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08-27-2013, 09:12 AM   #1
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Manual Focus lens explained

I have recently gotten into the manual focus lenses (due to price). I have a Sony Nex5N and a few pentax k-mount lens. I have a 50mm prime which is great but I also have a 70-200mm. There are lines on the camera that I do not understand these lines and am looking for a little insight on them. Can anyone help. Below is an example of the lines I see on my camera.


08-27-2013, 09:50 AM   #2
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It's the depth-of-field scale, a handy feature of older lenses but mostly absent from modern autofocus lenses. The reason for the curved lines is because depth of field changes as you zoom the lens. Zoomed all the way in, as shown, the lines all converge to a small range because the depth of field is small at any aperture at that focal length. Zoom it out and you start to see big differences in how aperture changes the DOF.
08-27-2013, 09:50 AM   #3
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The lines are a DoF scale. But they are for 135 FF so they are more or less meaningless on APS-C.

Using DOF Scales
08-27-2013, 11:07 AM   #4
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QuoteOriginally posted by boriscleto Quote
The lines are a DoF scale. But they are for 135 FF so they are more or less meaningless on APS-C.

Using DOF Scales
The DoF lines are correct no matter what format you're shooting on, be it 35mm film or a Q. At a certain distance and certain aperture you're going to have a certain DoF.

08-27-2013, 11:09 AM   #5
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I wouldn't say they are meaningless, just wrong :P With the resolution of modern digital sensors, I generally consider the zone of acceptable focus to be at least one stop shallower than indicated by the lens's DoF scale. Maybe on some of the new 24~MP sensors, it's more like 2 stops shallower. But you can still use the scale as a guideline along with your own definition of "acceptably sharp", just shift the f/numbers in your head.

Also, to the OP, the red curved line represents the infrared index. Since infrared doesn't focus at the same point as visible light, it requires an adjustment in focal distance if you want to shoot infrared shots. If your camera is unmodified, you would focus normally, note the focal distance on the lens then turn it until it'd lines up with the IR index. The shot will then appear to be out of focus in the viewfinder, but when you press the shutter, the resulting image will be in focus. (hopefully)

Last edited by maxfield_photo; 08-27-2013 at 12:34 PM.
08-27-2013, 12:11 PM   #6
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QuoteOriginally posted by boriscleto Quote
The lines are a DoF scale. But they are for 135 FF so they are more or less meaningless on APS-C.

Using DOF Scales
Huh? DOF doesn't change with sensor size. The ONLY thing that changes is how much of the circular image projected by the lens is recorded in the image. Period.

Okay, I'll modify my post to agree with maxfield_photo. That's because 'focus' is a point and DOF is defined as 'acceptable range of focus', meaning not so far out of focus as to be unacceptable.

Last edited by JimJohnson; 08-27-2013 at 12:16 PM. Reason: agree with another poster
08-27-2013, 12:24 PM   #7
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QuoteOriginally posted by boriscleto Quote
The lines are a DoF scale. But they are for 135 FF so they are more or less meaningless on APS-C.

Using DOF Scales
QuoteOriginally posted by JimJohnson Quote
Huh? DOF doesn't change with sensor size. The ONLY thing that changes is how much of the circular image projected by the lens is recorded in the image. Period.

Okay, I'll modify my post to agree with maxfield_photo. That's because 'focus' is a point and DOF is defined as 'acceptable range of focus', meaning not so far out of focus as to be unacceptable.
Ok, while DOF at the image level on a sensor does not change, when you look at the whole DOF issue, it is based upon acceptable sharpness when enlarging to 8" x 10". Therefore you enlarge a cropped sensor image more to get to 8 x 10 so yes, indirectly because DOF is defined for a certain size print it does change with sensor size.

The red line, which no one has addressed is the infra red focus which is shifted as the lens performance for focusing changes with long wavelength light
08-27-2013, 12:43 PM   #8
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Going all the way back to my first camera in the 70's that had a DOF scale, I never tried to do critical work depending on those engraved lines. After all, those lines represent somebody else's idea of 'acceptable' focus. (8X10 enlargements and so many grains of silver or whatever rules be damned ) If DOF was critical, I stopped down the lens and looked for myself.

My most common use for the DOF scale was to pre-focus by setting one side just a teeny bit past infinity and using the other side to determine how close my subject could come and remain in focus. If I wasn't comfortable that DOF was deep enough, I would stop down and adjust the shutter. And then if the shutter was too slow, make a tough decision. Today, I can probably make that compromise easier by just spinning up the ISO.

08-27-2013, 03:32 PM   #9
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One should also mention that this lens offers 2 markings for aperture setting, a white and an orange one.
The white one is valid for 70mm, the orange for 210mm. In between you must interpolate.

Now-a-days this is only important if you use an Auto flash (thyristor), where you have to set the flash to the aperture used on the camera.

I wrote some more about this lens using the booklet delivered with the lens when I bought it new in 1985 (?), have a look at
Tokina SD 70-210mm F4.0-5.6 Lens Reviews - Tokina Lenses - Pentax Lens Review Database
and my forum name.

EDIT Not visible in the picture of the OP:
Additional to the white lines and the red line, there is also a blue line. This is only valid if you are in the (pseudo) macro range, which is also marked at the distance scale with a blue banding.

Last edited by RKKS08; 08-27-2013 at 03:39 PM.
08-29-2013, 08:28 AM   #10
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Thanks guys this has been super helpful. I actually prefer my manual lens to the autofocus one I have now. I think it make me feel like I know what I am doing when I adjust my lens myself.
09-15-2013, 04:06 AM   #11
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In film days, I used hyperfocal settings a lot in street shooting, being reasonably happy with a shot that was in focus to an acceptable degree where it counted. I must admit that dSLRs have made me lazy, but I recently started using DOF scales again to hyperfocal street shots (using old 50mm-ish manual lenses and others) and find they still work fine. The laws of physics haven't really changed in 20, 30, 40, 50 years.
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