Lens theory question.

Feezor · Jun 30, 2010

Hey guys I have a question I dont know if it is a stupid question but I figure I will ask and see if anyone understands what I mean.

If you have a lens and the lens is say 80 mm diameter versus the same lens that is a 40 mm diameter when you are at an aperture of say F2 will the 80 mm be letting in more light due to the lager diameter (assuming everything including the lens is the same the only difference is the lens diameter). I wondered this the other day and I wouldn't think it would but could not put an answer as to why it would not.

So there is my question anyone want to get scientific and answer that?

Josh66 · Jun 30, 2010

Read this thread and see if it answers your question.

http://www.thephotoforum.com/forum/...o-gallery/207695-what-f-stop.html#post1942181

Feezor · Jun 30, 2010

Not sure if the answer is in the math equations but I suck at math. Sorry I guess I need it "dumbed" down a little more.

LarissaPhotography · Jun 30, 2010

if I'm understanding your question right, if you switch lenses and keep the same settings, your image will be exposed the same (obviously you'll see more with the 40mm than with the 80mm)

Josh66 · Jun 30, 2010

When you say 80mm diameter vs. 40mm diameter, do you mean the actual diameter of the aperture? Or the physical diameter of the outside of the lens?

Or the focal length...?

I'm not sure that I really understand your question...

Feezor · Jun 30, 2010

Okay made a 2 second picture to maybe help my question

The circle on the top represents the 80 mm lens looking directly at you at F2. The circle on bottom represents the 40 mm lens looking directly at you at F2. The red part (the lens aperture) is larger in the larger diameter lens so would that let in more light than the smaller lens opening?

Maybe that clears up my question.

Josh66 · Jun 30, 2010

OK - I get your question now. I think, lol.

I assume that when you said 80mm diameter you really meant a focal length of 80mm...? If you just mean the filter size, that doesn't necessarily mean anything.

OK, so assuming you have 2 lenses - one with a focal length of 80mm, the other with a focal length of 40mm...

At f/2 (on both lenses) although the actual size of the opening in the two apertures would be different, they would let the same amount of light through.

With both at f/2, the 80mm lens would have an aperture diameter of 40mm (f/2= the length of the lens divided by 2). The 40mm lens would have an aperture diameter of 20mm.

Even though the aperture diameters are different, they let in the same amount of light because the length of the lens is a factor too.

If you are just talking about the physical size of the lens (not the focal length), then there's no way to tell. You would have to know the focal length.

But...say you have two 85mm lenses. Say, the Canon 85mm f/1.8 and the 85mm f/1.2 L. The 1.2 version is physically much larger. But, if you set them both to f/2, since they are the same focal length, the same amount of light would be coming through.

Hopefully the answer to your question is somewhere in there... :lol:

Feezor · Jun 30, 2010

O|||||||O said:
But...say you have two 85mm lenses. Say, the Canon 85mm f/1.8 and the 85mm f/1.2 L. The 1.2 version is physically much larger. But, if you set them both to f/2, since they are the same focal length, the same amount of light would be coming through.

That answers my question. It lets in the same light. Same focal length same aperture just different physical size.

And if I understand correctly then it will let in the same amount of light. But I guess I just dont understand why. I am just thinking if you have a larger physical size the larger physical size would for some reason let in more light. I know it does not but I dont understand why.

I am confused too so it is hard to explain my own question when it confuses me haha.

Feezor · Jul 1, 2010

I think I understand now, thanks everyone

Josh66 · Jul 1, 2010

Feezor said:
I guess I just dont understand why. I am just thinking if you have a larger physical size the larger physical size would for some reason let in more light. I know it does not but I dont understand why.

Yeah - I know what you mean. I can't really explain that part...

That's a little above my pay grade, lol.

I have a feeling it has something to do with the larger lenses absorbing more of the light passing through them...? I don't know ... that was just a guess. :lol:

You know, it's probably just because the 1.2 L version is faster. It can open up to f/1.2 instead of f/1.8. To do that it would have to be a little bigger (I assume).