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06-29-2015, 04:44 AM   #1
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Soft images at f8 and infinity focus

Hi everyone,

I'm a long-time reader and first-time poster here on Pentax Forums

I'm having some focus issues with my Pentax lens. I own a film Pentax MX with a Pentax-M 50mm f1.4 lens.
Yesterday, I shot some landscape photos in super bright weather.
I set the aperture to f8 and my shutter speed was always above 1/500s of a second. I also manually-focused the lens to infinity because shooting far-away landscape.

When I saw my developed photos today, the images were soft. What's wrong here? Surely I didn't have camera shake.
My camera + lens has no problems taking sharp photos of closer subjects.

What's going on here? Is there something wrong with the lens at infinity focus?

Thanks

06-29-2015, 05:17 AM   #2
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I have two lenses where infinity focus is off. It basically means that your actual infinity is before the Infinity mark. What you are seeing, I think, is your lens actually focused past infinity.

Last edited by ChristianRock; 06-29-2015 at 07:33 AM.
06-29-2015, 05:40 AM   #3
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QuoteOriginally posted by canteaus Quote
I set the aperture to f8 and my shutter speed was always above 1/500s of a second. I also manually-focused the lens to infinity because shooting far-away landscape.
Are you sure the camera stopped down the lens correctly? Because a photo can look soft if its f1.4 focused at infinity. The other problem might be actual infinity focus - the lens' focus ring might be miscalibrated, or the lens mount might be a little off, thus preventing the lens from reaching infinity. One other possibility with a miscalibated focus ring is that the lens might focus past infinity.
The other thing is the film and developing itself, which might not be as "sharp" as you expect if you used modern DSLRs with digital sharpening. Especially if there was some atmospheric haze or something.
06-29-2015, 05:47 AM   #4
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I'm definitely shooting at f8 - there's no mistake about that.

Perhaps ChristianRock is right, maybe my infinity marking is past infinity.....

I guess the easiest way to test this is to find a digital Pentax SLR and test my lens?

06-29-2015, 06:17 AM   #5
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Focusing at infinity can be tough with longer lenses. A magnifying eyepiece might help. But nothing beats live view with magnification

Adam
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06-29-2015, 06:36 AM   #6
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QuoteOriginally posted by Adam Quote
But nothing beats live view with magnification
From the post I assumed OP used the lens on a Pentax MX film camera, but I may be wrong.
If OP used the lens on DSLR: Make sure you use M mode with a fixed ISO number, as the camera will shoot wide open at any other mode (they all default to Av, with no possibility of stopping down). LV with peaking and digital zoom can help a lot, since modern DSLR viewfinders are not designed for MF.
06-29-2015, 07:43 AM   #7
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QuoteOriginally posted by canteaus Quote
Yesterday, I shot some landscape photos in super bright weather. I set the aperture to f8 and my shutter speed was always above 1/500s of a second. I also manually-focused the lens to infinity because shooting far-away landscape.
As others have noted, just setting the lens to the infinity mark may not really be infinity. Many lenses actually focus beyond infinity.

Also, just because a landscape is 'far away' does not mean it is all at infinity. Is all of the image soft? Is any part of it sharp? It is possible the foreground is soft because you focused too far away and the background part is actually sharp. Focusing at infinity means the foreground will be soft.

Another thought is atmosphere. Long distance shots especially in the summer are often soft because of atmospheric haze.
06-29-2015, 08:48 AM   #8
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Sharp and distance are two words that seldom occur in the same sentence in the world of photography and focus is generally not the reason. As jatrax noted, the reason is air...air full of junk and moisture and temperature-related refraction.

One other consideration is that of sorrow of the pixel peeper...


Steve

06-29-2015, 09:42 AM   #9
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Thanks for your help everyone.

I was shooting the 50mm lens on film, with my Pentax MX.

The subject was indeed, "far away" about 900 meters, so infinity should be right.

However, as many have mentioned, my lens might not be properly calibrated and I may be focusing beyond infinity. It was also a sunny day so maybe the weather had something to do with it.

I'm going to take some scientific shots with precise measurements and I'll report back with photos~ thanks again!
06-29-2015, 10:16 AM   #10
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Btw, were you using a lens hood? It won't improve sharpness itself, but it can help a lot with overall image quality. And on very sunny days, you might think about using a UV filter. UV filters are useless on DSLRs, but they have a use with film cameras, so you can read up about that

And no, 900m does not necessarily mean infinity. It might be just a hair before infinity, but still
06-29-2015, 12:03 PM   #11
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Have you ruled out a problem in the developing chain? Were there other photos on the roll that turned out sharp?

QuoteOriginally posted by Na Horuk Quote
And no, 900m does not necessarily mean infinity. It might be just a hair before infinity, but still
You'll be hard pressed to find an object actually "at infinity". The depth of field is also absurdly massive at long focus distances, there should be plenty of room for error here (especially at f/8).
06-29-2015, 07:17 PM   #12
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QuoteOriginally posted by canteaus Quote
I was shooting the 50mm lens on film, with my Pentax MX.

The subject was indeed, "far away" about 900 meters, so infinity should be right.
A 50mm lens at infinity on film? Which 50mm and how are you assessing sharpness? If with anything other than a magnifying loup on the negative and something like Adox CMS 20 film, you are wasting your time and even then, the use case is sort of unusual.

FWIW, your best bet is to use the viewfinder focus aid to determine focus, not the focus scale on the lens. If the two do not compare, you can adjust the lens focus scale and infinity stop, though I would not do so against the focus screen. Instead, you collamate the lens and/or mount shims to the focal plane using a ground glass or focus screen*. Likewise, if you choose to collamate the focus screen, you do so against the focal plane.


Steve

* I know a tech who uses a split-image focus screen for this purpose.
07-01-2015, 11:56 AM   #13
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At "really bright weather" (blue summer sky around noon) you should keep in mind that film IS sensitive for UV light, but the lens is not corrected for these frequencies. The effect is mostly making the picture looking very soft, like a slight haze .

I had to learn this back in the sixties and seventies, specially in high mountains, at the sea, and in winter in snowy landscape.
But you should use a SMC or similiar coated UV filter, and ONLY in connection with a lens hood. Otherwise you may worsen the problem.

Of course, with digital cameras you won't need that filter. UV frequencies are higher than the upper limit the sensors do accept.

Last edited by RKKS08; 07-01-2015 at 12:20 PM. Reason: typing
07-02-2015, 01:16 PM   #14
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QuoteOriginally posted by canteaus Quote
I'm definitely shooting at f8 - there's no mistake about that.

Perhaps ChristianRock is right, maybe my infinity marking is past infinity.....

I guess the easiest way to test this is to find a digital Pentax SLR and test my lens?
Your MX should have a split screen that would allow you to see when the lens is in proper focus. That's how I found out that my 24mm lens was focusing past infinity.
07-02-2015, 10:27 PM   #15
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QuoteOriginally posted by ChristianRock Quote
Your MX should have a split screen that would allow you to see when the lens is in proper focus. That's how I found out that my 24mm lens was focusing past infinity.
Just one quick question then. Is there anyway that my split screen might not be properly calibrated to my lens?

Is my split screen seeing what my lens sees?
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