Teleconverters and fstop?

JohnN

JohnN

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Just a quick question - if you pop a converter on and it takes it from say a 5.6 to 8, does the lens still act like its an 8 - ie possible sharper images?

Or does it still act like a 5.6, thus needing something like f11 to get the sharpness of f8?

I hope that made some semblance of sense!
 
JohnN said:
Or does it still act like a 5.6, thus needing something like f11 to get the sharpness of f8?
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This...but you might have to stop down a little further to overcome the degradation caused by the T/C

Bob
 
Ah, cheers Bob - I guess using a TC becomes an exercise in quick and dirty maths :)
 
At f/8 you'd still shooting wide open on the lens, it's just the focal length has increased due to the converter but the aperture size hasn't, so the f/number is reduced.

Lenses sharpen up when stopped down because the aperture is smaller, and the TC doesn't affect the aperture size.
 
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I recently got a Kenko DGX 1.4x teleconverter and put it on my 70-200 f4 L lens. I had a 2x but I couldn't get AF with it so I went for the 1.4x instead. The few photos I've took with it so far have had pleasing results. They have been nice and sharp with the lens wide open giving me an f5.6 aperture at 280mm FL. More than pleased as it is compatable with the majority of my lenses, even the 50 f1.8 and the 24-105 f4 L works well with it where other makes of tc's weren't compatable with them.
 
squishy said:
At f/8 you'd still shooting wide open on the lens, it's just the focal length has increased due to the converter but the aperture size hasn't, so the f/number is reduced.

Lenses sharpen up when stopped down because the aperture is smaller, and the TC doesn't affect the aperture size.
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You might want to revisit that theory Scott

Bob
 
Canon Bob said:
You might want to revisit that theory Scott

Bob
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Why? Lenses have a sweet spot at smaller apertures because as the aperture gets smaller it only uses smaller portions of the centre of the optics which should be the least flawed part of the optics. But diffraction limiting will set in past some point, which is why there is a sweet spot around usually f/8-11

I might not have put my point across very well, I tend to get my words in a knot when I try to explain any of this stuff coherently :bonk:
 
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If a 600/4 is "sweet" at f/8 (apparent aperture diameter of 75mm) then you expect a 24/1.4 at f/1.4 (apparent aperture diameter of 17.15mm) to wipe the floor with it optically speaking.

Bob
 
Canon Bob said:
If a 600/4 is "sweet" at f/8 (apparent aperture diameter of 75mm) then you expect a 24/1.4 at f/1.4 (apparent aperture diameter of 17.15mm) to wipe the floor with it optically speaking.

Bob
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I mean smaller relative to it's wide open aperture.
 
squishy said:
I mean smaller relative to it's wide open aperture.
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Yes, I understood that Adam. The point I was making was that if you can produce a "sweet spot" 75mm diameter for one lens then it should be childs play to produce one less than 1/4 of that diameter for another lens.

Bob
 
Except the optical layout of a 600mm and 24mm will have to be very, very different, since they are producing vastly different fields of view.

It's not about the absolute size of the aperture, but it's size relative to the size of the elements of the lens, since it's the centre most portion of the optics that should be the least flawed.
 
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I'll have to come back to this tomorrow Adam, it's getting late over here and I'm liable to write something silly if I continue now.

Bob
 
scottthehat said:
:thinking:
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The beauty of apertures in terms of f stop is that they can be compared across all lenses because any given f-stop will let in the same amount of light, regardless of the lens focal length. The (aperture) f-stop is proportional to focal length divided by SQRT(aperture area), or something similar.
So the longer the focal length, the larger the aperture area needs to be to let in the same amount of light, and thus to produce the same f-stop number.

Still confused? :p
 
Canon Bob said:
The point I was making was that if you can produce a "sweet spot" 75mm diameter for one lens then it should be childs play to produce one less than 1/4 of that diameter for another lens.
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Except that the shorter lens gathers light from a wider field of view, has to bend the light further so is more susceptible to abberations (i.e. anything which causes a point source in the subject to not be focused at a point on the image plane) which help to destroy contrast and hence perceived resolution/sharpness.

A lens is potentially sharpest wide open, it's just that the inherent abberations in the design counteract that. Stopping down reduces the abberations so improving the perceived sharpness. Then diffraction takes over as you continue to decrease the aperture and this again starts to reduce the sharpness.
 
arad85 said:
Except that the shorter lens gathers light from a wider field of view...
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That's precisely it Andy and why the statement about using "the centre of the optics" is wrong as it would result in a changed FoV (similar to having a smaller sensor)

arad85 said:
A lens is potentially sharpest wide open, it's just that the inherent abberations in the design counteract that. Stopping down reduces the abberations so improving the perceived sharpness....
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That's about sums it up. The smaller aperture means that the light path from the opposite sides of the diaphragm become more equal in angle and distance and the effects of the aberrations become more suppressed as a result.

To de-tech it somewhat.....
Take a penalty (12 yds) into an open goal and you're going to find it quite straightforward to score without much accuracy or power. Take the same kick from the touchline (12 yards out from the corner flag) and it's going to be more difficult. If however the pitch was reduced in width by 50% (stopped down) then the required accuraccy and power would both be reduced.

Bob
 
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DoctorBob said:
The beauty of apertures in terms of f stop is that they can be compared across all lenses because any given f-stop will let in the same amount of light, regardless of the lens focal length. The (aperture) f-stop is proportional to focal length divided by SQRT(aperture area), or something similar.
So the longer the focal length, the larger the aperture area needs to be to let in the same amount of light, and thus to produce the same f-stop number.

Still confused? :p
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sorry bob,i understand that i was just having a moment about the fact that a TC doesn't affect the aperture size.
 
Canon Bob said:
That's about sums it up. The smaller aperture means that the light path from the opposite sides of the diaphragm become more equal in angle and distance and the effects of the aberrations become more suppressed as a result.


Bob
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Whatever the reason behind it, you're now agreeing with my original point that it's the smaller aperture relative to the wide open aperture that causes lenses to sharpen up, which is what you were disputing in the first place :bonk:

You even seem to understand the principal behind it better than me, I've just lost track of why we're arguing this point now
 
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Apologies then Adam.....I interpretted your statement "Lenses sharpen up when stopped down because the aperture is smaller" as meaning that a smaller aperture will be sharper than a larger one....ie, physical aperture size will determine sharpness.

My quest to attain fluency in French has pushed me towards seeing everything too literally.

Bob
 
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