OT - Zoom changes Iris Numbers?

TomHHI wrote on 1/8/2005, 8:01 AM

The f# is the diameter of the lens opening (iris) divided by the focal length. With a constant iris diameter, zooming in increases the focal length, thereby increasing the f#.

Chienworks wrote on 1/8/2005, 8:04 AM

The "f" number is a ratio of the diameter of the lens opening to the focal length of the lens. Any given f number represents the same amount of light being delivered per unit area of the image. This of course means that a longer lens needs a larger opening to give the same amount of light. For example, a 15mm lens with a 7.5 opening is f2. A 100mm lens would need a 50mm opening to still be f2.

Now, a zoom lens, on manual, doesn't necessarily change the iris size when zooming. So when you zoom in, the same size opening becomes a smaller ratio compared to the focal length. Therefore, less light gets through, and this is reflected by the f number going up. Smaller f numbers mean less light. You would hvae to open the iris manually to match the ratio needed by the logner focal length as you zoom.

Grazie wrote on 1/8/2005, 8:30 AM

Sorry guys . .I kinda knew this, and I guess you knew I knew this too . . but, please count to 10 here . .how does the camera know to change the numbers when I'm in manual? What is going on internally that "pushes" the f numbers up as I zoom in?

If I'm in manual surely any ref to taken note of the light coming "in" has been removed - that's what manual means - surely? . . . ok, I'll stop calling you surely . . But do you see my quandry? If in manual what is going on that allows f numbers to be raked up? There has to be another feedback to the f settings "display" to do this. If this is the case then there must be an internal circuit that is therefore "responding" to light . . this to me must mean it is reference that which "would" have been used for the auto side of things?

I promise, I'll see the light soon . . .

Grazie

Chienworks wrote on 1/8/2005, 9:07 AM

I'm sure a lot of this depends on the design of the camera and the lens. One thing to keep in mind though is that even though you're in manual, that doesn't mean the camera isn't still keeping track of everything. The camera can still be very aware of the zoom setting, the iris setting, the exposure, the focus (or lack thereof). It's just letting you make the adjustments rather than doing it itself.

In your particular model, it could be that the camera is aware of the focal length and the size of the aperature, does the math for you, and displays the result.

Grazie wrote on 1/8/2005, 9:24 AM

Now THAT'S what I've been wanting somebody to confirm for me . . Kelly you know me too well now! - Now I've thereefore got some sub questuions as a result . .. I'll work on them.

THANK YOU Kelly ..

Grazie

rmack350 wrote on 1/8/2005, 11:02 AM

Just to cloud the waters...

On manual zoom lenses, the iris stays put no matter where you are in the zoom. The lens also stays put at the f number you set it at.

I've notice on the specs of a few electronic still and video cameras that they specify two maximum f-stops, one at wide and one at maximum zoom. While most of the higher end lenses I've seen don't allow you to open up wider than what's usable at the longest focal length, consumer cameras have low ligh sensitivity as a selling point. Because they are usually fully automatic, they could get away with this sort of behavior and it allows marketing to make greater claims about low light performance.

I may have missed it but I've never seen a 1st AC have to pull iris during a zoom. I surely would have heard grumbling about this over my 12 or so years pushing dollies and listening to grumbling. Much of this was on 16 and 35mm shoots where you would pretty much never see auto iris used, if it was even available. And I think I would have heard it with video shooters as well since they never used auto iris and usually were shooting wide open.

More exotic zoom lenses might well allow this behavior but normally you don't get to open up past what the lens can handle at max zoom. (At least that's what I believe, others with more experience may just say "yeah, I knew that, and we just dealt with it.")

In your case you're going to have to either keep your practical widest iris number in your head or set exposure on the zoom, while you get your focus.

Rob Mack

Chienworks wrote on 1/8/2005, 12:13 PM

Zoom lenses that keep a constant f-stop number are harder and more expensive to produce. In order to save money, the manufacturer could produce a lens that doesn't retain a constant iris ratio and count on the auto features of the camera to correct for it.

Heck, a true zoom lens keeps accurate focus throughout the zoom range, and this is harder to produce too. I've seen some cheaper zooms that don't maintain focus because the camera's autofocus will correct for it. My cheapy Panasonic PV-DV900 is this way. Switch it to manual focus and zoom in ... it gets totally blurry.

Grazie wrote on 1/8/2005, 12:40 PM

Okay - When in Manual everything and "fixed" iris, at say 1.6, as I Zoom in the actual f numbers in the display rack up.

