focal length

<p>hello everyone, <br>

i started thinking recently about moving to medium format. it's not gonna happen over night as it's very expensive to get digital hasselblad (i'm thinking within a year or maybe even two) - i know i could start with film and move to digital, but (un)fortunately i'm a "digital child" and i never even owned a film slr... currently i shot with canon (30D, 5D, 1D mkIII + lenses and other equipment). my question is about lenses in medium format... is there any formula to convert FF focal lengths to MF? for example what 24mm/50mm/85mm/135mm that i use on 5D would be on H3D 31mpx. i specifically want to get this camera (it's cheapest hassy at the moment) but it has a little bit smaller sensor than the other H3Ds and i honestly have no clue how to convert focal lengths... any help would be greatly appreciated...<br>

greg</p>

<p>To my knowledge the H3D uses a sensor with 36mm x 48mm (giving a digonale of 60mm) ... the 5D has 24mm x 36mm (giving a diagonale of 43mm) ... so the conversion factor is 60/43 ... about 1.4</p>

<p>The field of view of a 24mm on the 5D is therefore produced with a 24x1.4=33mm on the H3D.<br>

(fov-50(5D) would be 70(H3D) and so on)<br>

This is not entirely true, since the 5D uses a sensor with a width to heigth ratio of 3:2 whereas the H3D uses a ratio of 4:3. The above mentioned conversion is only true for field of view over the diagonale.</p>

<p>Brian, Rainer,<br>

thank you guys for your quick response...<br>

Rainer, i think the H3D 31 has a slightly smaller sensor than the 39mpx and 50mpx versions... so by what you wrote, i understand one can't convert exactly the focal lengths because of different sensor ratios... <br>

anyway i have an idea now...<br>

thanks again,<br>

greg</p>

<p>The so-called "normal" focal length is one which has a nominal 45 degree field of view (measured on the diagonal). This would be about 43 mm on a 35mm film camera, whereas the traditional "normal" lens is 50 mm. By this convention, a 35 mm lens would be "normal" on a DSLR with a 1.5x cropping factor.</p>

<p>It becomes more difficult to compare two cameras with different aspect ratios. The traditional way is to use the diagonal measurement of the image. Depending on how you used the camera, you might choose to compare the diagonals if optimally cropped to fit a 2:3 ratio (35mm rules), a 4:3 ratio (645 rules), or a common intermediate (e.g., 4:5, because that is a common aspect ratio for prints).</p>

<p>The H3D-31 has a cropping factor of 1.3x (<a href="http://www.hasselbladusa.com/media/1342793/uk_h3dii31_datasheet_v2.pdf">http://www.hasselbladusa.com/media/1342793/uk_h3dii31_datasheet_v2.pdf</a>). This is because the sensor is slightly smaller (33x44 mm) than other Hasselblad backs (37x49 mm, 1.1x). The "normal" focal length for 645 is traditionally 80mm. Dividing by the cropping factor, a "normal" lens for the 31MP version would be 60mm.</p>