Fast lenses for night action shots

sifro wrote:

rlx wrote:

Wow. I like that dance video!

Super!

For a long video sequence of a 2m tall frame I would try a manual f/1 lens. The focal length is your choice. The focal length has no effect on the DOF if one keeps the same framing (but the distance to the subject changes). Of course the background looks more blurry with the longer lens.

I know these are going to be 2 very stupid questions, but...

1) If we are talking about framing, why are we talking about apertures rather than focal lengths? I would think that focal length is crucial to determine how to fit a 2m tall person into my shot, while aperture is just about lightning and how blurry I want my background to be.

I meant that the dancers might look sharp enough even at f/1 if the frame is 2m tall at the subject location. However just part of one dancer will look sharp if framing more tightly (say 2/3 of body height). To still have both dancers in focus with tight framing one needs to use a larger f/stop. I added that the dancers that are in focus (the DOF) is not dependent on the focal length used if the framing stays the same.

2) If background looks more blurry with longer lens, why do you say that focal length has no effect on DOF?
Isn't blurry background = shallow DOF? Or am I missing something?

You are asking the question because you have a single prime to play with. If you had the 50-140 that is f/2.8 at all focal lengths you could check it and convince yourself that the background blur increases with the focal length but the DOF stays the same. I didn't tell the scenario yet. The scenario is that all pictures are of the same couple of dancers and all pictures are framed the same (say half body) and the aperture is fixed at f/2.8. Of course you need a large dancing hall since one has to walk somewhat between the 140mm and the 50mm focal lengths shots to keep the same framing of the subjects. The distance to the background should more than twice the distance to the subject to make the test meaningful so it might be better to make the test outdoor.

For short video sequences or stills I would try an AF lens with AF-C wide or zone tracking or AF-S if you can use backbutton focusing and keep the focus on your moving subject up to date. I feel manual would do but with fewer keepers and a bit more work.

For short video sequences or stills I would try an AF lens with AF-C wide or zone tracking or AF-S if you can use backbutton focusing and keep the focus on your moving subject up to date. I feel manual would do but with fewer keepers and a bit more work.

Oh yes, I didn't use AF-C with tracking last night and I highly regret it.
On my next post I'm going to post some of the pics I took and one of the take-aways is that I need to study the AF system of my camera better