f2.8 lenses versus standard lenses (f3.5-4.5 etc)

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tezza73uk

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Terry
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I have recently picked up a Sigma 18-50 f2.8 for added clarity and speed when shooting weddings or in general hobbyist photography.

Have I assumed correctly that if I choose an aperture of f8 in Aperture Priority mode this lens will allow more light to the sensor than my Nikon 18-70 f3.5-4.5?

Thanks in advance,
Terry. :)
 
You are right that the lens is capable of letting in more light than your f3.5, but not if it's set at f8. f8 will be the same on both your lenses.

If you use aperture priority and set the aperture to f2.8, you will be able to get much faster shutter speeds with your new lens.

Also....

You will get a shallower depth of field too, which can, in some cases be an advantage over your old lens. But not if that'/s not the effect you're looking for.
 
^^^ Like badger said.

F/numbers are a ratio of aperture diameter and focal length, so f/8 is f/8 is f/8, regardless of lens, or camera, or format or whatever.
 
Thanks Badger, Hoppy.
 
Not sure if its what you are asking... but f stop numbers mean lower numbers for wider aperture..

Also, the real advantage of wide constant aperture glass is in focusing because the camera opens up the aperture all the way for focusing to take place. More light in means a better view for the AF sensors so more accurate results.
 
tezza73uk said:
I have recently picked up a Sigma 18-50 f2.8 for added clarity and speed when shooting weddings or in general hobbyist photography.

Have I assumed correctly that if I choose an aperture of f8 in Aperture Priority mode this lens will allow more light to the sensor than my Nikon 18-70 f3.5-4.5?

Thanks in advance,
Terry. :)
Click to expand...

Hi and welcome.

Others have already explained apertures above so I won't, but please tell me you're not shooting weddings for money if you haven't grasped apertures yet!
 
tezza73uk said:
I have recently picked up a Sigma 18-50 f2.8 for added clarity and speed when shooting weddings or in general hobbyist photography.Have I assumed correctly that if I choose an aperture of f8 in Aperture Priority mode this lens will allow more light to the sensor than my Nikon 18-70 f3.5-4.5?
Thanks in advance,
Terry. :)
Click to expand...

Just to pitch in because it looks like this is new to you.
the f2.8 number means this is the maximum aperture throughout the focal range of 18-50. There are advantages to this. But you don't need to always shoot at f2.8 just because you can. Image quality may be greated at f5.6 or f8. you can test this yourself very easily.

The other lens you have is f3.5 to f4.5 This means that at 18mm the max aperture is 3.5 and at 70mm zoomed in, the max aperture is 4.5
this is because as you zoom closer, the cheaper lens with the variable aperture essentially becomes a longer tube down which light is lost. Imagine looking down the tube of a toilet roll and then a kitchen roll. The image you see will be tighter but less light will get to your eye. Weird example but there you go.
The minimum aperture, helpful for long exposures and landscapes maybe, is normally about f22, but also can vary from lens to lens. People don't normally care as much about the maximum F- number

If you're taking shots at a wedding, for a friend I presume, then indoors the lower F stop would be handy but a flash is better for nice quality photos. also if you are outdoors and go for f2.8 (just because you can)...then the image may be softer and the whole group might not be in focus. Especially if you have two people just behind one another.
 
Badger UK said:
You are right that the lens is capable of letting in more light than your f3.5, but not if it's set at f8. f8 will be the same on both your lenses.
Click to expand...

As above, also worth noting, the 2.8 will probably give you a better shot at the 3.5 / 4.5 range of the other lens as this (2.8 lens) will not be at it's maximum F stop as this (F3.5 / 4.5) lens would be (I think thats clear :D)
 
Thanks again everyone. I have an understanding for aperture, DoF etc and have used it in macros, baby shots etc to isolate elements in the image.

What I was unsure of was how an f/2.8 could be 'better' than an f/3.5. I had a misconception that it was 'better' but scientifically thought it couldn't be!

Thanks for confirming that and for suggesting other benefits of an f/2.8.

