f-stop and focal lenght relationship

SonOfPerdition

Hi folks,

In the last 18 months i've gotten more and more interested in photography. I've spent a lot of time lurking on this forum and other online forums and reading some beginner books to get me started, and practicing as much as possible. I've gotten the hang of the basics of the technical side of exposure and can generally work my way around the nikon d60 i use. Composition is slowly getting there, but thats for another day. :O)

The reason i've started this thread is to ask a technical question regarding focal length and f-stop numbers and the amount of light hitting the sensor/film.

From what I've read online, the f-number is a relationship between focal length divided by aperture diameter. I just wanted to clarify if my interpretation is correct that the same settings using 2 different focal length primes results in 2 different exposure levels.

Example, If i take a picture a 35mm prime and a 50mm prime in manual mode, and all settings are equal between both pictures (iso, aperture, shutter speed, and lighting conditions are identical) .. does the 50mm lens expose more than the 35mm and produce a brighter photograph?

Thanks,
SOP

DaireQuinlan

SonOfPerdition wrote: »

Example, If i take a picture a 35mm prime and a 50mm prime in manual mode, and all settings are equal between both pictures (iso, aperture, shutter speed, and lighting conditions are identical) .. does the 50mm lens expose more than the 35mm and produce a brighter photograph?

Thanks,
SOP

In short, no. That's the reason for the relationship between the focal length and the aperture diameter in the first place, it means that f/2.8 on a 30mm lens is precisely the same exposure as f/2.8 on a 300mm lens (notwithstanding different light loss inside the lenses themselves etc etc which is minimal)/

charybdis

You're assuming the physical size of the aperture is the only factor influencing the transmission of light through the lens. If there were no optical elements involved, a lens of longer focal length would admit more light than a shorter one at equal f-numbers purely because it is a physically larger opening. However, lens design is more complicated than this as different kinds and arrangements of glass and optical elements admit different amounts of light. So, even two lenses of the same focal length and aperture but with differing designs can admit slightly different amounts of light.

The problem is that f-stops are not a particularly accurate measure of exposure control, but are accurate enough for our uses. They are, however, useful for controlling relative exposure in a single lens whose optical composition does not change, making the aperture diaphragm the only dynamic part of the optical assembly and giving it control of the amount of light admitted.

Lenses for motion picture cameras (where exposure is often more critical than in still photography and is primarily controlled though aperture) are branded with t-stops ("t" for transmission) so that different lenses of different focal lengths and apertures can be compared purely on the basis of how much light they admit.

nilhg

SonOfPerdition wrote: »

Hi folks,




Example, If i take a picture a 35mm prime and a 50mm prime in manual mode, and all settings are equal between both pictures (iso, aperture, shutter speed, and lighting conditions are identical) .. does the 50mm lens expose more than the 35mm and produce a brighter photograph?

Thanks,
SOP

Just to clarify, if the ISO, f-ratio, shutter speed and lighting are the same then the exposure should be the same or very close.

If the ISO, aperture diameter, shutter speed and lighting are the same then then you wont expect to get the same exposure.

The confusion arises because we all use f-ratio=aperture but technically I'm not sure that's entirely correct.

SonOfPerdition

thanks for the replies, that clears that up nicely for me.
I have been puzzling over it for a couple of months now.

magicbastarder

the way i've always (probably incorrectly) reasoned it, is that f2 on a 50mm is physically wider than f2 on a 30mm, but the wider angle of view on the 30mm means it's collecting correspondingly more light, and they roughly cancel out.

jpb1974

Check out the inverse square law

http://en.wikipedia.org/wiki/Inverse-square_law

http://www.photographictherapy.com/2009/01/19/photo_inverse_square_law/

jpb1974

I'll quote a chap from another site -

The same aperture e.g. f2.8 will always give the same exposure, irrespective of the lens, and assuming other factors are equal (shutter speed, ISO etc).

Looking at a sample calculation - I am assuming f4 and 100mm and 200mm lenses for simplicity :

100mm lens : f4 gives a diameter of 25mm. Total area of the 'hole' = 490.9375 sq. mm
200mm lens : f4 gives a diameter of 50mm. Total area of the 'hole' = 1963.75 sq mm (or 4 times bigger)

(area = pie*radius squared)

So, you're correct, the hole is much bigger.
However, now the inverse square law comes into play. So, the 200mm lens is twice as long as the 100mm and therefore only 1/4 of the light reaches the sensor (or film) as for a 100mm with the same 'hole' size. So, it exactly balances the larger size of the 'hole' or aperture. Four times bigger but twice as far away = same amount of light.