chromatic aberation!!

A.Gibson

A.Gibson

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400
Name
Anthony Gibson
Edit My Images
Yes
ive recently got the siggy 1.4x tc to go with my siggy 70-200 2.8, i was messing about with it last nite in the lounge using my flash and i really was impressed with how sharp it still is with the tc attached there doesn't seem to be any change in iq at all to my eye (i know there will be some but i cant notice it.lol) but this morning i got it out again to have a little play, not really taking pics of anything in particular but i was shooting straight up under a tree through the branches with the following settings,between 1,1000th and 1,800th sec, f4, iso640 or on some iso200 and wb set to auto and all shot at 280mm and after a few shots i noticed that the CA was really quite bad around each branch, took the tc off and shot again with the 70-200 wide open and it was gone :shrug: shuld this be happening with the 70-200 and 1.4xtc? i also noticed that if i set it to around the f8 mark the ca isnt there at all.:thinking:
when shooting normaly (not into harsh light) there is also no CA at all, is the tc faulty do you think?:shrug:

thanks
Gibbo..:thumbs:
 
A faulty T/C?. Anything's possible Gibbo but the test you're performing is a damn good way to induce CA. The CA almost always comes from the lens rather than the T/C but the T/C will accentuate it. Wide open on a zoom (and at either end of the zooms range) means you're pushing things to the limit with a high contrast target such as branches against the sky. I'd also guess that the periphery of the shot is worse than the central portion.

In short, I think you've discovered why a 400mm lens is usually better than a 200mm lens with a 2x T/C....or in your case, a 300mm lens is better than a 200mm lens with a T/C.

Shoot with "cautious settings" and you'll get reasonable results I think.

Bob
 
good good just glad i dont need to take it back, im sick of taking things back recently.lol
yeah it is not as bad in the center but it is mostly all over the image. i would love a 400mm prime but dont think the bank would be to pleased with me.lol
the results at f8 shooting into the harsh light are spot on its just when wide open it magically appears. i dont really want to get the 2x tc as it WILL degrade the image a lot thats why i went with the 1.4 tc and its made very very very little differance in iq and the focus speed is about the same to so to be honest i really am pleased with it.

thanks for the quick reply bob:thumbs::thumbs:

Gibbo..
 
TCA and LCA will never be a function of shutter speed, generally, chromatic aberration is usually worse as you move further off the optical axis. Hence in a system of lenses, if the exit pupil (aperture) is large, one usually ends up with more TCA/LCA

Translated for photography.. wide open lenses will normally exhibit more chromatic abberation.

The ABBE or "V" value for each individual lens in the system will also effect the amount of aberration, as will the form of each lens

Translated for photography.. a good manufacture will match the ABBE values of individual lens elements to lower TCA/LCA (one counteracts the other - often called APO lenses). This is hard maths, which is why, along with the advent of superb computing power, CNC free-form generated lenses lenses have been made from plastics (appaling V values) whilst still exhibiting excellent off axis performance. A very good lensmaker uses all the tricks in the book to provide a relative lack of distortion within a wide range of f-stops. However, if we all want thinner, lighter, cheaper, smaller lenses, something must give a bit

There is another issue to consider at the other end of the aperture range - diffraction this is why many modern lenses have a sweet spot. If you consider the extremes (pinhole cameras) then diffraction is a major issue
 
thanks richard that will explain why it seems to go at around the f8 mark.
this is why i love this forum, less than an hour after he first post and already two great reply's and also ive stopped worrying that i have a faulty tc.lol
thanks guys i shall be aware of this when i go out next time.:thumbs:

Gibbo..
 
Yeah thought there was something on lightroom to sort it ill give it a whirl later.
Thanks
gibbo.
 
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