1

I know it depends on brand to brand and model to model, but in general terms, what is the shortest focal length I can have before the black zone (A) starts to form, or it is barely noticeable? (B)

For full-frame 35mm and a cropped sensor?

I don't mind distortions or aberrations, just the black zone.

enter image description here

Edited.

Here are 2 fish eye lens shoots.

The first one has the totaly circular surrounding.

On the second one the exif info reads 8mm on a croped Canon Rebel sensor and the circle is more to the edges.

https://www.flickr.com/photos/zbowling/2746925285

I´m just wondering if there is a generic parameter where you should expect this disc fo form.

(I'm not refering on lens hoods, filters or lenses designed for a diferent sensor format)

  • 1
    Is this a theoretical question, or are you trying to solve a specific problem? – mattdm Dec 19 '15 at 19:06
  • More of a theorical question, but with a practical use, for example when exploring to buy a ultra wide lens. – Rafael Dec 20 '15 at 1:24
  • A lens designed for some given format shouldn't show any vignetting on that format. So, buy a lens designed for the format you are going to use it on (or one designed for a larger format, if you can mount it - such a lens will have an even larger image circle) – osullic Dec 20 '15 at 12:10
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You'll only see that kind of vignetting in two cases:

  1. If you're using a circular fisheye, where the lens is designed to have an image circle small enough to fall completely inside the frame borders.

  2. You're using a lens that was designed for a smaller format on a larger one (i.e., most typically, an APS-C ultrawide zoom lens on a full-frame sensor).

Because you haven't given us enough specifics, we don't know how the lens maps, which lens design or formats we're talking about, so in this case, no, there is no "generic parameter" to go by to tell you what the magic focal length is where there's no vignetting any more. It depends, and will have to be ascertained by trial and error.

For example, we know that the Tokina 11-17/2.8 DX fisheye lens tends to begin vignetting on full frame around 15mm, but has more of a weird butterfly-shaped image area, because a ton of people have had to try it and report back on their success to the interwebz (including Ken Rockwell). Meanwhile, the Sigma 12-24 that's designed for full-frame—has no vignetting even down to 12mm.

The Canon EF 8-15 f/4L IS USM is a unique lens in that it's designed for full frame, and intended to be a circular fisheye at 8mm, but a full frame one at 15mm. And we know that the vignetting mostly disappears at 14mm from reviews.

On a crop, of course, the use of a circular fisheye designed for full-frame changes. But then there is the Sigma 4.5mm circular fisheye for crop that exhibits the full image circle within the frame, rather than just dark corners as the full-frame Sigma 8mm does on crop, as your two example images show.

  • You're referring to an ultrawide zoom lens in your answer. Is it any different for an ultrawide prime lens? – gerrit Apr 27 '18 at 15:40
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The size of the image circle depends on the lens. Specialty lenses like some fish-eye lenses aside, lenses are constructed in such away that the image circle covers the whole sensor/film area. So there is no such black area, but this is one aspect why ultra-wide lenses are difficult to design and expensive.

The only circumstance where you may see such a black area is if you use a lens with a sensor which is larger than what the lens was designed for, for example if you use a lens for a crop sensor on a full-frame or 35mm-film SLR.

3

In general terms, as you ask, well... there is no general answer to this at all. It entirely depends on the lens design, the sensor size, and, possibly, on obstructions mounted to the front of the lens like lens hoods and filters.

For example, my phone has a 3.8mm focal length, and shows no sign of darkening in the corners. However, the Canon 1200mm lens — that is, 1.2 meters! — has noticeable vignetting when used wide open. Or, here, where you see black corners with an image taken with my 23mm lens, simply because I incorrectly used the lens hood for my 56mm.

On the other hand, if I put either of my lenses in front of the sensor the size of that in my phone camera, the corners would be fine — and that's true of the 1200mm as well. But if I put my phone camera's lens in front of a DSLR full-frame or APS-C sensor, you'd just see a tiny circle in the middle and mostly "black ring".

1

The "Black Ring" you are referring to is commonly called "light falloff" or "Vignetting".

It effects lenses of all focal lengths and is generally reduced as you stop down the aperture.

Poorly fitting hoods or stacking multiple filters can increase vignetting.

  • 1
    I think the question is referring to the actual edge of the image circle, not to the light fall-off that occurs near the edge. The illustrations in the question look like a hard line as would be seen when using a smaller aperture, not the soft feathering as would be seen with a large aperture. – Michael C Dec 19 '15 at 21:24
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The short answer is this: Focal length and the size of the image circle are not related. They are separate aspects in the design of a lens.

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Take a look at Roger Cicala's comments, of lensrentals.com, on some of their wide-angle lenses, such as the Sigma 8mm ƒ/3.5 EX DG (Nikon). Near the bottom of the page, he shows how the image circles of different wide angle lenses will appear to both crop-sensor and full-frame bodies.

He shows how the image circle of a 15mm on full frame isn't cropped, whereas on a crop body, the image circle at 10mm isn't cropped. Note, these are just examples, not hard rules (otherwise, the Nikon 14-24mm lens wouldn't be so spectacular at 14mm if the image circle cropped). But certainly, 10mm on a full-frame body crops substantially, and at 8mm, it's a full circle. There's no use going wider than 8mm on a full frame, because the image circle would just be using less of the camera's sensor.

His example image of 8mm on a crop sensor looks exactly like your example Apple Store image at 8mm: substantial cropping of the image circle at the corners, but not fully circular.

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