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Summicron and f stops

KCS

By KCS
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KCS

KCS

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Newbie alert... I have an M6 and a summicron 50/2, and I'm new to film... I'm also used to having my DSLR handle things. Now that I have to think more, I'm a little confused about how f stops are indicated on the lens. Is one stop f2 to f2.8? Or is one stop f2 to the unnumbered intermediate "click/stopping point" on the lens between f2 and f2.8?

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lars_bergquist

lars_bergquist

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Well I expected this to come up some day.

 

Aperture numbers are relative to the focal length. They are the second (lower) term of a division: 1:2 (or f:2) means focal length divided by the effective diameter of the hole that lets in the light is 1:2 i.e. half the focal length. But on the aperture ring, only the last term is written out, for reasons of space. Now, following this reasoning, the aperture (or 'speed') of 1:4 is one quarter of the focal length. If the focal length is 50mm, then the hole at f:4 is 50mm/4 = 12.5mm.

 

But what lets in light is not the diameter, it's the hole—i.e. its area! Now if we want each change of a full f-stop to exactly double, or halve, the light that gets through, we cannot use the sequence of 2 – 4 – 8 – 16 ... because these steps would actually change the amount of light by a factor of 2x2 = 4. Instad of multiplying (or dividing) the stop number by the factor of 2, we have to use the factor of the square root of 2 = 1.414 ... —consult your pocket calculator. Hence, we get the sequence 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 ... where, as you can see, every number is 2x the *second* previous, and 1.4x the immediately previous.

 

This way, each full stop does exactly double or halve the light. Why should we want that? Because this is what the marked shutter speeds do. They too are abbreviated fractions of seconds: 1/125 – 1/250 – 1/500 – 1/1000 and so forth. This means that you can change speeds or apertures without changing exposure, which is of course the product of the length of exposure and the size of the hole.

 

Consequently, 1/125th of a second at f:8 is the same exposure as 1/250th at f:5.6 or 1/500th at f:4 and so on. Nifty! Also, 1/125 at 5.6 is always the SAME exposure, irrespective of focal length. It is the same with a 21mm wide angle lens as with a 400mm tele cannon. No need to recompute it.

 

Now print this out and read it several times.

 

The old man from the Age of Manual Cameras

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lars_bergquist

lars_bergquist

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Im waiting for the mathematician to chime in with two pi little r squared equals pie big R squared. Or, that your aperture ring has half click stops.

 

Where/how do I open up those little doolacky digits on a MS keyboard?

Sorry, I had to do this, because entire generations of 'photographers' have grown up knowing exactly nil about elementary things like apertures and shutter speeds. Now one turns up and wants to know ... should we not help him?

 

There is more joy in heaven over one sinner that repenteth than over ten card-carrying saints.

 

As for pi and root signs etc: on a Mac, there is a character display and insertion facility in the top bar. There is something like it in Windows, but I don't know where and I don't want to know. However, in spite of the fact that the Web and the HTML standard are supposed to originate at CERN, you cannot normally use these characters here though they are usable on your own screen and in printouts. A root char is replaced with a ?. See, I tried!

 

The old man from the Age of the Mac Classic

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luigi bertolotti

luigi bertolotti

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A great explanation Lars. Thanks a lot for that.

 

It also makes sense why long focal length lens are usually slower; and why for a zoom lens, the minimum aperture increases as we zoom out.

 

AFAIK, this isn't always true: I'm not an expert on zoom lenses design, but seems to me that, probably depending on the zoom design used, a zoom lens can indeed be designed so that max aperture doesn't decrease when focal length increases: Lars, maybe if you play the role of "the old man from the Age of Voigtlander Zoomar" you can make clear this concept...:)

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KCS

KCS

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Thx Lars. I appreciate the detailed explanation, and I will need to read it several times :-) For a bit of background, I started with digital several years ago and recently realized how easy it has been to never try to understand the basics of photography. I can take great shots with a DSLR, and when I'm unsure of what will work, I just take more with different settings and hope that something works.

