# what exactly is a "fast" lens?



## vexed (Dec 22, 2001)

Jon said:


> Thanks for the great info SONET!!
> 
> :thumbup:


Sonet was much more descriptive than I would have been, I use it to take pictures of my daughter since she does not like to stand still. For a cheap lens it takes some great shots.


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## BLT (Jan 30, 2006)

vexed said:


> Sonet was much more descriptive than I would have been, I use it to take pictures of my daughter since she does not like to stand still. For a cheap lens it takes some great shots.


I have a nikkor series E 50mm 1.8 I bought on ebay for $35. I take fantastic pictures and is a joy to use.

The general concept of a fast lens is you can take pictures in lower light using a moderate shutter speed (generally 1 over the lens length 1/50 a second hand held fo the 50mm lens) and still have clear images. The narrow depth of field is iceing on the cake.


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## SONET (Mar 1, 2002)

vexed said:


> Sonet was much more descriptive than I would have been


I think you meant to say 'wordy'. It's a byproduct of trying to answer questions before they arise when I teach heh. :thumbup:


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## vexed (Dec 22, 2001)

SONET said:


> I think you meant to say 'wordy'. It's a byproduct of trying to answer questions before they arise when I teach heh. :thumbup:


Actually I thought you were being quite pithy.  It was a good reply especially for us photog noobs.


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## hockeynut (Apr 14, 2002)

vexed said:


> Yes, aka the thrifty fifty.


I picked one of these up today...will give it a shot (pun intended) tonight.


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## 85mm (Sep 2, 2005)

To me a fast lens is f/1.4 and up, er down. I love fast lenses and don't even own a flash. It's great for shooting in low light and for shallow depth of field for blurred out backgrounds.

shot at 85mm, f/1.2, 1/320s , iso 1600 using lousy street light









135mm, f/3.2, 1/500s, iso 200


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## Chris90 (Apr 7, 2003)

85mm said:


> To me a fast lens is f/1.4 and up, er down. I love fast lenses and don't even own a flash. It's great for shooting in low light and for shallow depth of field for blurred out backgrounds.
> 
> shot at 85mm, f/1.2, 1/320s , iso 1600 using lousy street light
> 
> ...


Those pictures are not fast enough to beat our company's image blocker.


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## Boile (Jul 5, 2005)

The way I see it, the term "fast" refers to shutter speed. A fast lens is one that allows you to take a correctly exposed picture using a fast shutter speed.
What's a fast shutter speed? For hand held photography, the rule of thumb is "the inverse of the focal length". So, if you're using a 200mm lens, you need to use 1/200 sec or faster. 
People like to use fast shutter speeds because it helps produce sharp images by eliminate blur caused by hand, equipment and subject motion. For a given exposure, using faster shutter speeds forces you to use bigger apertures (smaller f number). The result is a shallower depth of field, resulting in the subject being isolated from the distracting background, making for a more natural and pleasing picture. 
The beauty of referring to lenses by the term "fast" is that it automatically bundles focal length into the equation. That explains why a 200mm f/2.8 lens is considered fast but a 50mm f/1.8 is only so-so fast (for 50mm, a f/1.4 or f/1.2 would be considered fast).
In summary, a fast lens is one that allows you to do better (shoot at faster speeds) than the "rule of thumb".


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## Fast Bob (Jun 4, 2004)

Boile said:


> The way I see it, the term "fast" refers to shutter speed. A fast lens is one that allows you to take a correctly exposed picture using a fast shutter speed.
> What's a fast shutter speed? For hand held photography, the rule of thumb is "the inverse of the focal length". So, if you're using a 200mm lens, you need to use 1/200 sec or faster.
> People like to use fast shutter speeds because it helps produce sharp images by eliminate blur caused by hand, equipment and subject motion. For a given exposure, using faster shutter speeds forces you to use bigger apertures (smaller f number). The result is a shallower depth of field, resulting in the subject being isolated from the distracting background, making for a more natural and pleasing picture.
> The beauty of referring to lenses by the term "fast" is that it automatically bundles focal length into the equation. That explains why a 200mm f/2.8 lens is considered fast but a 50mm f/1.8 is only so-so fast (for 50mm, a f/1.4 or f/1.2 would be considered fast).
> In summary, a fast lens is one that allows you to do better (shoot at faster speeds) than the "rule of thumb".


