F/2s is the light amount the same across lenses?
Jun 1, 2016 15:56:41 #
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
Jun 1, 2016 16:21:18 #
wdross
Loc: Castle Rock, Colorado
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
The short answer is yes. One lense might be actually f1.98 while another might be f2.02 in reality, but the total difference between any of the f2 lenses will be way less than a 1/3 of a stop.
Jun 1, 2016 16:50:59 #
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
The f-stop is the ratio of the physical diameter of the aperture itself to the focal length of the lens. So at f/2, or any other f-stop, the physical diameter of the aperture can vary depending on the focal length, and the larger the diameter there is more light that can enter.
Jun 1, 2016 16:54:59 #
mwsilvers
Loc: Central New Jersey
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
No, it depends on the light transmission ability (Tstop) of the lens in question. Ideally, the Tstop should be equal to the widest aperture, but that is rarely the case. According to DXOMark, the Tstop of the Canon 35mm f/2 IS USM is 2.0, the same as the widest aperture, regardless of the body it's mounted on. On the other hand DXOMark indicates the Tstop of the Canon EF 100mm f/2 USM is 2.2 to 2.3 depending on the body it's mounted on. In other words the amount of light transmitted for that lens is up to 1/3 stop less then the 35mm f/2 IS when both lenses are set to their widest aperture.
Jun 1, 2016 17:27:41 #
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
If everything is mathematically correct, then it's pretty close. BUT, things are rarely accurate. As has been said, there can be a good margin of error. There is no standard that I'm aware of for a standard deviation.
I can't fathom that ANY company rounds up, always down and I'm sure it's quite deliberate to make a lens appear faster than it actually is.
As has been said, if DXO accounts for this then things could possibly getting better, but in older lenses they could definately be a bit slower than marked.
Just as a long zoom lens can be as much as 10% short, also making them appear a bit faster. ;-)
SS
Jun 1, 2016 22:39:02 #
Jun 2, 2016 05:45:42 #
billnikon
Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
If you could put all three lenses on your camera at the same time and take a shot say at F2 at 1/30 sec. (provided this was the correct exposure) ALL pictures would be identical for exposure.
Jun 2, 2016 07:10:30 #
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
Yes, at least in theory, f# is actually fractional ratio to the focal length,
eg: f2on a 50mm lens: aperture=1/2 50mm or 25mm
long time since I learned that, BTW that also explains in part at least diffraction at small f#s on short lenses.
Bob.
Jun 2, 2016 08:34:43 #
wdross wrote:
The short answer is yes. One lens might be actually f1.98 while another might be f2.02 in reality, but the total difference between any of the f2 lenses will be way less than a 1/3 of a stop.
This is why in the film days MANY photographers tested and calibrated their equipment (per the Zone system and the like). Finding Tri-X pan to be actually 200 on their camera with a given lens, or Plus-X to be 100 with a different lens. The more glass the less transmission as well. I believe manufacturers are really only guaranteeing or calibrating the lens based on pupil size and focal length not actual light transmission.
Jun 2, 2016 08:50:10 #
wj cody
Loc: springfield illinois
lamiaceae wrote:
This is why in the film days MANY photographers tested and calibrated their equipment (per the Zone system and the like). Finding Tri-X pan to be actually 200 on their camera with a given lens, or Plus-X to be 100 with a different lens. The more glass the less transmission as well. I believe manufacturers are really only guaranteeing or calibrating the lens based on pupil size and focal length not actual light transmission.
i agree, have always calibrated the lens and light meter on my cameras; that is an absolute for large format field and view cameras. the zone system is a big help in doing this.
Jun 2, 2016 08:50:45 #
Jun 2, 2016 15:09:50 #
it depends on the amount of light entering through the lens, a 62mm gives you less light than a 77mm which allows a huge jump in the amount of light entering through the lens and it goes that way with all of the lens size's
folkus wrote:
Is the amount of light entering the camera the same for 28mm, 50mm, and 90mm if each is set at F/2 and all other factors are the same? Thanks.
Jun 2, 2016 15:18:42 #
But I guess the OP was really wonder about how for example a 28mm f/2 lens with its tiny aperture can transmit the same amount of light as an 200mm f/2 with much larger opening.
Jun 2, 2016 16:37:56 #
wj cody
Loc: springfield illinois
BebuLamar wrote:
But I guess the OP was really wonder about how for example a 28mm f/2 lens with its tiny aperture can transmit the same amount of light as an 200mm f/2 with much larger opening.
it has to do with the transmission distance of light until it arrives at the film plane or sensor of a digital imaging device.
Jun 2, 2016 17:19:25 #
The f-stop, N, is the ratio focal length and aperture diameter. So the larger the focal length the larger the aperture diameter for constant N. So more total light is admitted. However, it is light intensity, light energy per unit area that matters for film/sensor exposure and this is unaffected by f-stop.
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.