 

My reason for getting a manual camera was precisely to find the holes in my understanding, force me to slow down a bit, bang my head against the wall a bit, and actually learn something.

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lars_bergquist

lars_bergquist

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Lars,

 

What is Stockholm greater than? :rolleyes::D

 

(For those who don't understand the question look below Lars's avatar.)

 

Pete.

Greater Stocholm is bigger than downtown Stockholm, just as Greater London is bigger than the 'traditional' London.

 

The old man who immigrated here

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lars_bergquist

lars_bergquist

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Thx Lars. I appreciate the detailed explanation, and I will need to read it several times :-) For a bit of background, I started with digital several years ago and recently realized how easy it has been to never try to understand the basics of photography. I can take great shots with a DSLR, and when I'm unsure of what will work, I just take more with different settings and hope that something works.

 

My reason for getting a manual camera was precisely to find the holes in my understanding, force me to slow down a bit, bang my head against the wall a bit, and actually learn something.

Very sensible. The 'photographic' side of the M8 (as distinct from the digital side) is deliberately kept to the classical minimum: Aperture, shutter speed, focusing. I do very often use my M8 on manual, and sometimes indeed without referring to the exposure meter. In backlit situations for instance, I may know the proper exposure for a normally lit subject and base my choice of exposure on that, because I know I am more intelligent than the meter. I am better at judging subjects than the meter; the meter is better than I am at measuring absolute light levels.

 

The exposure histogram in the 'INFO' display is an excellent aid in learning intelligent exposure, if you compare it to the picture itself. The aim is to be sure enough to mostly find 'chimping' unnecessary (good for battery life too!) but 'chimping' is an excellent learning aid.

 

The old man from the Age of Manual Cameras

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  • 15 years later...
Sanso

Sanso

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@lars_bergquist I grew up shooting manual / film cameras and never stopped. I think there is some oversight here on @KCS question about ‘unmarked half stops’ on the summicron. I recently purchased a summicron lens and have for the first time in my career see a lens with stops between stops. I have never seen this on any of the many lenses I have owned over my many decades of shooting and collecting cameras. I think the question is: are these unmarked clocks between stops actual half stops? 

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Jeff S

Jeff S

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On 6/5/2024 at 10:19 PM, Sanso said:

@lars_bergquist I grew up shooting manual / film cameras and never stopped. I think there is some oversight here on @KCS question about ‘unmarked half stops’ on the summicron. I recently purchased a summicron lens and have for the first time in my career see a lens with stops between stops. I have never seen this on any of the many lenses I have owned over my many decades of shooting and collecting cameras. I think the question is: are these unmarked clocks between stops actual half stops? 


Well, this discussion is over 15 years old and, sadly, Lars has passed on, while KCS hasn’t posted on the forum since 2013.  Still, even then, many lens manufacturers used, and still use, half-stop clicks. The Zeiss 50 equivalent even uses 1/3 stop clicks.  That said, I think Lars covered the actual meaning behind the markings rather well.

Jeff

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Sanso

Sanso

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Thanks @Jeff S ! I appreciate your reply and sorry to hear about Lars. Yes his reply was excellent. I was just wondering if those half stops are usable stops? I’m used to knowing by feel that I’ve moved from f11 to f5.8 for example but keep looking at my lens and noticing it’s sitting in random half stops. 

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Jeff S

Jeff S

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5 hours ago, Sanso said:

Thanks @Jeff S ! I appreciate your reply and sorry to hear about Lars. Yes his reply was excellent. I was just wondering if those half stops are usable stops? I’m used to knowing by feel that I’ve moved from f11 to f5.8 for example but keep looking at my lens and noticing it’s sitting in random half stops. 

Fully usable, just need to count differently by feel, if that’s your approach.

Jeff

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