[Mitch401]

I went to kart track on Saturday night and I noticed that a few photos were half blurry and the other half really sharp, is this due to the F2.8 and a small margin for error? I half focus on a mark or rock on the track and pan back with the kart. I use a K3 with a Tamron 70-200 and a Pentax AF360FGZ Flash unit. In the attached photo the front of the kart is focus but the driver and rear isn't.
Settings were Shutter Priority, F2.8, 1/180 sec, metering mode pattern, AFC, iso 800 and AF point was spot in the centre.
Thanks Brad

Attached Images

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[VoiceOfReason]

Could it be a misaligned element? Look at the guy back and behind the first one.

[calsan]

I wouldn't have thought 1/180 was fast enough shutter speed - so maybe it's just the flash stopping the action. As the Voice of Reason says, it's very weird that the front right and rear left of the mid ground are in focus.
Try shooting a less challenging subject in daylight with the same lens at F2.8. I'm sorry to say, this is a brick wall / newspaper taped to the wall test.

[fgaudet]

I've been looking at it for a solid 5 minutes and I can't figure out what went wrong. Part of the driver SHOULD be in focus because both front tires are kind of in focus (better than the driver anyway) but what really throws me off is the fact that the kart in the back is also kinda in focus, even the tire wall behind the second kart are somewhat in focus. This is a huge depth of field for a shot at f/2.8.

It just makes no sense to me, you've got me beaten

[Mitch401]

And here it is when going the other direction.

---------- Post added 03-27-2014 at 10:32 AM ----------

When I turn the Flash on it overrides whatever shutter speed I have in the camera to 1/180 or lower.

[johnyates]

Looks like the work of French photographer Jacques Henri Lartigue

[Imageman]

Pardon my ignorance about pentax autofocus systems and pentax shutter curtains, I don't use autofocus.


I have considered flash issues and discounted that based on both in and out of focus areas being flash illuminated.
I have also discounted lens element misalignment, in the following image the effect is reversed, lens misalignment would be consistent in both directions, i.e. in one the front would be in focus and in the other the rear would be considering the directions of subject in each image are reversed. Whereas in both directions the front is sharp.


I am therefore suspicious that because in the first image, as the rear of the gokart moves out of focus, the other gokart moves into focus and its a slow shutter speed, there may be some kind of focus hunting happening.


Could the lens lose focus as you pan or never achieve satisfactory focus lock, and then it hunts for it, and by coincidence during hunting, the focus moved back to include the other kart and the tyres. This perhaps is compounded by the slow shutter speed masking this hunting behaviour.


Im not sure of the direction of travel of your shutter blades. If the shutter travels horizontally in your camera different areas of the image are exposed at different times across the image allowing the kind of variable focus that this image displays.


The rapid panning and taking approach enforced by the fast action may not allow the lens to lock focus before exposure as you try to nail the shot, and if your lens tends to focus hunt, then the seeds of this issue are sown.


The basic focus tests described earlier will confirm the basic functionality of the lens and its focusing ability in a static subject, so are recommended.


Personally I have never been a fan of autofocus in critical high speed sports events and therefore suggest exploring manual prefocus for this type of image.


Increasing the shutter speed may also help reduce this effect if it is autofocus related although it would not cure it e.g. 1000th shutter speed would not allow time for hunting to affect the focus significantly before the shutter closes.

[crewl1]

Since it was using shutter priority the above explanation is very possible.

[fgaudet]

Your second photo is much better than the first one. But to me, the back wheel should be in focus more than it is... if you compare it's relative distance to other parts of the kart.

Either you had something transparent in front of a portion of your lens or you may have a loose or misaligned element in the lens. As it was suggested you should do some testing in a better lit settings at various focal length and various focus point (to get the rotating elements *if any* in your lens in multiple position).

so maybe it's just the flash stopping the action.

I don't know about the 360 but most flashes I have, when at full power, their discharge time is about 1/200, so not much action freezing there. But I'm with you on that one, for moving cars, I try my best to use at least 1/250... But no Pentax DSLR will fire a flash (in non-PTTL mode) if the shutter speed is above 1/180

---------- Post added 03-26-14 at 23:26 ----------

I am therefore suspicious that because in the first image, as the rear of the gokart moves out of focus, the other gokart moves into focus and its a slow shutter speed, there may be some kind of focus hunting happening.

I'm not 100% positive but I don't think the focus motor will engage once the mirror is up. But the Tamron has a stupid AF/MF clutch thingy that may cause you to lose focus and does behave very oddly when it's sitting not fully in either position, very weird setup.

It could be a lot of things and without test it is hard to figure it out... I though maybe a sticky aperture blades but wide open it shouldn't matter, unless the previous picture was at another aperture setting...

[devorama]

To me it looks like the plane of sharpest focus is not parallel to the image sensor. This is done intentionally in tilt lenses. It causes the plane of sharpest focus to be angled such that things of varying distance from the camera can be in sharpest focus at the same time. In the first picture, it looks like this plane intersects the front of the foreground car and the front of the background car. It would also explain why the tires in the background are sharper on the left side of the frame than the right, even though they appear to be the same distance from the camera. A misaligned lens element might do something like this.

My other thought was perhaps it's a vibration of the kart itself that is making the image blurry. The engines are in the back, so they shake more than the front of the karts. Hence more blur at the back of the karts.

[vonBaloney]

Could it be that the rear of these cars are just vibrating a tremendous amount more than the front of them?

[Mitch401]

I think it looks and sounds like pilot (photographer) error. Been taking some pics of a map with the same lens at f2.8 and all looks good. I notice when the karts were just rolling around in the formation laps the photos were ok (as attached).
I was doing half camera focus and then panning to grap a kart and was using the autofocus continuous mode and then firing. I think I was asking the auto focus to do to much. What settings would I need to get this flash to work at higher than say 1/250th.
thanks everyone.

[calsan]

Could it be that the rear of these cars are just vibrating a tremendous amount more than the front of them?

Bingo!!!
Looking closely, the blur at the rear of the karts is actually in a VERTICAL direction!

What settings would I need to get this flash to work at higher than say 1/250th.:

Just set the flash to high speed sync (HS with lightning bolt) and camera to Tv, dial in higher than 1/250. You do loose a lot of flash power doing this, so bump the ISO- will need to experiment to find the range and correct ISO.

Alternative is to do some trailing shutter sync, so the flash fires at the end of the exposure, giving you a blur then the cart brightly lit and sharp. Again, experiments are in order.

edit:
Another trick may be to use the flash wirelessly in HSS - use the built in flash to trigger it and place the flash unit itself closer to the action (effectively increasing the flash's range) - maybe off to the left side so it doesn't fire into the driver's eyes. This will also make the photos look more three dimensional.

[Imageman]

Calsan you seem to suggest its simple vibration of the karts.


Are you also suggesting that the tyres must be vibrating while stuck in the ground, some are blurred and some sharp.


I cant therefore agree that its due to vibration, something else caused this and I stick with my opinion until a better explanation comes along

[Mitch401]

Found another with the rear tyre sharp but the front blurry now.