long focal length vs. motion blur

SimplyMo · Jul 30, 2008

please correct me if im wrong:blushing: im trying to make sure i understand this thoroughly..

"A subject that is moving will appear more blurry if photographed with a telephoto lens."

this is true because a telephoto/long focal length lens magnifies a scene/objects more, causing the image to be larger. and, when a moving object(s) travels more/further across the frame it is more likely for the object to be blurry, because more of the motion is catured during exposure (especially during longer exposure, slower shutter speeds).

ohhh im terrible at explaining technical things clearly:???:... how did i do? am i correct? did you understand what i was trying explain?

thanks in advance

Big Mike · Jul 30, 2008

That's not correct. Blur is a result of motion and time. If the subject or the camera is moving there is the possibility of blur if the shutter speed isn't fast enough to effectively freeze that movement.

The statement could be a generalization though. Many zoom lenses have a smaller aperture at the long end of the zoom. F3.5 vs F5.6 for example. The smaller aperture will require a longer shutter speed, which will result in more blur (all else being equal).

SimplyMo · Jul 30, 2008

Big Mike said:
That's not correct. Blur is a result of motion and time. If the subject or the camera is moving there is the possibility of blur if the shutter speed isn't fast enough to effectively freeze that movement.

so basically its only because the longer focal length lenses require longer shutter speeds in order to admit enough light for a given exposure.. for example.. if a shorter focal length lens i set at one distance photographing a moving object with the aperture set at f/5.6..and the longer focal length lens is set at the same distance using the same aperture (f/5.6) shooting the same moving object... it will obviously need a longer shutter speed, therefore more motion blur is possible.. it has nothing to do with the greater magnification of the image (captured by the longer lens)?? <<which would cause the object to move further across the frame...

Big Mike · Jul 30, 2008

so basically its only because the longer focal length lenses require longer shutter speeds in order to admit enough light for a given exposure.. for example.. if a shorter focal length lens i set at one distance photographing a moving object with the aperture set at f/5.6..and the longer focal length lens is set at the same distance using the same aperture (f/5.6) shooting the same moving object... it will obviously need a longer shutter speed, therefore more motion blur is possible.. it has nothing to do with the greater magnification of the image (captured by the longer lens)?? <<which would cause the object to move further across the frame...

No, that's still wrong.

A 300mm lens at F5.6 and shutter speed of 1/30 will give you the same amount of 'subject blur' as a 20mm lens at F5.6 and 1/30.
The shutter speed is the only thing that affects blur (besides the degree of movement/motion). Focal length doesn't have anything to do with it.

That being said, a longer focal length with magnify blur from camera movement (camera shake). When you shoot with the camera in your hands, there will be some shake, and this shake can cause blur at lower shutter speeds. The rule of thumb is that you want a shutter speed that is a higher number (the reciprocal) than the focal length of the lens. So, for a 300mm lens, you would want to use a shutter speed of at least 1/300, in order to avoid blur from camera shake. A 50mm lens would only need 1/50.
Many will also tell you that you need to account for the 'crop factor' of your camera. So on most DSLR cameras, you would need to use something like 1/480.
This is for blur caused by camera shake. you can also reduce or eliminate this by using a tripod or other form of support...and of course, some people are shakier than others.

This is for blur caused by camera shake. Blur caused by subject movement is a different issue.

SimplyMo · Jul 30, 2008

thanks for the calrification bigmike

(and the time you put into it)
i understand it now..

i was just trying to reason why the book im reading says (quoted exactly word for word) :
"A long-focal-length lens magnifies objects and makes them appear closer to the camera. A moving subject will appear more blurry when photographed with a long-lens than it would if photographed with a normal lens used at the same distance."
(but this wasn't in reference to motion blur caused my camera shake..which i do understand--always use a shutter speed higher than the focal length--when hand holding camera-- which would cause the whole image to appear blurred/out of focus)
so i was just curious if that statement means the moving subject will be more blurry because the subject is moving further across the frame--due to being magnified....

anyway, thanks again