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03-19-2012, 04:25 PM   #1
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Could not control aperture of SMC Pentax 85mm F1.8 with Pentax K7

Hi all,

I need help to see what I am doing wrong with my recently acquire SMC Pentax 85mm F1.8. There is no selection on the lens to tell whether it can be set as auto aperture mode or manual mode. However, when I turn the aperture ring on the lens, I can see the aperture change accordingly. The fun part is when I put it in my Pentax K7 and select 85mm as manual mode, I can only shoot with widest open aperture. The aperture ring cannot set the aperture at all. Does anyone have the same lens that can help me in what I should do here?

Thank you very much,

Vietnam

03-19-2012, 04:36 PM - 1 Like   #2
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Set Using Aperture Ring to permitted in the Custom menu. In M mode use the Green Button to set the exposure, in all other modes it will only shoot wide open.
03-19-2012, 04:38 PM - 1 Like   #3
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It appears that you have the fully manual SMC Pentax ("K") lens". Assuming that there is nothing wrong with your lens, here's what should work for you:

You will have to go to the Custom Settings menu in your camera and find the menu item "Use of Aperture Ring" and set it to "permitted". When you use your lens you will have to operate it in manual ("M") mode as you say there is no "A" setting on your lens. When you use the aperture dial on the lens mounted on the camera, the aperture coupling lever will not be activated until you press the shutter release button, but you can test that your lens stops the way it should by using the DOF preview of your K-7.

By the way: It is a great lens that you have got - congratulations on that!

Last edited by Stone G.; 03-19-2012 at 04:45 PM.
03-19-2012, 05:18 PM - 1 Like   #4
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another possible problem are the electric pins on the K7, i had some bad experience with those too (my Auto Revenueon 55/1.2). Since the mount was coated and a manual lens so the pins didn't receive information from the lens. That means it will stay fully open no matter what aparture i set it on. Try to put a small piece of aluminium paper on the pins and screw the lens on, or (the way i did it) use a piece of sandpaper and rub it on the mount until the coating gets off. Mong rang em phu duoc anh xP

03-19-2012, 06:22 PM   #5
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Thank you all! Up till now, I thought I've got the bad one. Yes, this is the total manual lens and can only see with DOF preview like Steen has suggested! I've got the "Use of Aperture Ring" turn on already; however, I did change the shooting mode to aperture priority which will keep shooting at f1.8. Here are my sample at f1.8 and another at f4.0 straight out of camera with no adjustment. The aperture can only be switched during shooting! I guess I have to do trial and error to get to the right exposure. However, this is definitely worth the time and the money I'm invested in it!

Cám ơn bạn, xP. Lens này là lens manual nên không thể chọn khẩu độ bằng máy được!

f1.8:


f4.0



03-19-2012, 10:45 PM   #6
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Here are some more sample images that I've played around with this new lens. This is awesome manual lens that I've been playing with! No one will take it away from me from now on for sure!

f1.8 ISO 200:


With Kiron 2x tele converter:
03-20-2012, 12:00 AM   #7
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QuoteOriginally posted by vietnam74 Quote
Thank you all! Up till now, I thought I've got the bad one. Yes, this is the total manual lens - - - I guess I have to do trial and error to get to the right exposure. However, this is definitely worth the time and the money I'm invested in it!

Cám ơn bạn, xP. Lens này là lens manual nên không thể chọn khẩu độ bằng máy được!

Not quite: This is where the use of the green button in "M" mode comes in. When you have chosen your aperture, press the green button and the light meter will work to get the proper exposure value for you.

Glad to hear that your lens is fine. Happy shooting!
03-20-2012, 07:30 AM   #8
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there are a few things to remember when using manual aperture (K and M lenses)

Aside from the green button , which will set the shutter to match the aperture and light reading for the lenses when in M mode, using the DOF preview will activate the metering and show the exposure (+/- ev meter) with your current settings. this is useful if you don't want the green button to change the actual settings

Also consider that the K 7, while better than the K10and K20 when metering with manual aperture lenses, it is not as good as the K5 or *istD.

please see the attached chart which I have used to explain the sometimes eratic exposure of different cameras

although it is getting a little messy as I add new cameras and focusing screens, you can clearly see that especially for fast manual lenses, the exposure can seem eratic



you should test your lens/camera combination by taking shots at each aperture detent, and then pressing the green button. I use a block wall under uniform lighting conditions, but any uniformly lit uniform colored surface will do. paved roads, and sidewalks are quite good also.

