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04-14-2011, 12:00 PM - 1 Like   #1
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very basic shutter speed tutorial....

A few year back I wrote this explaination on shutter speed-how it effects your shots, and depth of field.This is very basic but could help those not only new to pentax, but new to DSLR photography as well
Here is the link

Question Aperture/Shutter relationship!!? - Digital Scrapbook Place

I posted it to my wife's account... part one is the second post down, user name is hattitudes.

part 2 is around 14 posts down. It seems to have helped the people on digital scrapbooking place, hope it helps someone here. You likely need to read part one to help understand part 2
Never did get to ISO part yet though

please tell me what you think if it helps
If enough people like it I will redo it for this forum

If anyone wants to read it just to help get the message to Jr. members better, please critique it and let me know your feed back


cheers

04-14-2011, 12:16 PM   #2
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good job on the explanation for newbies. it's pretty easy to overwhelm them with tech talk. you avoided it nicely while still conveying the basics
04-14-2011, 02:15 PM   #3
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keep it goin,newbies like me need it as easy as possible.i might need to go over it several times with camera in hand to get it to stick .but this is just what i've been looking for.
04-14-2011, 02:35 PM   #4
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Nice explanation Randy, covered the basics pretty well. You going to explain the 3rd leg? (ISO) waaayyyy back when that was the hardest concept for me to get.

NaCl(nice clean concise tutorial)H2O

04-14-2011, 02:37 PM   #5
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Nice. I recently explained it to my grandson and used the analogy of a tap and a bucket that needs to be filled with water.

Open the tap a little (narrow aperture) and it takes a long time (long shutterspeed) to fill ('correct' exposure); open it wide and it goes quicker.
Close the tap too early and the bucket will not be filled (underexposure); close it too late and the bucket overflows (overexposure).
04-14-2011, 03:38 PM   #6
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QuoteOriginally posted by NaClH2O Quote
Nice explanation Randy, covered the basics pretty well. You going to explain the 3rd leg? (ISO) waaayyyy back when that was the hardest concept for me to get.
That one's easy. ISO=100.
04-15-2011, 04:58 AM   #7
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ISO is easy; it's the size of the bucket (higher ISO is smaller bucket)
04-15-2011, 05:38 AM   #8
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QuoteOriginally posted by sterretje Quote
ISO is easy; it's the size of the bucket (higher ISO is smaller bucket)
sterretje, I LIKE that - the "size of the bucket" analogy. Then you might illustrate it all with a tap running with light rather than water:

To take a photo, you must fill the bucket with light. You do this by picking a bucket and a tap, and then opening your tap long enough to fill the bucket.

Shutter speed would be the amount of time you keep the tap open.
Aperture would be the pipe size (pipes ranging from 1/1.2mm down to 1/22mm)
ISO would be the size of the bucket you fill (high ISo = small bucket)

With a big bucket (low iso) you need to keep the tap open long to fill the bucket. You can reduce this amount of time (use a faster shutter speed) if you pick a tap with a big pipe (small f-value).

So, in poor light conditions, you pick a small bucket to fill (high iso), because there isn't much light to tap. You also need to keep your tap open longer (slower shutter speed), and you need to pick the biggest pipe available. I think I might use that, next time I need to explain these concepts to a newbie.

(EDIT: I just realized you can use this to explain noise at high ISO settings too. Noise can be "splashing" - so if you pick a small bucket, there is likely to be more "splashing" than with a big bucket. Unless you are lucky enough to have a k5, the new "splash-free bucket" :P If you have good noise reduction software, you basically have a nice cloth with which to wipe off the splashes afterwards! )
--------------

Randy, I like your toilet paper explanation too, it's perfect for illustrating why you get movement blur with slow shutter speeds. Great when people take the time to explain basic concepts well.


Last edited by MetteHHH; 04-15-2011 at 05:42 AM. Reason: Added the bit about noise at high ISO
04-15-2011, 05:39 AM   #9
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as far as it goes, the analogy works pretty well.

