Lens features I don't understand or I think are missing.

Peter69

Peter69

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Does anyone else wonder why there maybe a need to focus beyond infinity? Can some optical expert explain to me why all lens (I think) go past the infinity mark by so much?

Also, is itcjust me or would be hand to have a little pip at the lenses hyperfocal point? Ok, it won't be easy on a zoom.

I may be talking out the top of my head but thats never stopped me asking stupid questions before.
 
Well, infinity isn't as fixed as you think it is, and lenses are built within tolerances. If they were designed to focus to infinity and you bought one that front focused a little, you'd have a lens that doesn't focus to infinity. Then there's the zoom changing focus, etc. etc.

There's no such distance as 'hyperfocal' because it changes with aperture.

But someone smart will be able to give technically correct answers, I'm a bit self-taught so I could be completely wrong.
 
I have always found lenses that focus beyond infinity a bit annoying - most nikon af-d and ai-s lenses seem to have a "real" infinity stop but the newer af-s lenses seem to go beyound infinity.
 
The 2 infinity marks are to allow for temperature change. If you think of the lens elements when focused at say 30 mtrs and at 5 centigrade. And then at 30 centigrade. The eliments will be in a different position in relation to each other.
 
Phil V said:
There's no such distance as 'hyperfocal' because it changes with aperture.
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And also with sensor size.....many lenses can be used on cropped or full frame bodies

vetonbike said:
The 2 infinity marks are to allow for temperature change. If you think of the lens elements when focused at say 30 mtrs and at 5 centigrade. And then at 30 centigrade. The eliments will be in a different position in relation to each other.
Click to expand...

It also removes the need to build bodies with a super accurate register distance....a small change in register distance becomes a much larger change in the subject plane.

Bob
 
Lenses USED to come with a little button that limited the focus to where you set it - it was almost what has been asked for as "a hyperfocal button". This was back in the days of 35mm or 50mm fixed lens, then if was set the focus distance to 20 feet, f8 and be in the right place. Some photographers taped their focus ring to the hyperfocal distance - it was literallly taped so it couldn't move. If you wanted to do other stuff, many had a second lens of the same focal length without the taped up focus ring.

Why do lenses have more focus ability than stopping at a set infinity? - Infra red is another reason beyond what has already been said.

I sometimes think modern photographers would benefit from learning with some film, manual cameras and lenses for a day or three.
 
Lensflare said:
Why do lenses have more focus ability than stopping at a set infinity? - Infra red is another reason beyond what has already been said.
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I'm not sure Infra red would have any bearing on the position of the infinity stop as IR light focusses more closely than light in the visible spectrum and the focus shift for IR is always negative....more so with longer focal lengths. Only true apochromatic lenses would focus the whole spectrum in the same plane.

A typical AF system is designed to position the lens within a certain area of acceptable DoF. On Canon bodies it'll be within 1/3 DoF for high accuracy AF points when using lenses with a max aperture of f/2.8 or greater and simply within DoF for lenses with smaller apertures. To aid faster focus acquisition it may be that the position falls short or long and the small amount beyond infinity allows this. High quality manual focus lenses (Carl Zeiss for example) are calibrated to stop at infinity....there is no additional headroom there.

Unless somebody knows different?

Bob
 
AF systems need to see either side of the focus position to know where the sharpest point is. I think that is one of the key reasons for a variable infinity stop, though not 100% certain. For sure it makes things easier though - for focus calibration, freedom to have a different infinity position according to focal length, temperature shifts etc - and I can't see any downsides really.

I think some Zeiss AF lenses have a variable infinity position. Not sure about them all without checking.
 
I always thought that the reason for focus beyond infinity on AF lenses was because it stopped manufacturers needing to calibrate infinity on every lens as it came off the production line like they did in the film days. AF gets round the need for a hard stop at infinity as the cameras electronics can happily set it but in the days before AF setting it by hand without a hard stop was pretty difficult!

Don't buy any of the lenses changing size with temperature nonsense if glass changed size that much windows just wouldn't work!
 
a1ex2001 said:
Don't buy any of the lenses changing size with temperature nonsense if glass changed size that much windows just wouldn't work!
Click to expand...

It's not the glass, it's the aluminium barrel. Aluminium expands by around 22 microns/metre/degree C. My 600/4 would therefore expand by around 130 microns with a 10 degree C temperature increase. Some of this expansion would affect the spacing of internal elements and also have a bearing on the depth of focus which may preclude infinity focus.
Taken to more extreme limits then a lens that's specified to work from -20 to +40 degrees (60 degree span in temperature) could have a total change in length >0.75mm (for a 600mm lens).

Bob
 
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