Understanding how "fast" a lens is?

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inlineadam

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Ok so this has been confusing me for a while.

How exactly do we tell how fast a lens is.

Is it by the aperture? (f/stop)

So, say a lens with 1.8 would be faster than say a 5.6..

I'm confused :thinking:

Currently use a 55-250 IS 4-5.6 for motorsport which would mean it's slow..?


:thinking::thinking::thinking: CHeers!
 
inlineadam said:
How exactly do we tell how fast a lens is.

Is it by the aperture? (f/stop)

So, say a lens with 1.8 would be faster than say a 5.6..
Click to expand...

correct
 
Yes you are correct in your assumption that it is by aperture. Lens speeds are to some extent relative to their focal length, the longer you go the slower they become. Your 55-250 f4-5.6 is average for a lens of that type, long fast lenses are expensive and also heavy. The front element gets larger to let more light in as size increases so by the time you get to 400mm and f2.8 you need to have some serious arm muscles :D I used my Sigma 150-500 f5-6.3 (currently for sale) for shooting at Siverstone and it was fast enough. So in general the equation is, long + fast=expensive. You can get good results with slower lenses though, this was one of my Silverstone shots with the Sigma

IMG_5215.jpg
 
To expand a bit, wider aperture lenses are referred to as fast because their light gathering abilities allows faster shutter speeds.
And as Artyman said, it's relative to focal length as well. For instance, a 50mm f/2.8 prime would be considered a moderately slow lens of it's type, but a 400mm f/2.8 is as fast as they come at that focal length.
 
Thanks very much for the Replies. Had a bit of a confused moment when I was looking at a 50mm 1.8 thinking how on earth it could be faster than the 55-250 I have. That's cleared everything up, cheers :)
 
the easiest way to think about exposure in photography is compare it to a tap filling a bucket.
As yo open the tap the hole gets bigger and more water comes out and you fill the bucket quicker.
Shutter speed is more like how long you have the tap open for.

You can fill the bucket either by having the tap only slightly open but for a long time.
Or you can fill it with the tap fully open for a short time.

In camera terms the shutter and aperture control the amount of light reaching the sensor.

A fast shutter speed needs a wider aperture to compensate for the short duration.

A 1.4 aperture is a large hole, so fast
A 5.6 aperture is smaller so relatively slow.
 
Nice sunny day.. person one has a 400mm lens at f5.6 and achieves a shutter speed of 1000 ... person two has a 400mm at f2.8 and also a shutter speed of 1000 (he set the iso higher) So on the day who is shooting the fastest?

My point being you cant just say 2.8 is faster..only that it has the capability to be faster

agree/disagree ?
 
Disagree, the 2.8 is a faster lens full stop. The person using the 5.6 lens has used faster "film" to compensate for the slower lens.

The overall system may have achieved the same shutter speed but the original question related to lenses only :)
 
I've got to disagree too Tony. The f/2.8 is a faster lens than the f/5.6 end of story!
And don't forget (not that you would) that it's a lot easier to lose the background at a wider aperture too. :)

cheers
 
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KIPAX said:
Nice sunny day.. person one has a 400mm lens at f5.6 and achieves a shutter speed of 1000 ... person two has a 400mm at f2.8 and also a shutter speed of 1000 (he set the iso higher) So on the day who is shooting the fastest?

My point being you cant just say 2.8 is faster..only that it has the capability to be faster

agree/disagree ?
Click to expand...

It's like saying one car has a 150hp engine, another a 200hp engine. The 200hp engine is more powerful, but you won't always use that extra power.
 
KIPAX said:
Nice sunny day.. person one has a 400mm lens at f5.6 and achieves a shutter speed of 1000 ... person two has a 400mm at f2.8 and also a shutter speed of 1000 (he set the iso higher) So on the day who is shooting the fastest?

My point being you cant just say 2.8 is faster..only that it has the capability to be faster

agree/disagree ?
Click to expand...


I would hope person two has a LOWER ISO, or there is going to be a badly exposed image by one of them.
 
My rule of thumb..

f/5.6 = slow (anything slower is comatose)
f/4 = normal
f/2.8 = fast
f/2 or f/1.4 = my preciousssss..
 
OK lets try again... only this time I will make the question bold :)

person one has a 400mm lens at f5.6 and achieves a shutter speed of 1000 ... person two has a 400mm at f2.8 and also a shutter speed of 1000 (he set the iso higher) So on the day who is shooting the fastest?
Click to expand...

schoolboy error on my part.. he set the iso lower :) but the question remains the same..So on the day who is shooting the fastest?
 
Yeah OK :lol: I do take the point.
But it's hardly what the OP was really asking is it?

Nice one though...
 
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KIPAX said:
OK lets try again... only this time I will make the question bold :)

schoolboy error on my part.. he set the iso lower :) but the question remains the same..So on the day who is shooting the fastest?
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Assuming the guy shooting at 5.6 has set the iso two stops higher then they would essentially be the same....
 
