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Why is max aperture info needed / used?
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PostPosted: Sat Nov 02, 2019 8:18 am    Post subject: Why is max aperture info needed / used? Reply with quote

I've been fiddling with some Tamron adaptall-2 adapters with several lens mounts and film bodies.

My initial question was: how Pentax K and Minolta bodies get the max aperture information that is needed for correct exposure calculation using full aperture meterig.

Then I realized: They don't and they don't need it. The body just reads the light value wide open and stop by stop correct the time value.

But: why do Contax and Konica bodies use the max aperture value. Both meter exposure wrong if they don't get the max aperture value?

Question Confused


PostPosted: Sun Nov 03, 2019 3:34 pm    Post subject: Re: Why is max aperture info needed / used? Reply with quote

kansalliskala wrote:
I've been fiddling with some Tamron adaptall-2 adapters with several lens mounts and film bodies.

My initial question was: how Pentax K and Minolta bodies get the max aperture information that is needed for correct exposure calculation using full aperture meterig.

Then I realized: They don't and they don't need it. The body just reads the light value wide open and stop by stop correct the time value.

But: why do Contax and Konica bodies use the max aperture value. Both meter exposure wrong if they don't get the max aperture value?

Question Confused


I don't know about Contax or Konica, but some Fujica bayonet slr's are the same, the AX-5 can take advantage of the feature by being able to use M42-screw lenses in open-aperture shutter-priority and program modes, using the appropriate (XD) adaptor ... possibly the only body that allowed that functionality with those older lenses?

Maybe the body "knows" how far to push the aperture linkage in for any given stop, provided it's been "told" where it's starting from?

Either way, I'm happy it did, 'cos it meant I hung onto the screw-mount lenses I'd accrued "back in the day" and am continuing to enjoy them now I've "gone digital" Smile


PostPosted: Mon Nov 04, 2019 8:26 am    Post subject: Re: Why is max aperture info needed / used? Reply with quote

kypfer wrote:

Maybe the body "knows" how far to push the aperture linkage in for any given stop, provided it's been "told" where it's starting from?


That is actually how Nikon pre-ai bodies work: you tell them the maximum aperture by winding the ring back and forth. Smile


PostPosted: Mon Nov 11, 2019 4:50 pm    Post subject: Reply with quote

Each camera brand has its own way of processing this information. With Canon FD cameras, for example, they don't read the actual light value unless they're in "stopped down" mode, something that was a holdover from the FL days, and which the FD cameras had to insure backward compatibility. Instead, a Canon FD lens has a stud on the back of the lens that indexes with the body and tells the body the maximum aperture value of the lens being mounted. As a Nikon user also, I've gotten used to rocking the aperture ring back and forth whenever I mount a lens.


PostPosted: Mon Nov 11, 2019 6:07 pm    Post subject: Reply with quote

Actually, the Minolta SLR does need to know the currently selected aperture on the lens in order to provide TTL metering in a simple way, because they are mechanical calculators that only work with a specific arithmetic. If there is no connection to the aperture ring, then the camera will meter wherever the aperture blades are which means you are using stop-down metering and therefore your viewfinder goes dark if you are not wide-open.
As for the other brands, I don't know what they do. But for Minolta, the XD does not even need to know the minimum aperture value of the lens in order to provide shutter-speed priority, because the final-check metering will compensate the result regardless, and you can set an intermediate apertures as a limit to avoid diffractive aperture values reducing the sharpness which means the lens' MD tab doesn't connect with the camera's MD tab. But, it is only because of the CPU and the MC tab and ISO value being provided that this is possible.


PostPosted: Sun Nov 24, 2019 5:05 pm    Post subject: Reply with quote

Contax and Yashica lenses don't pass the actual aperture value to the camera. They pass the maximum aperture and the number of stops the lens is stopped down. Just the way they do it, don't know why.


