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12-25-2013, 06:06 PM   #1
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Questions about F stops

With the 2 lens I have, what is the largest F stop that one could use for depth of field before it's useless to go higher?

1) kit lens 18-55mm

2) DAL 55-300mm


Thanks

12-25-2013, 06:15 PM   #2
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This might help: Online Depth of Field Calculator

Not sure what you mean by "useless", DOF just keeps getting larger as you reduce the size of the aperture (bigger f number). At some point diffraction increases to the point that the image is softer than acceptable but the DOF is larger.
12-25-2013, 06:16 PM   #3
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We're going to need a little more info.

Are you looking for the 'minimum depth of field' at any focal length?

I don't know what 'useless to go higher' means. 'Worse image quality?'

Depth of field is always small at low (wide open) F stops, and always large at high (aperture closed) F-stops.
12-25-2013, 06:17 PM   #4
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It depends on the zoom (FL) and what you focus on A Flexible Depth of Field Calculator

12-25-2013, 06:18 PM   #5
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QuoteOriginally posted by Michael Piziak Quote
With the 2 lens I have, what is the largest F stop that one could use for depth of field before it's useless to go higher?

1) kit lens 18-55mm

2) DAL 55-300mm


Thanks
I cannot speak about these lenses in particular but generally, you should not try to stop down your lens to a very small aperture (such as f/16 or f/22) because your images will start to suffer from diffraction effects unless you use very high-quality lenses. If your kit lens is like the kit lenses of other manufacturers, I would not stop down the lens to f/16 or smaller unless you absolutely have to. You could look for lens reviews for these particular lenses and see what other owners of these lenses have to say about them. There may even be some reviews on here. Another thing that you could do (and possibly the most ideal) is to take these lenses out and shoot a test picture at each focal length and at each aperture then examine the images on your computer (assuming you're shooting with a digital camera).
12-25-2013, 06:24 PM   #6
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Used on an APS-C camera, any lens will be diffraction limited by f/16. That is to say, you will have more in focus at f/16 and f/22, but the stuff in focus will be less sharp that at a lower f/ number. So I think the answer to your question is f/11, if I understand it correctly.
12-25-2013, 08:21 PM   #7
dms
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Once you are diffraction limited--around f/11 on Pentax dslr, around f/16 on a full frame, your lens ability as regards resolution will drop inversely proportional to the f number. Thus on Pentax dslr at f/32 the resolution will be 1/2 of what it is at f/16.

If the resolution is also at the limit of the sensor, the 1/2 resolution is 1/4 the MP--so 16MP becomes equivalent to 4 MP.

But a very good lens likely can resolve quite a bit better than the sensor. So the actual effect on resolution is somewhat complex--also because both the lens and sensor resolution are involved--I believe square root of sum of squared resolution of each.
12-25-2013, 08:24 PM   #8
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I take back what I said about a better lens resolving more--once they are diffraction limited any reasonably good lens will resolve the same. That's why they are diffraction limited!

12-26-2013, 01:23 AM   #9
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Wow. I totally don't understand any of this!
12-27-2013, 05:43 AM - 1 Like   #10
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QuoteOriginally posted by davids8560 Quote
Wow. I totally don't understand any of this!
There is a physics property about light that shows it can slightly bend around edges. At large apertures this is not a problem because it represents only a small amount of the light passing through the lens, but as you stop down, this problem becomes more obvious, so that while depth of field increases at small apertures, the refraction begins to dominate so the picture looses sharpness
12-27-2013, 09:58 AM   #11
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@davids8560
Perhaps a practical but not entirely accurate answer could be that the higher the F-stop the greater the depth of field, but beyond F11 you start to lose sharpness even in the parts that are in focus. At about F22 and beyond the picture will become quite soft and above 32 it will look like its shot with a cheap camera.

Typically if you have a situation where you really need the depth of field you will sacrifice overall sharpness in favour of bringing the far out parts of the picture into some sort of focus by going F16 and beyond.

The point at which increasing F-stop makes the image less sharp does not depend on the lens - neither its quality nor its focal length or full open aperture. It depends only on the size and resolution of the sensor and how much softening you are willing to tolerate. The figures I mention above are applicable for a 16MP APS-C sensor such as used by the K-5, K-30, K-01, K-50.
12-27-2013, 10:09 AM   #12
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Thanks to all that replied - I learned a lot!
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