Help with F-Stops and exposure

spl

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Hi there, I see this is a basic question but I can't find the answer on my own. I have seen on various websites lists of f-stop numbers that correspond to full stops, for example: f/1.8, f/2.0, f/2.8, f/4

However my lens doesn't have an f-stop at 2. The 50mm Zukio Om series I'm using goes: f/1.8, f/2.8, f/4 ...

Some references suggest there is one and a third stops between f/1.8 and f/2.8, however there is no markings or colour difference on the lens to suggest either is a fractional stop.

So I guess my question is this: Are full stop numbers the same numbers for all lenses OR are the stops marked on my lens always full stops for my lens but not applicable to a different lens?
 

Dali

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A stop is a stop, being « full » or not. What is difficult to understand ? Why full aperture should be part of the std serie we use now (f/ 1.0 / 1.4 / 2.0 / 2.8 / etc...) If it is optically different ?
 

MattKing

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f/1.8 is a ~1/4 stop larger than f/2.0.
1/4 stop doesn't make a lot of difference, but if you had a lens that offered both, and the lens settings were accurate, and you exposed slide film to the same scene in the same light to each, you would be able to see a small difference.

If you want to do the calculation, square the f/stops, and then divide the results by each other.
 

RalphLambrecht

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guangong

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A simple answer with reference to the lens in question: the lens does offer 1.8, but then not enough room to inscribe f2 between 1.8 and 2.8. The 1.8 probably is a promotional ploy. Many of my lenses marked f2 actually open up slightly more...probably to 1.8, but who would ever need the tiny difference? As others have commented, just consider the 1.8 to be read as 2. Most high end lens makers wouldn’t bother.
 

MattKing

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guangong said:
Most high end lens makers wouldn’t bother.
Click to expand...
During the early SLR wars - think the 1970s into the 1980s - lots of them (Canon, Konica, Olympus, Nikon? + +) bothered, at least with 50mm standard lenses.
They did because they were all trying to gain market share in the advanced amateur market, and an f/1.8 lens could be sold as being faster than an f/2.0 lens to that target audience.
And they were faster - just not significantly so.
 

wiltw

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And there were some f/1.7 normal lenses on the market, too. Even NOIW there are a number of f/1.7 lenses from different manufacturers!
 

gone

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Even weirder than the imaginary "faster" lens stops is the way different lenses handle those f stops. The R 90 Elmarit I have is a 90 f2.8, but has a much, much shallower DOF than the Niikor 85 1.8 lens that it replaced. The R Elmarit's f2.8 wide open looks closer to the R 90 f2 Summicron in terms of the bokeh and DOF.

I often have to stop the Elmarit down to f4 for portraits or close up work, which sounds pretty strange, and even at f4 I have trouble getting the front part of the face and the nose in focus. There's nothing wrong w/ it, that's how Leica designed it, and at F5.6 it begins to get real sharp. The later R 90 2.8 lenses are a different design, and tend to be sharp even wide open.
 

MattKing

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AgX said:
It is 1/2 stop difference.
Click to expand...
2.0 x 2.0 = 4.00
1.8 x 1.8 = 3.24
4.00/3.24 = 1.2345 stops
f/1.63 would be 1/2 a stop faster.
QED
 

AgX

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The maximum aperture of a lens often is prestigious. And to achieve this it is common to offer just a half-stop gain.

Thus next to f1.4 lenses, which have a gain of a full stop to f2.0 (which once was a standard for high lens speed), there also are the cheaper f1.8 lenses.
And for these it is common to ommit the f2 stop.

Thus then with a lens with 1/2 stop click-stops it is 1.8 , 2.4 , 2.8 , 3.4 , 4 , 4.8 , 5.6 , etc.
Thus in this case between the first two click stops is a gap of 1 stop, instead of 1/2 stop. One should have this in mind when stopping down click stops. If the camera yields TTL metering one may meter to check if the second click stop is really at the half-stop position, or maybe eased out a bit to make the first click-stop step a bit smaller.
 
