What am I missing here?

Your all wrong. With some cameras when you turn on the flash the shutter automatically goes to 1/60 or 1/125 regardless of where you have it set. If that's not the case here, then in all dslr's the synchronization between the shutter and the flash unit is such that you get a quick open shutter, flash goes off, rear shutter blade moves. To get both blur and flash stop action in an image you have to set the flash / shutter to "rear sync". Then the front blade opens, exposes of course for the time you have set, then just before the rear blade moves the flash goes off. The strobe is synced to the rear blade instead of the front one. With a strobe that can do multiple timed flashes you can get a pretty cool shot with rear sync.
 
Your all wrong. With some cameras when you turn on the flash the shutter automatically goes to 1/60 or 1/125 regardless of where you have it set. If that's not the case here, then in all dslr's the synchronization between the shutter and the flash unit is such that you get a quick open shutter, flash goes off, rear shutter blade moves. To get both blur and flash stop action in an image you have to set the flash / shutter to "rear sync". Then the front blade opens, exposes of course for the time you have set, then just before the rear blade moves the flash goes off. The strobe is synced to the rear blade instead of the front one. With a strobe that can do multiple timed flashes you can get a pretty cool shot with rear sync.
It's called manual mode for a reason. If you are going to try and sound smart you should learn how to work you camera first.
 
If the op is still missing how this works, I actually taught this exact concept to my girlfriend who knows nothing about photography.

Think of the flash as your new shutter speed. Once you start using your flash the shutter really takes a back seat to the flash.

When a flash pulses it is much brighter than anything else in the scene, due to this it overwrites any existing light in the room.

Do this, turn your tv on. Take out your flash, and fire it into the ceiling. If you are using the on camera flash this will not work with a tv, but instead turn to the VCR. Flash directly at it and watch as you magically turn off the tv or VCR by overpowering their light source.

This is what is happening:
The flash pulse is fast, very fast. The flash pulse is also very bright. Brighter than most indoor lighting, however not brighter than direct daytime sunlight (in most cases). Your camera, as soon as you press that shutter button, is accepting any form of light into it. The more powerful the light source, the more it influences the image.

When your flash hits that moving fan (assuming it is an on camera flash) what happens is the extremely strong light source is lasting about 1/1500th of a second. I don't know what camera so this is different per model. Now, your shutter stays open for 1/20th of a second.

The most powerful light (being the on camera flash) will stay your main image, however you will get ghosting! This is what happens when you keep your shutter open longer than needed. Here is the real answer to your question now:

Ghosting: The culmination of all the light gathered during the shutter's open time.

This includes ambient, as well as your flash. Though because your flash is much much brighter than your ambient, the flash image lasts longer.

Try taking a picture of the fan with a shutter duration of 5 seconds. The flashed image will not be as strong as it was before.

In fact the longer the duration the less influence the flash has on the final image.

If you'd like to know more about this send me a pm.
 
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Your all wrong. With some cameras when you turn on the flash the shutter automatically goes to 1/60 or 1/125 regardless of where you have it set. If that's not the case here, then in all dslr's the synchronization between the shutter and the flash unit is such that you get a quick open shutter, flash goes off, rear shutter blade moves. To get both blur and flash stop action in an image you have to set the flash / shutter to "rear sync". Then the front blade opens, exposes of course for the time you have set, then just before the rear blade moves the flash goes off. The strobe is synced to the rear blade instead of the front one. With a strobe that can do multiple timed flashes you can get a pretty cool shot with rear sync.

I understand what you are trying to say.

Rear sync is not what is causing this though, rear sync corrects the reverse motion blur due to the flash popping at the start of the image exposure and not the end of the exposure.

Rear sync flash means- The shutter opens for its duration, right before the shutter is about to close the flash pops. This makes any motion blur come out the 'correct' way. Without this the blur due to flash is IN FRONT of the person moving instead of BEHIND.
 
Thank you all again. Thanks shmne for taking your time to type all that. I appreciate it. It is all correct and makes sense though what I was confused was not seeing enough motion blur on the second picture. You explained where the motion blur comes from. I know and agree with all that; but thanks again...

I did shoot the same scene on a daylight without flash at various shutter speeds. And all the way till 1/400th, I was still seeing slight motion blurs.
I will shoot the same wtih much faster shutter speed (faster than a sync speed) with a flash unit not pop-up flash and see at what shutter speed I will really freeze the motion.

Kami, I will also do the test you suggested in total darkness. I am pretty sure I can get super crisp image at much slower shutter speeds since there won't be any ambient light to create the second exposure...

So thanks again to everybody...
 
The motion blur you are seeing than is not at all from the ambient, the flash on your camera has a much longer pulse then you realize.

Same concept, slightly different reason.

The flash is probably around 1/1000th of a duration, at this speed the fan will have motion blur on the faster fan speed settings.

To get rid of this you just need to upgrade to a better flash, something with more of a 1/1500 would be fine for most uses.
 
I used cameras pop-up flash for the 2nd picture. I don't know if the flash makes difference on the blur. I have a Canon Speedlite 430EX. I will try that and see what difference I will see as far as the blur goes...
 
