Lens Help Please

Andrew Portess wrote:

I have a Canon A80 and am now considering a Digital Rebel DSLR. One
thing I am unsure of though is how to determine which telephoto lens
I would need for the DRebel. If I am shooting small birds at a
distance of 20 feet what would I need to ensure the image filled the
frame. Can somebody point me towards a good web site which explains
so that I can learn for myself.

Take the distance you are away from your subject (20 feet), and
divide it by the size of the frame you want to have (measured in
the same units as your distance from the subject, in this case feet).

Multiply this result by the size of the imaging sensor in millimeters
to get a reasonable approximation for the focal length of the lens
you will want.

You mentioned small birds, so let's use a subject size of one foot,
which should give you some margin around the edges of your frame so
you're not completely filling the frame with the bird. You are 20 feet
away. The Digital Rebel's sensor measures about 22.7 millimeters
across.

20 feet / 1 foot = 20.
20 * 22.7 millimeters = 454 millimeters

You will want a focal length around 400 mm if your birds are a foot
or smaller in size and you will be 20 feet away. Each pixel in your
output image will cover about 0.004 inches across at this distance
and focal length.

BJJB

Andrew Portess wrote:

I have a Canon A80 and am now considering a Digital Rebel DSLR. One
thing I am unsure of though is how to determine which telephoto lens
I would need for the DRebel. If I am shooting small birds at a
distance of 20 feet what would I need to ensure the image filled the
frame. Can somebody point me towards a good web site which explains
so that I can learn for myself.

Take the distance you are away from your subject (20 feet), and
divide it by the size of the frame you want to have (measured in
the same units as your distance from the subject, in this case feet).

Multiply this result by the size of the imaging sensor in millimeters
to get a reasonable approximation for the focal length of the lens
you will want.

You mentioned small birds, so let's use a subject size of one foot,
which should give you some margin around the edges of your frame so
you're not completely filling the frame with the bird. You are 20 feet
away. The Digital Rebel's sensor measures about 22.7 millimeters
across.

20 feet / 1 foot = 20.
20 * 22.7 millimeters = 454 millimeters

You will want a focal length around 400 mm if your birds are a foot
or smaller in size and you will be 20 feet away. Each pixel in your
output image will cover about 0.004 inches across at this distance
and focal length.

BJJB

Remember though that the Rebel does not a have a full size CMOS sensor so a
28-300mm lens will actually be like a 42-450mm. 1.5X multiplier.
"BillyJoeJimBob" wrote in message
...
Andrew Portess wrote:

I have a Canon A80 and am now considering a Digital Rebel DSLR. One
thing I am unsure of though is how to determine which telephoto lens
I would need for the DRebel. If I am shooting small birds at a
distance of 20 feet what would I need to ensure the image filled the
frame. Can somebody point me towards a good web site which explains
so that I can learn for myself.

Take the distance you are away from your subject (20 feet), and
divide it by the size of the frame you want to have (measured in
the same units as your distance from the subject, in this case feet).

Multiply this result by the size of the imaging sensor in millimeters
to get a reasonable approximation for the focal length of the lens
you will want.

You mentioned small birds, so let's use a subject size of one foot,
which should give you some margin around the edges of your frame so
you're not completely filling the frame with the bird. You are 20 feet
away. The Digital Rebel's sensor measures about 22.7 millimeters
across.

20 feet / 1 foot = 20.
20 * 22.7 millimeters = 454 millimeters

You will want a focal length around 400 mm if your birds are a foot
or smaller in size and you will be 20 feet away. Each pixel in your
output image will cover about 0.004 inches across at this distance
and focal length.

BJJB

Remember though that the Rebel does not a have a full size CMOS sensor so a
28-300mm lens will actually be like a 42-450mm. 1.5X multiplier.
"BillyJoeJimBob" wrote in message
...
Andrew Portess wrote:

I have a Canon A80 and am now considering a Digital Rebel DSLR. One
thing I am unsure of though is how to determine which telephoto lens
I would need for the DRebel. If I am shooting small birds at a
distance of 20 feet what would I need to ensure the image filled the
frame. Can somebody point me towards a good web site which explains
so that I can learn for myself.

Take the distance you are away from your subject (20 feet), and
divide it by the size of the frame you want to have (measured in
the same units as your distance from the subject, in this case feet).

Multiply this result by the size of the imaging sensor in millimeters
to get a reasonable approximation for the focal length of the lens
you will want.

You mentioned small birds, so let's use a subject size of one foot,
which should give you some margin around the edges of your frame so
you're not completely filling the frame with the bird. You are 20 feet
away. The Digital Rebel's sensor measures about 22.7 millimeters
across.

20 feet / 1 foot = 20.
20 * 22.7 millimeters = 454 millimeters

You will want a focal length around 400 mm if your birds are a foot
or smaller in size and you will be 20 feet away. Each pixel in your
output image will cover about 0.004 inches across at this distance
and focal length.

BJJB

Petr V. wrote:

Remember though that the Rebel does not a have a full size CMOS
sensor so a 28-300mm lens will actually be like a 42-450mm. 1.5X
multiplier.

The sensor size is already taken into account in the computation.
You don't have to worry about multipliers/crop factors at all using
the method I described.

BJJB

Petr V. wrote:

Remember though that the Rebel does not a have a full size CMOS
sensor so a 28-300mm lens will actually be like a 42-450mm. 1.5X
multiplier.

The sensor size is already taken into account in the computation.
You don't have to worry about multipliers/crop factors at all using
the method I described.

BJJB

Petr V. wrote:

Remember though that the Rebel does not a have a full size CMOS
sensor so a 28-300mm lens will actually be like a 42-450mm. 1.5X
multiplier.

The sensor size is already taken into account in the computation.
You don't have to worry about multipliers/crop factors at all using
the method I described.

BJJB