Complete noob questions!

[seward93]

Ok, so I am just getting into photography and I want to ask a few questons.

Can someone give me the details on ISO? I read this but I am still a little confused.

Also, what is this "f/3.5-5.6" in different lens decriptions?

Last question is about lenses themself. I don't understand what the 2 numbers are (ex. XX-YYmm, 118-155mm) Could someone explain these?


Thanks for the help in advance

 

[Jaszek]

ISO is the sensors sensitivity to light.
The f/ numbers are the size of the aperture. Different lenses have different apertures. Some are constant some are variable. if you have lets say 3.5-5.6 that means 3.5 is the largest it goes on the smallest focal length and 5.6 is the largest on the longest focal length.
the 18-55mm or others are the focal length of the lens. the mm stands for how far away the back lens element glass is away from the sensor(correct me if i'm wrong)

 

[Sarah23]

Ok, so I am just getting into photography and I want to ask a few questons.

Can someone give me the details on ISO? I read this but I am still a little confused.

Also, what is this "f/3.5-5.6" in different lens decriptions?

Last question is about lenses themself. I don't understand what the 2 numbers are (ex. XX-YYmm, 118-155mm) Could someone explain these?


Thanks for the help in advance

Iso is basically the sensitivity of your sensor. The higher the sensitivity, the higher shutterspeed you will be able to use (this is good in low light situation to avoid motion blur)

The f-stop is your aperture. That is like the "iris" of the lens. This number represents the widest your aperture can open for that specific lens. When it has multiple numbers (like 3.5-5.6) that means that when you are zoomed out all the way, at the widest your lens will go, your max aperture is 3.5. As you zoom in, since it has a variable aperture, the max aperture you can get will change. When you are zoomed ALL the way in, the max will be that 5.6. This is why lens like this are cheap and not used much by portrait photographer. Something with an constant f-stop of f/1.8 or something wide like that is going to be a better lens because your aperture will able to open up more, resulting in more light being let in, and you will be able to have faster shutter speeds. Wider f-stops also result in better bokeh (blur behind and in front of the subject)

The "mm" on your lens is your focal length. A 24mm is going to be a wide angle lens because it lets you get close to your subject while still getting a full body shot. something like the 70-200 lens is going to be longer, or "zoomed" in more, and will make you have to be farther away to get the same, full body shot. The actual number represents the legth between the glass and the sensor or something like that ( i would have to look it up again)

 

[AUZambo]

Another view on the f-number. It's the ratio of the diameter of the lens opening to the blade opening that allows light through.

For example, let's pretend the diameter of your lens is 5 inches. You have several blades in the lens that work together to close the opening as pictured below:

If the diameter of the opening of the blades is 2 inches, then your f-number is 5 inches/2 inches, or 2.5. If the diameter of the opening is 1 inch, then the f-number is 5/1 or 5.

The perfect F number would be 1 because that means the blades have opened up to the full 5 inches, but that's impossible as far as I know. The largest opening/f-# I've seen is 1.4.

Also, your longer lenses tend to have much smaller openings. Think of it like looking through a paper towel core. If you look through it like a telescope, alot of the light is blocked...but if you cut the length of the core to 1 inch then alot more light is let in.

 

[seward93]

Thanks for the answers everyone!

I'll post up anymore questions I may get here

 

[chrisburke]

I'll post up anymore questions I may get here

but before you do, make sure you do a quick search with the search tool on the forum, questions like these are on here by the dozen..

 

[bjorkfiend]

...

 

[AUZambo]

This information is not correct.

Less importantly, the f number does not directly correspond to the physical aperture created by the aperture blades. The ratio actually corresponds to the entrance pupil of the lens at the given focal length and aperture opening. For most intents and purposes this can be ignored and you may assume that we are talking about the aperture diameter.

More importantly, the f ratio is not in terms of lens diameter/entrance pupil but, rather, the focal length/entrance pupil.

If you'd like a more detailed read on the subject you can look at this: F-number - Wikipedia, the free encyclopedia

Yes...you are correct...but I figured what I said would be much easier to understand and it does explain (at least to me) why a lower f ratio signifies a larger aperture.

Just trying to keep it simple!