Is that big Apecture of 1.6-2.8 really needed?


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rt7snap

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Dec 26, 2005
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#1
I like take to take shots with lesser depth of field, (i think is big Apecture right?), sharp foregroud, blur background.

But im considering buying a canon lens with only f/4.5-5.6. (Canon EF 70-300mm f/4.5-5.6 DO IS USM)

I find that it can also shoot subject with blur background.

My qn:
What generally is the purpose of getting a lens with a big apecture, 1.4 , 2.8?

Constant
Btw, can help tell me what does it mean by 2.8 constant (does it mean, whatever the zoom ranges, it will be at 2.8, even in a bright sunlight setting?

Fast Lens
Oso, pls enlighten wat does it mean when people said this is a "fast lens"?

thank you folks!
Present 350D with kit lens
 

sk.images

New Member
Dec 9, 2005
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#2
Yes you can get the 'out-of-focus' effect with that lens, but you need to make sure that the subject (in focus) is a long way in front of any background and that you are relatively (compared to the distance between the subject and the background) close to the subject. To put it simply, yes you can get nice out of focus bacground, but not easily in many situations.

You are correct regarding constant aperture, i.e. the same aperture at all focal lengths.

Fast lens, means it has a large aperture, usually =< 2.8, basically that it requires less light (at the same shutter speed and ISO), another way to say this is that you can use faster shutter speed...
 

Aug 16, 2005
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Singapore, CanonGraphers.org
#3
To me, f2.8 constant is good so that if the focal length changes, the shutterspeed wont change.Thus I can regulate my shutterspeed.
Most photographers get big apperture lenses because of their ability to enhance the bokeh in shots for portraits.Or, for fast shutterspeeds in action freezing shots.
IMO, most big apperture lenses tend to be quite sharp too....
 

lsisaxon

Senior Member
Nov 29, 2004
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#4
rt7snap said:
I like take to take shots with lesser depth of field, (i think is big Apecture right?), sharp foregroud, blur background.

But im considering buying a canon lens with only f/4.5-5.6. (Canon EF 70-300mm f/4.5-5.6 DO IS USM)

I find that it can also shoot subject with blur background.

My qn:
What generally is the purpose of getting a lens with a big apecture, 1.4 , 2.8?

Constant
Btw, can help tell me what does it mean by 2.8 constant (does it mean, whatever the zoom ranges, it will be at 2.8, even in a bright sunlight setting?

Fast Lens
Oso, pls enlighten wat does it mean when people said this is a "fast lens"?

thank you folks!
Present 350D with kit lens
Usually, the depth of field is less with longer lenses. At 300mm, the depth of field is still very shallow even at f/5.6. However, when you zoom out to 70mm, you will find that at f/4.5 it may still be quite hard to get a good blur for the background.

However, at f/2.8, you would have an even shallower depth of field, and the blur effect would be more prominent.

Usually, with a bigger aperture lens, it is easier to focus because 1) it is brighter, 2) the shallower depth of field allows you or the AF sensor to distinguish the in focus point more easily. A fast lens is useful when you are in a situation where you cannot use flash, eg shooting for theatre, dance etc.

For a zoom lens, constant aperture means that as you zoom from the short end to the long end, the maximum aperture remains the same throughout the zoom range, the lens you mentioned will have a maximum aperture of f/4.5 when zoomed to 70mm and only f/5.6 at 300mm. It only dictates the maximum aperture, so you can still close down the lens to other aperture and the camera will maintain that aperture throughout the zoom range. For example, if you set the aperture to f/8, when you shoot, it will remain as f/8 at whatever focal length you zoomed to. But if you have set your aperture to f/4.5 at 70mm, then zoomed to 300mm, it will not remain as f/4.5 but become f/5.6 because that is the maximum aperture of the lens at that focla length.

For a constant aperture lens, since at whatever focal length, since the maximum aperture remains the same, you will not have this problem. You can still use a smaller aperture for shooting.

The reason a lens with a bigger aperture (smaller f-number) is called a fast lens is because for the same film speed, you are able to use a faster shutter speed because the lens allows more light in. That is why it is called a fast lens.
 

Apr 12, 2005
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#5
rt7snap said:
I like take to take shots with lesser depth of field, (i think is big Apecture right?), sharp foregroud, blur background.

But im considering buying a canon lens with only f/4.5-5.6. (Canon EF 70-300mm f/4.5-5.6 DO IS USM)

I find that it can also shoot subject with blur background.

