HD video is totally different.
it has two resolutions and only two resolutions. It doesn't matter how expensive your camera is, how big the
sensor is, even how many megapixels your camera
has. HD video still has only two resolutions. They
are simply called 1080 and 720 HD.
Photographers are used to cropping
their photos, taking part of an image and getting
rid of the rest, changing the shape and the format and so forth. You cannot do
that with video
1080 HD video refers to a video image
that is 1920x1080 pixels in size. 720 HD is
1280x720 pixels.
The mega pixels of your camera have
nothing to do with the resolution of the video
coming from your camera.
1080 video gives you an image size
approximately equal to two megapixels. 720 video
gives you an image size that is approximately equal to one megapixel.
With video, you are typically
shooting at approximately 30 frames per second. That
means your camera has to deal with thirty 1-2 mega pixel photographs per
second and keep doing it for however long you continue
to record the scene.
The resolutions refer simply to how
many pixels are in the image area, not to how large
or small the image will display on screen.
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A lot of video is actually shot at
29.97 frames per second, which is nearly 30 frames
per second, but it's based on an old standard that continues today for the
way that video is timed for broadcast.
At 30 frames a second, our mind puts
them together in a way that looks like
traditional video.
Film on the other hand has always
been shot at 24 frames a second…it is enough to change the way our brain
sees movement
We are so used to seeing films from
Hollywood shot at 24 frames a second that our
brain starts associating that frame rate with a film look.
60 frames per second is another
option, but is a more specialized speed often used
for sports to really smooth out movement.
~~~
shutter speed choice is very different from video to still photography.
If you
shot those 30 frames per second at 1/30th of a second, you end up with all of
that entire 1 second filled with images.
There
would be no gaps between each shot. Now suppose you shot at 1/1000th of a
second. The total time used would be 30 1/1000th of a second for the 30 frames,
meaning that 970/1000ths of that second, nearly the entire second, will be
blank. That puts a huge gap between each frame that causes the video to stutter
or chatter. Our eyes are not capable of creating smooth motion with that gap
and so action looks unnatural.
But normally for video, you will be
choosing a shutter speed between 1/30th and
1/90th of a second. You won't see much difference
between those shutter speeds. I also find that
you can get away with as high as 1/125th of a second if the motion is not too fast.
Some video purists feel that the
ideal shutter speed is 2 times the frame rate. That
would mean if you are shooting 30 frames per second, you would choose 1/60th of a second for the shutter speed.
~~~
you cannot shoot RAW with video on
a DSLR = need to be more careful with exposure
most DSLRs actually work best for
video in manual exposure.
frequently
things will be changing during the time you are recording that could make auto exposure change inappropriately.
flickering
or changing in brightness of the scene
you only have a narrow range of
shutter speed that you can use.
when you are looking at really bright
conditions, you're going to have to use small
f-sops.
If you want wide f-stops because you
want a limited depth of field, you have to cut the light and you're going to
need to get a neutral density filter
most cameras today do a very good job
and capture quality video at ISOs easily from 100
to 400 and even 800
the only other option is you have to
bring some lights in and add light in order to
get your exposure correct.
Zoom lenses are not very fast. That
means they don't have a wide maximum aperture. Single
focal length or prime lenses have become popular for video because they do offer a faster maximum aperture.
When you shoot wide open with a fast
lens, you get some really nice selective focus
effects.
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