Understanding focal length

Hey everyone....
any good links, I'm not all grasping this one yet?:o
Also, OT somewhat but my Pro1 has a 7.2mm-50.8mm f2.4-f3.5L but I thought it was called a 28mm-200mm? Then....where would I be in comparision using an XT with a 50mm f1.8 prime

okay. P&S's have very small sensors. thus use lens sizes that a 35mm couldnt use. the real mm length of the lens is 7.2-50.8mm, but since that really doesnt tell you much about what shots its going take, they also express the effective focal length it will give in 35mm terms. in this case 28-200mm.
so it would be like using a 28-200 lens on a Full frame or Film camera. sort of like how some people express a lens on a DSLR with a crop sensor as haveing a longer "equivanlent" focal length. in the case of the 50mm, it gives an "equivelent" (without getting into too many details that always result in photographic arguements) of 80mm on that XT, but the lens itself is still 50mm.

hope that helps,

To amplify what Bill said and to answer your question about the 50mm on an XT, since the XT is 1.6 crop, the FOV with the 50 is the same as 80mm on FF. Based on the focal lengths and equivalents you provided for the Pro1, it has about a 4:1 crop (200mm:50.8mm), so in order to have the same FOV as 80mm on FF (or 50mm on XT), you would be at 20mm on the Pro1.

If your Pro1 has the 35 mm equivalent of a 28-200 mm lens, you have to divided the numbers by 1.6 to get the equivalent focal length for your XT. That would be a 17-125 mm lens. Canon does not make a 17-125 f/2.4-3.5 lens. This is because it is easier to design quality lenses with a wider zoom range and wider aperture for small sensors because of the smaller image circle required.

You can get slower lenses that come close to this range, such as the Sigma 18-125 f/3.5-5.6 or 18-200 f/3.5-6.3 or you can use two lens if you need a faster aperture.

The closest two lens combination would be the Canon 17-55 f/2.8 and 70-200 f/2.8. This leaves a gap between 55 and 70 mm, but you can usually work around that. Hopefully Canon will come out with something like a 50-200 f/2.8 IS EF-S to better compliment the 17-55.

When you are at the short end of the lens on a P&S (7.2 or 7.3 depending on the camera) that works out to be about the equivalent of a 35mm lens.

TMR Design wrote in post #2236172
When you are at the short end of the lens on a P&S (7.2 or 7.3 depending on the camera) that works out to be about the equivalent of a 35mm lens.

not always. the S80 is 28mm for one.

Boy...I wish it didn't have to be so confusing..
I also saw something about "constant Aperture" I assume this refers to a "prime lens"

slyone wrote in post #2237653
Boy...I wish it didn't have to be so confusing..
I also saw something about "constant Aperture" I assume this refers to a "prime lens"

Not necessarily. A prime lens is a non-zoom lens, that is one focal length, such as 50mm f/1.4. It is a constant aperture lens. Constant aperture is normally used in describing a zoom lens whose maximum aperture does not decrease as focal length increases. You may see a lens referred to as, for example, 70-300 f/4.0-5.6. This means that at 70mm the max aperture is f/4.0, but as you zoom to longer focal lengths, the max aperture decreases, at some rate, until, at 300mm the max aperture is f/5.6. This is a variable aperture zoom. If the lens is, for example, a 70-200 f/2.8, then the max aperture is f/2.8 throughout the zoom range.

All primes are constant aperture, but not all constant aperture lenses are primes.

very good, Thanks for the brief clarification

ScottE wrote in post #2236130
Hopefully Canon will come out with something like a 50-200 f/2.8 IS EF-S to better compliment the 17-55.

55-200/2.8IS would be perfect for crop bodies!

//edit: Or even 55-125/2.8IS to make it smaller and compete with the Sigma 50-150/2.8
//edit2: giving it a 88-200mm 35mmFOV equivalent

runninmann wrote in post #2237717
Not necessarily. A prime lens is a non-zoom lens, that is one focal length, such as 50mm f/1.4. It is a constant aperture lens. Constant aperture is normally used in describing a zoom lens whose maximum aperture does not decrease as focal length increases. You may see a lens referred to as, for example, 70-300 f/4.0-5.6. This means that at 70mm the max aperture is f/4.0, but as you zoom to longer focal lengths, the max aperture decreases, at some rate, until, at 300mm the max aperture is f/5.6. This is a variable aperture zoom. If the lens is, for example, a 70-200 f/2.8, then the max aperture is f/2.8 throughout the zoom range.

