What's the difference?

[Martin Rickards]

Say between a 100 mm lens for 35 mm and a 100 mm for medium format? What gives one the coverage for medium format while another only covers a 35 mm frame? Just curious.

 

[Dan Fromm]

Design.

 

[Chan Tran]

I don't know either. I started to do some research. I think if you make the lens very simple with 1 element it would cover the normal angle of view.

 

[ciniframe]

What Mr. Fromm said. For instance. A 90mm f8 Super Angulon will cover 4X5 with some room to spare. The 90mm f4 Elmar for my Leica will not.
Now compare the two lenses. The Angulon has very large front and rear elements, even though it is f8 and the Elmar is f4. The lens for the Leica only has to project an image circle of about 46mm whereas the Angulon has to have an image circle of well over 150mm, probably more like 180~190mm to allow some room for movement.
Lenses are designed for the format they are mounted to.
I've demonstrated this to myself when I mounted a 105mm f4.5 Kodak Anaston from a 6X9 folder on my 4X5. At infinity the image circle was about 4.5 inches in diameter with the corners completely dark. Didn't think it would work but I just had to try it anyway.

 

[Leigh B]

To expand on Dan's "design" comment...

It's entirely a function of the angle of view. Wider angles cover larger film.

Envision the light coming from the lens as a cone, with its apex in the lens and its base at the film.
The cone's height is the lens' focal length. It does not change (for the purpose of this discussion).

The angle of view is the angle between the sides of the cone. This differs with different lens designs.
As that angle increases, the diameter of the base increase.

- Leigh

 

[tedr1]

With a 100mm lens for 35mm the coverage area is small compared to the focal length, the circle that covers the 35mm frame is about 43mm across, which is about half the focal length of the lens.

A 100mm lens for medium format requires a coverage circle approximately twice the size (depending on whether we are talking 6x6, 6x7 or 6x9) and the coverage circle is about the same as the focal length. There are two main challenges for the medium format lens that are more difficult than the 35mm lens: keeping the corners the same brightness as the center; keeping the corners the same sharpness as the center. Achieving these goals requires larger pieces of glass, different types of glass and more complex shapes of the curved glass surfaces. These increase the cost.

 

[Martin Rickards]

Yet there are many Tessar or Cooke triplets that easily cover medium format while 6/7 element double Gausses may only be suitable for 35 mm. Is it a function of aperture size or what?

 

[markbarendt]

35mm film format lenses are designed to sit very close to the film.

On a 35mm SLR the rearmost element is typically just forward of the mirror, maybe 1.5" or so. Lenses for larger format film cameras sit farther from the film.

The space between the rearmost element and the film limits the film size that the lens can cover:more space; more coverage.

 

[Sirius Glass]

Area coverage [covering 35mm versus 6x6], lens mounts, shutters on some lenses.

 

[film_man]

In simple terms, 100mm is the focal length, ie how far in front of the film is the lens when the image is focused (this is very simplistic, in practice this is true for telephotos but anything else gets complicated quickly). The coverage, meaning whether it can project to 35mm or medium or large format film is dependent on how wide the glass elements are.

 

[blockend]

On a 35mm SLR the rearmost element is typically just forward of the mirror, maybe 1.5" or so.

Even closer for rangefinders. The 35mm Jupiter 12 on my Kiev has a bulbous rear element that looks like it will hit the shutter blinds every time I fit the lens. It does hit some Contax fit rangefinder shutters and the clearance on the Kiev is tiny. Retrofocus Olympus compacts work on the same principle. OTOH the Taylor Hobson lens on my 5 x 4 can be feet away from the film plane with an extending bellows.

 

[markbarendt]

Even closer for rangefinders. The 35mm Jupiter 12 on my Kiev has a bulbous rear element that looks like it will hit the shutter blinds every time I fit the lens. It does hit some Contax fit rangefinder shutters and the clearance on the Kiev is tiny. Retrofocus Olympus compacts work on the same principle. OTOH the Taylor Hobson lens on my 5 x 4 can be feet away from the film plane with an extending bellows.

Yep.

 

[RalphLambrecht]

Say between a 100 mm lens for 35 mm and a 100 mm for medium format? What gives one the coverage for medium format while another only covers a 35 mm frame? Just curious.

the difference is only in the image circle it creates;the MF lens needs to cover a larger negative.You could overlay the two images and they would match.

 

[Martin Rickards]

the difference is only in the image circle it creates;the MF lens needs to cover a larger negative.You could overlay the two images and they would match.

But why does the mf lens create a larger image circle? what is the theory behind it?

 

[markbarendt]

But why does the mf lens create a larger image circle? what is the theory behind it?

If the angle that the lens projects toward the film is 60 degrees wide the further you are from the film the more coverage there is. 60 is relatively normal.

 

[Leigh B]

In simple terms, 100mm is the focal length, ie how far in front of the film is the lens when the image is focused (this is very simplistic, in practice this is true for telephotos...

Sorry, but that is incorrect by definition.

In a telephoto design, the rear lens node (from which the focal length is ALWAYS measured) is significantly in front of the rear node of the rear lens element. In fact it may be very far out in front of the front lens element.

The rear lens node may correspond to the rear node of the rear element on lenses of normal focal lengths.

- Leigh

 

[RalphLambrecht]

But why does the mf lens create a larger image circle? what is the theory behind it?

as somebody already said;that's in the design of the lens and the intended use of it.

 

[MattKing]

The question isn't why, it is how.
A 100mm lens for 135 vignettes the light before the corners of the medium format frame is illuminated. The lens elements are too small, and the lens barrel/aperture gets in the way.
The designers of the lens for 135 may decide not to simply enlarge it to work with medium format. There may be better ways to accomplish what they need.