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AgX

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Concerning the comment by Matt:

We got ranges of full-stop, half-stop and third-stop aperture values. The full stops of course are all the same in these ranges.

Also the mathematical values differ from the nominal ones.

The 1.8 value nominally is from the third-stop range. But as our lenses typically have click-stops at half-stops I subsumized the 1.8 value under half-stops too. (To make it not even more complicated.)

Of course theoretically we get by this a tiny bit less big gap between the first two click-stops than I described (5/6 instead of 1 stop). But our lenses got quite some play at the diaphragm anyway.



And for aperture calculating, I use just the nominal values and the factors 1.4 , 1.2 and 1.1
Close enough for me...
 
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guangong

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I was unaware tha iPad ‘so automatic correction feature garbled my post. Assuming that the f stops for Leitz and other high end lenses are accurate, often the lens aperture will open slightly beyond the f2 mark, probably to f1.8, but lens makers didn’t bother with marking. Difference is so small and could sway only uninitiated buyers. Also, faster isn’t always better. The Canon f0.9 lens was a flop at the market place, although probably coveted by collectors for its rarity.
Paraphrasing Carl Sandberg’s comment on cigarettes: All cameras and lenses are better than all other cameras and lenses.
 

Ken Nadvornick

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The formula is: f/stop = sqrt (2) ^ stop number
Or alternatively: f/stop = sqrt (2 ^ stop number)
Where: f/1.0 = stop number 0, increasing thereafter by whole stops

If f/1.8 is assumed to really be f/1.800000000 (unlikely), then it lies very close to 1+7/10 of a stop above f/1.0:

f/stop = sqrt (2) ^ 1.7 = 1.802500925

Or more closely:

f/stop = sqrt (2) ^ 1.696 = 1.800003859

The following is a generated chart showing fractional f/stop values from f/1.0 through f/32.0 in 1/10-stop increments. Ignore the right-hand column of zeros, as this is a non-standard use of this utility.



Ken
 

MattKing

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AgX is talking about those handy little click stops and marks on real world lenses.
A few of us are talking about f/stops (and in one case T/stops) themselves.
They are connected, but not perfectly .
 

Chan Tran

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wiltw said:
If you read the article which was linked in post #7, f/1.8 is 1/3 f/stop
Click to expand...
I would say 1.8 is 1/3 faster than f/2. 1.7 is half stop faster than f/2.
The way they rounded off number for the f/stop and shutter speed are not exactly mathematically correct.
 

AgX

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wiltw said:
If you read the article which was linked in post #7, f/1.8 is 1/3 f/stop
Click to expand...

Of course I know the various ranges of nominal aperture values. But for practical reasons, which I explained in my succeeding post, I handled F1.8 as belonging to the half stop range.
We can become very academic here, but the OP's question was one related to an actual scale, one with half-stops.
 

Chan Tran

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Most listed and I agree that 1.8 is 1/3 stop from f/2 and 1/2 stop from f/2 is 1.7.
 
OP
OP

spl

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Thank you all for great responses!

I do understand most of this, and I get that the f/stop number is related to the area/geometric property and has a log(2) relationship.

I guess what I'm getting out of this is as follows:

Full f/stop values go in the sequence: f/1.0 f/1.4, f/2, f2.8. This scale is standard because it begins at 1.

However my lens *clicks* at f/1.8, f/2.8, f4 ... and so on. This means that the difference between the first and second click is in excess of one f/stop

This means that if I go from the first to the second *click* I'm closing the aperture slightly more than a stop, and slowing the shutter by one stop will not result in the exact same exposure, so the rule of clicking the aperture and shutter in opposite directions but keeping exposure identical is not a correct assumption between the first two clicks on my lens because they are more than a stop. (Though probably not enough to get upset about)
 

Nicholas Lindan

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spl said:
the difference between the first and second click is in excess of one f/stop
Click to expand...

And don't be too surprised if it is less. The interval on the remaining f-stop clicks will be pretty accurate, but that first one is a bit of a wish-and-a-prayer on the part of the lens maker.
 
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