All primes are constant aperture, but not all constant aperture lenses are primes.

Let's not revive the old argument about the meaning of "prime lens". I know some people who will argue that your prime lens is the lens that you use most of the time. That could be a 50 mm f/1.4 or a 24-70 f/2.8 if that is the lens that you use more than any other. You have to be very carefull to use the phrase "fixed focal length" when talking about prime lenses around them.

KevC wrote in post #2248426
55-200/2.8IS would be perfect for crop bodies!

//edit: Or even 55-125/2.8IS to make it smaller and compete with the Sigma 50-150/2.8
//edit2: giving it a 88-200mm 35mmFOV equivalent

I was hoping that Canon would be able to make a 50-200 f/2.8 EF-S. They currently make a very good 70-200 f/2.8 for full frame bodies. With EF-S they do not have to cover as wide an image circle so I was hoping they could maintain the same quality and increase the focal length range. What ever they make it should start at 50 or 55 and go as long as feasible.

slyone wrote in post #2234700
Hey everyone....
any good links, I'm not all grasping this one yet?

A lens has a certain magnification power when focused on infinity. Lenses with a longer focal length have to be farther away from the film or sensor to focus at infinity, and thus create a larger projection of the image. Lenses with a shorter focal length are closer to the film or sensor when focused at infinity, and therefore create a smaller projection of the image.

The lens that magnifies the image similarly to how we see it is the so-called "normal" lens. By definition, the normal lens is the lens whose focal length is approximately the same as the diagonal size of the film or sensor frame. Thus, the normal lens for 35mm is the diagonal of the 24x36 format, which is about 44mm. The normal lens for the 15x23 APS format is about 28 or 29mm. The normal lens for a 3x5mm sensor that might be found in a digital point-n-shoot is about 6mm.

A 6mm lens on a 3x5 sensor will take the same picture as a 44mm lens on a 24x36 sensor.

Camera manufacturers market to the masses, not to experts. The masses know only about 35mm, and therefore have to have everything compared to 35mm to know what to think about it. They know that 28mm is moderately wide angle (because it's less than normal) and they know that 105mm is moderately telephoto. They know this because of what they habitually see when they look through those lenses.

A moderate wide on that 3x5mm sensor is 3.6mm. A moderate telephoto is 8mm. But if you write "3.6mm to 8mm zoom" on the front of the camera, nobody knows what it means. Most cameras with tiny sensors don't even tell you how big the sensor is, so even if you know how to determine the normal focal length, you don't have the information you need. So, the marketers write "28-105mm equivalent" on the camera. That means that the focal lengths of the zoom lens on that point-n-shoot will take the same pictures on that tiny sensor as a 28-105 zoom will on the 24x36mm format.

It really gets confusing when we use the same lenses for different formats. When I put an EF lens on a Digital Rebel, I'm using a lens designed for 24x36 on a camera that has a 15x23 sensor. Since the normal lens for that larger frame is longer, it stands to reason that the focal lengths will appear to be longer when used on a smaller format. Thus, to get the same effect as that 28-105, I would need something like 17-85. But the marketers expect people investing in a digital SLR to know the normal focal length for that camera, and therefore don't hold our hands by writing "28-105 equivalent" on a 17-85 EF-S lens. They expect us to know that 28mm is normal on that camera, and therefore that 17 is about 60% of normal, just like 28 is 60% of normal on the 24x36 format and 3.6 is 60% of normal on a 3x5mm format.

Believe me, if you spent only one day with a view camera under the tutelage of a view-camera photographer, you would realize how easy all this should be.

Rick "who thinks photography classes should start with a view camera" Denney

ScottE wrote in post #2248924
Let's not revive the old argument about the meaning of "prime lens". I know some people who will argue that your prime lens is the lens that you use most of the time. That could be a 50 mm f/1.4 or a 24-70 f/2.8 if that is the lens that you use more than any other. You have to be very carefull to use the phrase "fixed focal length" when talking about prime lenses around them.

Then they're referring to their "primary" lens, not their "prime" lens.

Jon wrote in post #2251536
Then they're referring to their "primary" lens, not their "prime" lens.

YOU tell them. I have given up.

THEY will tell you that "prime" means "first" or "best". Look it up in a dictionary.

I try to explain that for photography jargon "prime" means "fixed focal length", the purpose for language is to communicate, and nobody but them has any trouble understanding what "prime" means when describing a lens.