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Old Sep 7, 2010, 11:11 PM   #1
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Default Some basic lens questions

Looking at some older lenses:
Why can you get a 50mm as fast as 1.4, but anything less than 50mm about the fastest I have seen is 2, and most are 2.8?

K2000 body:
even after reading many web pages, I'm not sure I understand fully, does an "A" lens work on my body to detect the aperture setting, and only detect the setting or can change the aperture setting?

What's the difference in lenses with the metal "posts" sticking out (maybe I misunderstand, but that is so the camera body can detect the F stop?) vs the electrical contacts I am guessing the camera can tell the lens what to use?

I know an SMC F lens i bought seems to work just fine, but i wanted to try some older lenses, but it would be nice if the camera could at least detect the F stop I am thinking.

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Old Sep 8, 2010, 4:26 PM   #2
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OK, I'll try to divide my answer into digestable pieces.

With posts I assume that you mean the levers sticking out a few millimeters from the back of the lens.
Putting autofocus aside and concentrating on how it deals with f-stops, there are five basic types of SLR camera bodies.

The oldest type (before the sixties) doesn't have any light meter at all. You depend on your own experience or an external light meter to determine the expoure time and f-stop, the latter being set with a ring on the lens.

The second type does have an internal meter but can't handle the f-stop setting when it meters. You stop down the lens with the ring on the lens, in the viewfinder your picture becomes darker and the indicator in the viewfinder tells you if you have the correct f-stop for your chosen exposure time. But it's hard to focus with the dark picture in the viewfinder, so what you do is that you focus with the lens set to the largest f-stop, then stop down and do the light metering and change the f-stop or exposure according to what the indicator tells you.

Third type (and now we're in the seventies) allows you to choose an f-stop with the ring on the lens, but the lens remains fully open till you press the shutter button. One of the "posts" mechanically "tells" the camera body what f-stop you have chosen, electronically calculates what exposure that will give and tells you, with the indicator in the viewfinder, if your f-stop is correct or should be changed. Then, as you press the shutter button, the camera body, using the other "post" or lever, automatically stops the lens down to your chosen f-stop. Hence these lenses and bodies often includes the word automatic in their name.

As the fourth type, (and now we're in the eighties) we have the bodies with electrical contacts in the lens mount, and the corresponding contacts on the lens itself. These bodies doesn't rely on mechanical communication from the lens. Instead you put the lens in the A position on the aperture ring, and handle both the setting of f-stop and exposure time with buttons or wheels on the camera body. This allows the camera body to calculate an appropriate combination of the two, and voila! here you have the Progamme mode, from the Super A/Super Program body and later models. These bodies do have the mechanical levers too, allowing you to use older lenses with them.

And lastly we have the bodies that only relies on the electronic information and contacts. With these the aperture ring fills no function and was removed on a few lens models, and again on the DA lenses of today. But these cameras can't use older lenses, without the A setting. And the lenses can't be used on older bodies, that doesn't have the electronic contacts.

Todays digital Pentax cameras can handle both the mechanical and electronic mechanism for stopping the lens down, but can't handle the information from "post" number one so you can only use these "Pre-A"-lenses in Manual mode and you have to stop down (with the Green Button or DOF preview) to do your metering. This is pretty easy, though, so there's no reson to avoid old lenses when they are found at a favourable price. Some of them are optically superb, and there are no curses that would be strong enough if Pentax does what Canikon have done and take away that backwards compatibility we love so much.

If you had the patience to read all the way to here, I hope some of your questions have been answered.

Kjell

Last edited by bilybianca; Sep 8, 2010 at 4:36 PM.
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Old Sep 8, 2010, 6:34 PM   #3
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Quote:
Originally Posted by fofa View Post
Looking at some older lenses:
#1. Why can you get a 50mm as fast as 1.4, but anything less than 50mm about the fastest I have seen is 2, and most are 2.8?

#2 K2000 body:
even after reading many web pages, I'm not sure I understand fully, does an "A" lens work on my body to detect the aperture setting, and only detect the setting or can change the aperture setting?

#3 What's the difference in lenses with the metal "posts" sticking out (maybe I misunderstand, but that is so the camera body can detect the F stop?) vs the electrical contacts I am guessing the camera can tell the lens what to use?

#4 I know an SMC F lens i bought seems to work just fine, but i wanted to try some older lenses, but it would be nice if the camera could at least detect the F stop I am thinking.
HI FOFA,

I've numbered your questions, and will try to answer them accordingly -- probably more technically than you need, but hopefully you'll be able to find the answers that you're looking for. . .

