > Automate > Photomerge..." or "Edit > Auto-Align Layers..." as shown:”
Still, seems funny that the street light in one of the photos is missing the lamp. Not sure if and how the software deals loss of information.
Really simple and fast, at least for the two photo stack in my photo.
The software seemed to realign the images, even though the two photos looked to be at slightly different focal lengths.
The way the software usually aligns the images is to look for points and edges in the image and align them. This usually stretches or squashes some of the images (or parts of them) so that you lose the edges because they don't all wind up covering the exact same scene. If you have an exact stacked composition you want, leaving some extra space around the scene can be necessary.
Wow, that’s a lot.
Yes, the photo I edited became a bit scrunched, this became evident when I attempted to blend the sky exposure with the focus stacked ground exposure.
I used auto align to make the sky exposure work with the ground exposure.
I wonder if all of this auto blending and aligning has a negative impact on the photo. As I understand it, the lens correction software in Photoshop can negatively impact the sharpness.
Normally my comps have some space for cropping. Very helpful in this case.