Gabriele Turchi wrote:thanks rick
i was familiar with that calculator ,
what i was uncertain was if lenses made for other standard are named (focal length) in relation of the standard , but i guess not
ps: the FOV calculator is great , but a bit miss leading in this circumstance because for example lenses made for MFT (for example 8mm ) won't work on 35mm , so those will be the equivalent of the wider (without distortion ) lenses for 35mm
even tough i am not sure about 8mm lenses for MTF that have no distortion , but other members here said that exist
thanks to all
You just need to remind yourself that the focal length posted on the lens is the same focal length on any camera regardless of whether it is mounted on a full-frame 5D or a MFT or a S16 sensor. The FOV you capture on the different sensors is determined by the physical dimensions of the active sensor area. The smaller the sensor, the smaller the FOV for a given lens.
As you point out, not all lenses will have a large enough image circle to cover all sensors. For example, a Canon EF 25mm lens is designed to cover all sensors up to the full-frame size. But if you mount that 25mm lens on a true MFT camera, the FOV is halved on the smaller sensor and the image captured will be as if you had cropped the lens by a factor of 2. The image and FOV will look as if it was a Canon 50mm lens but remember it is still a 25mm lens.
A lens like the Canon EF-S 18-55mm is designed for the smaller APS-C sensor. Te image circle will not fill a full-frame sensor, but the focal length is 18mm regardless. The widest part of the lens is 18mm but the sensor has a smaller diagonal and results in a smaller FOV as if you were seeing a crop of 1.6 so you can think of the lens as behaving as if it were 29-88mm lens in terms of FOV.
On the BMCC as you know the FOV is reduced by a factor of just under 2.3 so a lens with a true focal length of 15mm will capture a smaller image due to the 2.3x smaller FOV. So the image FOV looks as if it was a 34mm lens mounted on a full-frame camera.
This is an oversimplification of comparing the image on various sensor sizes just to try to clarify the issue of FOV. It gets more complicated when you discuss perspective which is different than FOV. As I understand it, if you really wanted to duplicate the perspective you would see from a full-frame sensor on the BMCC's sensor, you would need to physically back away from your subject 2.3x the distance you had placed the full-frame camera. Hope I have that right. Not always going to be possible of course but may be practical for medium to close-ups. Or block your talent to suit the lens.
As for the very wide lenses, true they are often fish-eye lenses and that may be an effect you want. But if you want your vertical lines to look straighter, then check to see the lens specifies it is a rectilinear lens. Most rectilinear lenses will still have some distortion (perhaps you are familiar with the terms pin-cushion or barrel effect) but the best lenses will have no discernible distortion.
Rick Lang
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