One word, "strobe" effect. The web is an amazing intelligence magnet and it polarizes intelligent people into specific cliques. But, it has been my experience that intelligent people can and often do complicate even the most basic and simple problems. If anyone is familiar with a timing light in automotive realms then you might understand this. A "strobe" light is attached to a plug wire that triggers the light that corresponds to a mark on the harmonic balancer that spins at a high RPM. If the car is in "time", every time the "strobe" is illuminated the mark on the spinning balancer will appear to be stationary. The same principle applies here. If you look closely, you can tell that the fan is spinning in the clock-wise rotation by the "after" image. In the first image there is to much "information" being introduced to the film at that shutter speed. When the flash went off for that split second, it simply highlighted that one moment in the rotation of the fan. It is a simple principle really. If you have ever watched T.V. and you know that the image is produced by lines of light that cycle back and forth, up and down at a high rate of speed to fast for our brains to process as individual instances thus producing a moving picture. Simple, right. Have you ever tried to video tape that image with a camcorder? Then you know that when you play it back there are a bunch of lines that are a result of the camcorder not being able to record those lines of light at the same rate of speed that they are being produced.

Have I simplified this topic in a sufficiently complicated manner to sate everyone's intelligence?
 
Thank you all(Kami you too) for your delignet answers. I feel that you guys have the correct approach in answering my question but I am still having a hard time to understand how extra light from flash would overwhelmed portion of the blur in the 2nd picture.

Kami says that think of it as two images (one with flash and one with ambient light) overlaid over each other.

That's excatly what I was thinking but I am not seeing it when I look at it.
If I saw a crips image and same amount of blur all together in the 2nd picture, then I wouldn't ask this question.

I guess I will have to read and think more about this. Understanding exposure triangle was very easy for me and it was making alot of sense. But when I added the flash into the equation, things started getting more complicated...

I will try and take the same shot(w/o flash) during the day with all the blinds open and see how it will come up...

I think I understand why it isn't making much sense to you. Correct me if I'm wrong though. What you'd be expecting, based upon the answers you got here, is that the image that used the flash ought to look like a layered version of the two images you actually have here. That is, you'd expect relatively crisp fan blades (from the brief flash) to be overlaid on top of the same very noticeable motion blur of the fan blades in the first shot.

In actuality, though, the mere presence of the flash in this shot doesn't only briefly freeze motion... it also changes the overall balance of light in the shot altogether. For instance, when the flash went off, the ceiling was very brightly illuminated... In fact, the ceiling was so brightly illuminated in the moment that the flash went off, that the motion blur which you saw in your first photograph barely even exposed on top of the bright ceiling in the second photograph.

To simplify the math, let's pretend that your flash fires for 1/200 of a second (it would have fired much faster of course). Even though the flash only went off for 10% of the entire exposure, the camera probably collected 95% of its usable light during that short interval. In the remaining 90% of the time that the shutter was open, so little light was collected that you can't even hardly see the motion blur in comparison to the first shot. It's not only a matter of time, but a matter of proportion. The ceiling between the fan blades was captured so brightly when the flash fired that you can't even see the weakly exposed wisps of motion blur that would've occurred after the flash died off. The brighter light, in this case, ends up taking precedence over the comparatively weak and undetectable light.

In retrospect, I don't know if I made this easier to understand or actually just complicated matters... oh well... I gave it shot. ;)
 
Okay...okay... i think I've got it. Here's another way it can be explained.

To understand what's happening, break your single 1/20 sec. exposure down into 10 equal intervals of 1/200 sec each.

In your first shot, 10% of the usable light available to your camera is being collected every 1/200 of second. After the first 1/200 second, your camera collects 10% of the usable light, after the second 1/200 second your camera collects another 10%, after the third 1/200 sec your camera collects another 10%, and so on and so on. At the end of the 1/20 sec exposure, you have what can be thought of as ten 1/200sec exposures, all of which are equal in terms of the amount of light collected. When you layer these "mini-exposures", you get the pronounced motion blur.

When you used the flash, though, things change. In the first 1/200sec that your shutter is open, your camera probably collected 95% of its light. In the second 1/200sec, it only collected about a 0.5% of its usable light, and in the third 1/200sec it collected another 0.5% of its usable light, etc, etc.

When you overlay the ten 1/200 sec intervals in this case, the 9 later intervals barely even show up because altogether they only comprised about 5% of light that your camera collected.

Thus, even though the shutter remains open for the same amount of time in both shots, the brief leading flash in the second exposure was so bright that the remaining amount of time that the shutter was open couldn't possibly collect enough light to have a drastic effect upon the final result. That's why the motion blur seems to disappear even though the shutter remains open for the same amount of time... it didn't disappear, it's just been entirely overpowered to point that it is barely visible.
 
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Try this experiment. Turn the fan on high, lay on your back looking straight up at the fan now blink your eyes real fast and try to get the blades to stay stationary.
 
I think I understand why it isn't making much sense to you. Correct me if I'm wrong though. What you'd be expecting, based upon the answers you got here, is that the image that used the flash ought to look like a layered version of the two images you actually have here. That is, you'd expect relatively crisp fan blades (from the brief flash) to be overlaid on top of the same very noticeable motion blur of the fan blades in the first shot.

Yes, this is exactly what I was expecting to see.

And your approach to answer my question in both of your posts are very understandable.
You know most of the time (this happens in any forums. I am not talking about this specific forum or this specific thread. This is a general comment!..) you ask something specific and people answer different things. It is not because people don't know the answer (well sometimes they don't of course); it is generally because they don't read the question carefully or if they read it well and have understood it, they just cannot explain it well enough for the person who is asking the question.
Well I hope (again) people who replied to my question here don't get offended because I am not necessarily talking about them. There were some great answers in this thread and maybe not so great ones and maybe some unrealeted ones.
My thanks go to all because they took tehir time and tried. But if I had to pick the best answer, then I would pick yours with my eyes closed.
It is not because you typed 2 long paragraphs. It is because you carefully read and understood what I was specifically asking and then you took your time to develope a method to explain; to break it down for me.
I believe that knowing the answer is one thing and being able to explain is another.

So Thank you Coleman...:)
 

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