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08-21-2015, 02:39 PM   #1
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Are shutter speeds real?

I wonder if high shutter speed actually works as one might expect.

On film cameras, up to a certain setting (about 1/100 but more pricey cameras would go higher) the shutter opened whole. Above that setting, it merely formed a slot which moved across the frame and the width of the slot was what formed the effective shutter speed.

Because the speed at which the slot moved across the frame was actually pretty slow (of the order of 1/100 sec to move right across) that meant that a shutter setting of say 1/2000 was nothing like that. It was 1/2000 from the POV of exposure of the film but from the POV of freezing motion it was a fake - it was more like 1/300 and varied according to which way the target was moving.

I wonder if digital cameras suffer from some similar limitation because in some situations where a steady hold is almost impossible to achieve I can set my K3 to say 1/5000 but still get images blurred as much as they would be at 1/1000.

08-21-2015, 03:07 PM   #2
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I can wave my K-50 around with the shutter set at 1/5000 and set to hi-burst mode, every frame is sharp. So I wonder what's happening with your K-3 ?
Under what conditions are you shooting?
08-21-2015, 03:20 PM - 1 Like   #3
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I believe the shutter on your camera operates according to its design. The Wikipedia article on focal plane shutters has a good explanation:

https://en.wikipedia.org/wiki/Focal-plane_shutter

You are correct that above a certain speed (typically the X-sync speed), that the full frame is never fully exposed. That being said, the shutter opening is always a moving slit. This has implications, but motion blur is not one of them per se. The most common is stretching or compression of moving objects according to their direction relative to the curtain direction of travel. Stretching may be accompanied by blur in the stretched portions. Mixed motion (up/down/with/against) may turn circles into ovals, and so forth.

Example 1: The van in the photo below was moving (and slowing) in the direction of the shutter travel. The back of the van is stretched and lacks sharpness as a result. A faster shutter speed (narrower slit) may have remedied much of the stretching and associated blur. If the van had been traveling the other direction, it would appear less long than it really is, but sharper. Panning would have introduced additional interesting permutations. I believe the shutter speed was about 1/125s


Zorki 4K, KMZ Jupiter-8 58/2, Fuji Acros 100


Example 2: The wheel on the unicycle is actually round, but the shutter motion has rendered it to a slanted oval. Note that the rider is not blurred and neither are the spokes The unicyclist was traveling against the direction of curtain travel. I believe the shutter speed was probably 1/250s.


Pentax SV, ST 28/3.5, Ektar 100


So the short answer is that depending on the direction of camera and/or subject motion the rendering may not be true to the subject's actual shape and motion may not be stopped as expected.


Steve

Last edited by stevebrot; 08-21-2015 at 04:41 PM.
08-21-2015, 03:23 PM   #4
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Focal plane shutters still work like in the film days. On current Pentax DSLRs the fastest speed where the shutter exposes the entire sensor at the same time is 1/180s. For faster speeds you get a slot that moves across the sensor. The faster the speed the narrower the slot. But the exposed part of the sensor only receives light at the speed set, such as 1/8000s. You may see the rolling shutter effect, but I don't understand that you'd observe the blur which you mention.

08-21-2015, 03:29 PM   #5
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See this video for an excellent explanation of the OP's basically accurate description: How Does Your DSLR's Shutter Really Work?
08-21-2015, 03:33 PM   #6
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QuoteOriginally posted by peterh337 Quote
I wonder if high shutter speed actually works as one might expect.

On film cameras, up to a certain setting (about 1/100 but more pricey cameras would go higher) the shutter opened whole. Above that setting, it merely formed a slot which moved across the frame and the width of the slot was what formed the effective shutter speed.

Because the speed at which the slot moved across the frame was actually pretty slow (of the order of 1/100 sec to move right across) that meant that a shutter setting of say 1/2000 was nothing like that. It was 1/2000 from the POV of exposure of the film but from the POV of freezing motion it was a fake - it was more like 1/300 and varied according to which way the target was moving.

I wonder if digital cameras suffer from some similar limitation because in some situations where a steady hold is almost impossible to achieve I can set my K3 to say 1/5000 but still get images blurred as much as they would be at 1/1000.
Shutter speed has been one of the camera settings all mfg have focused on since the dawn of the semi-auto camera. I think your camera has a problem.
08-21-2015, 03:39 PM   #7
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QuoteOriginally posted by Ole Quote
For faster speeds you get a slot that moves across the sensor.
All speeds...

QuoteOriginally posted by Ole Quote
The faster the speed the narrower the slot.
The slower the speed the wider the slot. At speeds slower than maximum x-sync, the slot is wider than the frame.


