I think there are lots of people on this site to explain this to me. I am a bit confused here. I would like to know the difference between the Angle of view (Field of view) and Angle of Coverage.
Also the effect of them in the photography, which one causes the vignetting.
I appreciate your inputs in advance,
Cheers,
Mohammad
Posted by David Bostedo on August 26, 2012 at 10:25 AM
I don't think there's a difference Mohammad, if you're talking about a lens. Those aren't really standard terms and depend on how they are used. You could talk about the FOV or the coverage of a lens, and they are the same thing. You could also talk about the FOV of an image, and in that case, the size of your sensor or film comes into play.
As far as vignetting goes, assuming there's nothing outside the lens causing it (like a lens hood, or filters) then it's usually due to the lens design, especially at wide apertures. Usually as the aperture opens wide, the oblique light will be weaker from the center out to the edges and will not show as strongly in the image, leading to light fall off in the corners.
This could be a stronger effect on a lens with a wider aperture, and a wider FOV. Theoretically, I guess, you could create a lens that has a much larger image circle for the same FOV and you'd avoid the vignetting in the corners since you'd only be using the center section of the lens (tilt-shift lenses are sort of like this, since they create large image circles to account for movements). But most times, I think it's more a matter of the lens element design and minimizing it that way.
Posted by Mark Stahlke on August 26, 2012 at 11:03 AM
Angle of Coverage (AoC) determines the size of the image circle a lens projects. It's a function of the optical design.
Field of View (FoV) or angle of view, is that part of the AoC intersected by the capture medium. It's a function of focal length and format size.
Vignetting comes in two varieties. The first is light fall-off. The second is mechanical vignetting.
Fall-off typically results in a slight darkening of the corners and can be a pleasing effect. It's a result of the difference in the length of the light path between the corners of the medium and the center of the medium. Less light hits the image corners than the image center. It's more common in short focal length lenses.
Mechanical vignetting usually happens when the image circle is too small for the format size. The corners of the image turn black because no light hits them. This occurs when the AoC is smaller than the FoV.
Tilt/shift lenses and large format lenses require a larger AoC to accommodate the movements or else you get mechanical vignetting. In this case mechanical vignetting happens because the image circle isn't centered.
A practical example:
I have two 150mm f/5.6 LF lenses. One has a 105 degree AoC and projects a 386mm diameter image circle. The other has a 72 degree AoC and a 214mm image circle. Both of these lenses will work perfectly on my 4x5 camera and the FoV will be exactly the same.
I can use both of these lenses on my 8x10 camera and the FoV will be the same for both lenses but not the same as on the 4x5 camera. The 72 degree AoC lens will suffer severe mechanical vignetting because the image circle is too small for the 8x10 format.
Does that help?
Posted by Jim Bullard on August 26, 2012 at 2:44 PM
The angle of view is how wide an area the lens sees in front of the camera, for example a wide angle lens sees a broad angle of view while a telephoto lens sees a narrow angle of view. Angle of coverage refers to how wide the projected image area is behind the lens. This was of more importance in the days of view cameras since a narrow angle of coverage limited the camera's movements (swings, tilts, shifts, etc.). If you moved the lens or back too far you could move it out of the image circle behind the lens and get vignetting of the corners where it was out of the angle of coverage. It is of some concern with digital now that lens are being built specifically for APS-C sized sensors but only if you also shoot with full frame bodies using the same lenses. The image circle isn't big enough to fully cover a 35mm sized sensor.
David, Mark and Jim thank you for taking time to explain FOV in detail. I knew I could get the best results here at NPN. I have been searching google and reading different articles, after a while I got more confused. All three of you have put it in a different word and maid it so understandable to me.
Now, I can say the angel of view is the angel that a lens sees and the angel of coverage is what appears on a sensor. If you could use a APS-C lens on a full frame, the angle of coverage would not be big enough and would result in vignetting. Please let me know if I misunderstood you.
Discussion
4 repliesAlso the effect of them in the photography, which one causes the vignetting.
I appreciate your inputs in advance,
Cheers,
Mohammad
As far as vignetting goes, assuming there's nothing outside the lens causing it (like a lens hood, or filters) then it's usually due to the lens design, especially at wide apertures. Usually as the aperture opens wide, the oblique light will be weaker from the center out to the edges and will not show as strongly in the image, leading to light fall off in the corners.
This could be a stronger effect on a lens with a wider aperture, and a wider FOV. Theoretically, I guess, you could create a lens that has a much larger image circle for the same FOV and you'd avoid the vignetting in the corners since you'd only be using the center section of the lens (tilt-shift lenses are sort of like this, since they create large image circles to account for movements). But most times, I think it's more a matter of the lens element design and minimizing it that way.
Field of View (FoV) or angle of view, is that part of the AoC intersected by the capture medium. It's a function of focal length and format size.
Vignetting comes in two varieties. The first is light fall-off. The second is mechanical vignetting.
Fall-off typically results in a slight darkening of the corners and can be a pleasing effect. It's a result of the difference in the length of the light path between the corners of the medium and the center of the medium. Less light hits the image corners than the image center. It's more common in short focal length lenses.
Mechanical vignetting usually happens when the image circle is too small for the format size. The corners of the image turn black because no light hits them. This occurs when the AoC is smaller than the FoV.
Tilt/shift lenses and large format lenses require a larger AoC to accommodate the movements or else you get mechanical vignetting. In this case mechanical vignetting happens because the image circle isn't centered.
A practical example:
I have two 150mm f/5.6 LF lenses. One has a 105 degree AoC and projects a 386mm diameter image circle. The other has a 72 degree AoC and a 214mm image circle. Both of these lenses will work perfectly on my 4x5 camera and the FoV will be exactly the same.
I can use both of these lenses on my 8x10 camera and the FoV will be the same for both lenses but not the same as on the 4x5 camera. The 72 degree AoC lens will suffer severe mechanical vignetting because the image circle is too small for the 8x10 format.
Does that help?
Now, I can say the angel of view is the angel that a lens sees and the angel of coverage is what appears on a sensor. If you could use a APS-C lens on a full frame, the angle of coverage would not be big enough and would result in vignetting. Please let me know if I misunderstood you.
Million Thanks to David, Mark and Jim.