Mathematics and shutter speeds

[stargazeruk55]

Hi all, I have an interesting question for you

If a vehicle is travelling towards you at 40 mph, which i calculate at 58 feet per second, what shutter speed would you need to be able to get a sharp image????

Cheers, Martin

[jet_kit]

Oh, were life that simple!

There's more to this question than you've given us data on, so we have to make a few assumptions.

Let's assume that you don't want to get run over, so you're using a long lens, say 200mm on a DX format. The front average car will fill the frame at about 75ft at that focal length. Again, assuming that your aperture is around f11 the depth of field will be from 66ft to 86ft if you focus at 75ft. This means the image will be acceptably sharp as it moves through that 20ft DoF. So, in theory you could shoot at about ¼ sec and the object would be in focus. However, of course, an object moving at speed will be subject to speed blur at that sort of shutter speed and that will be a far greater problem than focus.

If you're shooting hand-held with a 200mm you should be shooting at about 1/200 sec unless you're using VR, in which case you could realistically get away with 1/50 sec if your stance and camera holding technique is good. Practically, I would tackle such a task at about 1/250 sec, then I would switch the AF off and pre-focus on the 75ft mark and wait for the car to enter my zone of focus, possibly shooting a burst.

Hope this helps.

[markgozz]

Chris I'm always so impressed with your detailed answers , you must of been slaving over a slide rule for hours to work that lot out.

Personally I would of cranked the aperture wide open , click onto dynamic focusing and fired off as many shot per second as I could

The difference between someone that knows what he's doing and me I guess .

Mark

[rbarry]

I second Mark on Chris's analysis, although VR won't remove motion blur from a moving object, it will only counter-act hand held jitter.

[jet_kit]

Originally Posted by markgozz

Chris I'm always so impressed with your detailed answers , you must of been slaving over a slide rule for hours to work that lot out.

Personally I would of cranked the aperture wide open , click onto dynamic focusing and fired off as many shot per second as I could

The difference between someone that knows what he's doing and me I guess .

Mark

Hi Mark,
It's just something us oldies had to learn to do in our heads on the spot. We had no AF, no VR, no auto-bracketing, no metering, no burst. And we had to wind the film on by hand!!!
The only this I miss on modern gear is the marks we had engraved on our lens to indicate the DoF.
Note that the f-stops are colour coded and correspond to the focus scale. The little red dot is the Infra Red focus mark (focus on the subject, then adjust the point to the red mark.). These days you have to consult tables - Why?

[stargazeruk55]

Thanks for your answer, which very much surprised me. I was expecting you to say use a fast shutter speed due to the speed involved.

I shoot motocross and my biggest challenge is getting the rider sharp as he comes over a jump.

This is a 'blind' setup really as you cannot see the bikes until they have started to take off and then the autofocus has to kick in, then VR and then shoot........many come out very soft or blurred, and thats of shutter speeds at 1/1000 sec!! I mostly shoot around F5 unless i'm using the 24-70 F2.8 to get closer, and i shoot continuous and single focus point .....

So i must be doing something wrong, in which case i shall give your suggestion a go Chris and see if it works for me

Thanks for the other comments gents, informative

It won't be till after Xmas when i go to the track again, see how i get on...

Cheers

Martin

[greenwing]

Originally Posted by stargazeruk55

autofocus has to kick in, then VR and then shoot.

Cheers

Martin

Martin, VR needs a half second or so to lock, so the sequence should be VR, focus, shoot.. I think you should probably turn VR off. I've seen things that say it's not effective over 1/500 shutter speed.

If you don't give VR that 1/2 second to get up to speed, it will cause blur rather than eliminating it.

Chris

[jet_kit]

Originally Posted by greenwing

Martin, VR needs a half second or so to lock, so the sequence should be VR, focus, shoot.. I think you should probably turn VR off. I've seen things that say it's not effective over 1/500 shutter speed.

If you don't give VR that 1/2 second to get up to speed, it will cause blur rather than eliminating it.

Chris

I agree. Turn off the VR, AF, the metering and all the other junk that just slows you down. Go manual, pre-focus on the jump. Take a couple of test shots of the crowd to get the exposure right. You could even set up on a tripod and fire a burst as soon as his front wheel comes into view.
Look at the pros around you and see what they do. I have a friend who does Rallying as a pro. He had a special bracket made for two Canons and a Video camera all bolted to a massive tripod and linked to a single shutter release. He sets everything up ½ hour before the first car is due sits down on his folding chair with a cup of coffee and just pushes the button everytime a car comes into his field of view. Simple.

[mapleleaf]

I would ignore the car altogether far to dangerous and go and shoot some flowers.

[jet_kit]

Originally Posted by mapleleaf

I would ignore the car altogether far to dangerous and go and shoot some flowers.

I'm with you Mapleleaf. If it moves it's far too quick for me to photograph.