Converting MM to Feet

[Dale Hutchinson]

I've been reading this and the 300D forum for quite a while now. I've seen just about every type of question answered and answered well. I just bought a DRebel and am researching lens.

My question is how does one go about converting MM to feet. In other words what is the equivilant of a 50MM lens in feet? I know a 300MM lens will reach farther than a 100MM lens but I can't get a gauge on how to calculate the distance.

Thanks,

 

[Wilbert Chan]

Regards,
Wilbert Chan

 

[jackal]

In other words what is the equivilant of a 50MM lens in feet? I know
a 300MM lens will reach farther than a 100MM lens but I can't get a
gauge on how to calculate the distance.

Don't bother with converting focal lengths to imperial units. The important detail is that longer focal lengths have narrower views than shorter ones.
http://www.usa.canon.com/eflenses/lens101/focallength/chart.html

You might want to convert minimum focus distance or the physical length of the lens, but leave the focal length alone.

 

[Brian Kennedy]

My question is how does one go about converting MM to feet. In
other words what is the equivilant of a 50MM lens in feet? I know
a 300MM lens will reach farther than a 100MM lens but I can't get a
gauge on how to calculate the distance.

The focal length of the lens refers to the flange-to-focus distance of the lens at infinity focus, which is a complicated way of saying that longer lenses (higher numbers) make for a more closeup view, as long as film size remains the same.

Try this link: http://www.photo.net/making-photographs/lens

Scroll down to the "How long a lens do I need?" calculator. This calculator is for 35mm cameras. If you are using the 300D/10D/D30/D60, then divide the focal length by 1.6 because the sensor is smaller than a 35mm frame.

--
Brian Kennedy
http://www.briankennedy.net/

 

[jackal]

The focal length of the lens refers to the flange-to-focus distance

The respondent appropriately re-phrased the question, which is the important part, but I would pick nits with this definition. For starters, "flange" refers to the lens mount, not an optical point. Here's an alternative definition:
http://www.usa.canon.com/eflenses/lens101/glossary/index_d.html#focal

However, that's probably more confusing than helpful. Take the EF 600mm f/4 -- it's only 456mm long. I like what the rec.photo.moderated lens FAQ has to say:

Q. What is the meaning of focal length? In other words, what about
a 50mm lens is 50mm?

A. A 50mm lens produces an image of a distant object on the film that
is the same size as would be produced by a pinhole 50mm from the film.

 

[Brian Kennedy]

Q. What is the meaning of focal length? In other words, what about
a 50mm lens is 50mm?

A. A 50mm lens produces an image of a distant object on the film that
is the same size as would be produced by a pinhole 50mm from the film.

That's right; sorry if I caused confusion. The reason I used the term flange focal distance (ftf) is because that's generally how most of us think of it, where the flange point is at the lens board on a large format camera -- not the lens mount for a 35mm camera. IOW, a 300mm lens requires 300mm of bellows for infinity focus. Same goes for a pinhole, as you say.

Hope this doesn't confuse the whole issue for the original poster .

--
Brian Kennedy
http://www.briankennedy.net/

 

[DRG]

I've been reading this and the 300D forum for quite a while now.
I've seen just about every type of question answered and answered
well. I just bought a DRebel and am researching lens.

My question is how does one go about converting MM to feet. In
other words what is the equivilant of a 50MM lens in feet? I know
a 300MM lens will reach farther than a 100MM lens but I can't get a
gauge on how to calculate the distance.