I did have a point.. the answers to the OPs question just seem to be a flat this is faster.. I wanted to point out to someone/anyone who may not realise.. that its all about the shutter speed.. its not a clear cut case of every time you use a 2.8 it will be faster than a 5.6 say...
 
Flash In The Pan said:
Assuming the guy shooting at 5.6 has set the iso two stops higher then they would essentially be the same....
Click to expand...

but how can that be if the 2.8 is faster?

thats my point... what we know-we know.... others are trying to find out.. so i just wanted to try and make it clear :)
 
Analogy: I have a Kawasaki ZZR 1100, top speed 175MPH my mate has a Honda 90cc top speed 40MPH. My mate on his 90cc bike is in top gear high revs, he is doing 40MPH. While me on my powerful 1100 I am in a low gear very low revs, I am doing 5MPH. My mate is riding faster than me, but I am 100 per cent sure, I am on the faster bike.
 
jonbeeza said:
Analogy: I have a Kawasaki ZZR 1100, top speed 175MPH my mate has a Honda 90cc top speed 40MPH. My mate on his 90cc bike is in top gear high revs, he is doing 40MPH. While me on my powerful 1100 I am in a low gear very low revs, I am doing 5MPH. My mate is riding faster than me, but I am 100 per cent sure, I am on the faster bike.
Click to expand...

we know.. but your mate was riding faster on the day

its like people are trying to miss the point :)
 
I though the original question is "how fast is a lens"

Not how fast is my shutter going...

in the context of peeps making statements like "you need a fast lens" they mean lower Fnum -- End of chat...
 
vizzair said:
I though the original question is "how fast is a lens"
Click to expand...

correct and we have 100% established that a f2.8 lens is faster than a f5.6 for example... i think everyone knows that...

however to explain to anyone who doesnt understand what "faster" means I attempted to show that it isnt always faster it depends how you use it.. it has the capability to be faster..

surely anyone reading the whole thread can see how it went? :)


vizzair said:
in the context of peeps making statements like "you need a fast lens" they mean lower Fnum -- End of chat...
Click to expand...

oh right.. your closing the conversation then? OK ... :(
 
Seriously.. I give up. :( I just wanted to point out that using a faster lens does not mean it will always be faster.. all lens can peform at the same speed...

seems I am wasting my time really ... too many people like to turn everything into an argument :(

PS look at the subject line... thats all I was trying to help the OP do :)
 
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I know the answer, but can't explain it well :lol:, does this do;

The wider you can open the aperture, the less time the shutter needs to stay open to get the light in.

I agree with KIPAX, the two lenses will be as 'fast as each other' - just the 5.6 jobby will be noiser as the ISO will be upped?

Joe
 
^Joe said:
I know the answer, but can't explain it well :lol:, does this do;

The wider you can open the aperture, the less time the shutter needs to stay open to get the light in.

I agree with KIPAX, the two lenses will be as 'fast as each other' - just the 5.6 jobby will be noiser as the ISO will be upped?

Joe
Click to expand...

Yes and no......

Take the scenario where you have identical cameras, both with ISO maxed out and you need a shutter speed of say 1/500 to stop the subject from being blurry.

Being able to shoot at f2.8 as opposed to f5.6 will allow you to carry on shooting and the guy with the f5.6 lens has the options to pack up and go home, settle for under exposed and/or blurry images. There may be an increase in the amount of noise due to under exposing but that's not really relevant ;)

In that scenario the faster lens will have an advantage unless you need a big depth of field ;)

What I believe Tony was saying (Tony, please jump all over me if I've read this wrong) is that if what/where your shooting should dictates whether you need a "fast" lens or not and/or how fast that lens needs to be.

For example, if you shoot cricket matches in the summer it's unlikely that shooting at f5.6 is going to cause you any problems (bearing in mind if the light gets that bad they stop playing ;) ) but for rugby/football played in the winter and/or under floodlights you need all the light available an may be forced into needing a lens that can open up to f2.8+
 
Russ77 said:
Yes and no......

Take the scenario where you have identical cameras, both with ISO maxed out and you need a shutter speed of say 1/500 to stop the subject from being blurry.

Being able to shoot at f2.8 as opposed to f5.6 will allow you to carry on shooting and the guy with the f5.6 lens has the options to pack up and go home, settle for under exposed and/or blurry images. There may be an increase in the amount of noise due to under exposing but that's not really relevant ;)

In that scenario the faster lens will have an advantage unless you need a big depth of field ;)

What I believe Tony was saying (Tony, please jump all over me if I've read this wrong) is that if what/where your shooting should dictates whether you need a "fast" lens or not and/or how fast that lens needs to be.

