EDIT: Scratch that - I won't be dragged off topic. My terminology is correct.

Drab said:
You're describing focal length breathing (focus breathing) and not all lenses do it.

no

as you seem to need technical details here they are;

http://scubageek.com/articles/closeup_lens.pdf

from the RRS website "Remember that as you focus the lens, magnification changes. To maintain your desired magnification, you need to move the camera to obtain the correct focus point"

the best way to shoot macro is usually to choose a ratio that is appropriate, say 1:1, then move the camera until where you want to be in focus is in focus.

I am not talking about focus breathing and I suggest you do some research before telling me that I am. Do you even own a macro lens? I do that is why I decided to try to help the original poster by showing them the use of a rail (which I also have).

Focus breathing is when the lens "breaths" out a bit when focussed at the closest distance.

With a macro (micro) lens it works in reverse. As you focus closer to the lens the magnifiaction ratio changes causing the subject to increase in size. RRS quote "This is an exercise in frustration without a macro focusing rail."

Drab said:
You're describing focal length breathing (focus breathing) and not all lenses do it.

fwiw, most lenses change focus when zooming too

Gareth said:
you will see the image zoom in as it focuses.

You're describing focal length breathing (focus breathing) and not all lenses do it.

because of the way macro lenses work, when you focus closer to the subject, the subject becomes larger. i forget the jargon, and this is a really bad expanantion.

Think of a focussing rail as zoom for your macro lens. You don't use it for getting more in focus and putting the focus where you want it. You can do that without a rail.

You use a rail for composition.

If I screw my (RRS) rail forward a very small amount, say 2mm, the whole composition of the shot will change at very close distances. I will have the same shot minimally changed before I re-focus, but after re-focussing the shot will "zoom" in significantly. If i don't like it I just screw the rail back and refocus. Try moving your tripod 2mm foward and then back to the exact same postion it was in prior. You will then see why you need a rail.

If you don't understand;

1. put your camera on the table in live view.
2. choose an object to use, place it close in front of the camera, focus.
3. move your camera forward 1cm, refocus on the same point manually without bumping the object or the camera.

you will see the image zoom in as it focuses.

back when I was doing a fair bit of macro/micro photography, I used a focus rail because I often could not focus with the lens - I was working so close that lense focus wasn't enough. As Drab said, instead of getting the camera and tripod to JUST the exact position, then ultra-fine tune withlense focus, with a rail you make the fine adjustments over a wider range. Some rails also allow lateral adjustment, which is useful for composition.

Pierre said:
I bought a Nikon 105mm VR and tried focus stacking in CS5. I don't mind purchasing a focus rail but I was wondering if just playing with the focus ring would not give you the same result.

A - Nice photo.

B - Can it give you the same result? Yes. Is it an easy way? No. When you're using a fixed focal length macro lens one must move the camera itself in order to shift the focal plane more than a few mm. Focus rails allow one to do this on axis easily. Moving the tripod / yourself is no where near as easy.

I bought a Nikon 105mm VR and tried focus stacking in CS5. I don't mind purchasing a focus rail but I was wondering if just playing with the focus ring would not give you the same result.

I had some success with sharp-edge subjects at ridicule small aperture but when it came to flowers with more reasonable aperture, CS5 do not seams to know what to do with the strong bokey changes of the close and the far side of the flower as the focus point moved. Would a focus rail be better at keeping relative size of the different parts from one shot to the next?

Or instead, the focus rail is just a more precise way to bring the focus in the right spot and gives no other advantages over manual focus for stacking purposes.

I read that stacking from the far to the near gave better result and I was wondering why.

Also, with 4 gig of ram, CS5 complains he does not have enough memory to stack more than 3 layers, so I have been stacking my 9 layers in group of 3, and finally stack of the 3 merged groups and I was wondering if this could be the cause of my problems. I found that CS5 was making some mistake as to what is sharp when the flower had subtle details.

The below required quite a bit of post stacking manual manipulations: