Question about fast lenses

[ziggy©]

The Canon 70-200mm IS f2.8 lens costs about £900 whereas the F4 non-is version costs around £400.

I know that the f2.8 lens is more expensive because it works better on low light situations and of course the IS. I know that f2.8 would be very usefull in low light but is it not the case that it will only be usefull if you want low DOF because f2.8 wont give you a lot of DOF.

The same goes for the f1.4 lenses. A lot of people recommend the 17-55mm f2.8 over the 17-40mm f/4 because of its low light performance at f2.8. I dont understand how they could be usefull unless you want to shoot macros with low DOF at low light. Or am i misunderstanding something?

 

[JohnD76]

The more expensive lenses tend to have a far better build quiality and use more expensive coatings and elements, which tends to allow for sharper images.

The best bet it to go into a shop and try both out and if you are happy with the F4 then go for that and keep the difference. If you can hire both lenses for a weekend this may be a better appoach since this will allow you to have a good play and decide which works for you.

 

[andrewc]

In a given moment the lens at f/2.8 passes twice the light as the other at f/4. During focusing the lens is wide open and this makes focussing more accurate in low light conditions. The lens only stops down when the shot is fired.

So basically, if you are working in low light conditions, a f/2.8 or better is the desirable option.

 

[MatBin]

Correct exposure is a combination of aperture and shutter speed for a given Iso rating. To get a correct exposure you balance one against the other, sometimes you need an aperture less (bigger hole, smaller number) than what your lens is capable of if you want a certain shutter speed e.g. 1.8 and your lens is an F4, at this point you have two options, slower shutter speed or higher Iso. A slower shutter speed may not be appropriate and a higher Iso might result in a 'noisy' image. So, a faster lens allows the higfher shutter speed. lower Iso combination to work and give a correct exposure.
Additionally an f1.8 lens should be sharper at say f4 than an equivalent focal length f4 lens.

Matt

 

[eccles]

A larger aperture gives you a reduced depth of field. This means that whatever your shooting aperture, because you autofocus at full aperture your autofocus will be more accurate, and in most cases faster as well.

 

[AliB]

The f2.8 is useful in low light because it allows you to keep a decent shutter speed especially if you are shooting a moving subject like people. You need to keep the shutter speed to about 1/60 sec or higher to avoid subject movement which will blur the image.

The only way to do that is to shoot with fast glass and/or high ISO. It is much preferable to keep the ISO as low as possible to maintain image quality.

To get the DOF right you need to have some understanding of the range in focus at specific distances. i.e. my 50mm f1.4 lens will give me very shallow DOF up close but if I am shooting people then I know that at 10ft I have a 1ft DOF so 50cm in front and 50cm behind. With the 70-200mm f2.8 at the same distance at 100mm I only have half of that so 50cm in total, 25cm in front and the same behind. But if I double the distance to 20 ft I have 2 ft DOF

It's useful to know where your DOF lies with your lenses and it's as much about the look and feel of it but knowing that distance is a factor is important.

 

[stuart anthony]

depends on what you shoot really,
the f2.8 will be better in low light conditions but only if you set it at 2.8. if you shoot landscapes like myself then f2.8 would rarely be used. but if you shoot portraits then F2.8 would give a more blurred background.
if your budget is aprox £900 then why not purchase the 70-200mm F4 IS L.
the 2.8 is also very heavy unlike the F4

Regards
Stuart

 

[Musicman]

The Canon 70-200mm IS f2.8 lens costs about £900 whereas the F4 non-is version costs around £400.

Can you let me know where I can get a new 70-200 f/2.8L IS for £900? The cheapest I can find in the UK is £1480.

I have one already, but I'd be interested to know what other bargains they might have

The 70-200 f/2.8L non-IS can be had for a whisker under £1000, which is probably a better comparison with the f/4L non-IS (£498 is the cheapest UK price on camerapricebuster.com)

Joking apart...

Aside from the low-light capability wide open and the DoF characteristics of a fast lens discussed above, you'll usually find that the performance (sharpness mostly, especially toward the edges of the frame, but also flare and overall image contrast) of a lens starts to improve markedly around a stop or two down from its maximum aperture.

So, an f/2.8 lens at f/4 will normally perform significantly better than an f/4 lens when its at the widest it can get.

the-digital-picture.com has a very useful lens comparison feature that allows you to see how different lenses perform across their aperture and focal length ranges.

