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10-03-2013, 10:33 PM   #1
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Why the F-- Display on M 50 1.7?

Hi-

I purchased a M 50 1.7 and it appears to be in very good condition, but why when I put it on either my K-01 or K200D does the camera display "F- - " where the aperture should display?

It's a mystery to me and it would be great if anyone has a reason and/or solution to this.

Many thanks.

10-03-2013, 10:48 PM   #2
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Pre-"A" lenses lack the electrical contacts that tell the camera what the minimum and maximum apertures of the lens are. Since the "crippled" K-mounts of the digital cameras also lack the aperture control lever there is no way for the camera or the lens to communicate aperture setting information. Thus the "F--"

You have to use stop-down metering with non-"A" lenses.

https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html
https://www.pentaxforums.com/forums/pentax-dslr-camera-articles/100260-manual...ic-lenses.html
10-03-2013, 10:51 PM   #3
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Only A/FA/DA lenses have the electronic contacts in the lens base to report aperture data to the camera. That's the difference between PK and PK/A lenses.

Plenty of info on using PK lenses in the manuals or on line.

The M 50/1. is a wonderful lens and worth the effort to learn how to use it to best effect.
10-03-2013, 11:29 PM   #4
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A brief tutorial for using an M lens on modern Pentax DSLR:

1. Put the camera in M metering mode. Don't worry, you won't actually have to "go full manual", but this is the only metering mode you can use for M lenses.
2. Set the ISO manually (auto ISO does not function in M mode). After setting the ISO, you are basically using aperture priority with your M lens.
3. Set the aperture on the lens using the aperture ring.
4. Compose your shot and press the green button. The aperture will automatically close to the selected aperture and take a meter reading. The shutter speed will be automatically set at this point.
5. Shoot.

Note, you can use the camera dial to change the shutter speed or just change the aperture for exposure compensation, after you have metered using the green button.

10-04-2013, 01:05 AM   #5
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Just to add a bit here. Without the electrical interface, there is not way for the camera body to know what you the photographer set the aperture ring on the lens to (f number). The result of this is that within the EXIF meta data that is stored within the image, a default f value of 2.8 is used.

So, what does this mean to you. You turn the aperture ring on the lens to f8, use the green button to meter the light, and snap the image. You then down load the image on to your PC and start taking a look at it with your post processing software. The post processing software indicates that the image was taken with an aperture of f2.8, but you just know that you used f8 on the image because you wanted a really good depth of field, and f8 is the sweet spot for the lens. Well, your both right, the image was taken with the f8 aperture, but the camera body did not know this (since there was no way to communicate this information from the lens to the body), so to have some value to put in to the EXIF data field for the image, the body used f2.8.

Also, after taking the image, you chimp it on the back screen, putting up all the information along with the image (the [info] button). The camera body reports the aperture was taken at f2.8 while you turn the camera around and look at the lens, saying that you set f8 on the aperture ring. Same reason as above.

10-04-2013, 02:11 AM   #6
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QuoteOriginally posted by interested_observer Quote
Just to add a bit here. Without the electrical interface, there is not way for the camera body to know what you the photographer set the aperture ring on the lens to (f number). The result of this is that within the EXIF meta data that is stored within the image, a default f value of 2.8 is used.
On my K-7 the aperture gets recorded as 0 (as it should) and the focal length is simply whatever you'd input for the SR value.
10-04-2013, 07:37 AM   #7
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Thanks everyone for your thorough answers, very, very helpful. I'm going to use it on my K200D for a while, I need to put some "thinking" back into my photography and get back into practice mode. Using this fine lens this way will help do that.
10-04-2013, 10:14 AM   #8
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I have a similar lens and decided to use it on my SLR, which can handle it. It would be worthwhile to get an A lens at a camera swap for cheap. You can then use the aperture setting in your DSLR for full manual control or even auto if you want.

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