Macro using a Focus Rail

[NikonianRich]

Hi, I'm looking to try macro photography and have seen a few videos of photographers using a focus rail. Now the concept looks pretty straightforward but (and please excuse me if this is a silly question!) surely as the camera and focus point is moved in closer, the object will become larger in each frame? So the question is, will the stacking software take this into account without issue? I have downloaded a free trial of Zerene Stacker.

Thanks in advance.

 

[480sparky51 50]

Short answer... yes.

But you still need to take enough frames to ensure all the parts of the subject are in focus in at least one of them.

 

[smoke665]

surely as the camera and focus point is moved in closer, the object will become larger in each frame

Short answer... yes.

To answer this we need to break down your question. If you move the camera, (changing the distance to the subject), the object doesn't magically change size, it just fills a smaller angle of view in the frame making it appear larger/smaller "perspective".

With Focus Stacking, you're changing the "Focal Plane", the location in 3D space where the camera is seeing the sharpest point. If you're using the lens to change the FP you aren't actually changing the distance to the subject. However, many lenses out there experience what's called "Focus Breathing", where changing the focus on the lens can actually alter the effective focal length of the lens, thereby changing perspective. Using a focus rail eliminates "Focus Breathing" because you aren't changing the focus ring, but results in image scaling do to distance change. Regardless of which method used, most modern software uses algorithms to detect, align, and scale images to maintain a constant subject size, even when focusing changes the view/perspective.

 

[NikonianRich]

Thanks for the replies, I'll give it a try and let you know how I get on.

 

[petrochemist128 153]

To answer this we need to break down your question. If you move the camera, (changing the distance to the subject), the object doesn't magically change size, it just fills a smaller angle of view in the frame making it appear larger/smaller "perspective".

With Focus Stacking, you're changing the "Focal Plane", the location in 3D space where the camera is seeing the sharpest point. If you're using the lens to change the FP you aren't actually changing the distance to the subject. However, many lenses out there experience what's called "Focus Breathing", where changing the focus on the lens can actually alter the effective focal length of the lens, thereby changing perspective. Using a focus rail eliminates "Focus Breathing" because you aren't changing the focus ring, but results in image scaling do to distance change. Regardless of which method used, most modern software uses algorithms to detect, align, and scale images to maintain a constant subject size, even when focusing changes the view/perspective.

If you are using a focus rail you are changing the subject distance.
This was the only option when focus stacking software first came out (it was written for microscopy where the only focus control is by movement of the subject distance).

I've never had any issues with image size changing, having first used stacking back in 2910 (Combine ZM) so I assume even the early budget (shareware) software compensated. Or it's simply not an issue because the portion of the subject that is in focus is always the same distance from the lens & so at the same magnification.

I did find that the focus control on my microscope was too coarse for getting sharp slices to combine when using the 40x objective, 5 different images across a flies ankle was the best I could manage & getting a decent stack would probably need nearer 50.

 

[Rickbb76 15]

I've done them with a rail and stacked as many as 20 images, not seen any sizing issues. It's best done in the studio or a totally windless day. All that fiddling about with the knobs and such. Any breeze or tripping over the tripod and the stack may be ruined.

Side note, check your camera/lens manual, it may have it built in for you, no rail required.