It actually might be clear enough for people who are so far down the $#!++~% that they can't see the end of the roll

I also like the analogy someone posted about ISO being the size of the bucket.

Where I think it is over simplified is depth of field, because it is not just background blurr, but foreground blurr also.

This gets into the level of complxity but, while the analogy of the hand in front of the tube works for slow shutter speeds, and shutters well in front of the focal plane, it will ultimately fail when you need to explain flash sync. and focal plane shutters. Maybe, at some point, you will need to explain that it is possible to separate your fingers (like Mr Spock with the Vulcan Greeting) and pass a slit over the front of the tube.
04-15-2011, 06:33 AM   #10
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Taking a photo is like filling a bucket by standing in the rain (a shower of photons.) The shutter is the lid on the bucket. Exposure time is how long the lid is off the bucket. The harder it is raining the less time it takes to fill the bucket....exposure time needed decreases as brightness increases.

Increasing ISO makes the bucket shorter but not narrower; the shorter the bucket the fewer drops it takes to fill it up.... a short bucket's lid must be off for a short time for it to fill.

If the lid is off too long the bucket will overfill - it has been overexposed & what slopped over the edge is lost - the highlights are as white as they can possibly be.

Since raindrops fall in random places and times the shorter time the bucket is exposed to the rain the bigger the % variation in the number of raindrops that enter it; ie. increased noise.

The physical size of the camera's sensor is like the diameter of the bucket. A Full Frame camera's sensor (bucket) is 1.5 times wider than a Pentax DSLR's, but it is the same depth. Therefore it takes the same time to fill but there's less % noise.

A compact P&S camera has a much smaller sensor (bucket) so there's a lot more noise for given exposure time (how long the lid is off the bucket....ie how long the shutter is open.)

The bucket analogy also works for megapixel count; The overall sensor size is akin to the overall size of a bucket which is subdivided into smaller buckets (like an egg crate); the megapixel count is how many smaller buckets line the big bucket and the above logic holds true for the small buckets too.
04-15-2011, 06:39 AM   #11
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Beautiful, newarts! Is that your own, or did you find it someplace? It's perfect.

Spillage as blown highlights is a great detail.

How about aperture? Can we put a bottleneck on our bucket, and vary the diameter of the neck? The wider the aperture the wider the neck?
04-15-2011, 07:15 AM   #12
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QuoteOriginally posted by MetteHHH Quote
Beautiful, newarts! Is that your own, or did you find it someplace? It's perfect.

Spillage as blown highlights is a great detail.

How about aperture? Can we put a bottleneck on our bucket, and vary the diameter of the neck? The wider the aperture the wider the neck?
Thanks, it is based on a weaving of other's comments with a few additions.

Sure, aperture can be seen as the size of the bucket's neck .... alternately, apertures are like donut shaped disks over the bucket's top - you can see the variable aperture opening when you look into the lens.!

Your addition is a really good one, thanks.

Depth of Field in the bucket/rain analogy might (inexactly) be thought of as follows:

*The longer the lid is off the bucket the greater distance the raindrops would have fallen from high in the sky. In effect, the longer the lid is open the deeper into the sky you are looking.

*Exposure time is proportional to aperture therefore Depth of Field is also proportional to aperture.

Last edited by newarts; 04-15-2011 at 07:24 AM.
04-15-2011, 07:32 AM   #13
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QuoteOriginally posted by newarts Quote
Taking a photo is like filling a bucket by standing in the rain (a shower of photons.) The shutter is the lid on the bucket. Exposure time is how long the lid is off the bucket. The harder it is raining the less time it takes to fill the bucket....exposure time needed decreases as brightness increases.

Increasing ISO makes the bucket shorter but not narrower; the shorter the bucket the fewer drops it takes to fill it up.... a short bucket's lid must be off for a short time for it to fill.

