Fisheye conversion lens

[gjtoth]

As one who has used them in the past, the attachment lens I think you're talking about produce mediocre to terrible images. Generally, CA is very prevalent, sharpness & focus is, in most cases, extremely difficult to achieve. Not too bad if you like fuzzy rainbows, not so good otherwise.

[VTphotog]

Originally Posted by Shinnen

Hi VTphotog,
Yes. I see that. So, what is quoted on the lens is the maximum apertures for wide open and fully zoomed?
...... john

Yes, that is correct.
One thing to be careful of when using a fisheye type lens, and especially the conversion lenses, is extraneous light sources. Since the lens is picking up light from such a wide area, sometimes lights we don't really notice or think of as being a problem, can create some strange effects.

[Shinnen]

Hi,
This is a learning experience for me. I started by trying to throw something together from a box of lenses, collected, over the years, from cannibalized cameras and lenses; but the best I can get is about 120 degrees. This doesn't really have the extreme distortion of the ones I've seen on the net.
By the way, what's the difference between a semi and a (regular) fish eye lens? The semi's I seen on the net look pretty good.
... john

[Ozzie_Traveller]

Originally Posted by Shinnen

Hi,
Yes, alright. The camera I'm thinking about trying says 1:2.6-5.5 I've never understood the aperture thing, because the aperture settings on the screen allow 2.6-8.0 Why the discrepancy?
Thanks,
..... john

G'day mate

May I "confuse" you a bit more ...

Aperture numbers - just like shutter speed number are fractions

A shutter speed of 1/500 second is often shown as '500', and an aperture of 1/16 is shown as 16, but with an 'F' in front to separate it from shutter speed numbers and because the fraction comes from the Focal Length of the lens in use

Aperture numbers are a fraction, where the fraction comes from the diameter of the hole the light goes thru vs the mm's of the lens in use. For example - if a 100mm lens had an aperture hole of 25mm, then 25/100 = 1/4 = F4

As you zoom a lens, the mm's change but the hole the light comes thru does not change - leading to an alteration in the fraction

Try this sketch for size ...


So - if your lens was a 100-300mm zoom, and at the 100mm position the aperture was F4 [from above], then at the 300mm position the math becomes 25/300 = 1/12 = F12

We are lucky today in that our lens designers can make lenses which are actually shorter than the 300mm stated. The optical design is called 'telephoto' rather than the original 'long' lens. This means that the resulting fraction is less than 1/12 [from above], possibly down to 1/6 to 1/9.

Therefore the information on the lens barrel will say something like "F = 4 : 9" ... meaning as you zoom the aperture changes from 1/4 the mm's to 1/9 the mm's of the lens

Does this help at all?
Phil

[Shinnen]

Alright. It's starting to come into focus. It's more complicated than I thought; but after rereading everyone's posts it's starting to sink in. I have not been taking into account that the focal length changes with zooming (a rookie mistake). That's were my confusion is wrt to the lens numbers.
I have a chance to buy a Pentax Takumar Spotmatic M42 lens, from an old film camera. The seller says that it has an adjustment that will allow it to be used on many cameras. It's a screw on, so I should be able to adapt it to one of my cameras. I will try it before buying; but would like an opinion as to whether this is a good (fisheye) lens.
Thanks,
..... john

[Ozzie_Traveller]

G'day John

The 'old' M42 threaded lenses can be used on almost any camera these days via the use of an M42 to [canon/nikon/pentax/m43 etc] bayonet adapter

The 'fisheye' part comes from the mm's of the original lens - if this old lens is on the 8mm to 16mm region is will be wide angle to very-wide angle. If the old lens is over 24mm then it will merely be a regular wide angle lens

One thing to double check with any adapters is the need to focus to infinity ... I have one particular adapter whose thickness is 'one paper sheet too thin' and thus the lens focuses beyond infinity - ie: with lens set to infinity & the stuff on infinity goes out of focus, and I have to back off a bit to get it right. With this adapter, 'infinity' comes into focus with the lens on about 30m

Hope this helps
Phil

[Shinnen]

Hi Phil,
First - Thanks for your help in understanding the lens numbers. It did help.
I won't pretend to understand what you're talking about wrt focusing of this lens. I think there are two things I want to ask you.
1. You mention an adapter. Will I have to get a bayonet adapter to get it to fit my 52mm or 58mm lens thread?
2. Would the best way of checking the focus of this lens wrt to my camera(s), be to hold it up to the camera and see if I get a sharp picture?
...... john
P.S. He wants $40 for the lens. It sounds like a great price to me, if it works.

[TCav]

Focusing shouldn't really be much of an issue. When a lens has a short focal length, as is the case with fisheye lenses, the depth of field is huge, so everything is usually in focus.

It's actually hard to get something out of focus when using a fisheye lens.

[BBbuilder467]

I'm completely lost trying to follow this conversation.

A fisheye "conversion" lens screws on the front of another lens and the aperture is irrelevant. The .5x, .7x is the conversion factor. A 50mm lens would seem like a 25mm lens at .5x and 35mm with .7x.

You need to know the focal length and the filter thread diameter as a baseline.

An M42 lens is a lens that adapts to an interchangeable lens body in place of the original lens. You can't just screw it on the front of another lens.

What camera are you using? It seems from your posts in the past, you have P&S cameras.

[TCav]

Originally Posted by TCav

Focusing shouldn't really be much of an issue. When a lens has a short focal length, as is the case with fisheye lenses, the depth of field is huge, so everything is usually in focus.

It's actually hard to get something out of focus when using a fisheye lens.

What does happen often, though, is that conversion lenses tend not to have great optics, so field curvature is a frequent problem. As a result, soft corners will often occur. This isn't a focus problem as much as it is an optics problem. And if the lens the conversion lens is attached to has a problem with soft corners, the conversion lens will compound it.

Conversion lenses are most often used for video, which has a much lower resolution than still images, so the field curvature isn't as noticeable.