For example, if you shoot cricket matches in the summer it's unlikely that shooting at f5.6 is going to cause you any problems (bearing in mind if the light gets that bad they stop playing ;) ) but for rugby/football played in the winter and/or under floodlights you need all the light available an may be forced into needing a lens that can open up to f2.8+
Click to expand...

The way I read his example was that it was a lovely sunny day and you didn't need to have your aperture number so low, as the light was plentiful.

I fully agree that in the low light, it's a different situ in that you need a faster (lower f) lens.
 
If someone would invent reverse ND filters we wouldn't have this problem. :)
 
The problem with people trying to understand these things in anything beyond their practical terms means that you have to deal with mathematical ratios and the physics of light and optics.

The f number is indicative of the size of the aperture compared to its focal length. More specifically, it is the focal length divided by the diameter of the aperture that gives you the f number. For this reason, it is not too hard to get lower f numbers at relatively low focal lengths compared to long focal lengths. Just to give an idea of how this works in reality, take a 50mm lens at f/2 and a 200mm lens at f/2. The 50mm only needs a 25mm wide aperture opening to achieve f/2, wheras the 200mm would need a 100mm wide aperture opening to achieve f/2.

If you look at it the other way, if these two lenses had physically the same size aperture of 25mm across, the 50mm would still be f/2, but the 200mm would be f/8! A full 4 f-stops slower!

The real practical usage of these f numbers, and why we use them in photography, is that we don't need to work out how changing to a different focal length with a different size aperture will affect the shutter speeds and iso choice, as any lens that collects x amount of light at any aperture will collect x - the same - amount of light on any other lens at the same aperture.

You can test how this all works without really understanding any of the above anyway. Just take your shortest lens, either a zoom or prime, and just point it at a light source which won't change too much (pc monitor is good for this!), and look at the shutter speed the camera gives you. Then switch to your longest and see what shutter speed it gives you, it should be identical.

In practice, this may not work in exactly the same way, as in a scene, different objects will reflect different amounts of light from different sources, but generally speaking, if you set your shutter and aperture manually for the correct exposure, those settings will work for any lens capable of that aperture that you would want to use.

I think that about explains it, but feel free to PM me if I've not been clear about anything in particular!
 
http://www.talkphotography.co.uk/forums/showpost.php?p=4405309&postcount=7

A little bit drunk last week, I made a suggestion elsewhere on the board about colating frequent questions ... i didn't express it as I had intended, wanting only a reference list of common questions, rather than a section.

This thread kind of makes me want to go back to that suggestion to clarify what I meant.

One of the replies to my suggestion pointed out how the Basics section was meant to cover frequent, BASIC questions, but this thread demonstrates how such a question can be lost.

The OP was satisfied by post #7, but here we are 20+ posts on with more complex arguments that are beyond the OP and myself for the time being.


If you want to argue with other togs about the complexities that are beyond the likes of me, can't you do it in another thread :shrug:


Seriously, I appreciate all the time and experience you guys pour into this forum, but it gets a bit heavy for me with a basic (budget) kit :D
 
Decided to bring this back up due to some more queries.

I understand now that the lower the aperture the faster shutter speed you can use. ( obviously )

I'm wondering; when we say a lens is fast, is that just because we can use a faster shutter speed on the camera, so in effect, most lenses are just as fast as each other?
 
inlineadam said:
Decided to bring this back up due to some more queries.

I understand now that the lower the aperture the faster shutter speed you can use. ( obviously )

I'm wondering; when we say a lens is fast, is that just because we can use a faster shutter speed on the camera, so in effect, most lenses are just as fast as each other?
Click to expand...

'Fast' is a relative term, a bit of jargon really. A fast zoom would be perhaps f/2.8, whereas a fast prime might be f/2 or lower. The opposite term is slow, perhaps f/4 or higher? Something like that.

Of course you can use any shutter speed with any lens, but the one with the lower f/number will allow a faster shutter speed or higher shutter speed, by which we actually mean a shorter duration shutter speed.
 
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HoppyUK said:
Of course you can use any shutter speed with any lens, but the one with the lower f/number will allow a faster shutter speed
Click to expand...

I understand thanks, just wanted to clear it up.

So essentially it's not "how fast a lens is", it's how fast a lens can be used :thinking:
 
KIPAX said:
correct and we have 100% established that a f2.8 lens is faster than a f5.6 for example... i think everyone knows that...

however to explain to anyone who doesnt understand what "faster" means I attempted to show that it isnt always faster it depends how you use it.. it has the capability to be faster..
Click to expand...

But in your example the f/2.8 lens IS faster, and IS being used faster. If you look at both settings used, f/2.8 and 1/1000 and f/5.6 and 1/1000, in terms of light entering the lens and hitting the sensor, the f/2.8 is quickest.

All Mr f/5.6 is doing is making his sensor more sensitive to the available light hitting the sensor to match the exposure of the other guy, and the amount of light hitting his sensor is less, hence the need for higher ISO.
 
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