Aperture Speed

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Fabian_P

New member
Hello all,

I'm a newly registered member, but I've been reading these boards for a while now. I'm a student out of Boston with a strong passion for cinematography and I'm happy to be here!

That aside, I was hoping someone could break down what lens/aperture speed means. I've recently purchased a 35mm adapter and I'm now in the market for some lenses. From what I understand, f1.4 lenses seem to be the best place to start. I have a fair understanding of lenses and adapters, but I remain completely stumped as to what a "fast" lens means.

Thanks!
 
Thanks for replying rob, but I understand that much... What I'm asking is what it means for a lens to be faster.
 
An aperture of F1.4 - allows to shoot with less light, but primarily the difference in image is the shallower Depth of Field (DOF). The bokeh is even greater, which can emphasize certain points of the composition (focus).

At wide focal lengths (eg: 20mm), DOF is greater - that meaning the area that is 'in focus' is larger.
At longer focal lengths (eg: 105mm), DOF is shallower - less distance in focus.

The aperture (speed) increases these characteristics, allowing a wide shot shot at F2 (for eg) to still have a shallow DOF, as opposed to the same focal length shot at F8 (more distance in focus).
 
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Like I said, the smaller the number the faster the lens... So a lens that has a maximum aperture of f1.4 is "faster" than another lens with a maximum aperture of f2.8.. And with that, the characteristics of a faster lens is the ability to shoot in lower light and with less depth of field.
 
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The meaning of a lens speed comes from it's use with shutterspeed. A faster lens means that you can use faster shutterspeed (which loses more light) and vice versa.

In practical term, it's not so much light efficiency or DOF that you're shooting for, it's sharpness. No lens is at its sharpest when it's wide open, ideally you want to stop down about 2 stops. On a lens rated at f2.8, you'll have to stop down to around f4 to get maximum sharpness; but on lens rated at f1.4, you get maximum sharpness at around f2.8, which is better for light efficiency.

Another thing to keep in mind is that f-stop do not apply across lenses with different focal length. For example, a 24mm f2.8 does not let in the same amount of light as a 135mm f2.8. This is because f-stop is defined as the focal length over the diameter of the iris. Lenses with shorter focal length will have a smaller iris diameter, which lets in less light.
 
Charles Lowthian said:
Nikon F2.8 zooms :happy:
Click to expand...

Even the Nikon zoom would benefit from stopping down a few stops. The point being that a lense might be decently sharp wide open (my 135mm f2.8 FD is surprisingly sharp), resolution can always be improved by stopping the lens down.
 
Huy Vu said:
Even the Nikon zoom would benefit from stopping down a few stops. The point being that a lense might be decently sharp wide open (my 135mm f2.8 FD is surprisingly sharp), resolution can always be improved by stopping the lens down.
Click to expand...


Once you get into the "Should I buy a lens or a new house" category, like the Zeiss Master Primes, you can shoot wide open at full sharpness. The Zeiss lenses are designed to shoot with perfect sharpness at T1.3

While on that note, however...

T-Stops and F-Stops are very similar. Think of it as the difference between Practical and Theoretical light transmission in the lens. The F-Stop of a lens is determined using simple mathematics, while the T-Stop takes into account the amount of light that is lost when it passes through the lens (all that glass has to absorb some light). Professional lenses are rated in T-Stops since they are more accurate.

Generally speaking, the difference between the FStop and the TStop of your average lens can be anywhere between a third of a stop to as much as an entire stop (Meaning that an F2.8 might actually be a T4.0 on the lens because of how much light the lens absorbs).


Think about it logically as well. The longer the lens, or with zoom lenses as well, the slower the lens. A Prime lens like a 25mm or 50mm can normally open as far as T1.3 or T1.4 because there is a small number of glass elements in the lens. A longer lens, like a 200mm prime or a 150mm zoom (those numbers are examples), will generally only open to a T2.8, or more likely a T3.4. This is because there are many more elements of glass inside the lens, which absorbs more light which in turns makes the lens "slower".
 
You know that sharpness isnt always needed? Sometimes a softer, shallower lens is whats called for. DOF / sharpness are tools like any other part of cinematography.
 
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