[robbo145]

Hi,

The scenario is:-
If I have two lenses, one has a max aperture of F2.8 and one has max aperture of F5.6. If both lenses are stopped down to F8 for instance will the amount of light metered be the same for the two lenses?

In other words will the "quicker" lens remain quicker when stopped down to the same value when compared to the "slower" lens?

I'm trying to justify to myself buying a faster lens but may not want to shoot often below the F5.6 range.

Cheers,
Ian

[allymac123]

A 'quicker' lens is only an advantage when used at apertures larger than possible with other lenses.

So only in the range of F2.8 -> F5.6 would you see an advantage.


F8 will always be the same. It is a universal constant.


On a slightly more complicated level, f2.8 lenses are almost always better quality in terms or sharpness, CA etc at all apertures.

[tontoshorse]

Originally Posted by robbo145

Hi,

If both lenses are stopped down to F8 for instance will the amount of light metered be the same for the two lenses?


Cheers,
Ian

yes, but aren't you going to consider depth of field either?

Edit: must type quickerrrrrrrrrrrrrrrrrrrr

[chrisw321]

At f/8, the same amount of light will be let in to both lenses, so you will have exactly the same shutter speed with both. HOWEVER, there will be some differences:

Generally speaking (as long as you don't go smaller than f/16 etc), stopping down will give a sharper image. So when they're both at f/5.6, the f/2.8 lens will be sharper, as it is 'stopped down', whereas the slower lens is 'wide open', running at it's limit. By f/8, the difference between the two lenses will probably be minimal.

Also, the lens remains with the aperture blades wide open, then close to the right size just before the shot is taken. So the f/2.8 lens will allow much more light to hit the sensor and focussing screen during focussing, so it should be able to focus more quickly and more accurately.

Hope this helps a little!

Chris

[robbo145]

Thanks all for the info

Got my first DSLR a few months back and have been experimenting at motorsports events. I have to say I find the shots taken at higher f-stops (larger DOF) better suited to this subject but this could all change with a 'quicker lens'

[RobDickinson]

At f8 the aperture is the same size with respect to the focal length.

This does NOT mean the same amount of light will pass through the lens and hit the sensor.

That depends entirely on the quality of the optics and manufacturing of the elements, and number. Though I wouldnt worry about this too much.

A good prime can be 1/3rd stop lighter than a zoom at the same focal length/aperture.

The fast lens will also AF quicker and look brighter due to being wide open apart from when actualy taking the shot.

And typicaly faster lens's are better made so will ultimatly resolve more detail/colour/contrast etc at any aperture that isnt diffraction limited.

[dazza74]

Is there a strong argument then that optically it's pretty pointless spending a fortune on say a 2.8 lens if you only plan to do landscape photography from F8 or smaller?

Not having owned an expensive own brand 2.8 lens in my life it's not a question I can answer myself so would be interested in others opinions

[lmccauley]

Originally Posted by dazza74

Is there a strong argument then that optically it's pretty pointless spending a fortune on say a 2.8 lens if you only plan to do landscape photography from F8 or smaller?

Yes, if that's the only difference between two lenses you are considering. Of course, it rarely is. Low distortion, CA and flare will be more important considerations.

[Spod]

You don't say what camera you're on but with some Canon bodies if you have an f/2.8 or faster lens the central focus point becomes more sensitive.

[RobDickinson]

Originally Posted by lmccauley

Yes, if that's the only difference between two lenses you are considering. Of course, it rarely is. Low distortion, CA and flare will be more important considerations.

Yeah as said a fast lens is usualy all round better.

And , prehaps rarely, f2.8 for landscapes is a realy good tool.

[lmccauley]

Originally Posted by RobDickinson

Yeah as said a fast lens is usualy all round better.

And , prehaps rarely, f2.8 for landscapes is a realy good tool.

Absolutely. I took this shot around sunset at f/2.5, 1/4 sec.

[dazza74]

Really works that imccauley, I should experiment more with wider apertures