A question of Aperture.

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Michael
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Right,

This may be a stupid question, but I hope it isn't.

If the f-number is found by focal length divided by the size of the iris, then a 50mm set at f1.8 would have an iris diameter of roughly 28mm.

So if the focal length then increases it makes sense that the f-number would increase (Like it does on my zoom lenses).

So why on a lens such as the 24-105L does the largest aperture remain at f4?

Surely if it can reach f4 at 105mm, it should be able to achieve f0.9 at 24mm with the same diameter Iris?

I hope someone can clear this up for me! (and i hope my ramblings make sense)

Regards, Mike.
 
As Mark said - constant apertures are what we want:thumbs:.

And although the physical iris may be able to open up proportionally, there's no guarantee that the rest of the internals of the lens are good for the larger aperture, look at the design of an f1 lens compared to that f4 zoom - it's clear that for the aperture open to 0.9, the front element would need to be larger, lens body wider etc.
 
Janitor. said:
Right,

This may be a stupid question, but I hope it isn't.

If the f-number is found by focal length divided by the size of the iris, then a 50mm set at f1.8 would have an iris diameter of roughly 28mm.

So if the focal length then increases it makes sense that the f-number would increase (Like it does on my zoom lenses).

So why on a lens such as the 24-105L does the largest aperture remain at f4?

Surely if it can reach f4 at 105mm, it should be able to achieve f0.9 at 24mm with the same diameter Iris?

I hope someone can clear this up for me! (and i hope my ramblings make sense)

Regards, Mike.
Click to expand...

Basically, it doesn't always work like that in a zoom.

The f/number is the relationship between the relative sizes of the entrance pupil and the aperture, as seen through the optical system. This can change in a zoom, and not always in an obvious physical way.

Different designs play the game differently, and perhaps it's ironic that consumer zooms are the ones to benefit most from the lower f/number that is theoretically possible at shorter focal lengths.

Higher grade zooms tend to go for the more desireable option of maintaining constant f/number, but it's obvious from the lenses we have that low f/numbers are generally the hardest thing to achieve. There are only a few that better f/2.8, and those are for Olympus and the 4/3rds format that is only a quarter the image area of full frame which makes optical life a lot easier.
 
I asked basically the same question about this time last year. I think the gist of it was "it doesn't work like that" and "if you used the same size diameter at wide end as the long end, it'll be rubbish". I guess you have to really look into how these lenses are constructed to get a satisfactory answer.

found the thread http://www.talkphotography.co.uk/forums/showthread.php?t=294056
 
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