Before the aperture

I am looking at my lenses. They all have a 1 before the aperture. Correct me if I'm wrong: The f-stop numbers represent the biggest aperture at minimum and maximum zoom. I don't understand what the 1 signifies.
Example: 1:4-5.6

In math the one is always assumed so "f/4" is the same as "f 1/4". The f-stop is based on the focal length, replacing the "f" in the aperture with the focal length. A 100mm lens with an aperture of f/4 has an aperture of "100 * 1/4" or "100/4" or 25mm.

KRB, I understand what you're saying. I'm not sure if we talking about the same thing. The lens says: 70-300mm 1:4-5.6. Going by what you're saying, am I supposed to multiply the 1 in "70-300mm 1:4-5.6" by the focal lengths listed? I'm not sure what exactly that'll give me, besides the focal length.

since your lens doesn't have a fixed aperture, you have to apply the focal length accordingly to what the maximum aperture is possible. ie: at 300mm on your lens, your maximum aperture would be f/5.6, or 300*1/5.6 or 300/5.6 meaning the aperture is roughly 54mm.

Muuraija wrote in post #7724782
KRB, I understand what you're saying. I'm not sure if we talking about the same thing. The lens says: 70-300mm 1:4-5.6. Going by what you're saying, am I supposed to multiply the 1 in "70-300mm 1:4-5.6" by the focal lengths listed? I'm not sure what exactly that'll give me, besides the focal length.

Think of it like a ratio. 70:4. Divide that down until the most basic part, and you get:
70:4 --> 35:2 --> 17.5:1

At that point, you have the physical size of the aperture. So, at 70mm, the aperture is roughly 17.5mm. At 300mm (300:5.6), the aperture is roughly 53.6mm

This all gets weird with contemporary lens design, but that's where it all started, and why it's on your lens.

hth

On the same page now! Thanks, everyone.

Photography is all about math, whether or not you like it or use it

It gets complicated REALLY quick too.

I am finding... After the explanations, things I learned back in the 9th grade are coming back to me. I was looking through a book and I found out that camera makes 100mm is 160mm. It doesn't outright say what the 1 is.

bjyoder wrote in post #7724813
Think of it like a ratio.

That is exactly right. Ratios are expressed as a:b.

For instance, 1:2.8 on the lens means that you replace the one with the focal length to find the aperture size (the size of the actual iris opening), as has been said. When you see it expressed as f/2.8, it's telling you that very fact: the one is replaced by the focal length, represented by f.

An 85mm f/1.2 has a maximum aperture opening of 70.8mm, which is why the front element (and the lens itself) is large. An 18-55 3.5-5.6 has a maximum aperture opening at 55mm of just 9.82mm, which is why the front element (and thus, the lens) can be much smaller.

The camera in my avatar wears a Canon 50mm f/0.95, which has a maximum iris opening of 52.6mm, the largest aperture of any consumer 50mm ever produced (matched by a Leica 50/0.95).

Brett wrote in post #7725112
The camera in my avatar wears a Canon 50mm f/0.95, which has a maximum iris opening of 52.6mm, the largest aperture of any consumer 50mm ever produced (matched by a Leica 50/0.95).

Nice. I know I'm going off-topic, but isn't it interesting that no-one else made such a fast lens in all that time (about 40 years) between the Canon Dream and the new Leica Noctilux?

Muuraija wrote in post #7724884
I am finding... After the explanations, things I learned back in the 9th grade are coming back to me. I was looking through a book and I found out that camera makes 100mm is 160mm. It doesn't outright say what the 1 is.

The part that I bolded above is a very wrong interpretation of what you read.

What the book should have said is that the field (or angle) of view of a 100mm lens on an APS-C format camera (like a 40D or any of the Digital Rebel series) is equivalent to the field (or angle) of view of a 160mm lens on a 35mm film camera (or a so-called "full-frame" digital SLR).

This has absolutely nothing to do with the aperture values or the way they are represented.

SkipD wrote in post #7726384
The part that I bolded above is a very wrong interpretation of what you read.

What the book should have said is that the field (or angle) of view of a 100mm lens on an APS-C format camera (like a 40D or any of the Digital Rebel series) is equivalent to the field (or angle) of view of a 160mm lens on a 35mm film camera (or a so-called "full-frame" digital SLR).

This has absolutely nothing to do with the aperture values or the way they are represented.

I understand. the point I was trying to make is that when the book refers to the focal length and aperture, it never addresses the 1. Just says on a crop body, the focal length of the lense should be fultiplied by the crop facture to get the focal length on a FF sensor...

Muuraija wrote in post #7727087
I understand. the point I was trying to make is that when the book refers to the focal length and aperture, it never addresses the 1. Just says on a crop body, the focal length of the lense should be fultiplied by the crop facture to get the focal length on a FF sensor...

Look at it from a the lens's perspective.

The mm is just distance from the back element, that's a constant number no matter which body you put it on. Now how your body's sensor adjusts to that lens is a different story, but has nothing to do with the lens and focal length.

What changes is the ratio of sensor size to 35mm, nothing else.

Muuraija wrote in post #7724691
I am looking at my lenses. They all have a 1 before the aperture. Correct me if I'm wrong: The f-stop numbers represent the biggest aperture at minimum and maximum zoom. I don't understand what the 1 signifies.
Example: 1:4-5.6

Its just one of the nomenclatures used by the various manufacturers to express the same info.

for example.

f/4 can be expressed several ways.

a) f/4
b) f4
c) 1:4

just know that on your lens which is a f/4-5.6, at some part of its range it will have a maximum aperature of f/4 (the wide end), at other parts of its aperature it will have f/5.6 (the long end), and in between these two ranges it will max out somewhere in the middle... generally f/4.5 and f/5.

do not confuse 1:4 with f1.4, they are not the same.

f/1.4 would be expressed as 1:1.4 in the "ratio" nomenclature.

nureality wrote in post #7727705
do not confuse 1:4 with f1.4, they are not the same.

f/1.4 would be expressed as 1:1.4 in the "ratio" nomenclature.

There have been some CRAZY posts on PentaxForums.com over this very issue, and how it relates to the so-so SMC-A 50mm f/2 and the AMAZINGLY awesome SMC-A 50mm f/1.2