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I need to video (but same q for photos) an interview using a Sony A7 and the kit 28-70mm f3.5 - f5.6. I want the shallowest depth of field I can get.
If I use 70mm the lowest aperture I can use is f5.6, but at the widest 28mm I can use f3.5.
If I use 28mm-35mm at f3.5-f4.5 and get as close as I can to the subject will the image be distorted (if so how)?
Yes, the image will normally be distorted, in that objects close to the lens will look larger and those farther away appear to be smaller.
For the human face viewed straight on, this is not at all flattering, as the nose will look (a tiny bit) larger than it should, the eyes and mouth smaller, and the ears and head even smaller (in relation to a 50mm view on any full-frame camera like the Sony A7 you mention, or in comparison to how we view the world around us with our visual sense (eyes and brain)). Since we pay attention to a speaker's eyes and the mouth, it would seem to add a risk of distractingthe viewer from the words, or looking odd or unattactive.
Depending on how close you get to the subject, in addition to the distortion, a shallow depth of field might mean parts of the face might end up 'soft' or blurred, and it may be that these would look better sharp.
Keep in mind that the not-in-focus elements are likely to look different depending on focal length - the 70mm or even 50mm length could be used in a set-up to produce a 'look' of subject versus background that might be what you are aiming for.
Here's an interesting discussion of achieving a shallow depth of field look with mid-range apertures. In the end, as a photograph or video is (primarily) visual, it seems like trying out the camera and lens combination is going to be the best way to work out how the distortion affects the image and whether your focal length will reach your aims!
To minimize the distortion of human face and to make depth of field shallow you need to use the longest focal lengths (70mm in your case). And of course wide open aperture.
Above recommendations are for camera/lens combination and do not depend if its video or still. But if you want to apply rule 180 for video (shutter speed) you may need ND filter
Don't use wide angle if you aim for the visuals of shallow depth of field. The point spread for a given distance from the focusing plane and the same focal opening is about the same regarding the physical object. However, with a wide angle lens, the backdrop is depicted at a much smaller scale in comparison with the foreground object, the result being that the "similar" blur covers a much smaller area on the image and thus leaves a lot more distracting content.
If you aim for a maximally defocused background, you should go into zoom as far as the camera allows before losing aperture as fast as gaining background magnification (assuming that your speed is still fast enough). This point of diminishing returns is different with different cameras.
Do a few experiments with objects placed similarly to how you expect them in the interview room. It's rather enlightening.
