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12-29-2011, 09:11 AM   #1
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Fixed Aperture, What Does It All Even Mean

Hey Everyone,

OK so I am pretty much just learning photography for the first time; I used my 200mm to take a photo of the moon yesterday for a double exposure. Many questions have since arisen with respect to my telephoto lens, and I would greatly appreciate some help, because google threads on fixed-aperture lenses seem to only pertain to zoom-digitals.

The lens: Pentax 1:4/200 SMC K-mount. It is a prime lens. Apparently fixed aperture?

The camera: Pentax ME (Has internal center-metering and auto shutter timing).

I had read that a good shot of the moon was f-11 at speed 1/film-speed (1/100 in my case). But when I went to use my camera this lens left me dumbfounded...

It has two sets of f-stop rings and neither moves.

What does it mean when the first ring has a big red dot at 32?

What does it mean when the second ring has a red diamond at 8?

Why does the lens have the name 1:4?

From what I can gather on the net by sifting through info on zoom-digital type lenses, the lens will always maintain an aperture of f-4? Then why the 32? Why the 8?

Also, since my camera determines shutter speed automatically and the moon was off center (center weighted meter) I could not get "auto mode" to register an eposure time other than "under" and ended up shooting in "B" mode with a guess of 1/10. I realise now I should have used 100x, but that is a little beside the point. I am curious if there is any way to change the exposure time drastically on a prime-fixed-aperture as you would when changing f-stops on a lens with variable-aperture.

As an added question I have seen some macro lenses listed as 200/4... Is this not the same designation as my telephoto lens?

Lastly, whats the point of fixed aperture?

I know I have just asked a lot here, but this ignorance is killing me. Thanks very much! Its my day 2 on this forum and I love it.

Dave

12-29-2011, 09:51 AM   #2
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Fixed means you can not change the size of the hole (aperture) the light comes through. Any lens with an adjustable aperture is, by definition, not a fixed-aperture lens. Mirror lenses are fixed, because they have no aperture blades to adjust.

For zooms, you are confusing fixed-aperture with constant-aperture. The (normally) better and faster zooms will stay at the constant minimum aperture (say, f2.8) as you go from wider to longer.

If you can not rotate the aperture ring it is probably on the "A" setting and requires pushing a little button to release it. Older lenses can have two rings, and no "A" setting.
12-29-2011, 09:53 AM - 1 Like   #3
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QuoteOriginally posted by David-C Quote
The lens: Pentax 1:4/200 SMC K-mount. It is a prime lens. Apparently fixed aperture?
There are two meanings. One applies to mirrors and other lenses without an aperture iris. Thus my 500/8 mirror is ALWAYS at f/8. The other applies to zoom lenses, which can have a fixed or floating MAXIMUM aperture. My Tomioka 55-135/3.5 has a fixed maximum aperture; an F35-70/3.5-4.5 has a floating maximum aperture. The term doesn't apply to a prime lens like your 200/4.

QuoteQuote:
But when I went to use my camera this lens left me dumbfounded...

It has two sets of f-stop rings and neither moves.

What does it mean when the first ring has a big red dot at 32?
It would help to know just which lens you have there. Can you find it in the lens review database? (Click on LENSES near the top left of this page.)

Is there an A between the 32 and the red dot? Then the dot is a button that lets you shift the ring into or away from the A (auto aperture) position.

QuoteQuote:
What does it mean when the second ring has a red diamond at 8?
The red diamond is usually an index mark for focusing.

QuoteQuote:
Why does the lens have the name 1:4?
Because the maximum f-stop is f/4. An f-stop is the ratio between aperture diameter and focal length. We don't need to know that diameter; we DO need to know the ratio. The f-stop f/4 means the iris diameter is 1/4 the focal length; the ratio is 1:4. Depending on the lensmaker or the user or the continent, a lens may be referred to as 200mm 1:4 or 200/4 or 4/200 or F 1:4 200 -- they all mean exactly the same thing.

QuoteQuote:
From what I can gather on the net by sifting through info on zoom-digital type lenses, the lens will always maintain an aperture of f-4? Then why the 32? Why the 8?
Your lens' maximum (widest) aperture is f/4; its minimum (tightest) aperture is f/32. And f/8 is one of the midway stops. The f-stop sequence would go: f/4-5.6-8-11-16-22-32. Every lens with an iris (this excludes mirrors, projector and copy lenses, etc) has a variable aperture -- assuming it's not stuck, of course!