#1. I'm no optics engineer, but for 35mm cameras, the 50mm FL was the easiest to design lenses for. Traditionally, the diagonal of the film frame was considered the "normal" lens (for 35mm film, this would be @ 42mm, but Leica optics engineers found 50mm an easier lens to design, so they settled on 50mm as the "normal" lens for 35mm film, and it stuck). Lenses shorter than 50mm need to be "retrofocus" designs to maintain the register distance (distance between the rear element of the lens and the "film" plane), and are harder to design (more expensive). Because of this design limitation, faster wide angle lenses need very large front elements to be both wide and fast, and critically ground large elements are very expensive. . . not a great answer, but it's all I've got. . . I think that size and cost are the main reasons why you don't see too many wide lenses faster than f2.8. I imagine new designs would be possible, but with the higher ranges in ISO speed available in digital cameras, newer lenses for digital bodies seem to actually be trending towards slower/more compact models. Not good for DOF control, but faster would be both unwieldy and prohibitively expensive.

#2. "A" lenses, Manual Focus with the electrical contacts on the lens mount, are the earliest series lenses where the camera body (e-dials) can control the aperture in the lens, meter the scene wide open, and stop down the aperture automatically for exposure. Earlier series lenses can still be used and metered effectively, but the electrical contacts are essential to Automated Exposure.

#3. On Pentax lenses, the metal lever at the back end of the lens mechanically controls the aperture blades. The electrical contacts on the mount of "A" and later series lenses serve a couple of different functions. On an AF lens, there are 7 possible pins, and on MF lenses, 6. Five of these pin positions only tell the camera body what the min and max aperture values of the lens are by either being shorted to the mount or insulated from the mount. Different combinations of shorted/insulated pins tell the camera body the aperture range of the lens. The remaining one pin on an MF lens communicates aperture information to and from the lens. On an AF lens, the 7th pin communicates lens info (possibly lens model, FL, focus distance, MTF, MF or AF mode -- depending on the lens series and model) from the lens to the body, and AF information to and from the lens.

In addition, there are two other contacts that may be located inside the lens mount. These are used either for Power Zoom functions on "F" or "FA" series PZ lenses or for SDM Auto Focus functions on later digital bodies and some "DA/SDM" series lenses.

Probably more info than you wanted to know, but between the "M" series lenses and "A" series lenses, Pentax changed the functionality of the aperture lever from just stopping down the lens to stopping down the lens to a standardized point. In earlier lenses, the f-stop chosen on the aperture ring was the only thing that determined how far the blades closed down. In the "A" series, this changed, and the travel of the aperture lever actuator in the body became calibrated to a standardized set of positions that corresponded to aperture blade opening diameters on the now standardized lenses. In other words, on "K" nd "M" series lenses, aperture lever travel was not calibrated, and on "A" series and later, the lever travel is calibrated to specific aperture values, or "linear".

#4. If you only get "A" series MF lenses and AF lenses, all of which have the electrical contacts on the mount, then you will get Auto Exposure and all the benefits that come with it:

Ability to use Program modes,
P-TTL flash,
all three metering modes
. . . and probably some other features that aren't coming to mind.

But you may lose out on a lot of great fully manual lenses out there, and there's some great glass that's usually very inexpensive for the optical quality. . .

Personally, I have chosen to use only "A" series lenses or later, and am glad to have the uniformity of function, as it makes shooting a lot more brainless for me, and that's a good thing. . .

Scott
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Old Sep 8, 2010, 7:14 PM   #4
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Thanks, I think both of you pretty much answered my questions. So if i want a lens to detect the F-stop, pretty much need the electrical contacts.
One thing I am looking for is a decent macro lens, and since macro mode is (so I read) better controlled manually, a total manual lens might not be that bad a deal. Plus I was looking for something in low light, hence my question on the 50mm vs. wider angle. I read somewhere F-Stop was a function of Focal Length divided by width (or something along those lines), so it seem, armed with that limited information, a 28mm would be easier to create a fast lens with. That was what was throwing me off. I figured there had to be some other reason, but I didn't run across it any where.

I do have one other question I can't seem to find an answer to, and fear it is as simple as the name, but i have been mistaken before.

What is a "soft focus" lens? I see some 80mm "soft focus" lenses, and was wondering.
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Old Sep 8, 2010, 11:50 PM   #5
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A soft lens is a lens that is designed to create a picture with lowered sharpness, even when it is correctly focused. But the softness isn't equally distributed, the picture is quite sharp in the centre and gets "softer" towards the edges. These lenses are designed specifically for portraits, giving a "dreamy" feeling to the picture and making signs of age a little less obvious, much apprecaited by models over 25 years old.

Kjell
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Old Sep 9, 2010, 1:32 AM   #6
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Actually, there are some very fast short cheap lenses, like 8mm f/1.2 for under US$20. But they're no good for film-format cameras -- they're used on small-sensor video cams. The image circle they project would only fill a small part of a larger frame.