My qn:
What generally is the purpose of getting a lens with a big apecture, 1.4 , 2.8?

Constant
Btw, can help tell me what does it mean by 2.8 constant (does it mean, whatever the zoom ranges, it will be at 2.8, even in a bright sunlight setting?

Fast Lens
Oso, pls enlighten wat does it mean when people said this is a "fast lens"?

thank you folks!
Present 350D with kit lens
Mathematically,
Front DOF = d &#8226; F &#8226; a2 / (f2 + d &#8226; F &#8226; a)
Rear DOF = d &#8226; F &#8226; a2 / (f2 &#8211; d &#8226; F &#8226; a)​
f: focal length
F: F number
d: circle of confusion diameter
a: subject distance (distance from 1st principal point to subject)


So DOF depends not only on the aperture size but also focal length, the COC and the distances.


Source : http://www.photozone.de/3Technology/depth2.htm


Try calculating the DOF here by imputing different values for the above in the table below : http://www.photozone.de/3Technology/demos/depth.htm


So your F/4.5-5.6 may not be able to get the desired DOF in certain situations.


====


General purpose of getting a lens with big aperture : DOF and also enable the use of faster shutter speed and/or lower ISO, especially in dim light shooting situations such as concerts/indoor performances/fashion shows etc. In some situations, if you don't have a fast lens, you may need to up the ISO and this introduce more noise in the pictures. At the extreme low light conditions, with a slow lens, your shutter speed to get a proper exposure may be too slow to freeze movement.


-===

Yes, constant aperture means that the camera is capable of having a maximum F/2.8 aperture in all its focal length of the lens.

Keeping in mind that the F number represents the diameter of the aperture relative to the focal length, the actual aperture size at F/2.8 at 200mm is actually very much (4x) bigger than F/2.8 at 50mm. The theoretical aperture diameter are :

F/2.8 at 200mm = 71.4mm
F/2.8 at 50mm = 17.8mm

So for some zoom lenses, the maximum actual aperture size is not large enough to have F/2.8 when fully zoomed in but can still easily have F/2.8 at 50mm. So they have a larger F number at longer focal length.

So constant aperture has nothing to do with the light condition. It's just stating the maximum aperture size the lens is capable of relative to focal lengths.

====
Fast lens. As explained by others, it's one with a small F number (i.e. large aperture relative to focal length) and it simply means it allows you to use bigger aperture to shoot a picture than one with a bigger F number. With a larger aperture used, it enables you to use faster shutter speed and/or lower ISO to achieve the same exposure when compared to one with a smaller maximum aperture lens.

A fast lens is especially needed when the light condition is so dim that you're unable to get the picture you want with a slower lens which either requires your ISO to be too high for your liking and/or shutter speed to slow to freeze movement or prevent handshake blur just to get a proper exposure.

==

BTW, I only have a prosumer Nikon Coolpix 5700 to my name and don't own any SLR or DSLR cameras. :lovegrin:
 

Pro Image

Senior Member
Oct 31, 2003
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#6
Clockunder said:
Mathematically,
Front DOF = d • F • a2 / (f2 + d • F • a)
Rear DOF = d • F • a2 / (f2 – d • F • a)​
f: focal length
F: F number
d: circle of confusion diameter
a: subject distance (distance from 1st principal point to subject)


So DOF depends not only on the aperture size but also focal length, the COC and the distances.


Source : http://www.photozone.de/3Technology/depth2.htm


Try calculating the DOF here by imputing different values for the above in the table below : http://www.photozone.de/3Technology/demos/depth.htm


So your F/4.5-5.6 may not be able to get the desired DOF in certain situations.


====


General purpose of getting a lens with big aperture : DOF and also enable the use of faster shutter speed and/or lower ISO, especially in dim light shooting situations such as concerts/indoor performances/fashion shows etc. In some situations, if you don't have a fast lens, you may need to up the ISO and this introduce more noise in the pictures. At the extreme low light conditions, with a slow lens, your shutter speed to get a proper exposure may be too slow to freeze movement.


-===

Yes, constant aperture means that the camera is capable of having a maximum F/2.8 aperture in all its focal length of the lens.

Keeping in mind that the F number represents the diameter of the aperture relative to the focal length, the actual aperture size at F/2.8 at 200mm is actually very much (4x) bigger than F/2.8 at 50mm. The theoretical aperture diameter are :

F/2.8 at 200mm = 71.4mm
F/2.8 at 50mm = 17.8mm

So for some zoom lenses, the maximum actual aperture size is not large enough to have F/2.8 when fully zoomed in but can still easily have F/2.8 at 50mm. So they have a larger F number at longer focal length.

