is there a 0.7 teleconverter?

[stu_the_flat]

Following a thread on teleconverters I was wondering if there where "wide angle" converters that where designed to focus a FX lens down on to a DX sensor? I can't think of a reason why such a thing does not exist?

Stuart

 

[Steve Smith]

Wide angle convertors do exist but they have to sit on the front of the lens rather than on the rear.

Because of this (and some other optical formulae which I don't fully understand) they do not reduce the light level the way rear of lens mounted convertors do.


Steve.

 

[HoppyUK]

But because they then become the primary image-forming objective, and are always relatively cheap, they are rubbish.

 

[stu_the_flat]

I know about them Its not what I mean. I'm talking about utilizing all the optics of an FX lens on a DX body. It would need to be really good glass to work properly! But you would also get more light being directed on to the sensor. which would be good.

 

[Steve Smith]

I know about them Its not what I mean. I'm talking about utilizing all the optics of an FX lens on a DX body.

I know. You are talking about an optic between the lens and the body which takes the full frame view of the lens and converges it a bit so it all fits onto a crop frame sensor.

A slightly convex lens will do this - but the image would then be out of focus so you would need to compensate by moving the lens closer to the sensor. Unfortunately the lens mount gets in the way.

I have done this with a large format camera which has the benefit of bellows and the ability to put the lens where you like. I used a 135mm lens with a +1 dioptre supplementary lens in front - it will work the same at the back (I tried that too).

What you are actually doing is making the lens a bit more wide angle than it was. In my example the 135mm lens is 7.4 dioptres (1/focal length in metres). Add the 1 dioptre of the extra lens = 8.4 dioptres. This makes the new focal length 119mm.

On a view camera with bellows, the lens is usually positioned at about the focal length away from the film. In the case of my 135mm lens it is around 135mm away from the film as expected. Add the supplementary lens and it needs to be a bit closer, probably at 119mm from the film just like a wider angle lens - which it has just become.

In my case, this converts the lens to give a wider view on the same format. This is the same thing as you are trying to achieve but in your case you are looking for the same image on a smaller format - but if you increased the format back to full frame, you would get a wider view.

So in summary, you could do it theoretically but you would need to remove the lens mount and move it all closer - and the mirror would probably hit it.


Steve.

 

[HoppyUK]

Edit: crossed post with Steve. Great minds etc

Ah, I see what you mean.

I don't have a problem with the theory, but since nobody has done it there must be a practical reason.

That is likely to be difficulty in gathering the incoming light cone which is going to have a wide angle and that's very hard to handle (one of the reasons teleconverter work best with longer lenses). Plus the additional optics are going to push the lens even further forward when the focal length change is naturally wanting to push the lens further back to focus.

And I guess the cost/benefit ratio wouldn't be too god, even if it could be done, relative to buying a new lens.

 

[PeteStewardson]

Following a thread on teleconverters I was wondering if there where "wide angle" converters that where designed to focus a FX lens down on to a DX sensor? I can't think of a reason why such a thing does not exist?

Stuart

The lens would have to be closer to the sensor so it's impossible to do as the lens mount is in the way.

Pete

 

[HoppyUK]

The lens would have to be closer to the sensor so it's impossible to do as the lens mount is in the way.

Pete

That could be overcome in theory - same principle as inverted-telephoto design used for all DSLR wide-angles.

I just think that by the time you've sorted all the problems, you will end up with a rather poor lens that costs as much as a new one.

 

[jgs001]

The concept exists in the world of Astronomy and refracting telescopes. They are focal reducers. The sit between the camera body and the rear of the telescope focuser (same as you would with a TC) and effectively reduce the focal length of the telescope by an amount. I've not got one (although want one), but I've seen them in various guises, 0.85, 0.6 and I think a 0.3.... I think the main issue being the design elements in camera lenses mean they are made to be as small as possible, for a specific focal length, whereas within a Refractor (or a reflector for that matter), there are no such constraints, and a 600mm focal length refractor is 600mm... a 1200mm refractor is 1200mm, from the objective to the plane of focus. The lens elements tend to be far simpler, a decent ED scope, is a simple doublet with exotic glass, an Apo is a triplet lens cell again with exotics, no other glass in the system.

 

[stu_the_flat]

Cheers For the answers guys.

I'm not sure I understand why the lens would need to be closer. surly that could be determined by the "angle" the infromation left the lens? as in raise the magnification from 0.7 to 08 and then place the lens further forward?

I can feel that I confusing myself as I type!

Stuart

 

[Steve Smith]

I'm not sure I understand why the lens would need to be closer.

Because you are reducing the focal length of the lens. Apart from telephoto designs (a subject for a different discussion) The focal length is also the distance from the film/sensor to the optical centre of the lens. You are changing the focal length so this optical centre needs to be moved back a bit. This means moving the whole lens back.

Another way of thinking about it is to think of just one convex lens. Focused at infinity, this lens converges parallel lines of light to a converging point. This is the point where you place the film or sensor to get an image in focus. If you add another convex lens you are making a lens which converges the light even more so the film or sensor needs to move closer to the lens to get to the focus point.

If you have a couple of magnifying glasses you can try this yourself. Use one to focus an image of e.g. a lit light bulb onto your hand or a wall and notice the distance between the lens and the focused image. Now do the same with two magnifying glasses held together. You should notice that the image to lens distance decreases and the focused subject gets smaller (the effect of a wider angle lens).


Steve.