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 Desert Dweller 07-18-2007 07:21 AM

velocity of subject vs ISO speed

technical question for the math geeks out there.

Say your subject is traveling at a certain velocity through your field
of view at a certain focal length and maximum aperture. And your camera
is not panning. The subject is so fast you need an extremely high
shutter speed. Even at this shutter speed you cannot get enough light
for a proper exposure. And you cannot create artificial light on the
subject. So you must increase the ISO. This means there is a
relationship between the ISO film speed and the velocity of the subject.

Given a certain ISO film speed, focal length, and amount of light, at
what velocity can your subject travel through your field of view before
you need to increase the film speed?

Is there an equation that describes the relationship between velocity of
your subject across your field of view and the ISO speed capable of
exposing it quick enough?

--
DD

 Alex Monro 07-18-2007 10:39 AM

Re: velocity of subject vs ISO speed

Desert Dweller wrote:

> technical question for the math geeks out there.
>
> Say your subject is traveling at a certain velocity through your field
> of view at a certain focal length and maximum aperture. And your
> camera is not panning. The subject is so fast you need an extremely
> high shutter speed. Even at this shutter speed you cannot get enough
> light for a proper exposure. And you cannot create artificial light on
> the subject. So you must increase the ISO. This means there is a
> relationship between the ISO film speed and the velocity of the
> subject.
>
> Given a certain ISO film speed, focal length, and amount of light, at
> what velocity can your subject travel through your field of view
> before you need to increase the film speed?
>
> Is there an equation that describes the relationship between velocity
> of your subject across your field of view and the ISO speed capable of
> exposing it quick enough?
>

It's not really a direct relationship between the subject velocity and
ISO speed, the lens aperture and light available comes into play too.

I guess if you wanted to derive an equation for eliminating motion blur,
you'd need to take in the pixel size, subject distance, and focal
length, so you could calculate the shutter speed so that the subject
would move less than 1 pixel in the image while the shutter was open.

 Roger N. Clark (change username to rnclark) 07-19-2007 03:03 AM

Re: velocity of subject vs ISO speed

Desert Dweller wrote:

> technical question for the math geeks out there.
>
> Say your subject is traveling at a certain velocity through your field
> of view at a certain focal length and maximum aperture. And your camera
> is not panning. The subject is so fast you need an extremely high
> shutter speed. Even at this shutter speed you cannot get enough light
> for a proper exposure. And you cannot create artificial light on the
> subject. So you must increase the ISO. This means there is a
> relationship between the ISO film speed and the velocity of the subject.
>
> Given a certain ISO film speed, focal length, and amount of light, at
> what velocity can your subject travel through your field of view before
> you need to increase the film speed?
>
> Is there an equation that describes the relationship between velocity of
> your subject across your field of view and the ISO speed capable of
> exposing it quick enough?
>
> --
> DD

Its a simple trig problem. Find out the size of a pixel
in your camera, and true focal length. Calculate the
angular size of a pixel. For example, a 6 micron pixel
and a 300 mm lens )6 microns = 0.006 mm):

a = arctan (.006/300)

Now for an object at some distance x traveling a velocity y,
compute the angular rate: arctan(y/x) (keep units the same,
e.g. at 20 meters traveling 1 meter/second gives the angular
rate (e.g. degrees/second).

For a sharp picture, the exposure time should be less than
the angular size of a pixel. Example: if the subject angular
rate were 1 degree/second and the size of a pixel with a given lens
is 0.001 degree, then you want the exposure time less than
1/1000 second (ideally about 4 times faster, so 1/3000 second).

Roger

 Desert Dweller 07-21-2007 03:10 AM

Re: velocity of subject vs ISO speed

Roger N. Clark (change username to rnclark) wrote:

>> Is there an equation that describes the relationship between
>> velocity of your subject across your field of view and the ISO
>> speed capable of exposing it quick enough?

> Its a simple trig problem.

Seems to make sense. I guess I "over thought" that one and was trying to
make it harder than it was.

--
DD

 Don Stauffer in Minnesota 07-21-2007 01:51 PM

Re: velocity of subject vs ISO speed

On Jul 18, 2:21 am, Desert Dweller <1...@invalidemail.org> wrote:
> technical question for the math geeks out there.
>
> Say your subject is traveling at a certain velocity through your field
> of view at a certain focal length and maximum aperture. And your camera
> is not panning. The subject is so fast you need an extremely high
> shutter speed. Even at this shutter speed you cannot get enough light
> for a proper exposure. And you cannot create artificial light on the
> subject. So you must increase the ISO. This means there is a
> relationship between the ISO film speed and the velocity of the subject.
>
> Given a certain ISO film speed, focal length, and amount of light, at
> what velocity can your subject travel through your field of view before
> you need to increase the film speed?
>
> Is there an equation that describes the relationship between velocity of
> your subject across your field of view and the ISO speed capable of
> exposing it quick enough?
>
> --
> DD

People have actually determined the mft of subject movement. A Perkin
Elmer engineer published an excellent paper back in the 70s showing
the mtf curve vs angular velocity. You have to change linear movement
velocity into angular velocity.

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