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Hasselblad lens was made for 120 or 220 roll film... if I successfully mount it to a full frame 35mm camera, how long is the lens? What it's effective focal length? I just don't know the math... Common Hasselblad lenses that interest me are the 80, 100, and 250... What's the reach on the 250 in particular?

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Hasselblad (250mm) lens was made for 120 or 220 roll film... if I successfully mount it to a full frame 35mm camera, how long is the lens?

250mm.

What it's effective focal length?

250mm.

That's because "effective focal length" is based on the angle of view obtained by a particular focal length when used with 36 x 24 mm full frame (a/k/a "35mm" 135 format) cameras.

If used on a medium format camera using the 6 x 4.5 (a/k/a "645") frame size with exposed dimensions of 56 x 41.5 mm, a 250mm lens would have an "effective focal length" of about 155mm, because it would give the same diagonal angle of view as a 155mm lens on a FF camera with a format size of 36 x 24 millimeters.

If used on a medium format camera using the 6 x 6 frame size with exposed dimensions of 56 x 56 mm, a 250mm lens would have an "effective focal length" of about 135mm, because it would give the same diagonal angle of view as a 135mm lens on a FF camera with a format size of 36 x 24 millimeters.

6 x 4.5 (a/k/a "645") is probably the most common frame size for 120/220 film. With other aspect ratios the 56mm width remains constant (because that's the maximum usable width across the short side of the film strip), but the length gets progressively longer along the long dimension of the film.

80mm = 80mm on FF camera, "effective focal length" of 50mm on 645 format camera.
100mm = 100mm on FF camera, "effective focal length" of 65mm on 645 format camera.
250mm = 250mm on FF camera, "effective focal length" of 155mm on 645 format camera.

The most popular Hasselblad cameras used 120 film exposed with a 6 x 6 frame (56 x 56 mm). Because the diagonal of a 6 x 6 format frame is longer, the conversion factor changes.

80mm = 80mm on FF camera, "effective focal length" of 44mm on 6 x 6 format camera.
100mm = 100mm on FF camera, "effective focal length" of 55mm on 6 x 6 format camera.
250mm = 250mm on FF camera, "effective focal length" of 135mm on 6 x 6 format camera.

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The focal length of a lens is a measure of its power to magnify. The focal length of a lens remains unchanged regardless. In other words, interchange a lens between different cameras of different formats and the image size of objects remains unchanged.

That being said, what changes is the angle of view. Think of a movie being projected on a screen, we substitute different screen sizes while the movie is being projected. The size of the projected image of objects does not change but the screen size change encloses different image sizes (formats).

The Hasselblad format is said to be 6cm by 6cm. The actual image on the film frame measures 56mm by 56mm. The diagonal (corner to corner) measure of this format is 80mm. If we mount an 80mm, the lens delivers an angle of view of 53° diagonally and 38.5° horizontally.

Mount this same lens on a full frame 35mm camera and this lens projects an image of a format size of 24mm height by 36mm length. The diagonal measure of this rectangle is 43mm. Because this format is smaller, only the central portion of the lens’s projected image can be utilized. In other words the screen size is reduced. Now the angles of view are reduced to 30.3° diagonally and 25.4° horizontally.

Mount a 100mm and the angle of view changes to 24.4 diagonally and 20.4 horizontally. Mount a 250mm and the angle of view diagonally will be 9.9° diagonally and 8.2° horizontally.

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    This would be a better answer if it actually answered the specific question asked. – Eric Shain Apr 14 '19 at 20:08
  • It's also be better if it didn't seem to be saying that the size of the image circle projected by a lens is changed by the size of the sensor placed behind it. – Michael C Apr 14 '19 at 22:42

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