Focus rails

[Gaz J]

Just bought a macro lens with a view to trying some hand held macro shots using flash. Whilst reading up a bit on macro photography in general I came across focusing rails.

I understand the principle behind them but my question is as you advance the rail say for example by 0.5mm each time without touching the focus does the image size change? If it does then does the focus stacking software resize each image to suit.

Thanks in advance.

 

[Canon Bob]

The plane of perfect focus will have the same magnification in each shot and it's the OOF areas that will change magnification slightly. When you stack the shots, it will use the parts of each image where the focus is good and hence the same magnification.

If you kept the lens still and took shots by changing the focus then the magnification would change as the focal length decreased shot by shot.

Bob

 

[IGADIZ]

The plane of perfect focus will have the same magnification in each shot and it's the OOF areas that will change magnification slightly. When you stack the shots, it will use the parts of each image where the focus is good and hence the same magnification.

If you kept the lens still and took shots by changing the focus then the magnification would change as the focal length decreased shot by shot.

Bob

Niiiiaaaaoooowwwwww!!!!.. that went right over my head. In english this time please!
(or, if you are confident... spanish will do nicely )

 

[Canon Bob]

Okay Ivan, we'll stick to English.....

Focus stacking uses multiple images to ensure that the whole subject is captured sharply....ie, all parts are within acceptable DoF.

This can be achieved in two ways.....refocus on a different area of the subject for each shot or move the lens progressively backwards or forwards between shots (typically on a focusing rail).

Moving the lens backwards or forwards will keep the magnification the same as there's no refocusing involved.
Refocusing will cause a change in magnification as the subject will be closer or further from the lens.

Example...
If a lens is 1:1 ratio at MFD then you focus slightly further away, it's no longer a 1:1 ratio....the image will have "shrunk" slightly.

Ref the change in focal length.....
Modern internal focus lenses reduce focal length as the lens is focussed more closely (FL is always specified when focused at infinity). Canon's 100/2.8 is 100mm at infinity but only 78mm when focussed at MFD. Refocusing for a stack would therefore change magnification and angle of view between shots.

Spanish?

Bob

 

[cosmicma]

Okay Ivan, we'll stick to English.....

Focus stacking uses multiple images to ensure that the whole subject is captured sharply....ie, all parts are within acceptable DoF.

This can be achieved in two ways.....refocus on a different area of the subject for each shot or move the lens progressively backwards or forwards between shots (typically on a focusing rail).

Moving the lens backwards or forwards will keep the magnification the same as there's no refocusing involved.
Refocusing will cause a change in magnification as the subject will be closer or further from the lens.

Example...
If a lens is 1:1 ratio at MFD then you focus slightly further away, it's no longer a 1:1 ratio....the image will have "shrunk" slightly.

Ref the change in focal length.....
Modern internal focus lenses reduce focal length as the lens is focussed more closely (FL is always specified when focused at infinity). Canon's 100/2.8 is 100mm at infinity but only 78mm when focussed at MFD. Refocusing for a stack would therefore change magnification and angle of view between shots.

Spanish?

Bob

thats very interesting i was talking about this just the other day and was trying to work out the advantages of a focusing rail and if there was any magnification involved
thank you for clearing that up i was thinking it was to do with camera angle ( setting it to 45 degrees or so ) so the distance from the object would remain the same
now you have explained it becomes pretty obvious

 

[ard-tullaich]

Very infomative post will go and try this method. New to macro so this sounds like good advice