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12-11-2009, 01:10 PM   #1
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depth of field

Hi,

I just received a used pentax-M f/1.7. I shot some test photos to see the difference between setting a aperture of f/1.7 and f/22. Is it normal that there is no difference of depth of field between these two extreme values of aperture. (I think it's not...). Then I look to the lens and when I turn the aperture ring everything looks normal ( the lens mechanically works. Can someone tell me why I get those results?

12-11-2009, 01:13 PM   #2
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Yes, you should certiainly see a difference.

First, could you post the pictures you took? Also, depth of field is most pronounced up close, the farther away you focus the less obvious changes in depth of field are. If you take a close-up of - say - a flower, you should easily notice the difference between F1.7 and F22.

I take it that by 'everything looks normal' you mean that if you take the lens off the camera and move the aperture ring, the aperture gets bigger and smaller. Also, does the camera meter differently at F1.7 compared to F22? For example, do you need to use a fast shutter speed at F1.7 and a slow one at F22? (because this should be the case)
12-11-2009, 01:31 PM   #3
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I think the problem is in Av mode... the camera does meter the same way for both aperture values then I get the same depth of field. When I use M mode, I meter with the green button, the shutter speed become different and then I see a difference in depth of field.
12-11-2009, 01:35 PM   #4
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Make a quick check of the aperture setting of the lens to confirm you are have it on the 'A' setting.

12-11-2009, 01:37 PM   #5
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Sorry David, I don't get what you mean.
12-11-2009, 01:39 PM   #6
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Pics
f/22


f/1.7


I don't see the difference...
12-11-2009, 01:45 PM   #7
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M-series lenses will always be on f1.7 when you use them in Av mode. You can only use the other aperture settings in manual (M) mode.
12-11-2009, 01:46 PM   #8
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QuoteOriginally posted by miss_alexx Quote
Sorry David, I don't get what you mean.
Scratch my first comment. You can't use AV mode with M lenses-- I think that's the only issue.

12-11-2009, 01:46 PM   #9
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QuoteOriginally posted by DavidWasch Quote
Make a quick check of the aperture setting of the lens to confirm you are have it on the 'A' setting.
M-series lenses do not have an 'A' setting.
12-11-2009, 01:46 PM   #10
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That is not a good subject for this test.

Try a ruler down it's length, at an angle.

Like this:

http://www.flickr.com/photos/12589183@N06/1523100812/
12-11-2009, 01:48 PM   #11
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I did not know that...
But is it normal that when I set the aperture to f/22.. then I click on the green button, it always bring me a shutter speed of 0,4'' not matter the light... then I got some under exposed photos?
12-11-2009, 01:50 PM   #12
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QuoteOriginally posted by miss_alexx Quote
I did not know that...
But is it normal that when I set the aperture to f/22.. then I click on the green button, it always bring me a shutter speed of 0,4'' not matter the light... then I got some under exposed photos?
Sadly metering with older lenses isn't always accurate, especially at small apertures.
12-11-2009, 01:50 PM   #13
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QuoteOriginally posted by miss_alexx Quote
I did not know that...
But is it normal that when I set the aperture to f/22.. then I click on the green button, it always bring me a shutter speed of 0,4'' not matter the light... then I got some under exposed photos?
If I understand what you're saying, yes.

Smaller aperture (f/22) = less light, but more DOF.

The reverse is true, larger aperture (f/1.7) = more light, but less DOF.

If there is not enough light for your camera to compensate with ISO, and you're set to aperture priority mode (I'm assuming, from reading your statement), then you're going to get underexposed as you exceed your ISO limits with the shutter speed set too fast.

Last edited by matiki; 12-12-2009 at 01:09 PM. Reason: typo
12-11-2009, 01:51 PM   #14
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QuoteOriginally posted by matiki Quote
If I understand what you're saying, yes.

Smaller aperture (f/22) = less light, but more DOF.

The reverse is true, larger aperture (f/1.7) = more light, but less DOF.

If there is not enough light for your camera to compensate with ISO, and you're set to shutter priority mode (I'm assuming, from reading your statement), then you're going to get underexposed as you exceed your ISO limits with the shutter speed set too fast.
You are so not helping.
12-11-2009, 01:54 PM   #15
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QuoteOriginally posted by kevinschoenmakers Quote
You are so not helping.
How is that?
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