Is there really a “best aperture”. Doesn’t it all depend on what you’re shooting and how?
Sometimes, the answer comes from left field:
What is the best aperture for video?
It’s f/2.8, or T2.8 if you are thinking T-stops.
I’ve studied the work of hundreds of cinematographers, and the T-stop they light to most of the time is T2.8. This corresponds to about f/2.5 or thereabouts, but you can assume it’s f/2.8, for the sake of your sanity.
Why do cinematographers pick T2.8?
The aperture number, called f-number, decides how big the aperture opening is. The higher the number, the smaller the aperture, and the lesser the light hitting the sensor.
Cinematographers always had to fight the low sensitivities of film stock. They were working with an Exposure Index of 50 or 100 most times, even today. The corresponding ISO is about double the exposure index, according to Kodak.
They didn’t have the ISOs modern cameras have.
This meant they had to open up the aperture for good exposure or use a lot of light. Too much light would have cooked film sets and the actors, well over legal tanning limits. They had to strike a good balance between practical lighting and what is tolerable. T2.8 was a good happy medium.
Depth of field
When you open up the aperture, you get shallower depth of field:
If you stop down or close down the aperture, you get deeper depth of field:
Movies like Citizen Kane had to use tons of light, for their deep focus look. If you want everything in the frame to be in focus, you must stop down the aperture. This cuts light by a tremendous amount.
Every step in the aperture scale cuts one stop of light, or double the light.
The other end of the spectrum is shallow depth of field, where you have no clue where the actor is. To get this effect you open up the aperture to its highest rating, sometimes T2 or T1.3 on traditional cinema glass – if you had the option.
These lenses were available back in the day when cinematographers used T2.8. The Godfather was shot with Bausch and Laumb Super Baltars that were T2.3:
Taxi Driver was shot on Zeiss Super Speeds that were a T1.4. Some scenes in Taxi Driver were shot wide open, especially the night shots, you can see the distinctive triangular bokeh, which was not always popular:
So why didn’t these cinematographers open the aperture all the way if they could have?
There’s no free lunch. Lenses in those days weren’t as advanced as today, so if you opened the aperture all the way, you had three major problems.
Flare is not just those cool streaks of light you see, it’s also a loss of contrast. Imagine going through all the trouble of learning about film stock and lighting, only for something stupid like lens flare to ruin your shot through lack of contrast.
The lenses weren’t that sharp wide open. This is true of almost every lens on the planet, with a few rare exceptions. Lenses are not the sharpest when wide open. The general rule of thumb is the lenses have to be stopped down by two stops for best sharpness.
E.g., if you had an f/1.4 lens, you had to stop down to f/2.8 for best sharpness. This also meant if you used different focal lengths, the sharpness across the entire range matched. This is the biggest problem with cheap lenses that open to f/1.4 or higher today. They don’t match across the entire set.
Great cine lenses like the Arri Master Prime series open to T1.3 across the entire range. You’ll never find an equivalent photo lens series that all open to the T1.3, at any price.
The Master Primes are known to perform very well at T1.3, they are sharp and cinematic. But even these lenses are sharper stopped down to T2.8.
The last reason cinematographers stopped down was probably the most important one.
Shallow depth of field also meant focus pullers had a terrible time following actors. Remember, they didn’t have monitors in those days. Focus pullers had to use all their experience and concentration to keep a moving actor in focus.
Today, there are aids, but unfortunately higher resolution has balanced that out. Focus mistakes are way more obvious in 4K, and even so in 8K. So whatever advantage monitors and electronic systems have introduced, higher resolution has taken away.
It’s still hard to focus moving subjects at T1.4 in 8K.
T2.8 provided cinematographers the best balance between shallow depth of field and deep focus. You could make out the background, so you were still in the story. This trend is still followed today by most cinematographers. Nobody wants a completely shallow depth of field, otherwise it will look like it was shot in a studio.
Deep focus is tough
The third reason to pick T2.8 is, very few people can afford deep focus.
When I shot Man May Love, I stopped down the Arri Master Anamorphics to T8, and compensated by raising my ISO to 6400, which is the limit for the Red Monstro. Even then I felt I didn’t have enough light.
There is another problem with stopping down the lens. When you cross a certain threshold, typically f/11 for Super 35mm sensors, you lose sharpness again. This is called diffraction, and the explanation is quite technical.
All you have to know is, just because you can stop down to T22 doesn’t mean you should. You’ll get softer images.
Just like with everything else in life, a happy medium is the best for the aperture as well. And it’s not like cinematographers sat down and decided these things cerebrally. It came about organically, through practical limitations of cinematography, cameras, lenses and lighting.
By fixing their preferred aperture, cinematographers got the ability to stay consistent with their lighting and exposure. They could achieve the great effects we love so much, even in the high-stakes, high pressure environment of a Hollywood film, because they were able to keep some things constant.
This is why, over a hundred years of cinema, T2.8 is the most preferred aperture for normal lit scenes.
For low light night scenes, cinematographers typically preferred T2 for many years. That is no longer that relevant today due to the improvement in ISO and sensitivity in digital cameras.
So, T2.8 or f/2.8 is the best aperture for filmmaking, if you’re feeling democratic.
Look at the classic popular zoom lenses for cinema, they are close to T2.8, T2.95 or T3.
Even photography zoom lenses are consistent across all brands at f/2.8. Look at B&H, even they let you find lenses with apertures above f/2.8 or higher, because they know that’s the threshold.
You can open the aperture as wide as you want, or you can stop down as much as you want. Not everybody sticks to T2.8 all the time. There are always exceptions.
But if you were to take a vote, T2.8 wins.