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06-29-2012, 12:55 PM   #1
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How accurate is the aperture reported by each manufacturer?

Not sure if this has been discussed before but I can't seem to find anything on PF so here goes.

I'm wondering if the aperture reported on each lens is consistance and accurate across brands, focal length and speed. etc. mainly the old MF lenses.

I guess my question is < is the f2.8 lens really a f2.8 or it's somewhere between f2.7 and F2.9 for example?>

What is a normal margin of error for those aperture numbers, if any?

The way I underdstand is the aperture is controlled by aperture blades but looking at the way the aperture blades function, while I'm no expert, I doubt it's very accurate even for the same lens of the same brand let alone across brands.

Can anyone shed some lights?

06-29-2012, 02:11 PM   #2
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There is the question of accuracy. That would be expected to vary between brands and even between copies of a particular lens. There is also variation (imprecision) between exposures. The matter of accuracy used to be one of the big arguments in favor of stop-down metering back-in-the-day when the Pentax Spotmatic was a popular tool. Once open-aperture metering became the standard, people pretty much decided that issue was not particularly important.


Steve
06-29-2012, 04:39 PM   #3
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There are all sorts of variables regarding how much light hits the sensor or film. My guess is there are industry standards regarding such things, but you might find them surprisingly lax. The F/stop system we use is a mathematical measurement, not a measurement of transmission. It is possible that a diaphragm mechanism is sloppy or inaccurate. Different glasses have different transmission properties (yes glass absorbs light). Also, different coatings affect transmission. When multicoating was first available, Modern Photography tested the Pentax 50mm 1.4 as transmitting more light than an Olympus 50mm 1.2 lens and according to my Spotmatic meter my SMC Takumar 85-210 4.5 lens showed as being 1/2 stop faster than my Tamron 70-210 f/4 lens it was replacing. In the movie industry they use lenses marked in T-stops (measured transmission) to eliminate variables. For us slr folks, the internal meter compensates for transmission differences but the diaphragm mechanism can cause exposure problems.
06-29-2012, 05:01 PM - 1 Like   #4
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There are three aspects to this question,
first, how accurate is maximum aperture, which has had lens makers claim different things within the limits of standards largely for marketing purposes. They used to test this sort of thing, in lens reports, along with focal length as tested, but lately that seems to haves disappeared.
Second, how consistent is the aperture as you stop down. For example my sigma 70-200/2.8 is within +/-1 count on greyscale from F2.8 to F32 which I consider excellent when thinking that 45 greyscale is one stop. My Tammy 28-75F2.8 h the exposure drift gradually a complete stip from wide open to stopped down, seems the mechanism is not perfect
Third is light transmission, my Ricoh 135/2.8 is one stop slower at all apertures than my other 135mm lenses, but the rendering of out of focus remains consistent at eqch aperture meaning it looses 1 stop in transmission.

So, the real point is test your gear and learn how it behaves.

06-29-2012, 05:23 PM   #5
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+1 Lowell on testing the old manual lenses. I recall doing this one time on some zooms, and found the much maligned plastic, loose, large, SMC Pentax_A Zoom 1:3.5~4.5 28~80mm here is maybe more like a 1:2.8 and fairly sharp too.
When I get time I will do it again and record some images.
Also, take off the filter before testing ap setting.
06-29-2012, 05:37 PM   #6
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I have plenty of old lenses that are visibly and obviously inconsistent when stopped down all the way -- often the last two stops vary a bit, i.e. the hole is not always the same size (or shape). They seem much more consistent at the more open apertures, which is generally where they are used so I don't worry about it much.
06-29-2012, 07:22 PM   #7
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As long as it's an f-stop, it can't be that accurate. If photo lens makers wanted to be accurate, they'd use the t-stop instead.
06-29-2012, 07:35 PM   #8
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QuoteOriginally posted by nater Quote
As long as it's an f-stop, it can't be that accurate. If photo lens makers wanted to be accurate, they'd use the t-stop instead.
Can't do that because then DOF calculations would be off, which do depend on actual f-stop no matter if some lenses have more light loss than others. F-stop is a ratio of physical things, not a certain amount of light. In film school, many of our cinema lenses had both f-stops and t-stops on them, which was very helpful for lighting...