So constant aperture has nothing to do with the light condition. It's just stating the maximum aperture size the lens is capable of relative to focal lengths.

====
Fast lens. As explained by others, it's one with a small F number (i.e. large aperture relative to focal length) and it simply means it allows you to use bigger aperture to shoot a picture than one with a bigger F number. With a larger aperture used, it enables you to use faster shutter speed and/or lower ISO to achieve the same exposure when compared to one with a smaller maximum aperture lens.

A fast lens is especially needed when the light condition is so dim that you're unable to get the picture you want with a slower lens which either requires your ISO to be too high for your liking and/or shutter speed to slow to freeze movement or prevent handshake blur just to get a proper exposure.

==

BTW, I only have a prosumer Nikon Coolpix 5700 to my name and don't own any SLR or DSLR cameras. :lovegrin:
Wah piangz......so in-depth and cheem!

Very detailed....very......very detail indeed. Even when I was studying in Melbourne, my degree course did not cover so in depth. Good one!:thumbsup:
 

Apr 12, 2005
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#7
Pro Image said:
Wah piangz......so in-depth and cheem!

Very detailed....very......very detail indeed. Even when I was studying in Melbourne, my degree course did not cover so in depth. Good one!:thumbsup:
Wah liao. Don't suan me.

I'm still a newbie who've started owning a digicam only 9 months ago and everything I know is just approximation and may not be 100% correct.

But then chronologically, I'm very old already. Graduated from NUS biz school in the late 80s. :lovegrin:
 

student

Senior Member
Jul 26, 2004
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#8
Clockunder said:
But then chronologically, I'm very old already. Graduated from NUS biz school in the late 80s. :lovegrin:
If you are very old already, then it makes me very ancient already. Graduated from US (University of Singapore before it became NUS) in the 1970s.

Coming back to this thread, there is a reason for "fast" lens or lenses with wide aperture.

An aperture of 1.6 is really quite nothing.

If you use a SLR you will appreciate the brightness of the viewfinder, and faster focussing speed. Otherwise, apart from creating out of focus effects in the foreground and background, you will need fast lenses (1.4 or faster) and fast films when you chose to photograph without flash in places with little light.
 

dRebelXT

New Member
May 14, 2005
1,636
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#9
rt7snap said:
I like take to take shots with lesser depth of field, (i think is big Apecture right?), sharp foregroud, blur background.

But im considering buying a canon lens with only f/4.5-5.6. (Canon EF 70-300mm f/4.5-5.6 DO IS USM)

I find that it can also shoot subject with blur background.

My qn:
What generally is the purpose of getting a lens with a big apecture, 1.4 , 2.8?

Constant
Btw, can help tell me what does it mean by 2.8 constant (does it mean, whatever the zoom ranges, it will be at 2.8, even in a bright sunlight setting?

Fast Lens
Oso, pls enlighten wat does it mean when people said this is a "fast lens"?

thank you folks!
Present 350D with kit lens
Hi,
here is a 2nd hand lens that might interest you. :)
http://forums.clubsnap.org/showthread.php?t=167556

Inn fact, EF70-300 DO IS isn't as good as the newest EF70-300 IS.
:)
 

rt7snap

New Member
Dec 26, 2005
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#11
jOhO said:
sorry, i hate to be grammar/spelling nazi, but it should be APERTURE, not APECTURE... :confused:
Thanks mate, tat's really a good correction. or i will keep spelling it wrongly.
Appreciate that!
 

jOhO

Senior Member
Apr 20, 2003
6,485
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Singapore
www.expressivelyjoho.com
#12
rt7snap said:
Thanks mate, tat's really a good correction. or i will keep spelling it wrongly.
Appreciate that!
u're welcomed! but it's spelt that way in (almost) every other thread here ahah.. i'm sure you would have noticed. :)
 

Mar 22, 2005
434
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16
#13
Just leaving a short note here:
To avoid exposure cunfusion, the aperture can be stopped down in bright light. "Constant" doesnt mean it is 2.8 FIXED and you cannot stop down. It just means that 2.8 is the widest aperture available at all focal lengths.

*****

Getting a "fast" lens have many advs. Here are 2 common advs:
1. Decent shutter speeds in dim conditions
2. Shallower depth of field (if it is what you want)
 

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