QuoteQuote:
I am curious if there is any way to change the exposure time drastically on a prime-fixed-aperture as you would when changing f-stops on a lens with variable-aperture.
Only a no-iris (or broken) lens won't let you change apertures. That said, there ARE ways to slow an exposure. The easiest is to... stop-down the aperture! Or add a ND (neutral density) filter; or if shooting B&W, use some heavy-cutting filter like a Red or Blue; or you could make a Waterhouse stop (look it up) but that doesn't really apply here; or you could just dial-down the ISO as much as possible.

QuoteQuote:
As an added question I have seen some macro lenses listed as 200/4... Is this not the same designation as my telephoto lens?
Exactly right. The differences are that 1) the macro lens focuses much closer; 2) the macro lens' optics are sharper; and 3) the macro lens costs much much more! That's a tele macro, good for field work, highly prized, highly priced.

QuoteQuote:
Lastly, whats the point of fixed aperture?
I think I answered that above. Cheers!

Last edited by RioRico; 12-29-2011 at 10:02 AM.
12-29-2011, 11:47 AM   #4
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Don't think Pentax ever made a 200mm f4 fixed aperture lens in K mount.Can you do pictures of the lens?
Jake

12-29-2011, 12:15 PM   #5
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QuoteOriginally posted by David-C Quote
It has two sets of f-stop rings and neither moves.
I am curious about this. Some early lenses, and some specialty lenses have two aperture rings and are called pre-set lenses. You set the first ring to the aperture you want to use and then use the other to first open the aperture for focusing and then to stop it down to the aperture you want to use. This is more common in M42 lenses than k mount though. It would help if you could post a picture of the lens showing the precise name or post the full name. 200 f/4 can be a lot of different lenses.
12-29-2011, 01:29 PM   #6
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QuoteOriginally posted by David-C Quote
It has two sets of f-stop rings and neither moves.

What does it mean when the first ring has a big red dot at 32?
If I understand correctly the lens only has one aperture ring, and it might be the depth-of-field scale that you think is the second aperture ring.

The red dot is a guide for putting the lens on the camera, and should be aligned with the red dot on the camera mount.

Most likely it is a K 200/4
Pentax 200mm F4 Reviews - K Prime Lenses - Pentax Lens Reviews & Lens Database
12-29-2011, 04:59 PM   #7
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Heres the lens, sorry the pictures are mirrored and garbage, using mac photo camera.
Attached Images
   
12-29-2011, 05:31 PM   #8
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Fogel: The link you posted is virutually identical as you can see... Only difference is that the "red dot in that pic is not on the 32, as is the case with my lens. Seems to me that the red dot is on a ring under the ring with the 4-32 numbers, so quite possibly this difference is a simple matter of assembly variation.

RioRico: Thanks so much for the detailed response, I really appreciate that you took the time to explain all of that. I now understand the concept of a ratio between the aperture diameter and focal length. I also understand lenses better in general, thanks. As you can see, my lens has an iris made up of 6 blades. Yet I am unable to change the diameter of the iris. No ring movement.

I have put in significant effort to see if either of the rings indicating f-stops will move and they absolutely do not. Might the lens be stuck? I have noticed that the ring with the f-stop numbers 4-32 does not sit immediately beside the ring with the red-diamond, as is the case with all of my other k-mount lenses. Perhaps this is indicative of a fixed iris aperture, but for ease of manufacturing Pentax used the machined rings coming off of the assembly line, even though they have different f-stops listed on them?

Why bother with an iris if it does not move? Why not just put a metal disc with a perfectly circular hole? Somehow, if it were a fixed aperture always set to max of f-4, I would expect to see only the number 4 on the rings. But then again I am noob. Furthermore, if you look at the pic you can see that the aperture diameter is very small, certainly not wide open.

jatrax: It would seem that I was mistaken to indicate two f-stop rings, as can be seen in the picture.


Thanks to everyone that has replied, I am anxious to get this sorted.

Dave

12-29-2011, 05:48 PM   #9
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QuoteOriginally posted by David-C Quote
effort to see if either of the rings indicating f-stops will move
I believe the ring that should move is the one between the two rings with markings on it.

