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03-25-2016, 02:42 PM   #1
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regarding f/1.2 and f/1.8 lenses

So I always was told that an f/1.2 lens was 1/2 stop faster than an f/1.4 lens (i.e. f/1.19) but it makes me wonder if such lenses aren't really closer to f/1.26 (only 1/3 stop faster than f/1.4). This could explain why so many people report very little difference between f/1.2 and /1.4 lenses in real world tests...

In an analogous way, are so-called f/1.8 lenses the reported 1/3 stop faster than f/2 lenses (i.e. f/1.78)? Or could it be that they really are just 1/4 stop faster (i.e. f/1.83)? I've always had this nagging suspicion that many f/1.8 lenses might actually be just "tweaked" f/2 designs in order to get them another 1/4 stop "better" marketing performance through some clever engineering. I do note that most f/1.8 lenses on the market seem to skip f/2 as an aperture choice and only allow a jump to f/2.8...

Thoughts?

Michael

03-25-2016, 02:47 PM   #2
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I always thought that f2 Taks were tweaked f1.8s to make them more 'budget'?
03-25-2016, 02:54 PM   #3
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QuoteOriginally posted by MJSfoto1956 Quote
I've always had this nagging suspicion that many f/1.8 lenses might actually be just "tweaked" f/2 designs
My "Lady Luck” (SMC Pentax-A* 135mm F1.8) often reports itself to be a F1.7

03-25-2016, 02:55 PM   #4
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I'd say that rounding is the main culprit. If you consider the T stop, many fast lenses actually transmit noticeably less light in practice than in theory. Thanks to stepless metering, though, these kinds of discrepancies are a non-issue with regards to exposure. As long as the camera knows that it's stopping down by exactly a third of a stop, and the wide-open light reading, the precise f value does not need to be known.

03-25-2016, 02:57 PM   #5
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I think these labels have become conventions. Just like with some other electronics
I remember reading that some 50mm lenses are closer to 53mm, but are labelled as 50 because.. well, 50 is simple, has tradition, and 53 seems a bit odd.
So it is possible that they use the same general design and tweak it one way or another just a bit, and then give it the more marketable label
03-25-2016, 02:59 PM   #6
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QuoteOriginally posted by Kerrowdown Quote
My "Lady Luck” (SMC Pentax-A* 135mm F1.8) often reports itself to be a F1.7
well 1/3 stop = f/1.78, so perhaps they didn't "round" but just "trimmed".
Or perhaps CaNikon are 1/4 stops while Pentax are 1/3 stops????

M
03-25-2016, 03:17 PM   #7
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The thing is 1/3 stop difference is visible for exposure. It is not really noticable for noise levels or dof.

In photography the unit is more like 1 stop and while camera leave the choice to go as low as 1/3 stop difference you can't be more precise for apperture/iso.

But here you speak of 1/4 stop instead of 1/3stop or 1/2 vs 1/3 stop so in reality we speak respectively of 1/12 or 1/6 of a stop difference. Something that is completely irrelevant in practical terms.

The thing is sure the appertures work a bit like psychological prices like 99$ when you see f/1.8 or f/1.9 it is still a very fast lens, bellow f/2. f/2.8 is fast because it is bellow f/3 while f/3.2 is "slow" like for DA21, even fi there less than 1/3 of a stop difference with f/2.8.

But on the opposite there no reason neither to expect that f/2 lenses are really f/2. Likely some f/2 lenses are really more f/2.08 or f/1.95 but it doesn't really matter. It is rounded to f/2.

03-25-2016, 04:04 PM   #8
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again, the original thinking had to do with the proverbial f/1.2 lenses of yesterday.
I'm simply not convinced they are 1/2 stop faster than f/1.4 lenses.

M
03-25-2016, 05:17 PM   #9
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Send me one and I will test it for you?
03-25-2016, 06:02 PM - 1 Like   #10
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QuoteOriginally posted by Kerrowdown Quote
My "Lady Luck” (SMC Pentax-A* 135mm F1.8) often reports itself to be a F1.7

I would quickly dispose of this faulty lens !
I will PM you my postal address - shipping pre-paid!
03-25-2016, 08:23 PM   #11
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Since the aperture number is based on a ratio of the focal length, if focal lengths' numbers are rounded (or fudged) by 3mm or thereabouts sometimes, doesn't that in itself inject inaccuracy into the stated aperture number?
03-25-2016, 11:22 PM   #12
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It is my understanding that 1.7 is 1/2 stop slower than 1.4 and 1.2 is 1/2 stop faster with 1.8 being 1/3 stop faster than f/2. That being said, I remember that the SMC Pentax 50mm 1.4 lens was tested slightly faster by transmission (T-stop) than the f/1.2 Olympus lens by Popular Photography sometime over 30 years ago. Way back when I had my first Spotmatic I compared my old Tamron 70-210 f/4 to my new SMC Takumar 85-210 f/4.5 and found the Tak transmitted more light according to my light meter. "f" numbers are mathematically calculated and there are a lot of variables. Film makers have historically used t-stops for more consistent results. TTL lightmeters correct for irregularities but it could very well be that some manufacturers take advantage of that.
03-26-2016, 02:21 AM   #13
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QuoteOriginally posted by goatsNdonkey Quote
Since the aperture number is based on a ratio of the focal length, if focal lengths' numbers are rounded (or fudged) by 3mm or thereabouts sometimes, doesn't that in itself inject inaccuracy into the stated aperture number?
This go further for me, the focal length change with focussing distance in many cases and if zooms too change focal length. But the physical max apperture size doesn't move. There can be optical elements to compensate for that, sure. But to think that a lens said to have some max apperture has exactly this max apperture ratio at all focussing distance and focal length is naive.
03-26-2016, 02:25 AM   #14
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QuoteOriginally posted by pentaxus Quote
It is my understanding that 1.7 is 1/2 stop slower than 1.4 and 1.2 is 1/2 stop faster with 1.8 being 1/3 stop faster than f/2. That being said, I remember that the SMC Pentax 50mm 1.4 lens was tested slightly faster by transmission (T-stop) than the f/1.2 Olympus lens by Popular Photography sometime over 30 years ago. Way back when I had my first Spotmatic I compared my old Tamron 70-210 f/4 to my new SMC Takumar 85-210 f/4.5 and found the Tak transmitted more light according to my light meter. "f" numbers are mathematically calculated and there are a lot of variables. Film makers have historically used t-stops for more consistent results. TTL lightmeters correct for irregularities but it could very well be that some manufacturers take advantage of that.
Transmission is another thing than apperture really. We assume it is the same for simplicity, but light as to go through the glass, some of it is absorbed, some of it is reflected and this has nothing to do with the physical apperture. The apperture define the maximum light density that can go through the lens, not the effective light density that go through the lens.
03-26-2016, 09:15 AM   #15
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There's no denying that 1.2 lenses are physically larger, heavier, and with larger diameter elements. You've got to get something for all that extra glass!
But even if you don't need that extra little bit of light - a half stop doesn't amount to much these days, when you can just raise the ISO a tad - the fun thing is that the depth of field is noticeably shallower with a 1.2 used wide open.
I've had some fun fooling with a few 1.2's over the years, although sadly I've never had a Pentax one to test. The best one I used was a Nikon 1.2 Noct. All the others have had, let's say, a character that was different from a comparable 1.4.
The razor-thin depth of field used to be considered a liability. Nowadays it's a desirable look. And that's probably why a 1.2 is interesting these days, even if you don't need that extra bit of light.
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