Photo science question

[davidc]

Here we have two images. One will become my 365 shot for today (most likely), but I noticed something while processing them here at work (hence the dubious quality)

#1
f14
30 sec exposure
ISO 100

IMG_8983wb2.jpg

(525.76 KiB) Not downloaded yet

#2
f18
30 sec exposure
ISO 100

IMG_8985wb2.jpg

(475.59 KiB) Not downloaded yet

Can anyone explain why changing the aperture from f14 to f18 results in greater blurring of the wheel? All other settings were identical.

Both were shot on a stationary tripod, lens was 10-22mm @22mm for both, the wheel was moving constantly (as far as I could tell). Also, I have other shots that seem to show the wider the aperture, the less blurred the wheel pods are which supports the notion that the wheel was moving at a constant speed.

[Paul Heester]

Hmm interesting. As you state exposure times are the same. It must therefore be the wheel moving faster between both exposures. If people are too slow to embark/disembark they do have the ability to slow it down, I must therefore deduce Mr Watson it was the wheel itself

[Mike Farley]

Hmm interesting. As you state exposure times are the same. It must therefore be the wheel moving faster between both exposures. If people are too slow to embark/disembark they do have the ability to slow it down, I must therefore deduce Mr Watson it was the wheel itself

I agree with Paul. Looking at the cloud movement, the amount of blurring appears similar in each image and given that one shot was taken straight after the other, their speed probably did not vary by much.

What is also interesting is that the second image has taken on a yellow cast which is not present in the previous one. Do you shoot with white balance set to AWB?

[davidc]

Aha I wondered if you'd notice. Both were taken using a homemade 10 stop filter (I was hoping to get cloud and wheel blur, more than I ended up with - I know why I failed now).

The glass I've used has an extremely strong green cast which is tricky to remove, so much so I'll probably revert to black and white most of the time. But in this case I used a freeware RAW file converter which, as you can see, had very different result! I'm about to upload today's 365 shot fixed as best I can in cs4 and ACR.

Once I have a bit of time I'll be setting a custom white balance for it but this was a first experiment

As for the question, I think it must be the wheel but I have an f20 shot where the blur is even more pronounced taken 2 mins earlier and an f8 one take just after than which is still "sharper". So though I agree it's probably the wheel I am leaving room for doubt and I'm planning a more rigorous experiment

[davidc]

Apparently the wheel moves at constant velocity most of the time and only stopping to allow disabled access. Considering I've noticed blurring at varying parts of the cycle and the common theme is the aperture I'm definitely going back to setup some test shots

[Mike Farley]

I was at the Southbank earlier today and the Eye definitely stopped a couple of times for a few seconds on each occasion while I was there.