Sharpness issue - What happened?

[Kermit power]

I took these shots yesterday at Goodwood. Why is it that some parts of the cars are perfectly sharp whilst others aren't?

Obviously all of the car is moving at the same speed, and, especially with the Williams F1, there was only a small percentage difference in the relative distance from me to the various points of the car, so what actually causes some parts to be sharp and others not? How do I make sure I'm getting the right parts sharp?

I'm assuming it's not down to depth of field, as the Williams was taken at f11, so shouldn't be an issue.

[Simpo Two]

Kermit power said:

Obviously all of the car is moving at the same speed, and, especially with the Williams F1, there was only a small percentage difference in the relative distance from me to the various points of the car, so what actually causes some parts to be sharp and others not?

I'd say it was largely DOF - your focus point seems to be too far back - but even if you get that right relative speed can get you - that percentage difference is enough to cause the issue. I'm fairly sure the only way you can keep both ends of the car sharp during panning is to be side-on - though I stand to be corrected by any motorsport experts here.

Edited by Simpo Two on Sunday 5th July 10:07

[Stickers]

Simpo Two said:

Kermit power said:

Obviously all of the car is moving at the same speed, and, especially with the Williams F1, there was only a small percentage difference in the relative distance from me to the various points of the car, so what actually causes some parts to be sharp and others not?

I'd say it was largely DOF - your focus point seems to be too far back - but even if you get that right relative speed can get you - that percentage difference is enough to cause the issue. I'm fairly sure the only way you can keep both ends of the car sharp during panning is to be side-on - though I stand to be corrected by any motorsport experts here.

Edited by Simpo Two on Sunday 5th July 10:07

Correct - the DoF issue is one that cannot easily be overcome, due to the size of aperature that the speeds involved in motorsport normally demands.

My best results when panning were obtained using manually pre-focused points on the track that I knew were being used as the racing line. It's a lot easier these days with digital, having the ability to instantly review your results, making minor corrections as you go along.

[trackdemon]

I dont think these are DoF issues, the reason for the blur on the front of these is because the front of the car is moving at a different rate relative to the side which is in focus. Typically these shots work best when the front is in focus so you should always try to track that. Two 'solutions' to get the whole car sharp whilst still having some motion blur would be to shoot completely side on (although you may still find the front and/or rear slightly blurred) or to stand further back & use a longer focal length..... I'm sure somebody much better with words than I can explain exactly why this is so!!

HTH
Steve

[chris.mapey]

I'd say that the issue relates to the sped of focus, and panning.

What I'd suggest happend is that you were panning, and then framed the shot and pressed the shutter.

In the time that the shutter takes to fire, the focus locks on. Which would be fine for a slower moving item, but the cars were moving fast, and moved "out of focus" in that short time.

This would explain why the focus is sharp on the Audi door (moved a little way down the car) and the F1 car, moving at a much higher speed, has taken the focus to the rear of the car (ie moved a full car length in the time the shot was fired)

Very difficult to cure - try changing the focus mode of your camers / lens to continuous, rather than servo, which will prevent to focus locking on when you take the shot.

If this is not a possibility, then try using manual "trap" focus - focus on a point on the track before the car is there, and "trap" the car in focus when it gets there (if you see what I mean)

Hope that helps

Chris

[Simpo Two]

trackdemon said:

I'm sure somebody much better with words than I can explain exactly why this is so!!

I'll throw 'angular velocity' into the pot!

[Kermit power]

I'm surprised it's depth of field, on the Williams at least.

The sun was strong enough to let me shoot at f11, and it wasn't on a long focal length (70-200 lens at 97mm) so I'd have thought that issue would be eliminated.

[trackdemon]

Simpo Two said:

trackdemon said:

I'm sure somebody much better with words than I can explain exactly why this is so!!

I'll throw 'angular velocity' into the pot!

There we go...

Either way, its nothing to do with f-stop or DoF

[simon_j]

I find it helps if you set the AF point in the direction the car is moving. So if the car is going to the left set the focus point on the extreme left side if you want the front of the car to be in focus.





Or if you want to get the middle in focus set it to just the other side of the centre.





You can then vary how much of the car you want to blur by the shutter speed.

The alternative is leave the focus in the centre and aim slightly in front of the car.

[flat-planedCrank]

I don't think its down to DOF either - I'd say its focusing speed / accuracy (that's probably not helped by panning with trees in front of you )

For situations where focusing can be difficult you might want to try separating the AF from the shutter button (if you haven't already - think some Nikon gear has this as default?). Allows you to pre-focus then take the shot without the AF having to stuggle as you pan with fences, trees, etc..

simon_j said:

I find it helps if you set the AF point in the direction the car is moving. So if the car is going to the left set the focus point on the extreme left side if you want the front of the car to be in focus.

I'd agree, but for those with consumer-level equipment off-centre focus points can be less accurate for tracking moving objects - all depends on the camera model of course.

(not an issue if you have pro-level sports cameras )



Cheers.

[trackdemon]

I think the Audi shot would be better executed by pre-focusing then going manual, shot two should be easy enough for 'C' continuous focus to track and if your focus point is the front of the car then that'd deliver the right results.

[Kermit power]

Cheers for all the suggestions guys. I'll try changing the autofocus point next time out as the first thing.