35mm equivalent

[Roger Lee]

If I add a 100mm lens to my XH A1 via an adaptor(letus or otherwise) is my field of view still 100mm?

I also have Canon still cameras and understand the conversion factor 1.6 for an APS-C type of sensor...1.6 x 100mm = 160mm field of view.

From the Canon website for the existing lens:

Focal Length: 4.5mm - 90mm (32.5mm - 650mm; 35mm photo equivalent)

I don't understand how a 90mm lens becomes a 650mm lens although I can see the 650mm effect when I look through my XH A1 at maximum telephoto.

I don't quite understand this conversion with my XH A1. I do have Canon lenses and am considering an adaptor such as the Letus. But I'm not sure what field of view I will end up with.

Thanks.

Rog Lee

[Chris Barcellos]

Roger, generally you are shooting to get the same field of view that you would have using a 35mm film camera. The field of view you actually achieve will depend on the adapter you get. Some adapter designs are able to deal with edge vignetting better than others. The more vignetting, the more you will have to zoom with your camera lens to to get inside vignetting. Remember, your A1 will be actually shooting an image thrown by the 35mm lens on to a ground glass, and the image on the ground glass is what can have vignetting.

[Petri Kaipiainen]

Lets first look at the fixed lens "conversion factor": What we really are after is the angle of view, not focal length.

For historical reasons people are quite familiar with 35mm still camera optics. 50mm used to be "normal", 24mm fairly wide wide angle and something like 300mm a longesh telephoto.

The angle of view comes from the film/sensor size and focal lenght. Take a piece of paper and draw a line 35mm long on it. Then mark dots 24, 50 and 100 mm away from the center of the line perpendicular to it. Connect these dots with the ends of the line and voilá, you have exact horizontal viewing angles for 24, 50 and 100mm lenses with 35mm film or sensor.

Next: take a red pen and at the center of the 35mm line draw a 6 mm long line. This represents a 1/3 inch video sensor. Find the corresponding dots with the same angles as previous focal lengths. You will find out that to get the same angles you have to devide the 35 focal lengths with 6. So, for a 1/3" sensor camera like XH-A1 you need 4, 8 and 16 mm focal legths for the same view angles.

As different cameras have nowadays different sized sensors they must have very different focal length optics, but that creates confucion. If I just say "my XYZ-AB1 cam has 4-80mm zoom, aint't it great", you go "huh? What does that mean?" But if I say "my XYZ-AB1 cam has 20-400mm 35mm corresponding zoom, aint't it great?" you go "Wow, nice wide-angle!"

For that reason lenses are often described converted to equal angle 35mm focal legths, even though they are usually much shorter in reality, but the final result is the same. Of course it would be best to just state the angles: 90 to 2 degree zoom. That would be exact in every case and I think that is the way it is going to be 10 years from now (I hope).

Next adapters. With those you are shooting the image made by the adapter lens, so the angle is whatever the lens provides and the camera lens drops out of the equation. As the adapters usually use 35mm still lenses, that is what you get. Of course it is also possible to zoom into the image (to shoot only the middle part), then the angle is smaller, but you start to get ground glass detail too much. So, with 100 mm lens you can get no wider image than 100mm would give you on a 35mm camera (24 degrees diagonal), or narrower.

[Roger Lee]

Chris, Petri,

Thanks for taking your time to reply. I think I understand it now.

Thanks again.

Rog Lee