Can I use my still lens for cinematography?
Of course you can, but there may be some limitations. This post will cover some reasons you might also need to add some cinema lenses.
Focus Breathing
When a still lens claims to be a certain focal length, say 100mm, that spec is only valid for infinity focus. As you focus on closer objects, the focal length can change, typically to a lower value. At a few feet, your 100mm lens might actually be 80mm.
The is not an issue for a still photographer because:
1) your don’t need the longer focal length for the closer subject
2) a still image is, well, still and whatever the focal length is it is fixed.
However, if you are making a moving image shot, and racking focus between people or other subjects during the shot, the change in focal length becomes obvious because the background seems to ‘breath’ (that is it gets larger or smaller because the lens is, essentially, zooming)
People DO still use stills lenses and some lens makers have added breathing to their design criteria.
What to do about lens breathing
One approach it to reframe the shot while racking focus. This does not solve the issue but it does make the change to the background less noticeable. If you are racking back and forth, reframing can make the shot confusing.
Another approach it to refrain from racking focus and separate the shot into a shot/reverse shot series each of which are fixed. Traditional continuity editing exists to solve problems just like this one.
The only way to really solve this issue is to use a cinema lens that has an unnoticeable level of breathing. They are expensive and you will want to check out their performance in this regard.
If you want to add the production value of racking focus, you need the equipment to do that
Parquin Cinematography 