What am I missing here?

[iskoos]

Okay I have had this question in my mind for a long time. I finally took 2 sample shots to post here. I am hoping somebody will be able to explain this to me...
The two pictures are taken with the same exact settings at the same exact location/time. The only difference is the second one taken with the flash on. It is obvious I guess...

So the first one has a motion blur and is under exposed(obviously). Motion blur is self explanatory. The fan blades are spining too fast for the shutter speed (1/20 in this case) to freeze the action. So the result is a motion blur. No problem here...
The second picture has the same settings plus flash. Now, when I look at the 2nd picture, I see no motion blur. Okay, I know the statement that flash freezes the action but no matter what, the shutter stays open for the same duration (1/20 of a second). During this time, it is okay to have crisp fan blades but since the shutter stays open for the same duration, there should still be some blur in the image next to the crips fan blades. My thinking here is this:

If motion blur is caused by subject moving faster than the shutter speed, then there should always be a motion blur in the pictures that have shutter speed of 1/20 or slower.

I am pretty sure there is logical explanation to what I am missing here and I would appreciate if you can help me see it...

One request though; if you read all this and your answer is "There is no motion blur in the 2nd picture because flash freezes the action or similar kind..." then you didn't understand what I can't see...

Thanks



Fan w/o flash

Fan w/ flash

 

[IlSan]

Well, flash freezes motion

Ok, aside from that - I would venture out now and say, that due to the extra light provided by the flash the sensor can pick up the image "quicker" than with no flash, thus, erasing the motion blur.

It would be interesting to see what happens, if you slow down the shutter with flash and see, at what setting you actually get motion blur again.

That is pretty much how I would explain it:

More light on sensor, picture is "taken quicker" thus erasing the motion blur, whereas without flash, the sensor "takes more time" to capture the image, thus, leaving motion blur.

Buuuut - I'm not an expert here, just thinking aloud

 

[Flash Harry]

Flash exposed the scene correctly or slightly under, fan speed not quick enough and not enough ambient to record further movement. H

 

[IlSan]

@FH

Was I close?

 

[fokker]

Actually you do have motion blur in the second image - look closely. There are actually two seperate exposures going on in that photo; one from the flash and onefrom ambient light. The flash has 'frozen' the motion of the fan at one point because the actual pulse of light from the flash only lasts like 1/5000 second or something like that. In the rest of the time that the shutter is open the ambient light added to the exposure already created by the flash and caused a slight blurring of the blades around their leading edges, which you can see by comparison with the first photo, is exposed the same amount as the blur in the fan blades.

Does that make sense?

 

[kami]

I agree with fokker. The image captured by the flash impulse in a sense "covers" the blurred image that would have been created by just the ambient light. However the shutter speed was slow enough to capture whatever remaining image of the fan blades that was exposed by your ambient lighting. Thus you have a little motion blur left in the image.

Think of the flash captured image as a photoshop layer of a clear image added over the blurred image. If that makes sense.

Just for $hits and giggles, try re-taking the photo in total darkness with a really slow shutter speed and take it with a flash. I bet you'll have a crystal clear image.

One thing I find intruiging is: Why does the fan blade farther away have less blur than the fan blade closer to the camera?

 

[iskoos]

I would venture out now and say, that due to the extra light provided by the flash the sensor can pick up the image "quicker" than with no flash, thus, erasing the motion blur.

More light on sensor, picture is "taken quicker" thus erasing the motion blur, whereas without flash, the sensor "takes more time" to capture the image, thus, leaving motion blur.

Yes, flash freezes motion. I agree here)))

But I cannot much agree with the above approach. Yes, if there is enough light, it would be easier for the image sensor to record the subject but this does NOT change the shutter speed. In other words, just because the camera picked up enough light to correctly expose the image, it does NOT close the shutter any earlier than what is set.
So if the shutter is set to 1/20, and you take the picture of the sun, it will still stay open for 1/20 of a second no matter what...

And my argument was that if 1/20 shutter speed causing a motion blur, then no matter what the lighting conditions are, the motion blur should always be there...

Fokker mentioned that I do have a motion blur in the second image as well. When I look at it, I see some kind of blurness yes. But it is definitely not like the 1st image. Thus this still doesn't explain much.
Yes, I agree there should be double exposure going on in the 2nd picture. I agree that flash fires much much faster than the shutter speed. During flash, there should be a clear image of the fan blades and for the rest of the exposure time (after the flash fires) there should be the second exposure provided with ambient light. And this should cause all the motion blur in the 2nd picture. But I don't see that much motion blur. There is some motion blur in it but shouldn't there be the same motion blur in the 2nd picture as well?

 

[bazooka]

Fokker is correct and I think kami further explained it well. There IS an equal amount of motion blur in the 2nd picture, but it is being overwhelmed by the much brighter scene which was captured during the time that the flash was illuminating the scene, therefore it is almost invisible and looks very much like a shadow.

Let's setup a theoretical scene similar to yours but with important differences. First, make the fan blades white. Then remove the ceiling and shoot against a cloudy night sky (yes, the fan is magically floating in air, but whatever). The sky is obviously too far away for the flash to illuminate so it will stay black with or without flash. Now, if we repeat the same experiment with the same lighting, the motion blur will be much more noticeable because the fan blades are not being overwhelmed by the "flashed" background.

Kami, the fan blades in back have less motion blur because they are travelling slower in relation to the sensor. Imagine two race cars travelling the same speed from camera left to camera right and leave the camera at the same focal length. One is 10 feet away, the other is 1 a mile away. Which one moves faster in relation to the camera and which one will have the most motion blur? The nearest car even though they are both travelling at the same speed.

 

[Dao]

It is better if both photo have the same exposure.

 

[iskoos]

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...

 

[kami]

If I saw a crips image and same amount of blur all together in the 2nd picture, then I wouldn't ask this question.

Well, you also have to take into account that you took the picture in a room with 6 sides. Your flash just doesn't light up the fan, it bounces off the other walls which in turn contribute to the light. So you don't see as much motion blur as the first picture. Even the blur from the ambient light is slightly overwhelmed (by what I would presume) by the remaining light bounced from the other walls.

 

[Dwig]

...
That's excatly what I was thinking but I am not seeing it ...

Look carefully at the uppermost blade and you can see the shadow of the fan's pole on the ceiling even though it was blocked by the blade when the flash fired. Its as if you are seeing through the blade, but actually your are seeing the ceiling exposed after the blade moved away.

Since the first shot was underexposed, the blurred component in the second is rather weak when compared to the more properly exposed flash component. This makes the blurred ambient component hard to see in most of the image. Also, the blurred blades in the ambient component are soft enough to resemble "shadows" when seen along with the frozen flash component making them less noticeable for what they really are.

 

[Dao]

OP, I believe the effect you saw in your experiment similar to what people call "Shutter Drag".

 

[Araxx]

Can't give you a logical explanation, but in fact the second picture has no motion blur because of the flash. And only because of the flash.

 

[manaheim]

Dwig nailed it.

Here's another example I just took with this method yesterday...

1/10th second handheld at 38mm in a very dark room.

If you look carefully you'll see tiny bits of blur here and there, but there wasn't a lot of ambient light so by and large you have none.