QuoteOriginally posted by David-C Quote
Perhaps this is indicative of a fixed iris aperture,
This is not a fixed aperture lens. It is a constant aperture lens. Looks like a regular SMC 200 f/4. As RioRico has explained fixed aperture is only found on specialty lenses like mirror lenses. You can clearly see the aperture blades in the one photo.

If the ring between the two with markings does not move I would have to think something is jammed or broken. Does everything on the mount side look OK? Nothing bent or broken?
12-29-2011, 06:02 PM   #10
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Just scoped out the lens...

Not a ding, dent, scratch, dust particle... nothing. It's honestly as though it just came out of the box.

Incidentally, that Red Dot is definitely for mounting. It is the only lens I have with a red dot, but I see a red dot on the body as well.
12-29-2011, 06:22 PM   #11
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Its the highly-regarded K200/4 (see the reviews). It's a manual stop-down lens, not a preset. That red dot functions as both the mounting guide and the aperture index. The knurled aperture ring with the f-stop numbers 4-5.6-8-11-16-22-32 should turn against that index, and the iris blades should open and close. If not, it might be broken (sob!) and I don't know how to fix that.

Are you located anywhere near a non-chain-store camera shop? An experienced togger should be able to help, maybe just to tell you that you're not twisting the right parts. Yeah, It could be that easy!

Last edited by RioRico; 12-29-2011 at 06:32 PM.
12-29-2011, 06:25 PM - 1 Like   #12
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QuoteOriginally posted by David-C Quote
hat Red Dot is definitely for mounting.
Yes, although sometimes it is on the mount plate itself either on the flat or the side. Do the aperture blades move if you move the actuation lever?
12-29-2011, 06:51 PM   #13
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QuoteOriginally posted by jatrax Quote

This is not a fixed aperture lens. It is a constant aperture lens.
"Constant" aperture applies only to zoom lenses, not primes. On a prime there is no focal length range for the aperture to be "constant" through.

OP:

The aperture ring which should move is the bit closest to the red dot. The bit with the white dot is a depth-of-field scale and is just a reference tool which doesn't move. The bit with feet and meters marked on it shows at what distance the lens is focused and the distance markings should move as you focus the lens in and out to different distances. The ring that lies directly over the top of that, with a cutout so you can see the distance markings, should not move. If it does, it is a simple matter of loose set screws.....very tiny screws....which you can tighten with a very small jeweler's screwdriver. It has zero effect on lens operation, even if it is loose or totally missing from the lens, so don't sweat it.

The only ring which should move on this lens is the bit with the aperture numbers on it (4, 5.6, 8, 11, etc). Hold the lens firmly and try moving it around so different aperture numbers line up with the red dot. If it won't move, then regardless of the cosmetic condition of the lens looking it just rolled off the assembly line there is something wrong with the mechanical condition of the lens' innards. Remove the rear lens cap and try operating the aperture actuation lever. You'll recognize it quite easily....just a little lever sticking out of the back of the lens, perhaps with a non-moving little metal shield on one side of it. Try moving the lever with your finger. You should be able to see the aperture blades opening and closing. (Note: if you do this on f4, nothing is going to move. But your photo shows it stopped way the heck down, so something should move).

Last edited by Mike Cash; 12-29-2011 at 07:59 PM.
12-29-2011, 06:58 PM   #14
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JATRAX>> Great Success with the actuation lever! The aperture definitely opens when I move the lever. Not sure what this means for the future of the lens though, but at least something moved.

RioRico>> I just read over the review of this lens, and it does seem to be highly regarded. No action from the ring, so perhaps it is in fact broken (*huge sob* considering I have the disposable income of a 6 year old). I live in Toronto so there has got to be a "togger" (new term for me, thanks!) somewhere. According to a "Henry's" store there used to be a guy that lived behind the Henry's store, but he died. No dice there I guess. I also have an Akarex with a closed shutter to get fixed. So I will look around...

Thanks again to everyone, I'm at least pleased the mystery is sorta solved. Gotta get some tech help...

Kindly,
Dave
12-29-2011, 07:03 PM   #15
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Mike Cash>> Thanks, your post came up as I put up my reply. Considering the situation of, lever working but ring not working, would you suppose it could be fixed by a tech-guy, or would you think it and I are up a creek?
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