Think I'm gonna enjoy my time on TalkPhotography :clap:
 
The advantages of a fixed f/2.8 max aperture on a zoom lens that spring to mind are....

- Manual exposure is far easier and more rewarding when zooming the lens does not cause the exposure to change;
- More control over shallow DOF;
- Faster shutter speeds and/or lower ISO settings are possible - a big advantage for weddings!;
- AF is faster and more accurate and on some cameras you get cross point focusing on all focus points rather than just the centre point;
- Viewfinder is brighter;
- Manual focus is a little easier;
- Stopped down to f/3.5 or f/4, an f/2.8 lens will probably be sharper than a cheaper, slower lens with variable max aperture, when set to the same f/stop.
 
A good thread this as I'm toying with the idea of an f/2.8 zoom lens. I think its unlikely I'd be using the lens @ f/2.8 most of the time but I want to confirm there is general principle is that a faster lens also makes a sharper lens in general (comparing at same aperture settings), or it makes it easier to produce a sharper image as you can use a faster shutter speed and therefore reduce blur from camera shake?
 
kman said:
A good thread this as I'm toying with the idea of an f/2.8 zoom lens. I think its unlikely I'd be using the lens @ f/2.8 most of the time but I want to confirm there is general principle is that a faster lens also makes a sharper lens in general (comparing at same aperture settings), or it makes it easier to produce a sharper image as you can use a faster shutter speed and therefore reduce blur from camera shake?
Click to expand...

sort of, but not quite there yet. Your shutter speed will be the same,for a given appeture, if you use a 2.8 zoom or a variable appeture one so there is no affect there. One of the advantages though is because appeture doesn't change as you zoom so your shutteer speed won't slow as you zoom more.

they tend to be sharper at because by the time you've got to f4 you've already moved away from the max appeture and into the lens' sweet spot.

Cheers

Hugh
 
That's it in a nutshell.

An f3.5 at 50mm might give you 1/30 sec in a church.
An f2.8 at 50mm might give you 1/50 sec.
And an f1.8 might get you up to 1/60 or 1/80 sec.

(real world shooting here rather than mathematical theory here guys so bear with me)

There is a proportainal relationship between the aperture (size of hole) and shutter speed. The smaller the number, the bigger the hole, the more light gets in and the faster the shutter speed gets. (but you lose depth of field which is a whole separate subject)

So if you can grab a 50mm f1.8 then in the real world you can go up to 1/60 -1/80 sec which is just about enough to freeze a swaying bride. :) At f3.5 you are stuffed if the Vicar/registrar says no flash.

All assuming you have got the camera working at it's practical limits of ISO too of course.
 
kman said:
A good thread this as I'm toying with the idea of an f/2.8 zoom lens. I think its unlikely I'd be using the lens @ f/2.8 most of the time but I want to confirm there is general principle is that a faster lens also makes a sharper lens in general (comparing at same aperture settings), or it makes it easier to produce a sharper image as you can use a faster shutter speed and therefore reduce blur from camera shake?
Click to expand...

There is no general principle that says faster lenses are sharper, and you could actually argue the opposite. However, what happens in practice is that f/2.8 zooms are always premium designs. They are much more complex, better corrected, and usually a bit sharper. And of course they go to f/2.8, and that is the bit you are really paying for.

But there is a big cost for that - money, size, weight. I have two lenses with a very similar range - Canon kit lens, 18-55mm f/3.5-5.6 IS, and EF-S 17-55 f/2.8 IS. The kit lens costs about £120 I think now, and the f/2.8 is over £700. If I take identical pictures with both lenses, at say 30mm f/5.6 or f/8, you will absolutely not be able to tell them apart for sharpness. The f/2.8 lens is better at wider apertures, fractionally, and has a little less distortion (easily corrected in post processing) but that's it.

Just to clarify the f/number with variable aperture zooms, it only varies at the maximum aperture (lowest f/number) so once you're past that, as you would be in normal photography with f/5.6 or higher, then the f/number remains constant as you zoom as in terms of exposure.
 
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