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11-11-2011, 02:03 AM   #1
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How to calculate minimum shutter speed for moving objects?

Hi!

I have perhaps a little silly beginnerís question. Are there some rules of thumb or formulae how to calculate minimum shutter speed for moving objects? I presume that such factors as focal length of lens, distance to the object and the speed of object should be taken into account. May there is some formula how to calculate approximate shutter speed on the basis of theses factors?
As I know, there is a rule of thumb for a still object (if neither a tripod nor a shake reduction is used): the minimum shutter speed is recommended about the reciprocal of focal length. So, for the moving object the shutter speed must be faster than this reciprocal. However, the question remains open: how much faster.

I sometimes take pictures through the window of a moving bus. I have not tried this with my Pentax K-x yet. However, I tried this with my old P&S camera. The results were various. Usually, the pictures of quite near objects were blurry. However, the pictures of farer objects were more or less normal. In that time I shot in different auto modes. So I did not control shutter speed manually. Now, I would like to shoot in more controllable way and control shutter speed. Nevertheless, at first I have to determine the appropriate shutter speed before to try. In such situations the scenery usually changes quite quickly as the bus is moving. So there is not much time to experiment with shutter speeds. And light conditions also might not be perfect and might limit the shutter speed. For example, if the necessary EV is below 15.5 (such a high EV is only for a very sunny day), then the maximum shutter speed 1/6000 can not be used at aperture F8 and ISO 800. So the use the maximum shutter speed usually will not be a solution.

May be someone can suggest me the potential shutter speed for the situation when the bus moves with speed 90 km per hour, the object is located 100 m from the bus and the focal length of the lens is 18 mm (27 mm in 35 mm equivalent).

Thanks in advance!

11-11-2011, 03:17 AM   #2
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There is no rule, generally photographers prefer shutter speed of 1/500 for people in motion, but for some athletic shots or BIF I found only 1/1500 to be fast enough. You can however choose lower speed (like 1/100) and try panning. That often results in even better pictures than trying to freeze everything by super fast shutter.
11-11-2011, 04:37 AM   #3
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The general approach that I'd use is to take a picture, check the result and adjust shutter speed accordingly. This approach takes into account lens, speed and distance quite nicely. As much as I am a math geek, I'm expecting variables to change faster than I can compute.

Unless your old point and shoot was a film one, you probably have a whole library of data to work with if you're still curious. Shutter speed and other information are commonly stored in the jpeg file. Look up the term exif.
11-11-2011, 04:52 AM   #4
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There are some guidelines, but can't find them (my book) at the moment.

Theoretically, you can calculate it. Take e.g. a person (1.8 meter tall) moving parallel with the sensor plane at a speed of 1 m/s (3.6 km/h). If this person is 16mm high (height of sensor) on the sensor, he/she will move about 9mm in 1 second ( (1m / 1.8m) * 16mm ) on the sensor horizontally which is roughly 37.5% (9/24) of the sensor; in case of a K5, that's about 1800 pixels. At 1/1000 sec the person moves 1mm which equals 1800/1000 is about 2 pixels.

For absolutely no movement blur, you should be below 1 pixel

What is acceptable depends on how far you enlarge and the viewing distance; see also the second article below.

Note that the above person is moving relatively slow and that your bus at 50km/h does roughly 16m/s

PS

found some articles on the web
1) How to calculate a minimum shutter speed to yield an adequately sharp image of a moving subject | blog.patyuen.com
bit funny that in the table shutterspeeds don't increase when speed gets above 30MPH
and the direction factor should be reciproke in my opinion; a subject moving under 45 degrees towards you only moves at speed/1.4 parallel to the sensor basically halving the required shutterspeed instead of doubling

2) Shutter speeds to freeze subject movement
includes a calculator
note that numbers calculated don't match the table results of the other article
calculations using the imperial system result in infinity, so something wrong there as well

11-11-2011, 06:29 AM   #5
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There is no one size fits all, a lot will depend on the amount of light you are shooting in also.

If you want a rule, here is one. But rules are meant to be broken. This is only a broad guideline. It is supposed to give you a starting point. From there you can make smaller changes to your shutter speed up or down in 250 increments or so until you get a picture you like:

Freezing 5.6 rule.

"For fast moving subjects, on a Sunny Day,
set your aperture to 5.6 and
your shutter speed to ISO x 10
[Eg: for ISO 100, it would be a shutter speed of 1000].
[If it is a hazy or cloudy or overcast day, you will need different speeds or you will need to change the aperture or ISO]

If you are in a moving vehicle AND the subject is also moving, things get pretty rough, shoot at ISO x 20 and change shutter speeds up or down as needed...

I find these kind of rules as very good as a starting point. I almost always have to change things a little bit up or down to get the right exposure that I am satisfied with.

If you are really new at this, just keep things simple and put your camera on Shutter priority, 1/1000 and set it to auto iso range of 100-800 and let the camera decide the aperture and the ISO within the range given. Then check your pics on the computer and try different settings until you get a feel for what would work in different conditions.

Last edited by psychdoc; 11-11-2011 at 07:21 AM.
11-11-2011, 07:12 AM   #6
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The math is pretty simple.

To "freeze" motion we want the image to move less than one display pixel during the exposure.

t < (scene.width/subject.speed)(display.pixel.pitch/display.total.width)

For a display pixel/width ratio of .25mm/250mm =1/1000,

t < (scene.width/subject.speed)/1000

For example, a subject is moving at 2m/s across a 10m wide scene; how fast should shutter speed be to freeze motion?

t < (10/2)/1000 ~ 1/200 sec.

Or,
Guess how long it will take the subject to cross the scene; divide by 1000.

