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Old Apr 14, 2005, 4:29 AM   #1
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I'm looking at lenses for a Digital Rebel XT (EOS 350D). Something I'd not thought about is the speed of the lens as I note some of your considering in the various postings here.

Can someone explain to me how speeds are rated and what the differences are? How can one lens be distinguished from another in terms of speed?

Thanks in advance,

Gary
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Old Apr 14, 2005, 8:57 AM   #2
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The speed of a lens is rated by its aperture opening ie: an f/2.8 lens is faster than an f/3.5 lens. The smaller the number the wider the aperture/lens opens which means more available light into the lens and onto the sensor. What this allows is faster shutter speeds in lower light scenarios. The glass in the lens gets larger and heavier the faster a lens gets. The faster a lens the higher the price.

Canon's fastest lenses are in the f/1.2-1.4 area. An example would be to look at the prices of their 50mm f/1.4 vs their 50mm f/1.8, their 135mm's etc. Zooms normally have a varying aperture for example the 100-400 L IS USM varies between f/4.5 and f/5.6. At the lower end (100mm) the aperture opens to f/4.5 but zooming all the way out to 400mm the lens slows down to f/5.6. The very expensive Canon L glass will have a fixed aperture at say f/2.8 or f/4 but look at the prices....several thousands of dollars.

You can also compare third party lenses such as Sigma, Tamron and Tokina. Some of their lenses are as good as Canon's for 1/4 to 1/3 the price.

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Old Apr 14, 2005, 9:50 AM   #3
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"Speed" is a misnomer in lenses, but we continue to use it.

One common use for the word is as Trout described -- it's a synonym for "maximum aperture." The "wider" the aperture (i.e. the bigger the opening, i.e. the SMALLER the f-stop number) the "faster" the lens.

But lens speed may also be referring to focus speed -- some lenses focus much faster than others. Canon's USM lenses generally find focus more quickly than non-USM lenses. Sigma's fast-focusing lenses are HSM lenses.
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Old Apr 14, 2005, 5:00 PM   #4
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There's two other points associated with a 'faster' lens:

1. Because a faster lens has the larger aperture it also has less of a DOF - The benefit of this is one can use this feature (sometime called 'bokeh') to isolate the subject from its background
http://www.stevesforums.com/forums/v...287905#p287905

2. The flip side of this is: the aperture number is actually a ratio of the diameter of a lens opening over it's focal lenght -> ie a one stop increase means a doubling of a lens diameter which is why 'fast' lenses are almost always heavier by design... This is what the folks are paying for - the extra glass and the metal to keep it together :?
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Old Apr 14, 2005, 6:04 PM   #5
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Thanks for your replies which are a great help.

NHL - wrt DOF, how does the speed of the lens relate to the "aperture priority" setting found on most cameras?

Gary
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Old Apr 14, 2005, 7:29 PM   #6
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When in Av - Aperture priority, you can dial in the largest aperture and the camera will set the shutter speed accordingly:

o with an f/2.8 lens you can can 'dial-in' f/2.8 to 'pop' the subject from its background
o If the lens maximum is f/5.6 - you can't set the camera to f/2.8
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Old Apr 15, 2005, 6:09 AM   #7
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Thanks, NHL.

Does the camera "know" the specifications of the lens electronically and thus only allow appropriate settings to be chosen, or do I have to manually set these somehow?

Gary
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Old Apr 15, 2005, 6:30 AM   #8
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garywood84 wrote:
Quote:
Does the camera "know" the specifications of the lens electronically and thus only allow appropriate settings to be chosen
Yes - The camera 'recognizes' the lens or combination of, like when one inserts a teleconverter for example, the minimum aperture is increased automatically by 1-stop with the insertion of a 1.4x TC in between


Quote:
do I have to manually set these somehow?
Nope
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Old Apr 16, 2005, 7:02 PM   #9
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And I suppose the "max aperture" is actually a minimum value (the smaller the num, the bigger the aperture)? At first it sounds kind of confusing, but if it refers to how much light can go into a lens... then it becomes more understandable...

I was having a hard time understanding why ppl kept talking about f11+ when the numbers on the lens said something like f3.5-f4.5 . But now I get it...

But here's a question : Although there's an advantage in speed, is there a picture-quality decrease? Because here and there I've been reading some consumer reviews that "shots at 2.8 were horrible" and "pictures only looked good at f8" for various lenses. So in that case the why would the max aperture matter if the shot is going to be bad anyway? Just, "to be there??"

Second question: is there really a noticable difference in speed between 1.4 and 1.8? (i.e. the 50mm 1.4 or 1.8 lens). or is it like trying to compare a 3GHz with a 2.9GHz CPU? Linear increase in performance, exponential increase in price? :-)
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Old Apr 16, 2005, 7:09 PM   #10
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BoYFrMSpC wrote:
Quote:
I was having a hard time understanding why ppl kept talking about f11+ when the numbers on the lens said something like f3.5-f4.5 . But now I get it...
I'm still havinga hard time trying to understand whatyou mean here! I thought I'd understood this thread so far, but this paragraph just doesn't fit my understanding! Please can you explain how the f11+ you refer to fits in with the f3.5-f4.5 on the lens?

Interesting point on quality too, btw. Will be interesting to see what people come up with.

Gary
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