PostPosted: Fri Jan 07, 2022 12:42 pm    Post subject: Reply with quote

The Minolta SR mount has 2 different tabs that connect with the camera mount, the MC-tab and the MD-tab:

The MC tab connects with the MC lug on the camera mount (SRT onwards). This tab tell the camera how many stops slower the user has set the lens relative to the full open aperture at which TTL light readings are taken. Note this tab does not tell the camera the actual set aperture, only how many stops slower the set aperture is relative to the full open aperture. On SRT cameras this tab actuates a linear cable mechanism that, through use of a pulley-system, combines with ISO and shutter speed settings to move the lollipop in the viewfinder. On later cameras this tab moves a wiper over a logarithmic resistor track. The MC tab therefore allows for aperture-priority mode exposures, where only the stop-down from measurement at full aperture is required as information (at a given shutter-speed / ISO combination).

The MD tab connects with the MD switch on the mount (XD7/11 onwards). This tab tells the camera that the lens has been set at its minimum aperture, and also tells the camera which of three possible minimum apertures this is (f/16, f/22, f/32). This switch therefore has four positions: OFF/16/22/32. It is quite remarkable that Minolta managed to code four reliable switch positions in the space of rotating the aperture ring from the penultimate aperture to the smallest one. This does mean that MD lenses do not have a half-click stop between the two smallest aperture settings, to allow for unhindered operation of this switch. The MD tab allows for shutter-speed priority mode exposures, where it is now desirable that the camera indicates the calculated f-stop, which it can only do by combining the information of minimum f-stop (MD lug) & stop down from full f-stop (MC lug).

As a note of interest; following Minolta's introduction of the acute-matte focussing screen (essentially a very fine array of microprisms surrounding the conventional microprisms & wedge), the f-stop scale on the Rokkor lenses became non-linear at the very wide aperture end (faster than f/4). This was done to allow for the limited numerical aperture and angle of acceptance of this acute-matte micro-prism grid itself; for wider apertures the screen was no longer getting twice as bright for every extra f/stop . This explains the missing half f-stops (or sometimes even full f-stop) at the wide-end of the later Rokkor & MD lenses. The spacing of f-stops as encoded on the aperture-ring needed to allow for this non-linearity of metering when connecting the MC tab to a linear (cable) or logarithmic (resistor) sensor on the camera mount. This adjustment is lens-specific as it also depends on the position of the lens' exit pupil a well as the amount of vignetting wide-open, hence some zoom lenses even have this non-linear aperture ring up to f/5.6


PostPosted: Sat Jan 08, 2022 9:05 am    Post subject: Reply with quote

Thank you Mark.

Friends


PostPosted: Wed Jan 12, 2022 12:12 am    Post subject: Reply with quote

RokkorDoctor wrote:


The MD tab connects with the MD switch on the mount (XD7/11 onwards).
...
This does mean that MD lenses do not have a half-click stop between the two smallest aperture settings, to allow for unhindered operation of this switch.


Interestingly, some late MC-X lenses as well don't half the half-stop between the two smallest aperture settings.


RokkorDoctor wrote:

As a note of interest; following Minolta's introduction of the acute-matte focussing screen (essentially a very fine array of microprisms surrounding the conventional microprisms & wedge), the f-stop scale on the Rokkor lenses became non-linear at the very wide aperture end (faster than f/4). This was done to allow for the limited numerical aperture and angle of acceptance of this acute-matte micro-prism grid itself; for wider apertures the screen was no longer getting twice as bright for every extra f/stop . This explains the missing half f-stops (or sometimes even full f-stop) at the wide-end of the later Rokkor & MD lenses. The spacing of f-stops as encoded on the aperture-ring needed to allow for this non-linearity of metering when connecting the MC tab to a linear (cable) or logarithmic (resistor) sensor on the camera mount. This adjustment is lens-specific as it also depends on the position of the lens' exit pupil a well as the amount of vignetting wide-open, hence some zoom lenses even have this non-linear aperture ring up to f/5.6


This explanation may not be correct - the non-linearity on the aperture ring of MC lenses was there long before the acute-matte focussing screen became available (roughly ten years before!!). The non-linearity of the smallest aperture on MC lenses was introduced with the SR-T to compensate for the lens vignetting at full aperture - authors like Josef Scheibel ("Das Minolta Buch", Heering Verlag 1971) have explained this in detail. Scheibel was an engineer himself, working partly for Minolta Europe in Hamburg and partly for his own engineering company. I have no doubt his explanation is correct.