Here's the 70-200 f/4L at f/4.0 vs the 70-200 f/2.8L at f/4.0, both at 135mm

http://www.the-digital-picture.com/...meraComp=453&SampleComp=0&FLIComp=0&APIComp=2

You can instantly see the difference

That said, in the specific case of the Canon 70-200 f/4L IS compared with the Canon 70-200 f/2.8L IS (Mk1) the f/4L is an outstanding performer wide open, especially in the centre of the frame, and it's a much less clear cut advantage. The f/4L IS is a more modern design that's taken great advantage in design developments over the best part of a decade that separated them.

http://www.the-digital-picture.com/...meraComp=453&SampleComp=0&FLIComp=3&APIComp=2

compare the f/4 IS with the new Mk 2 f/2.8L IS and a lot of ground is gained back by the f/2.8

http://www.the-digital-picture.com/...meraComp=453&SampleComp=0&FLIComp=2&APIComp=2

Some other considerations - my old Canon 50mm f/1.8 suffered quite noticeable halation in bright light at f/1.8. With my 50mm f/1.4, I can shoot with much greater confidence that I'll get a decent image at the same aperture setting. The fact that it also has an 8-blade aperture diaphragm also helps with background blur compared with the 5-blades on the f/1.8 version when shooting at anything other than wide open - highlights tend to acquire a definite pentagonal shape at f/2.0 on the f/1.8 lens.

 

[Scarecrow]

Can you let me know where I can get a new 70-200 f/2.8L IS for £900? The cheapest I can find in the UK is £1480.

Actually the cheapest price at the moment in the UK for a 2.8 IS is £1199, or it was just over a week ago when I bought mine.

 

[stuart anthony]

Actually the cheapest price at the moment in the UK for a 2.8 IS is £1199, or it was just over a week ago when I bought mine.

could you post were this is please, making a purchase.
thank-you

 

[EOS_JD]

Speak to Kerso (send him a PM here) - He'll beat that price I'm sure

 

[grum]

The f2.8 is useful in low light because it allows you to keep a decent shutter speed especially if you are shooting a moving subject like people. You need to keep the shutter speed to about 1/60 sec or higher to avoid subject movement which will blur the image.

The only way to do that is to shoot with fast glass and/or high ISO. It is much preferable to keep the ISO as low as possible to maintain image quality.

To get the DOF right you need to have some understanding of the range in focus at specific distances. i.e. my 50mm f1.4 lens will give me very shallow DOF up close but if I am shooting people then I know that at 10ft I have a 1ft DOF so 50cm in front and 50cm behind. With the 70-200mm f2.8 at the same distance at 100mm I only have half of that so 50cm in total, 25cm in front and the same behind. But if I double the distance to 20 ft I have 2 ft DOF

It's useful to know where your DOF lies with your lenses and it's as much about the look and feel of it but knowing that distance is a factor is important.

Thanks for this - what are the calculations involved in working out DOF at given distances/apertures?

 

[Arkady]

...I know that the f2.8 lens is more expensive because it works better on low light situations and of course the IS. I know that f2.8 would be very usefull in low light but is it not the case that it will only be usefull if you want low DOF because f2.8 wont give you a lot of DOF...

Just because it's an f/2.8 lens doesn't make it useless for everything else - you still get all the other apertures as well...

 

[AliB]

Google Depth of field calculator and look out for depth of field master.

It's quite a complex relationship but well work looking up your lenses at their most commonly used distances and gaining some understanding of where the focus is within the DOF.

 

[Scarecrow]

Speak to Kerso (send him a PM here) - He'll beat that price I'm sure

That's where I got mine, so I reckon the price I paid is about as good as it's going to get.

 

[ziggy©]

Ok let me try an example. Lets say i want to take a picture of the San Francisco bridge. The f2.8 lens will allow me to shoot the bridge using a shutter speed that is fast enough for me to hold the camera but how can i ensure that the whole length of the bridge is in focus? Surely f2.8 will not keep the whole bridge in focus?

 

[pete.rush]

Ok let me try an example. Lets say i want to take a picture of the San Francisco bridge. The f2.8 lens will allow me to shoot the bridge using a shutter speed that is fast enough for me to hold the camera but how can i ensure that the whole length of the bridge is in focus? Surely f2.8 will not keep the whole bridge in focus?

You don't want to use f2.8 to capture the bridge, as at that aperture the depth of field will be very small, i.e. the part of the image which is acceptably sharp. Have a read of the links, I've found this site very useful

DOF
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
Aperture
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
Lens
http://www.cambridgeincolour.com/tutorials/lens-quality-mtf-resolution.htm

 

[Scarecrow]

Ok let me try an example. Lets say i want to take a picture of the San Francisco bridge. The f2.8 lens will allow me to shoot the bridge using a shutter speed that is fast enough for me to hold the camera but how can i ensure that the whole length of the bridge is in focus? Surely f2.8 will not keep the whole bridge in focus?

