Constant aperture question

fingerz

fingerz

Messages
1,100
Edit My Images
No
The F number is a ratio, right?

So f/2.8 means that at a focal length of 50mm the aperture is 17.86mm wide, since 50 / 2.8 = 17.86 (rounded up), right?

So in order to get an aperture of f/2.8 at 50mm the aperture has to open wider than it would at, say, 18mm, right?

Soooooo.... If the aperture can open that wide, why restrict it at the wide end of the zoom? Why not allow the photographer to have even wider apertures at the wide end? I admit that it makes life easy when you know that the maximum aperture is the same, regardless of focal length but you'd think at least one lens manufacturer would make a fast zoom with variable aperture.
 
But as the focal length increases, the apature may stay the same in size but the ratio (f) will change.
 
Fast lenses mean big money. Large max aperture lenses have to be engineered designed and polished to extremely fine tolerances to be any use at all wide open, particularly so with the increased distortion inherent in wide lenses, and as it is, they usually benefit from being stopped down a little.

I would guess also that most of the pressure on manufacturers is to produce fast tele lenses for the pro sports, reportage market where most of their sales must come from.
 
SammyC said:
But as the focal length increases, the apature may stay the same in size but the ratio (f) will change.
Click to expand...

Correct a mundo!

I think you inadvertantly answered you're own question james young man!

the maximum aperture of a zoom lens is always taken at the widest point of the lens. e.g. 35-420mm f2.8-f3.7. The actual aperture stays the same size, but as the zoom length increases so does the ratio.

I'm not sure if that is technicaly correct, but that's the logical conclusion based on your theory given above.
 
unless of course you're suggesting that being as though ther have been various f1.0 lenses available for variuos cameras, then why can't all lenses be f1.0 at the wide end. For the answer to that refer to ct's post above.
 
No, what I mean is that my Sigma 18-50 is f/2.8 throughout the zoom. The max aperture is f/2.8 at the 18mm end and it's still f/2.8 at the long end.

50 / 2.8 = 17.857

18 / 2.8 = 6.428

So we can see that the aperture blades are capable of opening up enough to create a hole 17.857mm wide. If that's possible, why not let them open that wide at the 18mm end also? Why restrict them to 6.428mm? If they allowed them to open to 17.857mm at the wide end you'd get an aperture of f/1 (or near as dammit).
 
Well they probably can but the lens just cannot produce an image of acceptable quality at that diameter so they narrow it.

It's like the 50mm 1.8 is a bit soft wide open. The 1.4 doesn't suffer from the same issue but costs a hell of a lot more. This is because the glass is better and doesn't distort with the large diameter.

:)
 
nah, they can do it. It's just a massive conspiracy to make you buy more lenses.

As a friend of mine pointed out last night. If you can buy 6Mp cam phones in japan then why can't you here? Think about it~?~

*devils advocaat mode/off*
 
Phones in Japan get TV too. Japan is basically a huge testing ground for this stuff. They're all guinea pigs and the stuff that sells gets replicated abroad. Although you'd think a 6mp camera phone would be a no-brainer.

Mind you I'm happy with my one due to the silent 'shutter'. Couldn't live with a phone that made a noise every time I took pics.
 
fingerz said:
. Couldn't live with a phone that made a noise every time I took pics.
Click to expand...

Just wish people had the same principles for ring tones ;)
 
fingerz said:
No, what I mean is that my Sigma 18-50 is f/2.8 throughout the zoom. The max aperture is f/2.8 at the 18mm end and it's still f/2.8 at the long end.

50 / 2.8 = 17.857

18 / 2.8 = 6.428

So we can see that the aperture blades are capable of opening up enough to create a hole 17.857mm wide. If that's possible, why not let them open that wide at the 18mm end also? Why restrict them to 6.428mm? If they allowed them to open to 17.857mm at the wide end you'd get an aperture of f/1 (or near as dammit).
Click to expand...


I saw this thread when you started it. I looked at your argument and had to agree with you.

I didn't post because I knew there must be a flaw in the logic but I could not see it. I think the penny just dropped (well maybe).

The focal length is the distance from the objective lens to the film/sensor. With a long lens you can see it with a ruler held along the barrel. With a wide zoom the objective is not actually positioned anywhere near the focal length distance from the sensor so internal optics manipulate the light to make the image. Because of this and the position of the iris you don't get the theoretical aperture you calculate.

Anyways that my theory :shock:

Now I'll await the :tonofbric
 
You must log in or register to reply here.
Back
Top