S


PostPosted: Wed Jan 12, 2022 12:10 pm    Post subject: Reply with quote

stevemark wrote:

This explanation may not be correct - the non-linearity on the aperture ring of MC lenses was there long before the acute-matte focussing screen became available (roughly ten years before!!). The non-linearity of the smallest aperture on MC lenses was introduced with the SR-T to compensate for the lens vignetting at full aperture - authors like Josef Scheibel ("Das Minolta Buch", Heering Verlag 1971) have explained this in detail. Scheibel was an engineer himself, working partly for Minolta Europe in Hamburg and partly for his own engineering company. I have no doubt his explanation is correct.
S


Scheibel is almost certainly right; I have just checked a very early MC lens which indeed already has some non-linearity at the large aperture end of the aperture ring.

However, I can't help but feel the non-linearity is greater on later MC lenses and MD lenses (maybe they have more vignetting?)

Microprisms, wedges / acute matte microprisms definitely have a finite angle of acceptance as far as wide-open large apertures are concerned, which will affect meter linearity; this may or may not have been a further consideration in the calibration of the aperture scales on later lenses. It would be interesting if some documentation re. this could be uncovered.

Non-linearity of TTL light metering at large apertures will definitely have given camera manufacturers some headaches when calculated f/# stops or shutter speeds needed to be shown in the viewfinder... Wink

stevemark wrote:

authors like Josef Scheibel ("Das Minolta Buch", Heering Verlag 1971) have explained this in detail


I do have Scheibel's book "70 Jahre Minolta Kameratechnik", which unfortunately doesn't mention anything about the f/# stops. I didn't realise he had written another book; I'll have to keep an eye out for "Das Minolta Buch" as that clearly contains useful other information. Fortunately I can read German Wink


PostPosted: Wed Jan 12, 2022 9:19 pm    Post subject: Reply with quote

RokkorDoctor wrote:
I didn't realise he had written another book; I'll have to keep an eye out for "Das Minolta Buch" as that clearly contains useful other information. Fortunately I can read German Wink


Scheibel has written many books:
http://www.scheibel.de/buchliste/buchliste.htm


... including several books about the MC/MD system:
SR-T series: Das Minolta Buch (Heering 1971)
XM / XE series: Das Minolta Buch II (Heering 1977); explanation on "non-linear" aperture ring see pages 62/63
XD / XG series: Minolta - faszinierende Fototechnik mit der neuen Minolta-Generation (Heering 1979)
X-700 / X-500: Minolta X-700 - Minolta-Programm-System (Heering 1982)

In addition, Josef Scheibel has written books about the later Minolta AF cameras, starting with the 7000 and ending with the Dynax 7D. However he did not write a book about the 9xi because he didn't agree with the general layout of the 9xi. The later (and very successful) return to a more traditional user interface on Minolta cameras may partly be attributed to his feedback.

This information is based on personal talks with former "Minolta Schweiz" people around 2010 (while I was writing my "Sony Full Frame book: http://www.artaphot.ch/minolta-sony-af/alpha-systembuch)

S


PostPosted: Thu Jan 13, 2022 10:07 am    Post subject: Reply with quote

stevemark wrote:

Scheibel has written many books:
http://www.scheibel.de/buchliste/buchliste.htm
S


Like 1 small Thank you!

Sounds like he was an influential engineer in the German market as far as Minolta was concerned!