File :-(, x, )
Question about zooms Anonymous
What is it about zooms (18-55, 55-200, etc) that requires them to have smaller maximum apertures at narrower angles? The kit lens from my d40 is f/3.5 at the 18mm end, but f/5.6 at the 55mm end. Why is this the case? What is it about the design of the lens that does this?
>> elf_man !!DdAnyoDMfCe
It's because f stop is a ratio. The longer the focal length, the wider the aperture has to be to maintain the same light level. At 18mm, f4 is actually a narrower aperture than f4 at 55mm. You can get zooms with constant f stops, but they're more expensive.
>> Anonymous
>It's because f stop is a ratio

Ratio between what and what?
>> elf_man !!DdAnyoDMfCe
Focal length over aperture. Pretty sure, been a while since I read up on it.
>> Anonymous !dMYYvF5Blo
Thats seems to be how I know it...

Also, why would you get a kit lens?
>> Anonymous
     File :-(, x)
>>102833

Let's say a 24-70, the ratio is 2.9. The aperture is 2.8

Obviously, there are exceptions and it's more like a rule of thumb than a real rule.

Also, if it ain't 1.2, it's made of fail.
>> elf_man !!DdAnyoDMfCe
>>102837
It's still kinda rare to see a d40 without the kit lens. At least, I think they finally started selling it without, it may still be bundle only.
>> Anonymous
why would you get a d40
it even feels like it should be called playskool my first dslr + fail
>> Anonymous
>>102838
That's not how it works. The ratio of the minimum and maximum focal lengths is not what makes the aperture. It's the ratio of the focal length to the diameter of the aperture opening. So if a lens is 50mm and the aperture is f/1.8 then the aperture diameter is ~28mm wide (the / is a division sign).

So in reference to the OP, consider the fact that the aperture needs to be larger to maintain a constant ratio to the focal length when you increase focal length. This is why cheaper zoom lenses have smaller apertures at longer focal lengths than at wider ones.
>> Anonymous
>>102841
I care nothing about what anyone else says about my camera. It does what I need it to do. I've learned well enough not to trust any advice I get from 4chan.
>> Anonymous
     File :-(, x)
>>102838
O RLY?
>> Macheath !8b4g0BkNZg
http://everything2.com/index.pl?node_id=1260276

Explains it well