I'm not calling for the explanation of the various types of zoom through glass or whatever - but my "displayed" numers get BIGGER! They start creeping up fom 1.6 > 1.8 > 2.X . and onwards . . . the numbers displayed. If, as I've "locked" the iris to 1.6 why or rather WHAT is making the numbes increase? The zoom toggle? Is there any reference back to the iris? Is it trying to tell me something? If it is trying to tell me something then I'll listen . .and act on it. If it is making a "best" stab at what the Iris setting should be - BY INDICATING THIS - then I'll take noitice. But how optically correct and with what validity or how much importance should I impart to the numbers being displayed? If they are a combinatiohn of the position of the zoom toggle and/or the glass moving then this is ONLY one side of the story/equation?

Grazie

Chienworks wrote on 1/8/2005, 12:44 PM

Well, we're all just guessing here. You'll probably have to contact the camera manufacturer to get a real answer.

Grazie wrote on 1/8/2005, 12:46 PM

Fair . . . Grazie

rmack350 wrote on 1/8/2005, 5:34 PM

Ugh! Seems like an important test to make before buying a camera. I hadn't really considered this. The inability to keep focus through a manual zoom bugs me more than an iris change.

Rob Mack

rmack350 wrote on 1/8/2005, 5:36 PM

Grazie,

Try the experiment in reverse. What happens if you zoom all the way in, lock the iris, and then zoom out? My guess is that the exposure will start at that higher setting and stay there. But I could easily be wrong.

Rob Mack

jaegersing wrote on 1/8/2005, 6:03 PM

Hi Grazie. I think the indicator is telling you the effective aperture at any given time. Even though you set it at f1.6 when zoomed out, the lens cannot sustain this when you zoom in, and the actual aperture gets smaller (f18, f2.0, etc.). The display is not telling you to change anything (not directly anyway), it is just telling you how things are right now. And what this means is that if you have an evenly lit scene and set exposure when zoomed out, you will be slightly underexposed as you zoom in. If it's possible to set for correct exposure of the zoomed-in subject, then you will not need to adjust anything as you zoom. Of course, in practice you will not have an evenly lit scene, so the initial exposure setting has a lot of possibilities and that's where you might need to make the best compromise for the exposure setting so that the image is OK right through the zoom range.

If you use an auto exposure mode, the camera would normally compensate for the reduced aperture by changing either the shutter speed or the gain of the CCD output signal before it is converted to digital. The exposure shift during zooming can be very noticeable and unwelcome, so manual is the way to do it.

Richard Hunter

farss wrote on 1/8/2005, 6:09 PM

Just digging through the memory a bit here. I remember a pro stills guy explaining to me that 'f' stops relate to DOF, not actual light transmission. So two lenses at the same f stop and focal lenght may have one or more stops of difference in the amount of light coming thru the lens.
Now he explained that video lenses, the good ones are calibrated in 't' stops. Two different lenses at the same t stop and focal length will let in the same amount of light but may have different DOF. His explaination was that traditionaly video wasn't concerned with DOF but was concerned with getting enough light.
Probably doesn't help one iota though.

You probably also need to factor in what the camera is doing with gain.
Bob.

Chienworks wrote on 1/8/2005, 6:14 PM

Bob, any good still camera lenses will have f-stops calibrated for light transmission ability, not for DOF. When setting exposure a light meter will tell you what f-stop to use and this setting works on any lens regardless of focal length, brand, etc. Otherwise, you'd need a conversion chart for every lens telling you, "if the light meter says f5.6, use ...", and that would be a nightmare.

Barry_Green wrote on 1/9/2005, 12:39 AM

Richard's answer is correct, as to what you're experiencing.

The lens is designed such that it has a maximum aperture of f/1.6 on the wide-angle end of the lens, and f/2.8 at the most telephoto (in the case of the FX1 and DVX, other cameras may be similar; I believe the XL2 is f/1.6 on the wide end and f/3.4 or so on the full telephoto end).

Zoom out to maximum wide angle, and then open the iris up, and it'll go all the way to f/1.6 (or "OPEN", which would equate to f/1.6... on the DVX, you'll see a listing for f/1.7, and then the next more-open setting just reads "OPEN").

Then, zoom all the way to full telephoto and try to open the iris as much as you can. You'll get f/2.8. The lens has a different maximum wide-open aperture depending on what portion of the zoom range you're at. And so when you're at max wide angle and you open the iris up, and then you commence zooming in, as you zoom the design of the lens won't be able to accomodate such a wide aperture, so it lets you know what the current widest aperture it can support is.

As far as f-stops and t-stops... f-stops are a mathematical relationship to the size of the iris versus the focal length of the lens. T-stops are "transmission stops", which are calibrated to accurately measure the amount of light actually transmitted by the lens. Various factors can come into play, such as the more optical elements the lens has, the more light gets "absorbed", and the less actually gets through. So you may end up with a lens that at f/2.8 is rated for T3.3... implying that there's a certain amount of light loss in the lens, and the actual transmitted light would correspond to T3.3 when the aperture is set at f/2.8.

So when calculating exposure, you want to use the T-stop. For calculating depth of field, you'd use the F-stop.