Recently a fellow tech & controlbooth.tv reader wrote in, “since I can achieve any particular field of view at various camera-to-talent distances simply by adjusting the focal length of a zoom lens (assuming a powerful enough lens); is there any benefit or downside to any one particular focal length/distance combination? For example, will the depth of field change subject to focal length and distance?”
OK, great question!
First, if you are unsure what the terms field of view, focal length and depth of field refer to click on the hyper-links provided. Second, let’s illustrate the question to make sure we are all on the same page. Here is a generic stage plot.
For the sake of this discussion we are framing a medium shot (2’9” vertical x 4’11” horizontal) of the guitarist downstage, stage-right. To begin we are also assuming the camera is located 50’ from the downstage lip which would put it a total of 56’ 11” from the guitarist. If using a 2/3”sensor camera at the 56’ 11” distance our lens focal length would need to be 112mm to achieve the medium shot.
At these given particulars (the camera to subject distance, the sensor size and lens focal length, as well as the field of view) an end result will be 5’ 10.25” of the upstage lip behind the talent (the totality of what can be seen in the background) will be visible. Practically speaking that means that drummer, the side facing amplifier, the monitor mixer and computer will [barely] be excluded from the shot.
So, how do these particulars impact depth of field? Well again, as the camera has a 2/3” sensor and if the aperture of the lens is set to F4 [a typical ideal range for entertainment specials aired on broadcast television is usually F4 – F5.6], the resulting depth of field will be 5’ 7.5”. Whatever is upstage or downstage of that Depth of Field box will be progressively out of focus.
Now, if we keep the field of view exactly the same (2’9” vertical x 4’11” horizontal, a medium shot at the guitarist) but instead move the camera fifteen feet closer to the stage the camera will now be 42’ 1.75” from the guitarist. To achieve the same medium shot at the now closer distance the lens’ focal length will need to change to 83mm (as opposed to 112mm before). Because we are using a wider focal length setting on our zoom lens and a closer distance to the talent the upstage background area seen will increase somewhat from 5’ 10.25” on the upstage lip to 6’ 2.375”, a gain of approximately 4”. The increased upstage visible area seen as well as the increased angle-off [due to the camera remaining on center-line to the stage but now closer in proximity] will result in some of the off-stage equipment being seen from time to time. Interestingly though, even though the camera location and focal length needed to achieve a medium shot has changed (we are now closer to talent and wider in focal-length), the depth of field for all practical purposes remains similar to the 50′ example stated before. The DoF changes from 5’7.5″ to 5’ 7.7”, an increase of 0.2 inches (approximately half a centimeter).
If we again move the camera fifteen feet closer to the stage maintaining the same field of view (2’9” vertical x 4’11” horizontal, a guitarist medium shot) the camera will now be 27’ 6.25” from the subject. To achieve the same medium shot at the now closer distance the lens’ focal length will again need to change, now 54mm. The upstage visible background will increase again from 5’ 10.25” (originally) to 6’ 2.375” (35′ to stage) and now 6’11” (20′ to stage), a gain of nearly one foot when compared to 50′ calculations! As before, even though the camera distance and focal length to achieve a medium shot has changed, the depth of field remains for all practical purposes as it was in the previous examples, now 5’ 8.1”.
It should be noted that the minute changes in depth of field [when FoV, aperture and sensor size are constant] is in part due to us using a relatively wide aperture (F4) in our hypothetical scenario. The smaller the aperture (the higher the F stop number) the greater the DoF changes will appear. However for most Houses of Worship [as well as broadcast television productions] it is normal to shoot at wider apertures and therefore I think the example and results are applicable.
Aside from field of view and depth of field discussed above, there is another factor to be considered in answering the tech’s interesting question. Zoom lenses have a focal length dependent performance “sweet spot.” In general, with most zoom lenses as you increase the focal length by zooming in the light efficiency of the lens decreases. In general focal length and light efficiency are inversely proportional [assuming the lens barrel diameter of the lens is constant]. This is why the speed (aka, the light efficiency) of most video zoom lenses is noted on a spec sheet as an F-stop range. This focal length dependent light efficiency phenomenon is called F-Drop (aka F-Stop Ramping). Therefore in general it is considered best to shoot at wider focal lengths where the zoom lens is more light efficient than to shoot at telephoto focal lengths [if possible considering the many potential constraints of a given shooting environment, seat kills, etc.].
But doesn’t wider focal lengths affect the look of an image? Yes! Indeed focal lengths do. Here is a YouTube video which illustrates this on the human face:
Conclusion: If always maintaining an identical field of view at the subject (in our example, a medium shot), moving a camera closer or further from talent will result in the depth of field remaining nearly constant. What will change however is the width of the visible area behind the subject, as well as the amount of optical compression (how far or near the area behind the subject appears behind the subject). Longer focal lengths make the subject appear closer to a background. Wider focal lengths make the subject appear further from a background. Longer focal lengths often make the background appear larger. Wider focal lengths often make the background appear smaller.