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08-21-2015, 03:43 PM   #8
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The shutter speed is the difference in time between when the shutter's 1st curtain starts to open and when the 2nd curtain starts to close, e.g. a 1/1000 shutter speed means that 1/1000 after the first curtain starts moving the second curtain will start to move as well. The fastest speed in which the first curtain will be completely open before the second curtain starts to close is the flash sync speed, which for Pentax DSLRs is 1/180 of a second. Faster than this and the second curtain will start to close before the first is totally open, creating a moving "slit".

No pixel will be exposed to light more than any other, and every pixel in a line parallel to the shutter blades will be exposed at the same time. So no there will be no issues with motion blur in the usual sense.

Instead the problem becomes the timing difference between the top of the sensor and the bottom of the sensor. The top of the sensor (where the curtains start moving) will be exposed 1/180s before the bottom, meaning each line is capturing a slightly different moment in time. Thus if the subject or camera is moving during exposure, depending on the direction of motion, there can be certain distortions, e.g. stretching or slanting. This is usually referred to as rolling shutter.

08-21-2015, 04:34 PM   #9
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QuoteOriginally posted by peterh337 Quote
in some situations where a steady hold is almost impossible to achieve I can set my K3 to say 1/5000 but still get images blurred as much as they would be at 1/1000.
Camera motion is deadly. If a steady hold is almost impossible, be prepared for results that are almost without blur.

How much blur is acceptable...5 pixels? Now consider how much pitch movement (degrees of arc) would be needed to generate 5px displacement at the sensor when using a normal lens. (Pitch is not yet corrected by the SR system.) Is that movement reasonably possible in only 2/10ms (1/5000s)? Uh, huh...

High shutter speed may do an acceptable job at stopping subject motion, but camera movement is another matter entirely.


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08-21-2015, 04:55 PM   #10
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QuoteOriginally posted by Cannikin Quote
The shutter speed is the difference in time between when the shutter's 1st curtain starts to open and when the 2nd curtain starts to close, e.g. a 1/1000 shutter speed means that 1/1000 after the first curtain starts moving the second curtain will start to move as well. The fastest speed in which the first curtain will be completely open before the second curtain starts to close is the flash sync speed, which for Pentax DSLRs is 1/180 of a second. Faster than this and the second curtain will start to close before the first is totally open, creating a moving "slit".
Great explanation.

The ability of the shutter to stop action is very complex and depends on several factors (amplitude, duration, and direction) as well as the type of shutter used, the lens mounted, blade/curtain speed, and the opening size. It is not just focal plane shutters. Leaf shutters are cleverly made to provide even illumination radially, but may create artifacts as well. How does "tunneled" DOF sound?


Steve

Last edited by stevebrot; 08-21-2015 at 08:18 PM.
08-21-2015, 07:54 PM   #11
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The average travel speed for vertical* travel shutter curtains in modern DSLRs is about 25m/s - so anything approaching or exceeding that speed in the frame will be prone to rolling shutter effects.

*for horizontal FP shutters travel at around 15~20m/s. Leaf shutters are even slower than that.

Last edited by Digitalis; 08-22-2015 at 01:38 AM.
08-21-2015, 08:30 PM   #12
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QuoteOriginally posted by Digitalis Quote
The average travel speed for horizontal travel shutter curtains in modern DSLRs is about 25m/s - so anything approaching or exceeding that speed in the frame will be prone to rolling shutter effects.
I should do some playing around with my Kiev 4A. The vertical shutter on it and its more legitimate cousin, the Contax II have painfully slow curtains despite supporting a 1/1000s shutter speed.


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08-21-2015, 09:59 PM   #13
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Well I learn something every day!

I go back to the OM1, 1970s, and on that the shutter was visible (and shut, IIRC) when you removed the lens.

I never saw a mechanical shutter (when e.g. changing lenses) on any DSLR so I thought that the shutter is implemented electronically, in the way the sensor is scanned.
08-22-2015, 06:31 AM   #14
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Take a photo with the lens off and watch closely.

There are lots of videos in YouTube that record the shutter with a high speed camera so you can slow it down.

Here's one on petapixel with way too much talking. It came up first in a Google search.

Watch How DSLR Shutters Work in 10,000FPS Super Slow Motion
08-22-2015, 07:24 AM   #15
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...or use the mirror lock-up mode. You'll see a focal-plane shutter.

The X-sync speed of all recent Pentax K-system DSLRs is 1/180s. This is the fastest speed where the entire sensor will be exposed at once. Higher shutter speeds are implemented through a slit in the shutter that travels over the sensor, which gets narrower as you increase the shutter speed.

In my experience, accuracy of very high shutter speeds is subject to sample variation, and can degrade with use. My first K-3 II (returned due to service advisory) slightly overexposed (by about 1/2 stop) at 1/8000s, but I don't see any difference between 1/2000s and 1/8000s on my current body. My K-5 would overexpose by about 2/3 of a stop at 1/8000s as well. On the other hand, my retired K-r would at one point underexpose by as much as 1-1/2 stops at 1/6000s (for some reason, this no longer reproduces).

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