[Film-format: 110-m4/3 or larger, like 135/FF, 135/HF (similar to APS-C and old commercial cine), 120/220 MF (645, 6x6, 6x7, 6x9, etc) and up]

Also were made some fast cheap film-format lenses other than 50-55mm, like a number of f/1.8's around 35-45mm. But these are for rangefinders, which don't have the SLR's mirror to contend with. Such fast lenses may extend back nearly to the film plane. Try that with a SLR... oops, crash.

The real trick to making big fast lenses is building-in optical corrections. Yeah, you can correct for optical aberrations -- but the more corrective elements you add, the fatter the lens is, and the 'flatter' the image becomes. A simple Tessar lens can give a prized 3D look just because it isn't perfectly corrected -- but Tessars also can't be made to be faster than ~f/2.8. Or so I have read.

Some film-format glass around f/1 exists. It's almost impossible to use for general photography -- hair-thin DOF, extreme softness and aberrations, etc. Stanley Kubrick got hold of some NASA spy-cam f/0.7 lenses to shoot Barry Lyndon. Take a look, see if you like it... see if you can guess why it wasn't a box-office hit.
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Last edited by RioRico; Sep 9, 2010 at 1:36 AM.
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Old Sep 11, 2010, 3:35 PM   #7
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from ...
"F-stop = focal length / aperture diameter. The notation, "f/8," implies (aperture diameter = ) focal length/8. The larger the f-stop number, the smaller the aperture. F-stops are typically labeled in the following sequence, where the admitted light decreases by half for each stop: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, ... Each f-stop is the previous f-stop multiplied by the square root of 2."
All of this means that say you have a prime lens, and your have a variable aperture, so that you can vary the amount of light you let through the lens. This is just like your eye, in a dark room it opens up wide, then out at the beach it closes down and you then squint, so as to cut down on the amount of light. The f stop number is just a labeling device so that you can correlate the size of the opening to the amount of light that is passed.

Now with lenses and their "speed" or ability to pass light, you get into some optical engineering complexities. If you want to let as much light in as possible you need a large aperture which translates to a low f number. However, you want to let the light in such a way as its corrected so as to be focused properly on the sensor. That takes glass, which is heavy and needs to collect the light, thus tend to be large (think of a funnel). But on the other side of the coin, large apertures translate into shallow or very narrow depth of field, which is usually not desirable for landscapes - which wide angle lenses tend to be used for. Also, all the high quality optically ground glass, is expensive and heavy.

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Old Sep 11, 2010, 9:24 PM   #8
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Barry lindon, is one of my favorite movies of all time. Also, the movie is absolutely gorgeous, the lighting is so ethereal and mostly used natural lighting for scenes to give it an authentic look instead of some studio overdone blockerbuster look.

Is this why you're saying it wasnt a blockbuster hit?

Its definitely a classic, and reveled for one of the best epics of all time. IMO it wasnt a "blockbuster hit" because its as long as two normal films put together, the average american barely has the attention spam for a 30 minute slap stick sitcom.
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Old Sep 12, 2010, 9:40 AM   #9
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OK, I bought a 2x Teleconverter, it happens to be a vivitar MC and has the electrical "nubbs" on that flat surface of the mount, and one horizontal moveing arm. So I figured it would turn my F or DA lens into basically an "A" lens. But somewhere along the lines I guess I am not using it properly, I was just jacking around with it in the backyard. It did ask for the FL for Shake Reduction when I turned the camera on. But I never saw an F stop setting in the view finder, and everything i took was blown out (EXIF data said F 4.5, but the F lens is a 4-5.6 and at full 210mm is only 5.6). So I guess I am not doing something properly. Is it because it does not know the lens F stop limitations (can't read the lens ID chip)?
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Old Sep 12, 2010, 12:50 PM   #10
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Does it have the drive for autofocus as well as the electrical pins, and are tehre the same number of electrical contacts as on the lens andor camera?
There are a number of TCs which were made for the PZ cameras (use electric zoom instead of screw drive) which are semi-compatible, but won't autofocus. The elecrical contacts usually are just straight through, and the data the camera sees is just from the lens. The light loss from the TC isn't accounted for unless you use it manually, with the green button to set exposure.
Other problems, sometimes, are that the arm which controls aperture can get bent, or bound up so it doesn't actually change the aperture on the lens. (this would account for the overexposure) Check the arm for free movement, and return (a small spring for this), then connect it to the lens, and operate the arm- you should see the lens iris change when you look through the lens.
IIRC, the contacts on the PZ types won't make connection correctly with the non-PZ cameras. Some of the little balls stick up, and some are inset - they mate with the opposite kinds on the camera. The pattern of which protrude and which don't is different, so this may be why the camera isn't reading the aperture - it thinks you have a fully manual lens. If this is the case, you will probably have to use it in fully manual mode.
Fully compatible TCs have a chip, which modifies the lens data, so the camera gets the corrected information.

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