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what does the f/2.8 or f/4L or f/1.8 mean in canon lenses?

Nav _ , Jun 19, 2006; 03:42 a.m.

I'm sorry if this is a silly question but I'm trying to understand the digital camera world..

I'm planning on getting the 24-105mm f/4L Canon lenses. However I'm trying to understand what the f/4L stands for. does it stand for the widest f-stop? and what does L mean? - therefore why is the 4L series better than the 2.8 - because as if you'd want to shoot under such a low aperture- I figure there's something I'm not getting here...




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Alistair Windsor , Jun 19, 2006; 03:58 a.m.

Yes. The f4 is the smallest f number (largest aperture).

In general f2.8 is "better" than the f4 though it all depends on the lens. The f2.8 lenses tend to be heavier and bulkier than their f4 counterparts.

The 24-105/4L IS is the only full frame standard L zoom with IS. I would argue that its 2.8 counterpart, the 24-70/2.8, is slightly better. Slightly faster for low light, shallow DOF for portraiture, and almost no vignetting at the wide end. It lacks IS and a little reach.

Ken Munn , Jun 19, 2006; 04:14 a.m.

To dispose of the L first, this indicates that is one of Canon's top notch professional lenses, built for optical and mechanical excellence, rather than to meet a consumer or 'prosumer' price point. Amateurs are allowed to buy them, though!

The number indicates the maximum aperture of a lens. Oddly, the smaller the f number the bigger the max aperture (the f number is actually the ratio between the focal length of the lens and the diameter of the elements in it - thus a 50mm lens with 50mm diameter glass in it would be f1.0, with 25mm glass would be f2, with 18mm glass would be f2.8, etc).

Practically speaking a bigger aperture means the lens captures more light. This means that you can - if you wish - use faster shutter speeds or slower film (or sensor) speeds in poor light for better results. Another upside/downside of shooting at large apertures is that the front-to-back area of a picture which is acceptable sharp is much shallower, allowing you to shoot - for example - a portrait against a distracting background by throwing the background completely out of focus while keeping the subject razor sharp. There are many other uses for differential focus like this.

Within the lens there is an aperture control mechanism (a diaphragm) which can restrict the size of the aperture, thus changing the f number from shot to shot. Typically whatever the maximum aperture of the lens you choose, you will be able to control the shooting aperture all the way down to f16 or f22 (that would be an aperture size of 2.2mm on a 50mm lens, for example.

Hope it helps - there are many photography 101 sites around the web which will explain (and illustrate) all this much better than I can.

NK Guy , Jun 19, 2006; 04:24 a.m.

Alistair Windsor , Jun 19, 2006; 05:24 a.m.

Ken's answer is substantially correct.

However when you look at a lens you will see it is not the size of the elements that determine the maximum f-stop. Otherwise a 17-40 would have elements of size 1 cm whereas the front element is slightly over 6cm. You often here it explained that the f-stop is the ratio is between the focal length and the size of the aperture. If you diassemble a lens and look at the physical aperture you will typically find the hole is much smaller than it should be (typically it buried deep within the lens to make the blades lighter). To a first approximation what matters is the size of the aperture when viewed through the front element. Especially with wide angle retrofocus designs you will find very large elements even when the lens is not particularly fast.

Alistair Windsor , Jun 19, 2006; 05:29 a.m.

The lens I referred to is the 17-40/4L. It has maximum focal length 40mm and maximum aperture f4. The simple calculation shows that the largest aperture should be 10mm at focal length 40mm and a massive 4.25mm at focal length 10mm. Actually the maximum aperture of the diaphragm is likely fixed but the apparent size changes since the actual diaphragm sits behind many of the elements and changes apparent size when you zoom.

Alistair Windsor , Jun 19, 2006; 05:49 a.m.

You often "hear" it explained ....

I can spell and I can type. Apparently I can't do both simultaneously.

Eric Reagan , Jun 19, 2006; 09:09 a.m.

If you're going to shell out that much money on glass get a good book to go with it. Understanding Digital Photography by Bryan Peterson and Understanding Exposure by Bryan Peterson. Both are great books by a great author. He regularly contributes to Popular Photography. Could be the best $15 investment you make in photography.



caleb condit , Jun 19, 2006; 09:13 a.m.

agree with second post

the 24-70mm 2.8 has an ability to shoot with a larger aperture and has less problems with vignetting. Typically, trying to cover too much ground with one lens is a bad thing. It becomes physically difficult to create a quality lens, trying to be all things to all people. I would go for the 24-70 as it's the same price, but a much better lens.

Look up in Wikipedia "aperture" and the L is the series of lens, IE Luxury :) Basically, the larger the number (f/22) the smaller the opening and the more area that's in focus and the inverse for the smaller number (f/1.8)

Alistair Windsor , Jun 19, 2006; 07:21 p.m.

Thanks for agreeing with my post. I am not sure I would characterize the 24-70/2.8 as much better than the 24-105/4. The 24-105/4 does have fairly hefty vignetting at the widest setting when wide open but is otherwise optically very good.

For a walk around lens I think the extra reach and IS probably makes the 24-105 a better lens. However for portraiture I think the 24-70/2.8 is a better bet. You could however get the 24-105/4L for an everyday lens and primes for portraiture.

My everyday lens set is based around f4 max lenses.

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