f stop numbers, why those exactly?

[Hanthx]

Hi guys

Quick question
What do the f stop numbers (2 2.8 4 etc.)
actually mean. Let me rephrase the question a little bit, why are those particular numbers inscribed on the lens. My teacher says that they were picked due to something "Historical". Im thinking its related to the calendar.? Any answers??
Scott

[#Pedro]

Hi Scott,
the stop number tell you, what part of the incomming light will come out of the lens at it?s outlet side. the stop 1:1,0 f.e. means, that all the light passes thru without any lost of brightness. 1:2,0 means, that the light is dimmed to the half of the incomming amount. If you have a camera with in-build lenses and a prism viewfinder system, it?s possible that this stop values are not real the stops for the film exposure, as the flicker-free viewfinder consumes another part of the light (usually 1 stop). So it depends on how the lens is marked, with the effective stops considering the film exposure or with the stops considering only the lens performance.
Pedro

[pcg]

Pedro, as usual, is on the money w/ his comments. Another way of saying what he's described is that the f-stops are a linear & mathematical notation for the amount of light allowed through the lenses. Think of the designations are fractions: f2 = 1/2; f4 = 1/4; f8 = 1/8th, etc. As the fraction becomes smaller, the opening in the lenses (the iris) becomes correspondingly smaller...

Beyond being a notation for light quantity, f-stops allow you to determine depth of field. For me, this is their real descriptive power. The bigger the f-stop (f1.8, for instance), the shallower the depth of field. Conversely, the smaller the f-stop (f16, for instance), the deeper the depth of field.

At f1.8, your depth of focus might be 2.5 feet. At f16, the camera might be in focus from 4.5 feet to infinity. [These are examples only, not based on an actual lenses! A number of Super 8 books from the '70's have depth of field charts.]

So understanding f-stops is terribly important if you want to be able to control focus & understand the effect of light on your exposure.

If you're simply shooting in auto-mode, none of this matters. But as your shooting becomes more subtle & conditions more dicey, you'll want to understand these tools so that you can respond w/ skill & control...

/Pat

[#Pedro]

Pat pointed it out much better. "Fracation" din?t come in my mind in English language.

Of course, the influence of the stop on the field of depth is very important. But unfortunatly very limited with movie cameras. With still cameras, you can control precisly the field of depth, openening f.e. the stop one step and setting the shutter speed one step faster at the same time. The exposure will remain the same, but the picture will point out only the focused detail of the szene, in order to emphasize it.

With movie cameras, you cannot determine the shutter speed this way. Normally it?s fixed by the framerate. Well, if the object is rather static, you could double the framerate in order to open the stop by one number, or close/open the variable shutter...

But in most cases, you will use the zoom lens to determine the depth of field. When you zoom in and keep the aperture at the same stop, the depth of field will become smaller, too. You need a separate depth chart for each zoom setting.

Another possibility to limit the field of depth is the use of neutral grey filters. They will limit the amount of light passing through the lens and the aperture will open to compensate this.

What?s very important about f-stops, expecially with our tiny format, is to look if you are shooting at a ideal f-stop (normally not the most open one) in order to get sharp pictures. I would strongly evite to shoot at totally open stop! The contrast and the impression of sharpness sucks significantly.

Pedro

[mattias]

good explanation, but to clearify the math: an f-stop of 2 means 1/4 of the "available light" comes through. the number is the length of the lens divided by the *diameter* of the aperture, and the *area* that lets the light through is of course proportional to this number *squared*.

cutting half the light (this is the definition of one stop) from an aperture of 1 leaves you at 1.4 (1.4^2=2*1^2). the light is cut to 1/4 (2 stops) for every number in the sequence 1,2,4,8 and to 1/4 for every number in 1.4,2.8,5.6,11. and since there's one stop between 1 and 1.4, to know all f-stops numbers you only have to know 1 and 1.4 and then double every other time to get the series:

1,1.4,2,2.8,4,5.6,8,11(note the rounding off),16,22,32,44,64,88,128,176,256 and so on...

[Nigel]

Don't forget in the movie industry we use
"T" stops. These are the true "f" stop for the lens. The T-Stops vary from lens to lens and they are the f-stop with light loss inside the lens factored in. So when you are shooting and using multiple lenses never use f-stops always use T-Stops or you will get yourself in trouble. Most T-Stops are in Red on a lens. Use your meter the same way. Good Luck

[#Pedro]

I think the Angenieux lens for Beaulieu is marked in T-stops
Pedro

[#Pedro]

I think the Angenieux lens for Beaulieu is really calibrated in T-stops...
Pedro

[Alex]

<BLOCKQUOTE><font size="1" face="Courier, Arial">quote:</font><HR>Originally posted by Nigel:
Don't forget in the movie industry we use
"T" stops. These are the true "f" stop for the lens. The T-Stops vary from lens to lens and they are the f-stop with light loss inside the lens factored in. So when you are shooting and using multiple lenses never use f-stops always use T-Stops or you will get yourself in trouble. Most T-Stops are in Red on a lens. Use your meter the same way. Good Luck<HR></BLOCKQUOTE>

Nigel, how much can you be off, if you use f-stops instead of T-stops...with the wide lattitude of negative...can you really get into that much trouble???

Or is it a certain area where you can get into trouble...like when you are wide-open, or almost stopped down all the way...?

-Alex

[mattias]

alex, shooting negative with modern lenses you're almost guaranteed not to get into trouble. on some of the older lenses though, f and t could be several stops apart. the worst i've ever seen was an arri lens where they differed by 1/3 or so. keep in mind also that the f-stop should be used for accurate depth of field calculations, since it depends on the actual size of the aperture and not on the amount of light.