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04-25-2010, 07:55 AM   #1
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A question about my new (old) nifty fifty...

I recently managed to get my hands on a 50mm f1.7 M off ebay for the small sum of just 35.

I have used it tonnes recently and it is ten times better than my kit lens and 50 - 200; sharp and contrasty.

However, I have noticed that when the aperture is set to the highest F Stop there is still a lot of background blur but I can get some really sharp pictures although the depth of field isn't that big despite the lens being almost closed. Conversely, when I open the lens right up it doesn't seem that sharp at all on the parts that are in focus.

Is this normal? I was always led to understand that a higher F Stop means a larger depth of field and my kit lens has a much lower maximum F stop and yet a huge depth of field in comparison.

It's mounted on a K200D. Also, for picture examples Hali Santamas Photography The picture of the piano keys on the first page is with the lens wide open and the two photos of the Sikh procession on the second page are with the highest F stop (F22).

Cheers,

Hali

04-25-2010, 08:17 AM   #2
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It has to do with photographer to subject distance...and subject to background (or foreground) distance. Plus, the FOCAL LENGTH you're at.

The best test is to set yourself at minimum focusing distance in front of, say, a flower, with something a few feet in the background. Now do shots at the different apertures with the different lenses, and see how they behave.
04-25-2010, 08:22 AM   #3
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you may be suffering from defraction when stopped all the way down. diffraction is the impact of light bending slightly past an edge (in this case the aperture) as you stop down the impact of defraction is a greater percentage of the total light passing through the aperture.

Wide open you may be experiencing focusing errors, because the Apparent DOF in the viewfinder is higher than when yo look at the same image on a big screen or zoom in,

if you want to continually use manual focus lenses yo will need a split image focusing screen.
04-25-2010, 08:22 AM   #4
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Cheers, I'll have a fiddle about when I get some time. Just out of interest, why would this be different on my limited nifty fifty to my 18 - 55 or 50 - 200 lenses? Is it something to do with the zoom?

04-25-2010, 08:31 AM   #5
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I am also just beginning to use the old manual lenses, so I could be wrong... But, perhaps all your pictures are taken with the lens wide open. You cannot control the aperture on a manual lens just by adjusting the aperture on the lens.

Put your camera in manual mode with the mode dial. Choose your aperture on the lens. Point your camera at a subject that will give you a good exposure and push the green button. This will open up the lens with the desired aperture and it will meter the light for you. No you are ready to take the photo! A good way to check that this works is that the shutter speed will go down dramatically in dimly lit room when you close the aperture (high F-number). Then you know you are on to something!

Hope this helps! If it doesn't. Post a picture with the exif data intact.
04-25-2010, 09:27 AM   #6
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QuoteOriginally posted by Hali Quote
Cheers, I'll have a fiddle about when I get some time. Just out of interest, why would this be different on my limited nifty fifty to my 18 - 55 or 50 - 200 lenses? Is it something to do with the zoom?
It is not to do with the ability to zoom, but rather the lack of an A setting on the aperture ring. The newer Pentax cameras lack a mechanical trace that tells the camera where you have set the aperture ring on the camera. The only semi reliable way to set exposure with a lens not on the A position is to select the aperture you want and then use your camera's method of setting the shutter speed, usually the green button or the AE-L button. Note carefully that the aperture will stop down to your setting only if your are in M mode, as noted above. Any auto mode will default to Av, and the lens will always be wide open.

If you have depth of field preview on your power switch, a circular mark past the ON position, holding the lever in that position (it is spring loaded) you can set the appropriate shutter speed with the e-dial, while watching your view finder screen to see where the camera thinks the shutter should be. Depending on the model, this will be a meter bar or numbers at the extreme right of the viewfinder information bar giving numbers. The numbers are the number of stops that the shutter speed is different from what the camera wants.

Unfortunately, depending on the camera model and the lens you have, the exposure may vary depending on the selected aperture. In this case, you need to find an aperture that meters accurately and use that, adjusting as necessary, or memorizing how far out the lens is on any given aperture and adjusting.

A more expensive solution to the variable exposure problem is to use a focusing screen from the *ist D series of cameras. The problem appears to be caused by the brighter screens on the newer cameras, such as my K10.
04-25-2010, 10:34 AM   #7
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There is a Distance scale on your lens. There is a distance scale on Most lenses. Corresponding to that scale are pointers to the f numbers. What that scale is telling you is the Range of reasonable focus at each given f number. For instance, if you're lens is focused at 1 meter. Your in focus range will be roughly 0.7 meters to about 1.2 meters at f22. If you focus to 3 meters, your range goes from about 1.5 meters to near infinity at f22. Focus at 0.45 meters (the closest distance on my A50 f1.7), now your acceptable focus range at f22 is from about 0.42 to about 0.48 meters. Get the picture?? Funny how they built that information right in, but nobody pays any attention to it anymore..

04-25-2010, 01:03 PM   #8
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Have you read the sticky thread in this forum on how to use manual lenses on a DSLR? I'm guessing those pictures you think were at f/22 actually weren't, because you weren't in M mode.

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