aperture to use for landscape with the Tamron 17-50 f/2.8 non-vc lens?

[Helen B]

Luckily, lens hoods have no effect on refraction. They simply reduce the amount of light from objects outside the frame.

 

[JG_Coleman]

Luckily, lens hoods have no effect on refraction. They simply reduce the amount of light from objects outside the frame.

Huh, guess I learn something new everyday. I always thought lens flare was an example of refraction... which seems to be a misunderstanding in terminology that I share with plenty of people (its remarkable how many people refer to flare as "refraction"). Needless to say, I had to immediately go googling for technical information about lens flare after reading your post and, not surprisingly, you're right... lens flare is not refraction, but reflection.

I thought it was funny though that every technical article online about lens flare is very clear about saying that lens flare is NOT refraction at all, yet just about every forum post, blog post, content-factory article that pops up in the search results use "refraction" and "lens flare" as if they are synonyms. This reminds me of the good old "PPI vs. DPI" debacle that some people get so worked up about.

So, AaronLLockhart, lens flare is just plain, old, not-scientific-sounding "reflection"... not "refraction" as I previously stated.

Thank you, Helen.

 

[Joshonator]

It should also be noted that the optimal aperture in terms of sharpness depends on the lens to some extent.

 

[AaronLLockhart]

So let me make sure I understand what's being said here. By what you are saying, diffraction will increase with longer focal length?

Nikon D5000 | 18-55mm AF-S VR | 50mm 1.8G | HB-47 Hood

 

[TCampbell]

And of course "diffraction" and "refraction" aren't exactly the same things either. The physics of diffraction are based on the assumption that the lens is optically perfect -- so we aren't really talking about how well the optical glass can refract light or whether one lens is better than another. "Diffraction" happens when light is forced through a tiny area and then needs to spread out. Refraction happens when light contacts a transparent surface/air transition at an angle (such as the two sides of a lens element) and the light bends as it passes through the transition.

 

[Nat.]

I don't think it does.

Here's an article on diffraction: Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Quote:

Technical Note: Independence of Focal Length
Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).

 

[table1349]

To the OP. A series of landscapes with the 17-50 at various f stops: f11, f16, f22. You decide what you think will work for your situation.

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[Joshonator]

From my understanding of physics, refraction cannot be controlled since it depends of how good the glass is. I think the glass is usually a fast medium and doesn't cause too much refraction (and if it does the only thing you can do is clean the lens or get a new lens). Whereas diffraction is directly dependant on the size the aperture hole.

Diffraction can cause light waves to overlap and interfere which in the case of light hitting the sensor, will leave a lack of detail (tiny gaps in light getting to the sensor).

 

[Helen B]

From my understanding of physics, refraction cannot be controlled since it depends of how good the glass is. I think the glass is usually a fast medium and doesn't cause too much refraction (and if it does the only thing you can do is clean the lens or get a new lens). Whereas diffraction is directly dependant on the size the aperture hole.

Diffraction can cause light waves to overlap and interfere which in the case of light hitting the sensor, will leave a lack of detail (tiny gaps in light getting to the sensor).

A lens focuses light because there is refraction at the air/glass and glass/glass surfaces. Refraction is not a bad thing in a lens, it is a good thing - essential in fact. The lenses we commonly use work because of refraction.

Destructive interference ('gaps') caused by diffraction is rarely a problem in normal photography because the light arriving at the aperture is not usually coherent - ie the 'overlaps' of light heading to two different image points caused by diffraction rarely cause destructive interference because the overlapping light in this case is not usually at the same frequency and in a constant phase relationship. The effect is generally to spread light that should have focused to a single point into a spread-out interference pattern with a central disk (the Airy disk) surrounded by rings (all of which is the Airy pattern). The loss of (otherwise possible) detail happens not because of gaps but because image points are spread into disks. Of course there are other factors in a real lens that will also cause a point to spread (such as lens aberrations, defocus etc) so diffraction simply sets a minimum possible amount of spread.

 

[AaronLLockhart]

A lens focuses light because there is refraction at the air/glass and glass/glass surfaces. Refraction is not a bad thing in a lens, it is a good thing - essential in fact. The lenses we commonly use work because of refraction.

Destructive interference ('gaps') caused by diffraction is rarely a problem in normal photography because the light arriving at the aperture is not usually coherent - ie the 'overlaps' of light heading to two different image points caused by diffraction rarely cause destructive interference because the overlapping light in this case is not usually at the same frequency and in a constant phase relationship. The effect is generally to spread light that should have focused to a single point into a spread-out interference pattern with a central disk (the Airy disk) surrounded by rings (all of which is the Airy pattern). The loss of (otherwise possible) detail happens not because of gaps but because image points are spread into disks. Of course there are other factors in a real lens that will also cause a point to spread (such as lens aberrations, defocus etc) so diffraction simply sets a minimum possible amount of spread.

So, ultimately it's unavoidable, even with a super clear glass?

Nikon D5000 | 18-55mm AF-S VR | 50mm 1.8G | HB-47 Hood