How far away you are from the bridge will factor in as well.

 

[Joe T]

You don't want to use f2.8 to capture the bridge, as at that aperture the depth of field will be very small, i.e. the part of the image which is acceptably sharp. Have a read of the links, I've found this site very useful

DOF
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
Aperture
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
Lens
http://www.cambridgeincolour.com/tutorials/lens-quality-mtf-resolution.htm

I think ziggy knows that if you read his post again.

ziggy, faster lenses need to be of better quality so that they maintain that quality even when wide open at f2.8 (or whatever). This means you get even better quality stopped down to f8, as well as the ability to use f2.8 when you need the light gathering capacity.

Hope this helps.

 

[EOS_JD]

How far away you are from the bridge will factor in as well.

Yes a very important factor is the magnification - distance to subject and focal length.

It will be a bigger factor than the aperture in that case.

Lets say you're 75m from that bridge with your 24-70 f2.8 lens. And it's getting dark and you need to shoot at high ISO 1/40th sec to save from camera shake. At 35mm 75m from the bridge, even at f2.8 everything from 57m to infinity will be in focus......

So shooting a bridge at f2.8 isn't so bad..... Although most sensible people would set the ISO low and use a tripod.

 

[HoppyUK]

Yes a very important factor is the magnification - distance to subject and focal length.

It will be a bigger factor than the aperture in that case.

Lets say you're 75m from that bridge with your 24-70 f2.8 lens. And it's getting dark and you need to shoot at high ISO 1/40th sec to save from camera shake. At 35mm 75m from the bridge, even at f2.8 everything from 57m to infinity will be in focus......

So shooting a bridge at f2.8 isn't so bad..... Although most sensible people would set the ISO low and use a tripod.

Not sure about those numbers, but assuming you are shooting with a crop format camera, eg Canon, then if you focus a 24mm lens at 10.7m (hyperfocal distance) then even at f/2.8 everything from 5.4m to infinity with be sharp.

I see the OP's dilemma, and I agree that shallow DoF is sometimes an undesirable side effect of using a low f/number. In which case, don't use it - raise the ISO, reduce the shutter speed with the aid of IS.

But very often you do want shallow DoF. It's an incredibly useful creative technique that, realistically, you can only get with a DSLR and a low f/number lens.

 

[EOS_JD]

Not sure about those numbers, but assuming you are shooting with a crop format camera, eg Canon, then if you focus a 24mm lens at 10.7m (hyperfocal distance) then even at f/2.8 everything from 5.4m to infinity with be sharp.

I see the OP's dilemma, and I agree that shallow DoF is sometimes an undesirable side effect of using a low f/number. In which case, don't use it - raise the ISO, reduce the shutter speed with the aid of IS.

But very often you do want shallow DoF. It's an incredibly useful creative technique that, realistically, you can only get with a DSLR and a low f/number lens.

Whilst I agree with most, the last part depends on what you're shooting. As shown above even at f2.8 unless you are pretty close (high magnificatoin of subject on sensor) f2.8 won't make a massive difference.


Even at f8, if I get close to my subject withthe background a short distance away, I can get nicely blurred backgrounds.

My point is that magnification matters more than fstop. That said, fast lenses provide better quality at most apertures and the ability to shoot at f2.8 rather than be stick at slower speeds really makes all the difference.

Fast lenses = better quality images.

 

[HoppyUK]

Whilst I agree with most, the last part depends on what you're shooting. As shown above even at f2.8 unless you are pretty close (high magnificatoin of subject on sensor) f2.8 won't make a massive difference.


Even at f8, if I get close to my subject withthe background a short distance away, I can get nicely blurred backgrounds.

My point is that magnification matters more than fstop. That said, fast lenses provide better quality at most apertures and the ability to shoot at f2.8 rather than be stick at slower speeds really makes all the difference.

Fast lenses = better quality images.

Not really true. While it's true that fast lenses tend to be of higher quality, but that's not because they are fast. It's because they are, generally, made to a higher overall standard (and priced accordingly) even if in practise there's not much in it.

There are plenty of examples where slower lenses are better than fast versions - as they should be because they are easier to design. One area in particular where slower lenses tend to perform better is in flare control. Here's a particularly dramatic example where the Canon 17-40L f/4 beats the much more expensive 16-35L f/2.8. Scroll down a liitle to the sunset pics and roll the mouse-over to compare http://www.kenrockwell.com/canon/lenses/17-40mm-performance.htm#flare

 

[EOS_JD]

Maybe it should have read fast lenses make better images (there's some images yuo just can't take with slow lenses).