If the lid is off too long the bucket will overfill - it has been overexposed & what slopped over the edge is lost - the highlights are as white as they can possibly be.

Since raindrops fall in random places and times the shorter time the bucket is exposed to the rain the bigger the % variation in the number of raindrops that enter it; ie. increased noise.

The physical size of the camera's sensor is like the diameter of the bucket. A Full Frame camera's sensor (bucket) is 1.5 times wider than a Pentax DSLR's, but it is the same depth. Therefore it takes the same time to fill but there's less % noise.

A compact P&S camera has a much smaller sensor (bucket) so there's a lot more noise for given exposure time (how long the lid is off the bucket....ie how long the shutter is open.)

The bucket analogy also works for megapixel count; The overall sensor size is akin to the overall size of a bucket which is subdivided into smaller buckets (like an egg crate); the megapixel count is how many smaller buckets line the big bucket and the above logic holds true for the small buckets too.
Pretty good, but it falls down in the area of noise.

The problem is not that raindrops are random over the sensor and therefore under uniform lighting the different sensor buckets fill differently. the problem with noise, is one of accuracy. Lets keep the remainder of your analogy, and say that for any one bucket, total exposure is the measured volume of water in the bucket with a ruler.

As light falls, we increase the ISO which is really a magnifying glass looking at a ruler with higher and higher magnification. we are attempting to measure smaller and smaller increments of depth, but have not changed the graduations on the ruler, and therefore we are "guessing" at the graduations inbetween. the "guess" is partially due to errors in magnification, from one bicket to the next, and also due to background noise and indicision of the measurement itself, such that the same amount of water in two adjacent buckets, which should be exactly the same yeild different measurement results.
04-15-2011, 09:10 AM   #14
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QuoteOriginally posted by Lowell Goudge Quote
Pretty good, but it falls down in the area of noise.

The problem is not that raindrops are random over the sensor and therefore under uniform lighting the different sensor buckets fill differently. the problem with noise, is one of accuracy. Lets keep the remainder of your analogy, and say that for any one bucket, total exposure is the measured volume of water in the bucket with a ruler.

As light falls, we increase the ISO which is really a magnifying glass looking at a ruler with higher and higher magnification. we are attempting to measure smaller and smaller increments of depth, but have not changed the graduations on the ruler, and therefore we are "guessing" at the graduations inbetween. the "guess" is partially due to errors in magnification, from one bicket to the next, and also due to background noise and indicision of the measurement itself, such that the same amount of water in two adjacent buckets, which should be exactly the same yeild different measurement results.
Lowell, while measurement precision plays a role, there is a fundamental noise associated with counting discrete things like raindrops or light. Such things are random in both space and time. Put a piece of paper out in the rain & watch the drops hit; each drop falls at a different place and at a different time.

The key thing is that where and when a drop falls is independent of where or when the last drop (photon) fell. It turns out that the noise (called shot noise) is proportional to the square root of the number of things counted.

This is usually expressed as signal/noise ratio = sqrt(n). People looking at photos see noise when the s/n ratio is around 10 and think the image is noise free when the ratio is above 40 or so; a ratio of ten means there were 100 photons collected by the bucket and a ratio of 40 means there were 1600 photons collected.

In the rain collection analogy a bucket of four times the diameter has 1/4 the variation between buckets that have their lids off for the same length of time.

In the context of silicon based photo sensors a perfect one micrometer by one micrometer square bucket is full when it collects around 1000 photons. Real sensors are only about 1/4 that efficient in part because color filters only allow 1/3 of the light to pass. (These are not precise numbers and depend on details.)

There is also noise contributed by heat, cosmic rays, electronic amplifiers; this is the noise you'll see after taking a photo with the lens cap on.
04-15-2011, 05:21 PM   #15
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Newarts

I think your last sentence is the biggest contributor along with measurement error. Remember noise is the difference between sells
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