what's the use of F2.8 constant zoom lens?


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#1
Hi,

i have been comtemptating for a while now of getting a constant F2.8 zoom lens eg Tokina 28-70 ProSV that's on offer... but could never find a compiling reason to do so...

i know the constant F value is fast and is bright... and good in taking low light situation... can someone quote a couple of example?

if i up the value of the lens on my camera to say F8... and compare this to a normal 28-70 at F8(non constant lens), what is the difference?

I also noticed that while the F2.8 is constant, using Shuttle priority, the value of time brinks on i up the value for nightshots.... does it mean the shot will be grossly underexposed.. so what's the use of a fast lens in thiis situation and how do i compensate for it to get a correct exposure...

and if i still take the pic when the value brinks, the exposure will be wrong, right?

Thank you in advance for teaching...
 

azone

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#2
In low-lighting environment, for the same shutter speed (e.g., 1/60), using f2.8 rather than f4 or above will allow more ambient light to be exposed in the pic and thus your background will not be so dark.

If you're gonna shoot at f8 all the time, then of coz there is no need to get a constant f2.8 lens.

In Shutter priority, with the f value blinking, you can use external flash to compensate.

But of coz, the lens must be able to produce reasonable quality at f2.8 to start with. No point getting a lens with constant f2.8 if the pic is extremely soft and not usable when taken at f2.8.
 

Zerstorer

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#3
Pop the lens on your camera, look through it and you will see one of the important differences. Fast lenses result in a much brighter viewfinder image, you will find taking shots and composing a more enjoyable experience. Moreover, your AF will work better.
 

#4
For me, in low light environment, my shutter speed will be very slow because I'm using ISO 100/200 slides. So if I use flash, the aperture I used will be 5.6. If I don't intend to use flash, to counter artificial lighting requires the use of filters, which will slow down the shutter speed even more (tripod). I don't really see much use, except in outdoor environment in the evenings, when it's unsuitable to use flash.
 

Prismatic

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#5
You usually have no idea how low light at the location where you are going to be. Having a fast lens is a safeguard as an alternative to pushing film. Since I'm alway shooting performances and concerts, F2.8 or faster lens have become an essential for me. The lighting conditions I I face are usually of the 1/125 at ISO 800 type, especially when flash is not allowed, I have to use settings like 1/60 at ISO 800 with f2.8 just to get a decent exposure.
 

jasonpgc

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#6
Most people who opt for fast constant aperture lens are people who shoot handheld. Meaning, they don't have time to set up tripod because the subject is moving. As such, they usually need a zoom lens to speed up composition and also a fast shutter to capture the action. For Zoom lens, the fastest aperture is usually f2.8. It is not surprising that most streetshooter who owns a f2.8 Zoom lens shoots handheld at f2.8 90% of the time, because they NEED the speed. I believe they'll shoot at f2 too if such a zoom is available.

Now, some of these people actually use Manual Exposure Mode. If they are not using a constant aperture zoom, the Exposure will be different when they Zoom with the lens.

For example,

If the person is shooting with a 28-105 f4.5-5.6 at 28mm f4.5, if he zoom to 105mm, the aperture will AUTOMATICALLY reduce to f5.6 . If he Snaps the shutter unknowingly, the picture taken at 105mm will be 1/2 stop under his expected exposure.

or

if a person is shooting with Aperture Priority, if he get 1/125s F4.5 at 28mm, If he Zooms to 105mm, the new shutter speed will AUTOMATICALLY change to 1/90s. For streetshooters, this may be the difference between a Blur and a sharp picture.

Now, if you're considering to get a constant aperture F2.8 Zoom lens. Always get one that performs at F2.8. There is NO POINT carrying all that extra weight if the lens SUCKS at F2.8. You bought the lens because you NEED the SPEED, because you NEED to shoot at F2.8.

If you really NEED such a lens, get the Real thing, save up for the L

If you are a Landscape shooter, get prime lens instead :bsmilie:
 

showtime

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#7
Michael Ang said:
Hi,


if i up the value of the lens on my camera to say F8... and compare this to a normal 28-70 at F8(non constant lens), what is the difference?
there is generally no difference in dof and brightness. but depending on the no of blades the apperture has, the bokeh would be slightly different.
 

TME

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#8
But doesn't using a constant f/2.8 lens mean better optics since there are less elements? I thought the quality of the image would be better since it is a constant aperture.....
 

TME

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#9
showtime said:
there is generally no difference in dof and brightness. but depending on the no of blades the apperture has, the bokeh would be slightly different.

How does the number of blades in the aperture affect the bokeh? The bokeh is generated by the diffusion of light about the blades when the aperture is stopped down right? So does different number of blades affect that? What is the usual number of blades? More means better or worse? I think most of my lens have about 5 blades?
 

Prismatic

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#10
TME said:
How does the number of blades in the aperture affect the bokeh? The bokeh is generated by the diffusion of light about the blades when the aperture is stopped down right? So does different number of blades affect that? What is the usual number of blades? More means better or worse? I think most of my lens have about 5 blades?
The number of blades in the lens affects the quality of the bokeh, typically, the more the better. Which is why most portrait lenses has 8 blades or more. The determinating factor is actually the shape of the aperture rather then the number of blades. When you have more blades, the shape of the aperture gets closer to being a circle which in theory is the ideal aperture shape.

