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01-31-2010, 03:30 AM   #1
Jonathan Mac's Avatar

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Apertures of M lenses: 28/2.8, 50/1.7 & 135/3.5


I've recently picked up some manual primes to play with on my K200D, as newer AF versions are either too expensive or very hard to find.

I have a question regarding the aperture settings close to wide open. All of the lenses' aperture rings generally click at full & half stops, ie 4, 4.5, 5.6 etc, although there is only one full stop between the smallest & second-smallest, ie the 50mm goes straight from F16 to F22.

My questions is regarding the apertures close to wide open though: for example the 50mm is F1.7, then 2 clicks to 2.8. I am sort of assuming that it makes sense for the one in between to be F2, but it could just as easily be 2.4, as they are both on the way to 2.8. Similarly, the 28mm F2.8 has two stops in between 2.8 & 5.6: are these 3.5 & 4 or 4 & 4.5? The 135 F3.5 has one stop between 3.5 & 5.6: is it 4 or 4.5?

I haven't been able to find this info on the web, so any help would be appreciated. As long as the photos turn out well I'm happy, but if someone asks what aperture one was taken at, I'd like to be able to tell them

01-31-2010, 06:46 AM   #2
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Usually it's a half-stop, BUT if your first stop IS a half-stop, the next one will be a full stop.

For instance, if your lens clicks at f2,8, something, then f4, that something is quite probably f3,5. If the lens clicks at f3,5, something, f5,6, that something is the full stop at f4.

You could use metering to see if this holds for all lenses. By exposing a static scene identically at two apertures, the relation between shutter speeds and aperture will guide you.
01-31-2010, 11:22 AM   #3
Just1MoreDave's Avatar

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QuoteOriginally posted by bdery Quote
...You could use metering to see if this holds for all lenses. By exposing a static scene identically at two apertures, the relation between shutter speeds and aperture will guide you.
This is the best answer because it will tell you what your lenses actually do. That may be different from other lenses.

A subject that will give a single peak on a histogram is helpful, like the white wall. Use one labeled setting to establish a baseline exposure, and note the histogram. Let's say it's 1/500 sec. at f2.8. That means f3.5 should be 1/350, f4 should be 1/250, f4.5 is 1/180 and f5.6 is 1/125. You can check your math by moving the aperture ring to aother labeled setting, changing shutter speed to the calculated speed and taking another photo. The histograms should match - not exactly, but pretty close. Then for the unlabeled setting, adjust shutter speeds until the histogram matches, and see what aperture corresponds with that speed.

I figured I could answer the question for my M50/1.7 and compare to other lenses. I have an M, two As and an F right here. Put the camera on a tripod, mount the F, set aperture ring to A, take a photo of an evenly lit white wall, adjust for a nice histogram, use Av and take a series of photos from f1.7 to f5.6. Do the same thing with an A lens, same results. Put on the M. Exposures match at f1.7 and f2.8 when I use the shutter speeds from the A or F lenses. Take some photos at the unlabeled click setting. Look at histograms to see what photos match. The unlabeled position is f2.0 on my lens. I went through the more complex process because it's possible to assemble the M50/1.7 and have the aperture out of adjustment. Now I know that it's set correctly.
01-31-2010, 01:35 PM   #4
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I've got all three of those lenses, and based on my experimentation, I'd say the first click on the 50 is actually around f/2.4. The first click on the 28 is f/4, and apparently that's why it is marked with a white dot (to show it is a full stop). The 135/3.5 seems to be somewhere between f/4 and f/5.6 - presumably the half stop, which Pentax usually calls f/4.5 even though mathematically it should be more like f/4.7


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