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07-13-2010, 02:24 AM   #1
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About zoom lenses with variable aperture

Many zooms do have a flexible wide aperture, let's say f=1:4.0 @ 18 mm focal length and f=1:5.6 @ 35 mm focal length.

Now, is there a way to find out or know before the REAL widest aperture at a zoom position anywhere between 18 and 35 mm? The camera tells me of course if I play a little bit, but my intention was to know before I even pick up the lens and mount it!

Basically, I am looking for something like a DOF table, but referring to widest aperture throughout the zoom range.

07-13-2010, 03:56 AM   #2
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Since F stop is just the ratio of the focal length to the diameter of the aperture, I think that as long as you knew the latter, you'd be all set to make a calculation.

F StopFocal LengthDiameterFormula
3.518mm18/3.5=5.142865.14286x3.5 = 18
5.655mm55/5.6=9.821439.82143x5.6 = 55

I don't think the diameter change is linear, so I wouldn't know how to proceed from here

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07-13-2010, 05:52 AM   #3
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It's something that will vary by model. AFAIK, the best you can do is to just post a thread asking people with that particular lens to tell you. If it's an older manual lens, things may not be quite so precise.
07-13-2010, 05:58 AM   #4
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Typically the aperture changes linearly as a function of focal length, and has to do with placement of the iris in the lens much closer to the rear of the lens so that most of the focal length change is already taken care of by earlier lens groups. It does not relate at all to the front filter diameter because this is simply selected on wide angle lenses to avoid vignetting.

But for an 18-55mm F3.5-5.6 It is clear that the 18mm corresponds to F3.5 and 55mm to F5.6, and I think you will find that the aperture changes to F4 at 28mm and F4.5 at about 35-40mm

07-13-2010, 10:41 AM   #5
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Note the actual f-stop may change continuously, not really in discrete steps as your camera has no chocie but to report. Even if your camera *claims* that one lens changes from f/4 to f/4.5 at 26mm and another changes at 31mm, that's just rounding off of the actual f-stops. With either lens, at 28mm, you're probably somewhere between f/4 and f/4.5. So it's really not worth obsessing about the actual change over "points" - because they probably aren't actually "point" at all.
07-13-2010, 11:01 AM   #6
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QuoteOriginally posted by Marc Sabatella Quote
Note the actual f-stop may change continuously, not really in discrete steps as your camera has no chocie but to report. Even if your camera *claims* that one lens changes from f/4 to f/4.5 at 26mm and another changes at 31mm, that's just rounding off of the actual f-stops. With either lens, at 28mm, you're probably somewhere between f/4 and f/4.5. So it's really not worth obsessing about the actual change over "points" - because they probably aren't actually "point" at all.
agreed, It would be somewhat interesting to take a series of shots with one of my FA variable aperture zooms, increasing focal length 1 mm or 2 at a time, and doing an exposure test with a uniformly lit surface. you could then extract from the exif the focal length, aperture and from the histogram, assess the variation in exposure. Since the lense coding with pentax only works in 1/2 stops, you should see some exposure errors / variations at the boundries, but then if they make the transitions at 1/4 stops off the exact settings your exposure would be always within +/- 1/4 of a stop. and the meter should sort that out. Interesting idea for someone with a 28-105 F4-5.6 and time to shoot and assess 35-70 shots.
07-13-2010, 12:30 PM   #7
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Someday I'll probably do such a test myself. Until then, I'm satisfied with the following observation: the viewfinder image doesn't obviously change in brightness at those "points" of change. If it really jumped by half a stop, I should be able to see it.

Another "test" that requires no real work: consider that the "points" of change will presumably differ depending on whether your camera is set to change aperture in 1/2 stops or 1/3 stops. At least, I assume that's true even if the lens coding only works in half stops (not sure what that means exactly).
07-13-2010, 12:36 PM   #8
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QuoteOriginally posted by Marc Sabatella Quote
Someday I'll probably do such a test myself. Until then, I'm satisfied with the following observation: the viewfinder image doesn't obviously change in brightness at those "points" of change. If it really jumped by half a stop, I should be able to see it.
the viewfinder does not "jump" change because the transition is linear and continuous, only the reporting jumps. that you should be able to see.
QuoteQuote:
Another "test" that requires no real work: consider that the "points" of change will presumably differ depending on whether your camera is set to change aperture in 1/2 stops or 1/3 stops. At least, I assume that's true even if the lens coding only works in half stops (not sure what that means exactly).
Your exposure will jump differently yes, as a function of your ev steps, but I am not sure how the camera interprets the lens aperture at maximum, because regardless of ev steps, the coding probably only changes in 1/2 stop increments, unless on newer lenses in the DA range perhaps, they have changed the communication on pin 7 to include other features as well, making the aperture coding pins somewhat redundant.

07-14-2010, 12:41 AM   #9
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Now I know...

Thanks for all the input! Let's hope the open-aperture TTL metering knows to round it close enough...

The funny thing is, that I always preferred constant or nearly constant aperture zooms - now I know why
07-14-2010, 04:45 AM   #10
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Some (or maybe all?) Canon cameras also put the real, effective f-stop into the exif, so in the exif you can find the rounded selected f-stop e.g. f2.8 and the effective f-stop e.g. f2.975. Unfortunately Pentax doesn't seem to include that in the exif, at least I haven't find it.
07-14-2010, 05:34 AM   #11
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QuoteOriginally posted by simico Quote
Some (or maybe all?) Canon cameras also put the real, effective f-stop into the exif, so in the exif you can find the rounded selected f-stop e.g. f2.8 and the effective f-stop e.g. f2.975. Unfortunately Pentax doesn't seem to include that in the exif, at least I haven't find it.
but what is the point unless this is calibrated with the remainder of the lens transmission factors.

One thing that I don't see any more in lens reports is the focal length as tested and aperture as tested.

It used to be that this was reported along with the image quality, but no longer
07-14-2010, 10:14 AM   #12
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Also, consider that metering itself is kind of a shot in the dark (OK, not literally) in terms of hoping to end up with the exposure the photographer wantsed; plus adjusting a fraction of a stop in PP if desired is trivially easy even for JPEG. All the more reason why these fractional difference in reported f-stop just don't matter.
07-15-2010, 01:55 AM   #13
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Yesterday evening, I've tried some F-and-above lenses with the *istD and the Z1p (in HyP modes):

using the dial to choose the widest aperture and zooming slowly, both bodies showed (congruently) the change of aperture on their displays.

For example, the SMC-F 35-135 starts at 35 mm of course with f=1:3.5, but between ca. 40 mm and 80 mm, you'll get f=1:4. From 85 onwards it is f=1:4.5.

I should keep tables of my variable aperture zooms (only 6 as far as I remember), because if you want to decide about a focal length for a specific photoghraphic situation - a moving object under certain lighting conditions, requiring a minimum shutter speed and thus a maximum aperture at a focal length -, it comes in handy to know the circa-aperture of the zoom lens at zoompositions x,y and z.
07-15-2010, 07:05 AM   #14
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Vranx

the question is, what is the actual exposure as you zoom?

while this shows the transition points, and as I suggested, these are probably at the mid points between exact F4 and F4.5, how does the exposure change at the sensor?

this is the other part of the test I suggested.
07-24-2010, 01:22 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
Vranx

the question is, what is the actual exposure as you zoom?
[...]
this is the other part of the test I suggested.
Hi Lowell,

this is the part I didn't understand entirely: will the EXIF show f-stop positions inbetween or do I have to conclude from minimal metering differences when shooting a calibrated object?

Please enlighten me :-)
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