When you have, let say, 5 blades, a point source in the bokeh will be blurred as a pentagon, similarly, a hexagon for a 6 blade lens. To some this may look a bit unnatural as every point in the distance is diffused as such. Though it is unlikely you will actually it clearly as such shapes.

There are some lenses in the Minolta range which has round apertures, but I have never seen the bokeh from these lense, prehaps Minolta users will like to comment.
 

ST1100

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#11
TME said:
How does the number of blades in the aperture affect the bokeh? The bokeh is generated by the diffusion of light about the blades when the aperture is stopped down right? So does different number of blades affect that? What is the usual number of blades? More means better or worse? I think most of my lens have about 5 blades?
Generally the more blades, the smoother the highlight blur. A 5 blade aperture will produce pentagons, while an 8 bladed one produces octagons. There's some personal preference involved, but generally, most folks prefer smoother (more circular) blurs. There are also seven bladed ones around; i actually prefer these bcoz i think the octagons look too 'regular'.

Another good reason to get a good 2.8 (ie usable at wide open): at wide open, the bokeh is circular, bcoz the blades don't come into play at all. (i find the bokeh of my 70-200/2.8L much much better at f2.8 than f4.0.)

There's another twist to consider in the bokeh thing: the blur in front of the focus plane is usually upside down from the blur behind it. This affects lenses with odd number of blades. Most drastic with 3 bladed diaphragms - you get right side up and upside down triangles in the same picture.
 

Zerstorer

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#12
One thing though, the aperture usually affects bokeh only when stopped down.

When wide open, most lenses have almost perfectly round openings and the aperture blades are not a contributing factor at this setting.
 

#13
jasonpgc said:
Most people who opt for fast constant aperture lens are people who shoot handheld. Meaning, they don't have time to set up tripod because the subject is moving. As such, they usually need a zoom lens to speed up composition and also a fast shutter to capture the action. For Zoom lens, the fastest aperture is usually f2.8. It is not surprising that most streetshooter who owns a f2.8 Zoom lens shoots handheld at f2.8 90% of the time, because they NEED the speed. I believe they'll shoot at f2 too if such a zoom is available.

Now, some of these people actually use Manual Exposure Mode. If they are not using a constant aperture zoom, the Exposure will be different when they Zoom with the lens.

For example,

If the person is shooting with a 28-105 f4.5-5.6 at 28mm f4.5, if he zoom to 105mm, the aperture will AUTOMATICALLY reduce to f5.6 . If he Snaps the shutter unknowingly, the picture taken at 105mm will be 1/2 stop under his expected exposure.

or

if a person is shooting with Aperture Priority, if he get 1/125s F4.5 at 28mm, If he Zooms to 105mm, the new shutter speed will AUTOMATICALLY change to 1/90s. For streetshooters, this may be the difference between a Blur and a sharp picture.

Now, if you're considering to get a constant aperture F2.8 Zoom lens. Always get one that performs at F2.8. There is NO POINT carrying all that extra weight if the lens SUCKS at F2.8. You bought the lens because you NEED the SPEED, because you NEED to shoot at F2.8.

If you really NEED such a lens, get the Real thing, save up for the L

If you are a Landscape shooter, get prime lens instead :bsmilie:

hi,

:bsmilie: :bsmilie: :cry: :cry: :cry:

can't afford a L lens..
 

#14
hi everybody,

just for your info... i finally took the leap of faith and got the Tokina 28-70 Prosv one on offer at CP....

well, i took a couple of shots on my cheap cheap EOS300... and the result is not too bad.. but it is not a L lens so dun expect L quality....

one thing though, it's heavy... had to buy a battery grip just to hold it steady... add to my cost..

focusing using this lens is much much faster than i expected... compared to my other cheapo lens anyway...

sorry i dun have a scanner can't show on the net...

still waiting for my 300D this Monday... hahaha

people will frown upon this lens because it's suppose to be just an average len...
others wiil say cheapo lens cheapo results..
but i say it's may be cheap... but it serves it purpose... while being on the cheap ...



anyway, thank you everybody for your valuable comments.. greatly appreciated...
 

jasonpgc

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#17
Michael Ang said:
it's on offer at CP for $475, comes with a standard package lens + soft case + hood and free gifts : Lens pen, Tokina UV 77mm filter, and a rather useless Optech Handphone case.. hahah
Its a very good price, congrats.

BTW, How much you are paying for your 300D?
 

showtime

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#18
TME said:
How does the number of blades in the aperture affect the bokeh? The bokeh is generated by the diffusion of light about the blades when the aperture is stopped down right? So does different number of blades affect that? What is the usual number of blades? More means better or worse? I think most of my lens have about 5 blades?
i dunno what is normal. but the number ranges from like you say, 5 to 9 (thats what i have seen)they might be more.
the bokeh is affected because the appeture is not a actual circle. the smaller the number of blades, the less smooth the sides of the circle. therefore the circles of confusion created when using different bladed appertures will look slightly different. more blades means smoother circles.
this is put very simply.
 

showtime

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#19
Zerstorer said:
One thing though, the aperture usually affects bokeh only when stopped down.

When wide open, most lenses have almost perfectly round openings and the aperture blades are not a contributing factor at this setting.
thanks for this insight.
 

#20
jasonpgc said:
Its a very good price, congrats.

BTW, How much you are paying for your 300D?

hi,

the 300D is retailing at 1999... but the launch is monday.. and i have told i will get it by tuesday... hopefully ard 50-100 lesser than retail price...

not sure yet
 

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