F stop question

[Mike410]

Hello,

Can someone please explain why lens speed is measured in f stops? I thought f stops refer to aperture values (depth of field). What does the DOF have to do with the speed of lens?

This question comes after reading an article on here about lenses with the following quote:

"This lens might be your kit lens, but it should preferably be fairly fast (a fixed f/2.8 if possible) to give you greater control over depth of field."

Why would a small f stop number (f/2.8) be considered fast?

Thanks in advance (I'm still learning

Mike

[bono]

This is quite easy. Lower f-stop-numbers mean that you can use the lens in lower light. The lens can deal with difficult light situations. With say f/1,7 you could reach 1/60s in low light conditions, with f/8 something about 1s. The lenses have to be optimized for that, which is expensive.

The cost of low f-stop-numbers is the depth of field. You will find only the focus point sharp, everything beside is blurred. The depth of field is also lowered by a larger camera sensor and the focal range of the lense and the distance of the object you take a picture of.

Depth of field decreases throug lower f-stops, larger sensors, longer focal range and objects that are nearby. This is the reason that macro shots have a very noticable sharp center and blurred edges.

Further reading: Aperture - Wikipedia, the free encyclopedia

[autofocus]

Originally Posted by Mike410

Hello,

Can someone please explain why lens speed is measured in f stops? I thought f stops refer to aperture values (depth of field). What does the DOF have to do with the speed of lens?

This question comes after reading an article on here about lenses with the following quote:

"This lens might be your kit lens, but it should preferably be fairly fast (a fixed f/2.8 if possible) to give you greater control over depth of field."

Why would a small f stop number (f/2.8) be considered fast?

Thanks in advance (I'm still learning

Mike

Mike, I cut this right out of Google...hopefully it'll give you a better understanding:

"F," I learned, could actually mean "fraction". The "4"
in "f4" actually stood for 1/4. The diameter of the
aperture at f4 would be 1/4th the focal length of the
lens. A 50mm lens set to f4 would have an aperature
diameter of 12.5mm (50mm X 1/4). A 1000 lens set at F22
would have an aperture diameter of 45mm (1000mm X 1/22).
So with this information, you could derive the formula:
F-Stop = A/FL; where A is the diameter of the aperature, FL
is the focal length of the lens, and F-stop is expressed in
it's fraction form.

The speed of the lens makes reference to how large the aperture will open, thus allowing more light in for a given exposure. The aperture also controls DOF whereas a large aperture will give you shallow DOF (as in F/2.8 vs F/5.6) Conversely, a small aperture, as in F/16, gives you deeper DOF. Lenses that have apertures that open to F/1.2, F/1.4, and F/2.8 cost more than those that don't. The benefit is being able to shoot in lower light conditions, and having more control over your DOF...that's what you're paying for

[inkista]

The best explanation of maximum aperture and what it means is this one: What is maximum aperture and why is it so important? Highly recommend reading it, as it should make the "faster" = lower f-number thing clear, in practical shooting terms.

Just to clarify a little on the explanation above (f doesn't stand for fraction, it stands for focal length), the f-number (#) is the ratio of the focal length (f) to the diameter of the aperture (a) opening:

# = f/a

So, if you multiply both sides by the aperture (a):

a# = f

And then divide by the f-number (#), you solve for the aperture (diameter):

a = f/#

That's why the aperture diameter is given as f/# (e.g., f/8).

The nice part about this notation is that it then doesn't matter what the focal length of the lens is. An aperture diameter of f/2 is still f/2 whether it's 50mm on a 100mm lens, or 25mm on a 50mm lens. It's actually less math to do it this way than to give the actual aperture opening measurement in millimeters.

[Dennismc]

Great post and thanks for the info. It finally enabled it to click on why my pictures of the ducks I have been taking are slightly blurred still.

I have been shooting them wide open for the lens, (1.8 on the 35mm and 5.6 on the siggy)

When shooting as I was yesterday in bright daylight, using the small aperture I was creating a very narrow depth of field so say only the head of the duck, which I had the focus on was in complete focus and the rest started to go OOF.

I knew this but it didnt really click until reading this. Thank you again!

[AgBeard]

The best presentation of the whole aperture saga I have seen of this is in the Interactive 123di Manual

You can download a demo copy from:

123di: The Most Complete, Comprehensive, Authoritative Digital Photography Interactive Guide.

[Mike410]

Thanks for the replies, lots of helpful info given! It really clears things up

[fxxrodriguez]

[QUOTE=inkista;953210]The best explanation of maximum aperture and what it means is this one: What is maximum aperture and why is it so important? Highly recommend reading it, as it should make the "faster" = lower f-number thing clear, in practical shooting terms.

Thanks for posting this link... a great learning curve for us beginners learning about maximum apertures, etc.