Flange distance on the K-3
Posted 16 October 2014 - 10:33 AM
I measured the distance between the mount and the gate and approximately it has a 46 mm length. This means the flange distance is almost 5 cm so a C-mount 25 mm lens will render it into a slight telephoto? Do I need a 20mm instead to make it a true normal lens.
Can someone explain? Am I right?
Posted 16 October 2014 - 10:51 AM
K3 (M42 vesion) has standard 45.46 mm FFD.
C-mount has 17.52 mm FFD.
This means that C-mount lens will work at very close distances (few centimeters) from object only, without any chance to focus to infinity or even a portrait.
FFD isn't related to a crop factor (depending on sensor/frame size).
Posted 16 October 2014 - 04:42 PM
I do own an M42 K-3 camera.
So, let me get this straight. If I use the C-mount lenses, it will be like a macro lens?
Why does the kit zoom lens work fine to infinity?
Do you recommend to use only M42 mount lenses?
Posted 16 October 2014 - 05:05 PM
1. If I use the C-mount lenses, it will be like a macro lens?
2. Why does the kit zoom lens work fine to infinity?
3. Do you recommend to use only M42 mount lenses?
2. Because kit zoom has correct FFD (flange focal distance) for this camera. To focus C-mount lens to infinity on an M42 camera you need to insert this lens about 28mm deep into the camera that is just impossible for reflex cameras, and useless for non-reflex ones since you'll lose access to lens control rings.
3. Yes, due to long FFD of M42 mount specification you are very limited in lens mount choice. T2 lenses (M42x0.75) will work fine with corresponding adapter though. So, you may use M42 and T2 lenses easily, and there are literally hundreds different lenses available in these mounts. So, you'll be able to find all what you need without going to C-mount.
Posted 17 October 2014 - 07:38 AM