Focal Length Confusionism

[Pere Casals]

The flange location is an arbitrarily-placed measuring point. If you removed the 300mm Sironar S from the shutter and put it in a properly spaced barrel with the flange at the very rear instead of in the center shutter, it would still have a 300mm focal length, and the nodes would still be in the same place relative to the optical system.

Mark, yes, while modern shutters are normalized for the critical dimensions allowing to mount the same cells in a compur or in a copal, it happens that the rear cell flange to the shutter flange may vary slightly.

For example for the copal 0 size that dimension is 5.7mm (see highlighted in red) while in the compur the dimension is 6.1mm, so difference is 0.4mm

Saying it clear, from the manufacturer's datasheet you can locate the Rear Node exactly, but if (for example) you replace a factory mounted Copal by a Compur then the FFD and flange to Rear Node distance is to vary by 0.4mm, in the 0 size lenses. Well, not much !!!



For Copal 0 it is 5.7mm.





For Compur 0 it is the 6.1mm value (the 4.5mm value is for 00 size)

[Mark Sawyer]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

[Dan Fromm]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

You can drag a dead horse to water but ...

[Mark Sawyer]

You can drag a dead horse to water but ...

...if you can get it to do the backstroke, you've really got something!

Yeah...

[Pere Casals]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

Of course Focal lenght won't change, but FLD will change, in particular by 0.4mm with the size 0 (copal vs compur)