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I'm mounting a Hasselblad Sonnar C 150 mm f4 on a 35mm sensor (43mm diagonal) camera.

What is the calculation to get the new angle of view and equivalent focal length? Does this also affect the aperture? for example if the 'crop' factor' ends up being 2, would I end up with a 300mm F2.0 lens?

Edit:

So far, the answers are:

  • No change. It's a 150mm and F4.0
  • It'll be a tele lens at about twice length of ~270mm, same aperture (or tele at F7.3)
  • It'll feel shorter length at about half ~80mm, half the aperture F2.0

I'm still in the dark (pun intended, twice) as to what's up.

The two pictures below, first was shot on the 150mm F4 blad mounted on 35mm sensor.

150mm C4 Medium Format

The second picture was shot using the 55-200mm APS (crop) lens on the 35mm sensor @ F5.6 and zoom at ~105mm. The shutter for the second picture had to be two stops slower to get the same (underexposed) picture.

55-200mm Sony DT (APS) at ~110mm F5.6

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A 150/4 on 35mm acts like a 150/4 on 35mm.

Full-frame 35mm (43.2mm diagonal) is the default frame of reference. Equivalence is usually about referring some other sensor/film size back to that 35mm default. As in "if I use X on my camera it'll be like using Y on FF 35mm."

But... the math in case you do care about the other equivalences:

Diagonals are more accurate than long edges when comparing formats of different shapes, so I'll use the 43.2mm diagonal of 35mm vs. the 79.2mm diagonal of the 56mm x 56mm—aka 6x6—'Blad.

On that 6x6 this lens had a 35mm equiv. focal length of (43.2mm/79.2mm * 150mm) = 82mm and an equivalent aperture of f2.2.

To go the other way just flip the math around (79.2/43.2 * 150)=275mm and f7.3. So using it on FF is like using 275/7.3 on a 'Blad.

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It will still be a 150mm f4 lens on a 35mm sensor. You will just be using a portion of the full image circle.

The field of view will be narrower than what you are used to on medium format, you can work out the equivalent "focal length" from the ratio of the lengths of the film/sensor. E.g. For 6x6 film the horizontal angle of view will be the same as 56mm/36mm * 150mm = 233mm lens on 6x6. Depth of field will also be roughly a stop (56/36 ~ 1.5) deeper.

On 35mm it will produce roughly the same framing/depth of field as a 230-240mm f5.6 on 6x6. This comparison is only meaningful if you think in terms of a specific format and focal length combination. Realistically it goes from a medium telephoto/portrait lens to a longish telephoto on 35mm.

  • I'm fairly sure you want to compare diagonals, not lengths. – Philip Kendall Jun 2 '15 at 18:59
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    Sure, it was intended as an example. You can work out diagonal or horizontal fov similarly. I've also got no idea what aspect ratio the OP is using/used to. – Richard Smith Jun 2 '15 at 19:03
  • @RichardSmith added some more detail and pics. – MandoMando Jun 4 '15 at 21:04
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Going from a 6cmx6cm to 35mm, expect a focal length equivalent to 294mm f/7.8 on the original 6x6 medium format sensor. (note reducing the sensor size makes it a darker lens, not a brighter one, sorry)

As sensor size decreases, depth of field will increase for a given aperture.

Check out the calculator on the following page. Enter 35mm, 6x6 f/4 and 150mm.

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

For a 150mm lens on 35mm the angle of view is about 13 degrees. (http://www.sweeting.org/mark/lenses/medium_format.php)

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