Sensor size & Focal Length + Depth of field

Posted on by

Home Forums Camera Sensor size & Focal Length + Depth of field

  • Creator
  • #172580
    Max A.

      Hello, Mr. Deakins and all the Cinematographers over this fantastic forum. I hope you Mr. Deakins and Mrs. James are well.
      I would like to ask you (Mr. Deakins, Mr. Mullen but also who wants to join the topic), if it is possible, your thoughts about the focal length paired with the sensor size.

      I know (if I remember correctly) that you had shot ‘Empire of light’ with an Alexa LF as well as for ‘1917’.
      if I remember correctly for ‘1917’ you choose a 40mm lens that resulted in the field of view something near a 35mm-lens of a 35mm full-frame sensor in order to have a similar feeling to some still photographs that you researched as inspiration for the movie.
      This kind of “calculation” is something that you did also on ‘Empire of light’?
      Let’s say that you had to shoot an OTS and for that kind of OTS you were going to shoot with a 32mm (on a super 35 sensor/stock) but now with an LF sensor your 32mm is something like 24mm (roughly) field of view (but not for the perspective distortion). In this case, do you maintain the 32mm focal length moving forward the camera to have the “right” composition, or do you use another lens (probably a 40mm)
      Furthermore, even the depth of field is differently related to sensor size.

      To be short, did you think about the sensor size when you had to choose the lenses during the shooting in order to have your desired field of view (same thing for the T-stop and depth of field)?

      I own a super35 sensor camera with a high crop factor (something like 1.56) so sometimes when I have to shoot something wide I have to use a really wide lens like 16mm in order to have a field of view something like 24mm.
      I won’t deny that sometimes it confuses me because it doesn’t let me choose the goal in a totally free way.

      Sorry if my English is not good enough and the post could result in confusion. Thank you in advance for your time and your availability.

      I wish you a peaceful day.

    Viewing 7 replies - 1 through 7 (of 7 total)
    • Author
    • #172592

        It’s just numbers, I wouldn’t waste too much time overthinking it. You get a set of lenses for a particular camera / format that range from wide-angle to telephoto and you work with those every day on the set, you don’t spend every day converting focal lengths in your head.

        If the conversion factor is 1.5X, where you’d use a 35mm in Super-35, you’d probably use a 50mm in Full-Frame (52.5mm is the exact equivalent to a 35mm when going from a 24mm wide sensor to a 36mm wide sensor.). However, the conversion from Open Gate in a regular Alexa to Open Gate in an Alexa LF is more like 1.3X.  So if you like a 32mm on a regular Alexa in Open Gate, you’d use a 40mm on an Alexa LF in Open Gate (41.6mm to be exact if the conversion is 1.3X — I’d have to look up the specs again for both formats to make sure that’s correct.)

        The difference in depth of field is due to the focal length and again, the amount you have to compensate is the same as the conversion factor, so if the factor is 1.5X then your larger format needs to be stopped down by 1.5-stops to match depth of field to the smaller format.

        If you shot in Super-16, you’d have a whole different set of focal lengths to deal with.

        Max A.

          Thank you very much for your answer and clarification Mr. Mullen.
          I agree and maybe I was overthinking that, sometimes is better to trust to what an eye sees in the frame rather than use energy doing math. Especially during a stressful day of shooting with a number of tricky situations and events.

          Great to know the proportion between sensor size and iris aperture in order to match the depth of field of different sensor sizes.

          Thinking a bit about the depth of field, therefore when we shoot on a full frame or even a large format sensor, do we need more light (so even a bigger light package) if we don’t want a rapid fall-off of the focus related to a big sensor?
          Maybe I’m overthinking again..

          Thank you again for your answer Mr. Mullen

          I wish you a peaceful day.


            Larger formats use longer focal lengths for the same field of view so have less depth of field, so yes, you have to stop down if you want to counteract that.

            Generally that means more light… but larger sensors tend to handle higher ISO settings because the increase in noise is less obvious, so you can partially compensate for the lower depth of field by increasing the ISO — you could, for example, go half and half, boost the ISO a little, boost the light level a little.

            However, some people shoot larger formats precisely because they like the shallower depth of field.


              IMO to understand lensing you have to understand the effect each lens has to you and by definition the effect you want the audience to feel.

              I know it sounds pedantic but it’s the reality. At least to me.

              You don’t start choosing lenses by choosing the lens itself. You choose first where to put the camera relatively to the action or the thing you want to shoot.

              The variables here are the distance to your subject and the angle you shoot from.

              Then you choose which focal length to use for your desire framing in that position.

              Position, angle (no lens related) and framing (lens related).

              Photographers and cinematographers who have been shooting for long time develop their own style because their approach and reaction to compose an image is usually always the same. It’s their vocabulary. Other great ones force themselves to vary project to project.

              Study photography, look for the camera position relatively to the action. Look what they want to say and how. Look around and see what lenses they used if you like.

              But their language and approach to create an image is way more important than to know exactly what focal lengths they used.

              If you know this then no matter the format or sensor size you will be able to adapt it on your own work.


              Max A.

                Thank you very much for your answer quijotesco. Although your reply could appear a bit pedantic I think it is very important. I think that the fact of where to put the camera related to the action is one of the most important aspects of storytelling and often it is underrated (also by me when there is no time to shoot).
                My question was a bit specific about technician aspects of physics and sensors, related to the field of view, perspective distortions, depth of field, and so to lens choice in order to better execute the theory that you rightly explain.
                Your point is crucial, and of course, triggers me so much interest because I really feel that is a big narrative tool for a DP and a filmmaker.

                Thank you again for your reply.

                I wish you a great day.

                Roger Deakins

                  For ‘1917’ we did shoot on a 40mm on the Alexa LF. To me this lens length was equivalent to between a 35mm and a 32mm on a standard format but that seems debatable. Some suggest that a 40mm is equivalent to a 27mm on an academy format. This doesn’t feel right to me and I only really am interested in what feels right. On ‘Empire of Light’ my favored range on lenses was between 32mm and 65mm but I sometimes shot much wider than that as well as longer. I was just judging by eye as I set a shot. Best not to get obsessed with numbers!

                  Max A.

                    Thank you very much for your answer Mr. Deakins! The lesson here for me is to trust in what I think is right about action/subjects, camera position, and lens choice related to the narrative purpose that I want for a scene.
                    Too often I overthink numbers and “scientific” info, but I start thinking that it is a waste of energy and a complication for a narrative approach to images.
                    Thank you again for your reply Mr. Deakins, can’t wait to see ‘EOL’! The second trailer (and the article with your interview and diagrams on BSC) makes me so impatient!

                    I wish you a peaceful day.

                  Viewing 7 replies - 1 through 7 (of 7 total)
                  • You must be logged in to reply to this topic.