Normal lens fo arir large format

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  • #215293
    rama lingam

      Dear master Roger what is the normal lens for large format. What lens gives more intimacy. Basically it’s less depth. If I want to 32mm coverage for large format what lens would you prefer. How large format 75mm, 125mm sees the subject different from normal super 35mm lenses

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    • #215294

        To calculate the equivalent focal lengths in terms of horizontal view, find out the width of the sensor area being used and divide the larger number by the smaller number to get the conversion (crop) factor.

        Generic figures of 24mm wide for Super 35 and 36mm wide for Full-Frame 35 means you divide 36 by 24 to get 1.5. So that’s the conversion factor. To get a matching horizontal view you’d multiply the Super35 lens by 1.5 to get the equivalent view in FF35. So a 32mm lens in Super35 has the same view as the 48mm in FF35.

        But to be more precise you need the actual widths of the sensor areas being used. Open Gate on a classic Alexa is 28.25mm and on the Alexa 35, it’s 27.99mm. Open Gate on the Alexa LF is 36.70mm. So if you were comparing a classic Alexa to an LF, both using Open Gate ARRIRAW, you’d divide 36.70 by 28.25, which is 1.3.  So a 32mm lens on a classic Alexa would have to be a 41.6mm on an Alexa LF if you wanted to match the horizontal view.


          It’s a misunderstanding when people commonly say that focal length creates perspective effects. Perspective on landscapes and faces is determined by camera position and distance to the subject. It’s not even a camera issue — take a walk in the mountains. You see a boulder on the hill you are standing on and there is a distant mountain peak. As you walk closer to the boulder it gets relatively larger compared to the distant mountain.  It’s the same thing with a face — get closer and the distance from your viewpoint to the nose changes but the distance from the nose to the ears is a constant, so the nose gets relatively larger compared to the ears as you get closer. What the focal length provides is the view of the subject, how tight or wide it is, the degree of magnification or expansion.

          So if you match distance and you match field of view between formats by using the equivalent focal lengths, you get the same image more or less. What’s different is the depth of field, because the larger formats need longer focal lengths to achieve the same view, there is less depth of field unless you stop down the lens (by the same amount as the conversion factor, so if the factor is 1.5X then you’d stop down the equivalent lens on the larger format by 1.5-stops to match the depth of field of the smaller format.)


            May i ask a about the aperture and equivalence? I’ve read many confusing position about that. For some It seems that aperture changes to consider the equivalent focal length, for others the sensor doesn’t influence the aperture because It’s a ratio based on actual lens size . Who’s right?


              From Wikipedia:

              The f-number N is given by:

              N = f/D

              where f is the focal length, and D is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by “f/”, which forms a mathematical expression of the entrance pupil’s diameter in terms of f and N. For example, if a lens’s focal length were 10 mm and its entrance pupil ‘s diameter were 5 mm, the f-number would be 2. This would be expressed as “f/2” in a lens system. 

              But focal length isn’t really a measurement of the physical length of the lens, it’s the distance from the optical center to the sensor plane when the focus is at infinity, and the aperture size is not the measurement of the size of the hole by measuring the back of the lens but is the “effective aperture” size as seen through the front element. So this is why lenses can vary in size even though they might all be a 50mm.

              But if you put a 50mm lens set to f/2 on cameras with different sized sensors, it’s still f/2 exposure-wise just as if you only had one sized sensor and then cropped in post to smaller areas.

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