03-20-2012, 08:46 AM   #9
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Ah! I'm not aware of the green button in my Pentax's camera. This is really good feature. I've tested it last night and wish it can give me the evaluation with the flash as well, just like the test button on the flash when we push it, it will fire the flash and get the exposure ready. Hopefully, this feature may incorporate into the newer Pentax!?? Thank you all for the tips.

Lowell, could you please tell me more the procedure of your test? How could you derive gray scale average exposure? Did you replace the focus screen of your Pentax K7 as well? Is that easy? Do you have the DIY type of instruction for this type of modification?

Below is what I did with bounce flash and focus on the rambutan. I never see best focus plane like this lens with no distortion at wide open f1.8 even with my 16-50mm or 50-135mm DA* lens! The starbucks mug is on the same plane as the rambutan and with f1.8, it is still in focus!

03-20-2012, 02:54 PM - 1 Like   #10
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test is simple

camera should be set to JPEG, neutral contrast, highlight and shadow detail protection off. Set EV steps to .5 stops not .3, because the lens has either full or 1/2 stop detents

Test target is simple, any uniform color wall, or surface, uniformly lit. best is outdoor, with target ouot of the shadows.

take a series of shots at each aperture detent, using green button to set shutter as the camera thinks best. I prefer spot metering, but ifg you have a split image finder use average metering. the split image finder screws up spot metering.

using your photo editor, measure the greyscale value on the histogram. I can't tell you how to do this in your editor, because I only use/know Corel PSP X through PSP X4. Most editors allow you to read the mean or median histogram value, in any color or greyscale. Some, including PSP allow you to use the selection tool and measure the greyscale value in the selected area only. This is useful, especially if you used spot metering, because it can avoid the impacts of lens vignetting.

Once you have the greyscale values, plot them in a spread sheet against aperture. each change of 45 around the median value of 120 is 1 full f stop. you will get a curve similar to my K7 curve.

Note, on pentax lenses the normal aperture arrangement is that the first and last detent are full stops, and the remainder of the detents are 1/2 stop increments.
03-20-2012, 06:37 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
Set EV steps to .5 stops not .3, because the lens has either full or 1/2 stop detents
Great tip, I often over look that small detail.

Last edited by mattt; 03-20-2012 at 06:48 PM.
03-20-2012, 10:43 PM   #12
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Thank Lowell! I'll work on that to get myself familiar with this great lens.
03-20-2012, 11:31 PM   #13
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@vietnam74:bác ở VN hay ở nước nào ạ...e cũng là mem Pentax ở VN đây. lens của bác là lens đời K hoàng toàn Manual(đời sau M42 và trước đời M), ko thể chỉnh khẩu trên body đc, các vấn đề khác các mem đă nói khá rơ, ko rơ vấn đề của bác đă ok chưa>>>bác có thể trao đổi với e qua YM trungdia@yahoo.com..thanks
03-21-2012, 06:08 PM   #14
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Chào smilingman82, Nam hiện đang ở Cali. Cám ơn bạn đă ghé qua. Ḿnh đă chụp thử lens rồi và thấy vừa ư lắm! Cũng đang tập tành chụp thôi chứ không có pro cho lắm nên cũng c̣n học hỏi để chụp h́nh đẹp hơn với Pentax K7 này. Bạn xài máy phim Pentax chắc là có nhiều lens xịn lắm hả? Kỳ rồi Nam về Việt Nam đi chụp chung với thằng bạn, gặp bà con ở bển chụp toàn là Canon và Nikon không hà! Smilingman82 chắc là hàng hiếm ở bển!
03-21-2012, 06:20 PM   #15
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QuoteOriginally posted by vietnam74 Quote
Chào smilingman82, Nam hiện đang ở Cali. Cám ơn bạn đă ghé qua. Ḿnh đă chụp thử lens rồi và thấy vừa ư lắm! Cũng đang tập tành chụp thôi chứ không có pro cho lắm nên cũng c̣n học hỏi để chụp h́nh đẹp hơn với Pentax K7 này. Bạn xài máy phim Pentax chắc là có nhiều lens xịn lắm hả? Kỳ rồi Nam về Việt Nam đi chụp chung với thằng bạn, gặp bà con ở bển chụp toàn là Canon và Nikon không hà! Smilingman82 chắc là hàng hiếm ở bển!
e cũng đang dùng K7 và cả Film ...hội Pen ở HCM và HN cũng có đó bác..bác mún quen a e ở VN th́ vào đây trao đổi cho vui
Box Pentax
đây là diễn đàn chính của a e pentax VN, bác về HN th́ cứ Pm cho e nha. e sẽ làm guide cho bác ..glad 2 see U
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