06-29-2012, 08:08 PM   #9
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QuoteOriginally posted by wombat2go Quote
+1 Lowell on testing the old manual lenses. I recall doing this one time on some zooms, and found the much maligned plastic, loose, large, SMC Pentax_A Zoom 1:3.5~4.5 28~80mm here is maybe more like a 1:2.8 and fairly sharp too.
When I get time I will do it again and record some images.
Also, take off the filter before testing ap setting.
I find my copy of the F 28-80(Non-SMC Autofocus) to be much brighter than my 18-135 when set to wide open as well. About as sharp too.

Must just be a quirk with those plastic lenses - Good copies are great, but they build them to the standard of a bad copy.(No disappointments then, eh?)
06-30-2012, 07:39 AM   #10
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Some times I wonder if a lens is very soft wide open are not really that fast at all, and should be re-labeled one stop down instead, ,
But fast is where the big bucks come in

I think quality control determines if a 1.4 is really a 1.4 or not

Thanks

Randy
06-30-2012, 02:30 PM   #11
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Mechanically, I found it amazing when looking at the lens internally:

The aperture pin at the back of the lens can move approx 1cm to cover f22 to f1.2.
The aperture ring follows an unlinear closing guide to give the same distance of each click.
The unlinear guide must allow the lens closed down fast enough to take the shot, hence cannot be too tight to allow iris move freely - too lose will result in inaccuracy.

And we are asking accuracy in 1/3 of a stop, for all lenses, from all manufacturer... if possible.
07-01-2012, 09:25 AM   #12
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OK so the general consensus here is the F-stop reported is not something one should rely on for accuracy.

Thanks all for the great insights.
07-01-2012, 10:48 AM   #13
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QuoteOriginally posted by ducdao Quote
OK so the general consensus here is the F-stop reported is not something one should rely on for accuracy.

Thanks all for the great insights.
It is a design feature, a selling point, ... there is a standard for it and the lens may not be off more than 1/3 stop at the most. That will be for center readings. Otherwise you would be able to sell a f/2 lens for the price of a f/1.4. Companies are not evil, they are not always trying to trick you and yes there are standards. The same holds for focal length by the way. You may not be off by more than 1% of the defined focal length - time to sue some zoom manufaturers :-)
07-01-2012, 12:27 PM   #14
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Lenses are probably close enough not to worry too much about, a wee tweak in PP can usually sort any issues.
07-01-2012, 01:56 PM - 1 Like   #15
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F-stops are purely geometrical, the ratio of aperture to focal length, regardless of actual light transmitted. Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), f-numbers do not accurately correlate with light transmitted. F-numbers corrected to measure light transmission rather than aperture ratio, called T-stops (for Transmission-stops), are sometimes used instead of f-stops for determining exposure.[7] A real lens set to a particular T-stop will, by definition, transmit the same amount of light as an ideal lens with 100% transmission at the corresponding f-stop.

Use of f-numbers leads to exposure inaccuracy, particularly for lenses with many elements. This is particularly problematic in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. To avoid the problem, lenses used in cinematography were bench-tested individually for actual light transmission and calibrated in T-stops, allowing fixed-focal-length turret-mounted lenses to be changed without affecting the overall scene brightness due to differences in transmission for the same f-number. Many modern cinematographic lenses are factory-calibrated in T-stops. In still photography, without the need for rigorous consistency of all lenses and cameras used, slight changes in exposure are less important, and are largely masked except for the highest-absorption lenses by film and sensor exposure latitude. Source = F-number - Wikipedia, the free encyclopedia
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