No, "fast" refers to maximum aperture.... a 200mm f/2.8 lens is *one stop* faster than a 200mm f/4.0, meaning that the extra stop will allow twice as much light to reach the film plane. Each stop halves or doubles the amount of light....increments of shutter speed work the same way.


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## Boile (Jul 5, 2005)

Fast Bob said:


> No, "fast" refers to maximum aperture.... a 200mm f/2.8 lens is *one stop* faster than a 200mm f/4.0, meaning that the extra stop will allow twice as much light to reach the film plane. Each stop halves or doubles the amount of light....increments of shutter speed work the same way.


Given the same focal length, sure, a f/2.8 is faster than a f/4.0. My explanation doesn't preclude that. A f/2.8 will allow you to use a one stop faster shutter speed than a f/4.0 would, given the same light exposure.

If "fast" refers to maximum apperture (exclusively), why not call it "big"? Why the term "faster", which clearly implies speed? I was just saying that speed and apperture are interrelated.
amount of light hitting the sensor = apperture x shutter speed

The question was "what's the definition of a fast lens?"
If I understand your definition, a 50mm f/1.8 is faster than a 200mm f/2.8? Very few people would consider a 50mm f/1.8 a fast lens. (it only costs $80 for a reason)


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## Cliff (Apr 19, 2002)

Boile said:


> The way I see it, the term "fast" refers to shutter speed.
> 
> (snipped)


In the context of a lens, fast denotes a large aperture. Focal length is already captured in the aperture, which is a ratio.

To the point of an earlier poster, I would not limit the categorization of a fast lens to any particular number because there is a strong dependency on the focal length of the lens. A 300mm telephoto with an f2.8 aperture is a fast lens while a 50mm f2.8 lens is not.


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## Cliff (Apr 19, 2002)

Boile said:


> The question was "what's the definition of a fast lens?" If I understand your definition, a 50mm f/1.8 is faster than a 200mm f/2.8?


Correct. It lets more light in. I could handhold in dim lighting with the 50mm while the 200mm would probably require a tripod or a flash (or image stabilization).



> Very few people would consider a 50mm f/1.8 a fast lens. (it only costs $80 for a reason)


Broadly speaking, an f1.8 lens is a fast lens. A 50mm f1.8 lens is an inexpensive lens. A 50mm f1.4 is an inexpensive lens relative to my 85mm f1.4, which in turn is inexpensive relative to a 300mm f2.8, but they are all fast lenses.


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## Boile (Jul 5, 2005)

Cliff said:


> In the context of a lens, fast denotes a large aperture. Focal length is already captured in the aperture, which is a ratio.


How exactly is focal length captured in the aperture? :dunno: 
The two are orthogonal concepts.



Cliff said:


> To the point of an earlier poster, I would not limit the categorization of a fast lens to any particular number because there is a strong dependency on the focal length of the lens. A 300mm telephoto with an f2.8 aperture is a fast lens while a 50mm f2.8 lens is not.


I agree with the last sentence. That implies that the term "fast" is NOT synonymous to maximum aperture.
I attempted to give a definition of "fast lens" that's not categorized on a number, but on the well known "rule of thumb for hand-held photography", which in turn marries focal length and aperture.
If a lens lets you do "better" than the rule of thumb, it's a fast lens. How much better? That's why we have fast and faster.


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## Cliff (Apr 19, 2002)

Boile said:


> How exactly is focal length captured in the aperture? :dunno:
> The two are orthogonal concepts.
> 
> I agree with the last sentence. That implies that the term "fast" is NOT synonymous to maximum aperture.
> ...


The f stop is the ratio of the focal length to the lens aperture.

I would suggest a little more research on your part as photographers have generally come to a meeting of the minds on the notion of what is and isn't a fast lens. Wikipedia has a decent article on lens apertures: http://en.wikipedia.org/wiki/Lens_aperture


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