Fish eye lens

[BradGC]

I have been looking at a few differemt lenses, and from what i see a Fisheye lens doesnt produce a square image (it comes out circle) whereas a diagonal fisheye comes out square, but what would be the main difference between a super wide angle and a diagonal fisheye? or are they basically the same thing?

[Redgum]

Fisheye and super wide are quite different. The "diagonal" term appears to be a marketing concoction or a term that someone has chosen to describe the different effect between full and cropped frame fisheye.
In essence the fisheye is designed to distort the image where a good super wide (like the Nikon 14-24) is designed to eliminate distortion and capture the widest possible angle. Learning to use a super wide effectively is an art in itself and reliant on good composition. This sort of lens is a valuable asset in many situations whereas the fisheye is designed for special effects.

[Tannin]

All lenses distort. Repeat, all.

With normal and telephoto lenses, and even with only moderately wide angle ones, this distortion is usually too small to be visible to the casual eye. But as you go to a wider and wider field of view, it becomes obvious.

In fact, it is not the lens which causes the distortion, it is the geometry of recording an in-the-round three-dimensional scene using a two dimensional recording medium (film or digital sensor). If you go wide, you cannot avoid distortion no matter what. The only choice you have is which sort of distortion you would prefer. You can use a lens which distorts lines but keeps sizes correct (fisheye) or you can have a lens which distorts size but keeps lines correct (rectilinear). You cannot have both. Ever.

(If this is hard for you to understand, consider the task of making a map of the world. No matter what you do, you cannot produce a (flat) map of the (round) world without distorting it. The wide angle lens distortion problem is exactly the same.)

(By the way, "lens distortion" has nothing to do with this topic. "Lens distortion" is the term used to describe the relatively very minor imperfections that result from the mistakes and compromises a lens designer makes. "Lens distortion", in other words, is the difference between the theoretical, ideal performance of a perfect lens and the actual measured performance of a real lens. We are not talking about "lens distortion" in this thread.)

A circular fisheye throws a circular image onto the film plane. They are generally used for scientific applications where the very wide angle and the scale accuracy are important. For example, suppose you want to take a picture of the whole sky once a day and then measure how much of it is covered in cloud. Only a circular fisheye can (a) capture the entire sky in one shot, and (b) do so without distorting the sizes of different parts of the picture (which would invalidate your measurements). For general photographic purposes, a circular fisheye is just about useless. If you owned one and took a picture with it six times in a lifetime, you would probably be guilty of over-using the lens.

A diagonal fisheye, just like any "normal" rectilinear lens, takes a rectangular crop out of the circular image gathered by the front element. Diagonal fisheyes are not the same as orthodox rectilinear ultra-wides. Like wide rectilinear lenses, they distort the round world to get it onto a flat sensor, but they preserve the size of objects at the cost of bending lines that do not happen to pass through the centre of the picture. (Rectilinear lenses, remermber, preserve straight lines but distort the size of objects that are not in the centre of the picture.)

Diagonal fisheyes are very useful indeed, and I recommend them to any photographer, especially if you already have a rectilinear ultra-wide, and/or if you have a desire to record the natural world more accurately. Yes, it's good to have both. Some ultra-wide shots only work well with a rectilinear lens; some ultra-wide shots only work well with a fisheye lens; many, perhaps most, ultra-wide shots work just fine with either sort - especially if you have taken a little time to work out which ways of taking the shot work best for you with the one lens or the other.

Fisheye lenses are not, repeat not, just for special effects. A fisheye is a very useful tool with which to record the world around you. See here for a few examples: http://www.ausphotography.net.au/forum/showthread.php?t=10256

[warhammer]

good explanation tannin.
i just got a 15mm canon fisheye and love it for all sorts of stuff, one of those lenses i have always wanted ever since i was a kid and seen the fisheye photo's in skateboard, surf and snowboard magazines. its a great lens in tight spots inside and even good for landscape, and it goes without saying anything board sports looks unreal.

[warhammer]

Here are two shots i have taken since i got my 15mm canon fisheye a week ago.
distortion is present but looks ok to me. i like the effect.

[Redgum]

Very nice. But both photos show the importance of getting your horizon straight with a fish eye. :)

[CabanSail]

This shot was taken with a Sigma 10-20mm at 10mm on a DX sensor.

http://photos2.media.pix.ie/FD/15/FD15401CAD994D6287ABC2170E235DF0-0000318539-0001967011-00800L-D160212BAACC42088F8FD66781ABE7DD.jpg (http://pix.ie/cabansail/1967011)

This shot was taken with my 8mm Fisheye on the same body.

http://photos5.media.pix.ie/09/D0/09D01BA6CF4C41128F4C2C1EA69EBAD5-0000318539-0001940133-00800L-6C46E0BEE687474D9D981AFB13E70A0E.jpg (http://pix.ie/cabansail/1940133)

The slight difference in focal length is not the defining issue here. I have taken two shots on with the camera on a tripod with the Sigma 10-20 and the Nikon 10mm Fisheye. Both had the same focal length but different angles of view (102 and 180 degrees respectively)

[Redgum]

I didn't think the Sigma was a fisheye lens? Just superwide.

[warhammer]

yer should have picked that up, simple straighten after would have fixed that. DOH
i think you might need your eye's checked on that night shot though, it was straight when i took it, being a night shot i used a level to help me.

[Redgum]

In that case the POV from the fisheye was too high. (pointing skyward). Tricks of the trade with fisheye but all in the eye of the beholder. :)