A Question on Aperture

htid

htid

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Chris
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I have the 300mm f4 and of course the 2.8 is considered better, but I don't understand why. I obviously know that 2.8 will allow more light in allowing for a faster shutter speed, but at the distances you use 300mm + lenses for (such as birds etc.) 2.8 is just too narrow a DoF to be useful, isn't it?

The only thing I can think is because (as far as I know) lenses are at their optimum when stopped down slightly, so my f4 performs best at 5.6 or 8 whereas the 2.8 performs best at 4 or 5.6. But 5.6 is still pretty low at 300, 400mm isn't it?
 
DoF increases with distance from lens, so the difference between f/2.8 and f/4 at 50m won't be so great as the difference at 5m.

And of course the 2.8 gives a marginally brighter viewfinder.
 
Don't forget that depth of field is not only relative to the aperture, but also the distance to the subject you are focused on. If I was shooting a bird at a distance of say 30m with 2.8 aperture there will be a whole lot more in focus than say shooting a portrait at 6ft with a 2.8 aperture.

For wildlife you need fast shutter speeds so the 2.8 is going to give you 2 more stops than the f4 version and trust me when you are pushing your kit to get the shutter speeds 2 full stops is a lot of light!
 
htid said:
I have the 300mm f4 and of course the 2.8 is considered better, but I don't understand why. I obviously know that 2.8 will allow more light in allowing for a faster shutter speed, but at the distances you use 300mm + lenses for (such as birds etc.) 2.8 is just too narrow a DoF to be useful, isn't it?

The only thing I can think is because (as far as I know) lenses are at their optimum when stopped down slightly, so my f4 performs best at 5.6 or 8 whereas the 2.8 performs best at 4 or 5.6. But 5.6 is still pretty low at 300, 400mm isn't it?
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I have the 300 f4 AF-S, and 500 f4 they are both sharp wide open.

The 300 f2.8 will focus faster as more light is available to the AF sensors.

The 300 f2.8 will take a 2xTC giving you a lower cost, and fair bit lighter weight 600mm f5.6.

I use the 1.4 and also 1.7TC on both of the lenses above, and have been impressed with the results wide open, decent light helps when using the 1.7TC but it still AFs' fine.

Personally I would rather have the compact 300 f4 AF-S, but that is because it gives me the option of a lightweight and compact telephoto kit when I do not want to haul in excess of 3kg of glass around
 
Lower fstop can also mean a softer picture. This is certainly the case with my 50mm
 
tc4 said:
Lower fstop can also mean a softer picture. This is certainly the case with my 50mm
Click to expand...
...but also completely irrelevant. A Canon or Nikon 300mm f/2.8 is about as good as any lens gets and the one adjective that you couldn't use is "soft".
 
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