Infinity Focus

I am confused by the concept of infinity focus. I was reading about astrophotography and there was a lot of discussion of how to focus on stars. Why isn't it as simple as turning the manual focus ring on the lens to the infinity setting?

Many modern autofocus lenses lack a hard infinity stop, as well as lacking a distance scale. This is more accurate than a hard stop, which might be inaccurate due to temperature expansion.

Turning the focus to infinity works on traditional fully manual lenses. Modern electronic lenses are not very good for any kind of more technical work.

Don

D Cox said:

Turning the focus to infinity works on traditional fully manual lenses. Modern electronic lenses are not very good for any kind of more technical work.

Don

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No, plenty of manual focus lenses focus past the infinity mark. Particularly the telephoto types.

FrancoD said:

D Cox said:

Turning the focus to infinity works on traditional fully manual lenses. Modern electronic lenses are not very good for any kind of more technical work.

Don

Click to expand...

No, plenty of manual focus lenses focus past the infinity mark. Particularly the telephoto types.

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Just because the lens can rotate past the infinity mark doesn't mean it has to. Don is talking about setting to the mark.

Gerry Winterbourne said:

FrancoD said:

D Cox said:

Turning the focus to infinity works on traditional fully manual lenses. Modern electronic lenses are not very good for any kind of more technical work.

Don

Click to expand...

No, plenty of manual focus lenses focus past the infinity mark. Particularly the telephoto types.

Click to expand...

Just because the lens can rotate past the infinity mark doesn't mean it has to. Don is talking about setting to the mark.

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Setting to the mark may not be infinity at low temps, at different apertures and on non parfocal zooms.

A lot of old manual focus zooms were not parfocal , so you needed to refocus as you zoomed and that also affected the position of infinity.AF lenses do that automaticaly (re-focus as you zoom).

Gerry Winterbourne said:

FrancoD said:

D Cox said:

Turning the focus to infinity works on traditional fully manual lenses. Modern electronic lenses are not very good for any kind of more technical work.

Don

Click to expand...

No, plenty of manual focus lenses focus past the infinity mark. Particularly the telephoto types.

Click to expand...

Just because the lens can rotate past the infinity mark doesn't mean it has to. Don is talking about setting to the mark.

Click to expand...

Which with telephotos is not accurate enough due to temperature effects, which is why they go beyond...

Autofocus lenses usually also focus past infinity to stop the motor getting strained when it hits a hard stop.

I'm fairly sure I have lenses that don't actually focus to the horizon, instead getting close enough that infinity is in the normal DOF range. Cropping or printing large from these lenses shows the focus is short.

Tom SilverSpring said:

I am confused by the concept of infinity focus. I was reading about astrophotography and there was a lot of discussion of how to focus on stars. Why isn't it as simple as turning the manual focus ring on the lens to the infinity setting?

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Only my oldest lenses (pre-2000) have an infinity mark and a hard stop. Later lenses have an "infinity line" which means "infinity focus is somewhere along this line". My most modern AF E-mount lenses don't have a distance scale at all because they're all focus by wire.

This nice article from Zeiss talks all about problems with obtaining a precise manual focus in DSLR AF cameras (many apply to mirrorless too). It also makes me want to run out and buy a Zeiss Batis lens with that cool LCD distance scale.