True "f-stop" ratings: We all assume that a lens rated "f1.8" is faster (allows more light to pass through to the film/sensor) than a lens rated "f2.0", which is faster than a lens...

wteffey Loc: Ocala, FL USA

We all assume that a lens rated "f1.8" is faster (allows more light to pass through to the film/sensor) than a lens rated "f2.0", which is faster than a lens rated "f2.8". But how do we really know? In other words, does a lens from mfg. "P" at f8.0 admit as much light to the sensor as a lens from mfg "T" or "S" at the same indicated f-stop? A leading mfg now has a camera with a constant f2.8 throughout the entire zoom range, a significant technical accomplishment for which a premium price is justified. But, and I am always mistrustful of big business, how do we really know that this camera's f2.8 is really faster than a competitors f3.5? And how do we know that the f2.8 is constant throughout the entire zoom lens? I have experimented with two prime lenses, one a f2.0 and one a f2.8. Under the same apparent conditions, the f2.0 lens allows a faster shutter speed than the f2.8, seeming to confirm that the f2.0 lens is actually "faster", but I believe my question is still valid.

Frank T Loc: New York, NY

You've hit on something that has been well known in the movie industry for years and back in film days they actually tested ever lens they used for this very reason. Now you know that it isn't just size that matters. Due to the different types of glass or plastic and the coatings, different lenses have different light transmission properties. So yes, an f2.8 lens can be faster than a 2.0. However, now that you know that, you'll need to accept that there is little you can do about it other than renting a lens first before you buy it because I don't think you're going to be able to find information about a specific lens relative to this phenomenon

wteffey Loc: Ocala, FL USA

I agree. Still, it might be interesting to know if Tamron's 4.0-5.6 admits the same light as Nikon's, but in reality it is the quality of the photo that matters.

Nikonian72 Loc: Chico CA

An aperture number is a mathematical determination. It is the diameter of the inner iris divided by focal length of lens. If the iris of a 100-mm lens is 50-mm in diameter, that aperture is labeled as f/2 (50/100 = 1/2 = f/2).

Transmission numbers, on the other hand, consider iris diameter, lens formula, coatings, blade count, blade shape (close to circle vs hexagon pupil), etc. T-numbers are sometimes used in extremely critical lens calculations.

photoninja1 Loc: Tampa Florida

Thirty years ago I had to test my cameras to verify aperture and shutter speeds. Even then apertures were pretty true. It was shutter speeds that were inconsistent, moreso in medium format TTL cameras like Bronica, Mamiya RB67, etc. Now there isn't enough difference to even think about, and the cameras are sensitive and responsive enough that the whole thing is moot. Not to trouble yourself about it!

Pablo8 Loc: Nottingham UK.

wteffey wrote:

We all assume that a lens rated "f1.8" i... (show quote)

The 'Test' as mentioned above, to be valid, the 'Target' should have been a constant size / area, constant tone, and an identical focal length of the lenses. With those criteria, the test should have shown valid results.

potmead Loc: 191miles North of London, England

Nikonian72, i think you have your figures confused, the f stop of a lens is the focal length of the lens divided by the diameter of the front lens element (which is the EFFECTIVE diameter of the iris when fully open)

ie 100mm lens 50mm diameter
100/50 = f2.0

This is why lenses with smaller apertures have large front elements, especialy long telephoto's

Graham

potmead Loc: 191miles North of London, England

Just to clarify my last paragraph,

when i say smaller aperture's i mean lower f stop numbers, which equates to a larger aperture

Graham

GrahamS Loc: Hertfordshire, U.K

potmead wrote:

Nikonian72, i think you have your figures confused, the f stop of a lens is the focal length of the lens divided by the diameter of the front lens element (which is the EFFECTIVE diameter of the iris when fully open)

ie 100mm lens 50mm diameter
100/50 = f2.0

This is why lenses with smaller apertures have large front elements, especialy long telephoto's

Graham

This is totally incorrect. Nikonian72 is correct. The diameter of the front element has nothing to do with the calculation of the f number.

GrahamS Loc: Hertfordshire, U.K

There will be very little variation in transmission value between reputable manufacturer's lenses of similar specification, all things being equal. What variation there is will be in tenths of an f-stop or .1%. The variation in shutter speed or the inaccuracy of the shutter timing mechanism, (although electronic shutter control is much better than the old mechanical controls,) is much greater.

potmead Loc: 191miles North of London, England

Actually, we are both wrong, the maximum aperture is a ratio of the focal length to the 'Entrance pupil' of the lens. This in neither the physical size of the aperture or the diameter of the front element, but the size of the image of the aperture as seen through the front element.

http://en.wikipedia.org/wiki/F-number
http://en.wikipedia.org/wiki/Entrance_pupil

But while the diameter of the front element may not feature in the calculations, it does indeed account for the difference in the maximum aperture.

Faster lenses have larger front elements!

Graham

GrahamS Loc: Hertfordshire, U.K

potmead wrote:

Actually, we are both wrong, the maximum aperture ... (show quote)

I quote from the linked paper from the Harvard Extension School:

The F-number of a lens is the ratio of its focal length divided by the diameter of the aperture.

http://cdn.cse7.org/2011/fall/lectures/3/aperture.pdf

I have been teaching this for 40 years!

jerryc41 Loc: Catskill Mts of NY

wteffey wrote:

We all assume that a lens rated "f1.8" i... (show quote)

I have no doubt that one f/2.8 is going to let in more light than another f/2.8, but does it really matter? There is more to choosing a lens than how much light it admits. An f/2.8 will let in more light than an f/4.0, and a high quality lens will take better pictures than a low quality lens.

It would be more of an intellectual/technical exercise to compare the light transmissioin of various f/2.8 lenses. Would the results of such a test make any difference in the real world? I think the results would show extremely small differences.

potmead Loc: 191miles North of London, England

i believe we are all agreed on the ratio between focal length and 'aperture' is correct.

however, is the 'aperture'
1, the diameter of the lens - as indicated in your paper,
2, the diameter of the physical 'iris' within the lens body.
3, the 'apparent' diameter of the 'iris' as viewed from the front of the lens .(Entrance Pupil)

I have a 400mm f6.3 lens which from the calculations has an aperture of 400/6.3 = 63.5mm

1, The front element is 59mm dia.
2, The 'iris' is smaller than 50mm as this is the diameter of the lens barrel.
3, The 'apparent' diameter of the iris is 24mm

None of these figures give the correct 'f6.3' number, How can this be?

I am now confused, i thought i understood this!

Graham

PhotoArtsLA Loc: Boynton Beach

F stops are a physical calculation as has been mentioned here. T Stops, which are what we like in the movie industry, are TRANSMISSION f/stops, which is the actual light hitting the film plane after the lens elements due their absorbing.

Can a f/2.8 lens be faster than an f/2? Not from any reputable manufacturer, which all use lens physics to indicate the speed of the lens.

T Stops are an extra hassle, thus lenses featuring T Stops are MUCH more expensive than any "standard" lens. Generally, cinema lenses with T Stops are also much better made, with much better materials, and essential additions like no physical stops on the f/stop ring for iris pulling and gears pre installed to work with follow focus systems. That's why a cinema zoom lens can cost $40,000 or more.

There. I tried a basic explanation with no math. Just wait 'til we start talking about the circle of confusion!