Question about f numbers

Maybe a silly question, but here goes:

Do all f numbers correspond regardless of which lens you are using? i.e. if you are using a 85mm 1.8 and use f2.0, will this need the same or less or more light (i.e. longer/same/shorter shutter speed) for correct exposure or, because it is a 1.8 capable lens, need less light (and so a faster shutter speed)?

f2.0 would require more light and a slower shutter speed than if you open wide at f1.8. if you bump iso up too that can help compensate for shutter speed too. thats the basics of it.

a f2.0 on a 85 would need the same light as a f2.0 on a 200mm or whatever else to make the same exposure

dang. you just made me loose my reasoning to get another lens because it is a 1.4 and I was trying to convince myself that at 2.0 i would get the same DOF but would need less light as it was a better lens.

Hey we all have our own arguments to why we should and should not buy things - that was mine . But thanks for the speedy reply.

well DOF differes from lens to lens if thats what your asking (i only had 6 hrs of sleep today and its been a long day so my minds not totally here =P). some lenses produce better ones than others. a f2 L lens would probably produce better looking dof or background blur comapred to a normal f2 lens but the more expensive L will require the same light at f2 as a regular f2 lens will. ok thats probably not related to what you were talking about but....ok im going to go take a nap hahah

heck go get yourself another lens =P forget what i said =P

no no you got my question right - 85mm f2.8 would give the same DOF as a 70-200 zoomed to 85mm f2.8. But I was trying to convince myself I should get the 85L (1.4) as I could get it to 2.8 with less light and less blur.

Like I said, I was just trying to convince myself that I need a new toy. I don't, but I want one .

Forget the lens rating, it is just a way of CONTROLLING the amount of light, as is the shutter speed, between the shutter speed and the size of the aperture you control the amount of light reachinbg the sensor or film. Any lens will require the same aperture setting for the same exposure

So yes its' the same for every lens

you can use the 'sunny f16 rule to help you visualise

i.e. if you have a bright sunny day with 100 ISO film the exposure will be ( near enough) 125 sec at f16

So ANY lens will be set to that or any variation of that i.e.
250th sec@f11
500th sec@f8
1000th sec@f5.6
2000@f4
4000@f2.8
etc.

A fast lens like the 85mm 1.8 might not be able to get a good exposure on a bright sunny day as the shutter speed might not be able to be fast enough at f1.8, so you might have to stop down

saqib wrote:
no no you got my question right - 85mm f2.8 would give the same DOF as a 70-200 zoomed to 85mm f2.8. But I was trying to convince myself I should get the 85L (1.4) as I could get it to 2.8 with less light and less blur.

Like I said, I was just trying to convince myself that I need a new toy. I don't, but I want one .

if you look at the reviews most say that unless you actually NEED the extra stops between the 1.8 and the 1.2 ( for either shutter speed or Bokeh) then the 1.8 offers as good IQ AT THE SAME APERTURES

. I don't per se NEED it, I just WANT it . But there are many other things I want so I guess I will just leave it for now.

Thanks for everybody's input

One other issue I think needs to be considered. I have the 24-70 f/2.8L and the 200 f/2.8L. They are both f/2.8 right? Correct, but they do not autofocus the same in the available light. The 200 autofocuses faster and more accurately. So just getting the light to the lens is not the whole story.

Don't get me wrong the 24-70 is a great lens, just the 200 is slightly better in low light.