Thanks,

Lenses don't have "reach" in that they don't capture only up to a specific distance. Rather, the focal length determines the magnification of a given object on the image plane. Combined with the size of the sensor, which determines what size area of the image plane is captured, and you end up with a certain field of view. Objects will look larger when captured with a lens of long focal length. However, the size of the object itself is important. A small object that is far away will require a longer focal length to fill up your frame compared with a large object. Perhaps you are asking the following question: to fill my viewfinder with a specific object at a certain distance, what focal length do I need? If so, we can answer that as follows:

On the focal plane, for a "distant" object (distance > > lens focal length):

Size of projected subject = (Actual subject size) * (focal length) / (subject distance)

You want this size to be equal to that of the sensor size, to fill up your field of view. Thus:

(sensor size) = (Actual subject size) * (focal length) / (subject distance)

which can be rearranged to yield:

(focal length) = (sensor size) * (subject distance) / (actual subject size)

If we focus on the vertical dimension, the sensor size is 15.1mm, so:

(focal length) = (15.1 mm) * (subject distance) / (actual subject size),

with the distance and sizes in any units you want as long as they are the same (both in feet, for example). The focal length result will be in millimeters (mm).

Thus, to capture the complete body (top of head touching top of frame, bottom of feet touching bottom of frame) of a 6ft. person standing 6ft. away, you'd need about a 15mm lens on a 300D camera.

David

 

[Dale Hutchinson]

I've been reading this and the 300D forum for quite a while now.
I've seen just about every type of question answered and answered
well. I just bought a DRebel and am researching lens.

My question is how does one go about converting MM to feet. In
other words what is the equivilant of a 50MM lens in feet? I know
a 300MM lens will reach farther than a 100MM lens but I can't get a
gauge on how to calculate the distance.

Thanks,

Lenses don't have "reach" in that they don't capture only up to a
specific distance. Rather, the focal length determines the
magnification of a given object on the image plane. Combined with
the size of the sensor, which determines what size area of the
image plane is captured, and you end up with a certain field of
view. Objects will look larger when captured with a lens of long
focal length. However, the size of the object itself is important.
A small object that is far away will require a longer focal length
to fill up your frame compared with a large object. Perhaps you are
asking the following question: to fill my viewfinder with a
specific object at a certain distance, what focal length do I need?
If so, we can answer that as follows:

On the focal plane, for a "distant" object (distance > > lens focal
length):

Size of projected subject = (Actual subject size) * (focal length)
/ (subject distance)

You want this size to be equal to that of the sensor size, to fill
up your field of view. Thus:

(sensor size) = (Actual subject size) * (focal length) / (subject
distance)

which can be rearranged to yield:

(focal length) = (sensor size) * (subject distance) / (actual
subject size)

If we focus on the vertical dimension, the sensor size is 15.1mm, so:

(focal length) = (15.1 mm) * (subject distance) / (actual subject
size),

with the distance and sizes in any units you want as long as they
are the same (both in feet, for example). The focal length result
will be in millimeters (mm).

Thus, to capture the complete body (top of head touching top of
frame, bottom of feet touching bottom of frame) of a 6ft. person
standing 6ft. away, you'd need about a 15mm lens on a 300D camera.

David

Thanks to all. I've gained the info I needed. And David, I had to break out my TI scientific calculator but by golly it works. 8^)
--
Hutch69

Flowerchild of the 70's

 

[ian neilson]

Hi, What is the math formula for converting 500mm lens to feet distance?

 

[cpharm86]

Hi, What is the math formula for converting 500mm lens to feet distance?

Send a pm to the op. After 13 years from the post he probably has it memorized.

 

[Mark B.]

Hi, What is the math formula for converting 500mm lens to feet distance?

1. This is a 13 year old post

2. The question makes no sense; focal length of a lens does not convert to distance. Focal length of a lens is the length from the point at which light rays converge on the lens to the medium that records the image. Here's a diagram to help visualize it.

 

[Kjeld Olesen]

Hi, What is the math formula for converting 500mm lens to feet distance?

1. This is a 13 year old post

... is that the US and the UK should had gone metric in the 13 years that have passed

--
Kjeld Olesen
http://www.acapixus.dk

 

[diness]

[No message]

 

[Mark B.]

Hi, What is the math formula for converting 500mm lens to feet distance?

1. This is a 13 year old post

... is that the US and the UK should had gone metric in the 13 years that have passed

I don't disagree at all on that. It's pitiful that the US is so stubborn.