different lenses, same fstop questions
 

On Sunday I did a green screen interview shoot along with an ex3. I used my kit lens (although I had several canons and nikons with me). I had what looked like fairly crisp focus to me, but the director complained. I realized that he wanted deeper dof so that the talent's entire head was totally in focus. They didn't light the subject enough ( my oppinion) so I had to jack up my iso to 9db and stop down to f7.1.
That setting gave hime the dof that he was looking for.
Here's my question- Assuming I would have had to stop down any lens to f 7 (at least), would there be any difference if I had used one of my faster lenses? I was at 70mm.
I could have used my Canon 17-55 f2.8 or 50mm f1.4, and moved closer, my Tokina for nikon 28-70 f2.8 from where I was, or even my nikon 105 f1,8 moved back. In other words, would a faster lens, at the same settings perform any better? I didn't do a test, but I assumed that any of the lenses would have to have been stopped down equally. Is this the case?
Bruce Yarock

Re: different lenses, same fstop questions
 

You'll get some small variations between lenses. A lens with more glass elements will loose a little more light than a lens with fewer elements, but the DoF will be essentially the same at any given focal length/aperture for all lenses.

If you move closer to the subject then it follows that for the same framing you would need to reduce the focal length in which case the DoF will increase with the opposite happening if you move further away.

Re: different lenses, same fstop questions
 

Thanks Allister.
I wanted to move closer and go wider, but ther wasn't enough room, with us sgaring a monitor, etc. But that's a good point to remember-closer plus wider = deeper dof.
Bruce Yarok

Re: different lenses, same fstop questions
 

How far back do you think you were?

Re: different lenses, same fstop questions
 

Doug,
I was about 8-9 feet from the subject. Acording to an iphone dof app, the dof at the settings I settled on(f 7.1,70mm and 9db gain), supposedly gave me 1.25 feet of dof. That was based on the Canon 5d, because neither of the dof calculator apps have the FS100 as a camera choice.
Bruce Yarock
Yarock Productions

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Re: different lenses, same fstop questions
 

One thing - you'd be better off using a preset for an APS-C camera than the 5D. That should be closer to the FS100s Super 35mm sensor than the 5D's FF. Also, if you're using lenses with F-stops (and not T-stops), then you will notice a difference even at the same settings. I can't remember which way it went, but when I had a Canon 16-35L2.8 and the 25-70L2.8, at the same focal length and F-stop, one was about half a stop brighter. Never hurts to check. Technically though, they'd have the same DOF at the same focal length though.

I agree with Alister -your "best solution" would likely have been a wider lens closer. Though as you get closer to your subject, your DOF will also decrease, so there's like a point of diminishing returns. For instance, a 16mm lens right in front of them may have had the same or less DOF because of how close you'd be. I know at times when I want stupid shallow DOF but don't have room for a 100mm+ lens that I can cheat and get a 50 or 70 real close to the talent (depending on the ability to use a really tight shot) and get it. Though I moved away stupid shallow DOF when the novelty wore off. :)