Last edited by newarts; 11-11-2011 at 07:17 AM.
11-11-2011, 09:20 AM   #7
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One guideline tip I got from a pro at an air show, was to get some motion on airplane propellers in flight (it is better to show some motion than to try to free the propeller motion IMHO) don't use shutter speeds over about 1/350th. Obviously there is a little trial and error to this but it's worked very well for me.
11-11-2011, 11:13 PM   #8
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I used to use a Pentax K-1000. I think the maximum shutter speed on that was 1/1000, though with the slow film ISO (ASA back then) numbers, I doubt I used the maximum speed often. WIth the stock 50mm 1.7 I could catch action at 1/500 or even 1/250. That required panning, but I shot track meets for our school newspaper with a 70-200 lens with that camera. I guess what I'm saying is that much of shooting moving subjects has to do with expectations and technique. If you want to catch hummingbird wings, then you would need a super fast shutter speed. For most photos, a little motion blur is just fine and adds to the sense of motion.

11-12-2011, 03:46 AM   #9
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Found my book. Below are the guideline values in Michael Freeman's "the manual of outdoor photography"; my copy is from 1983

PHP Code:


                filling         50%            diagonal
                 frame           frame          filling frame
--------------+---------------+---------------+---------------+
row boat      |     1/125     |   1/60        |   1/60        |
3km/h         |               |               |               |
--------------+---------------+---------------+---------------+
pedestrian    |     1/250     |   1/125       |   1/125       |
5km/h         |               |               |               |
--------------+---------------+---------------+---------------+
athlete       |     1/1000    |   1/500       |   1/500       |
20km/h        |               |               |               |
--------------+---------------+---------------+---------------+
cyclist       |     1/2000    |   1/1000      |   1/1000      |
30km/h        |               |               |               |
--------------+---------------+---------------+---------------+
running horse |     too fast  |   1/2000      |   1/2000      |
50km/h        |               |               |               |
--------------+---------------+---------------+---------------+
car           |     too fast  |   1/2000      |   1/1000      |
80km/h        |               |               |               |
--------------+---------------+---------------+---------------+ 



11-12-2011, 06:44 AM   #10
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(Time to cross frame)/1000

Estimate how long it will take the subject to cross the scene; divide by 1000.

I posted this earlier & want to hammer it home.

This strategy is completely general and accurate for things moving smoothly. You can probably make a good guess without looking through the viewfinder, and can certainly make a good estimate while looking through the viewfinder.

Learn how to pan the camera; it'll do wonders to freeze action while letting the viewer get a good feel for how much motion was involved in the actual event.
11-12-2011, 11:56 AM   #11
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Actually it works on the circle of confusion principle. It really has no difference than the 1/focal length rule of thumb, except that for motion you need to consider the focal length or magnification, subject distance and subject speed as a function of movement across thie frame. If the subject moves more than the 20micron CofC in thie shutter time, it is not frozen to be acceptably sharp
11-12-2011, 12:16 PM   #12
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QuoteOriginally posted by Lowell Goudge Quote
Actually it works on the circle of confusion principle. It really has no difference than the 1/focal length rule of thumb, except that for motion you need to consider the focal length or magnification, subject distance and subject speed as a function of movement across thie frame. If the subject moves more than the 20micron CofC in thie shutter time, it is not frozen to be acceptably sharp
That's right.

It was the basis for my estimate posted above (using a display resolution of 0.25mm for an 8x10" display). Similar to what's used in on-line DoF calculators.
11-14-2011, 01:52 AM   #13
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Hi!

Thanks for advice and information. I examined suggested shutter speed calculators and formulae. I thin that the formula recommended by the site Shutter speeds to freeze subject movement might be the most appropriate, at least for preliminary calculations. I a rearranged the formula a little to get shutter speed in seconds:

Shutter speed <= [[Circle of confusion (in mm)] / 1000] x Distance to object (in m)] / [Object’s speed x [Lens focal length (in mm)/1000]]

I input this formula in Excel and got sensible results. It interesting, that this formula yields the traditional “1/focal length rule” if the circle of confusion is chosen 0.02 mm, the distance to object is chosen 25 m and the object’s speed is chosen 0.5 m per second. So this rule has quite sensible foundation.
It also interesting that this website suggests that the smaller circle of confusion should be used than in standard DOF calculators, for example 20 microns (0.02 mm) instead 30 microns for full frame sensors and 13.3 microns instead of 20 microns for 1.5 crop factor sensors. So I presume that these reduced values for circle of confusion allow getting sharper pictures.
However, I noticed that the very online calculator of minimum shutter speed works correctly only if the distance is measured in meters and speed in m/sec. If other units of measurement are chosen, especially in miles/hour, then the calculator yields incorrect results.

I tried to carry out calculations for my situation with a bus. I assumed that the bus speed is about 90 km/h our 25 m/s and the distance to object is about 50 meters. As I have a Pentax K-x, I used circ of confusion 13.3 microns or 0.013 mm. I chose the focal length 18 mm (the wide “end” of my standard kit lens). According to my rearranged formula, I got that the shutter should be 1/675 or faster. So I can set the camera to TV mode and choose shutter speed 1/675 or something faster and try. Of course, to get a correctly exposed picture I also should set correct aperture and ISO as well as take into account DOF aspects. But it is a different issue. However, I think that it daylight time shutter speed 1/675 allows achieving normal exposure without extremely wide aperture or extremely high ISO.
If I want to shoot objects that are located 25 m from bus, then the minimum shutter speed is 1/1350. This shutter speed is harder to handle if it not very sunny day. If want to shoot objects with focal length 35 mm, then even for objects 50 m away the minimum shutter speed will be 1/1313. So it seems to be very complicate to shoot quite near objects from moving bus with larger focal length.

I hope that I have rearranged the formula correctly and that there are not errors.
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