the f: number of a camera with no lens

I was just doing some testing with a 35mm reflex with no lens.

 

Metering the light from a plain surface with no lens gave me the

exposure time of 1/250.

 

Metering the same surface with a 35mm lens gave the exposure time of

1/30 with an aperture of F:2.8

 

So I could conclude that using a camera with no lens gives us an f:

number of 1.

 

Is this correct? An F:1 aperture would allow as much light to reach

the film as if there was no lens at all? If so, Wow...that's a lot of

light :)

 

Cheers

When you meter with a lens, in some way, the lens communicates to the camera body what the aperture is versus what it will be when the shot is made. So if you're metering with an f/1.4 lens and have it set to f/8, the metering in the camera body has to have that information to function properly- specifically, has to know that when you shoot, the exposure will be 5 stops slower than right now.

 

When you meter without a lens, that information is not furnished. If you know how your camera body functions without a lens, fine, but otherwise, don't assume that it automatically is metering correctly for the circumstances. The camera body isn't designed to function without a lens, the owner's manual probably doesn't tell you whether it's functioning right or not, so I would guess the reading you get is fairly meaningless.

Hmm...I see

 

But I thought it was such a coincidence to be F:1, a special number, that I concluded that it was correct.

 

My camera is a manual one with no eletronic contacts, although I know there are mechanic contacts.

 

So you are saying that if I make other tests the camera without the lens it will meter differently, not as if it were an f:1 aperture?

 

Anyway I will do some futher testings. I will share it with you later.

 

Cheers

No, that's not how it works. The f number of a lens is nothing more than a number. It is the result of dividing the diameter of the iris opening (aperture) into the focal length of the lens. That's all. A 50mm lens at f/2 has an iris opening of 25mm. At f/1 the same lens has an aperture of 50mm f/number = focal length/aperture diameter. Yes, it is that simple. Now that you know the formula, you can work out any f/number for any lens or you can calculate the aperture's diameter from the f/number and focal length.

I know the formula and what the F: number means. But as far as we are talking about light levels and not focal length and mm's, the Nikon F-mount seemed to be F:1.

 

As our other fellow suggests, that behavior should not be the same within other lighting situations. Isn't it?

 

I would still think that some relation can be achieved. That way I could meter the light with no lens attached (not sure why I would want that, though. Lol).

 

Thanks for the responses so far.

In the Nikon school of the late 1960's, they showed you how you could use a translucent body cap, set the camera's meter like using a preset lens, and set the asa to force the rig to be a poor mans incident light meter. Since this was from the slide rule era, simple and practical, it must be declared as "impossible" :)! Some folks expoxied a semi transparent white easter egg to a body cap with a hole in it, to create a wider angular coverage. Old Pop photo had an article on doing this when the first TTL slr cameras came out.

I would guess it means the meter in your camera accepts light from

an opening angle equivalent to what f/1 would be on this particular lens. I don't think this is necessarily the same as the restriction the mount diameter places on light reaching the film plane. Also, there's nothing magic about f/1. In principle you could make faster lenses (and I think a few have been made).

Well, how far off the film plane is your camera's lens mount? Around 40 +/ mm, right? How wide is your lens mount diameter: around 40 mm also.

 

And there you have it: F/1. Of course ... opening diameter / focal length, length of lens = f number. Or 40 / 40 = 1. Eureka!

Uhlig, That makes sense...I guess :)

 

Although the early replys from our fellow members indicated that what I was writing didn't make sense, I think there is so possible logic in that.

A pinhole camera has no lens but the further the pinhole is from the film plane the larger and dimmer the picture is. So the equivalent f number is dependent on both the diameter and focal length and differs in every camera.

I won't bother you with numbers and formulas per se.<p>

 

But the glass in my f/1:1.2 lenses (50mm (EOS) & 55mm (FD) have front elements more than 1" across.<p>

 

What is the approximate width of the lens in question? Under 1"? If so, it <i><b>cannot</b></i> be an f/1 lens.