Lens Infinity Marker Confusion

This is driving me mad, so please bear with me!

I am having a bit of trouble determining exactly which part of the infinity mark on my 17-40mm L constitutes infinity, for those times when I want to manually focus to infinity and can't see to check the focus (like at night, for example).

From looking at the lens when I first got it I presumed that infinity was denoted by the forward-facing 'tail' on the marker next to the infinity symbol (the same as the lens is set in this photograph here), in the same way that a very similar marker is used to denote the 17mm position on the focal length scale. I had been using that setting, as shown in the linked photograph, but was never completely happy with what I was doing.

Recently, whilst taking some landscape photographs and auto-focusing on an area around half a mile away, I noticed that the camera was setting the focus past the beginning of the infinity mark to about half-way along the mark itself, which obviously suggests that infinity is at least at that point, but almost certainly further. As a test I took a few more photographs with the focus turned back to my previously-used infinity mark (see linked photograph above!) and they came out slightly soft.

So, as far as I can make out, the beginning of the infinity mark that looks as if it is denoting something specific is actually denoting nothing of any great significance and is misleading. Infinity itself appears to be somewhere past the beginning of the mark, but nowhere obvious.

I have read about the reasons why you can focus past infinity on lenses, but that isn't what I'm going on about. I'm on about where infinity is, rather than the fact that I am able to focus past it.

Any suggestions, comments or observations would be gratefully received.

Mike

Lenses can usually focus past infinity.

Theories abound on this. Some people might reply that it's catering for temperature variations, some might say it's because it prevents the AF function from slapping into the end stop and damaging the mechanism.

There's also the fact that the distance scale printed on the lens isn't completely accurate (slight variances in manufacture).

Tiberius, I realize that, as stated in my second to last paragraph.

Pete, I'm thinking that it shouldn't be too much to expect for infinity to be marked, upon which temperature variations (or whatever else) can act to necessitate further adjustment by the photographer. How things appear to be now are that there isn't anything to even suggest infinity - in fact the only mark that appears to be suggesting infinity is in fact doing nothing of the sort!

Mike

Oh well, it appears that I'm the only person in the world who wants a usable infinity mark on their Canon lens. That explains why Canon haven't bothered!

Mike

I second that ... if Canon is listening..

Ahh good. Thanks Cody, this campaign is really gathering some momentum now!

Mike

It says in the manual that it is compensating for temperature changes (the bit past infinity)

Yes, but what I'm saying is the bit that is shown as - or alludes to being infinity - from which you can go past, isn't infinity, which is misleading.

Mike

The hyperfocal distance of a 40mm lens at f/4 on a 20D is 21.1m. Why do you even want to try to focus at infinity by using the distance scale rather than your own eyes, or AF? What's wrong with focusing at around 21-30m and letting DOF take care of the rest. It's not like you will be able to spot whether an object at infinity is pin sharp or not with a wide angle lens. It's hardly going to spoil the shot.

For all practical focusing purposes, any distance beyond 21.1m is the equivalent of infinity when using a 40mm f/4 lens on a 20D. If you close down the aperture or zoom wider then the hyperfocal distance becomes even closer than 21.1m. So for this lens, where would you have Canon put the infinity marker - 21.1m, 30m, 50m, 100m, 1,000m, 1000,000,000,000,000,0​00,000m? None of those is anywhere near infinity, but does it really matter?

If I've got my reasoning wrong then please correct me.

tdodd wrote in post #5750997
The hyperfocal distance of a 40mm lens at f/4 on a 20D is 21.1m. Why do you even want to try to focus at infinity by using the distance scale rather than your own eyes, or AF? What's wrong with focusing at around 21-30m and letting DOF take care of the rest. It's not like you will be able to spot whether an object at infinity is pin sharp or not with a wide angle lens. It's hardly going to spoil the shot.

For all practical focusing purposes, any distance beyond 21.1m is the equivalent of infinity when using a 40mm f/4 lens on a 20D. If you close down the aperture or zoom wider then the hyperfocal distance becomes even closer than 21.1m. So for this lens, where would you have Canon put the infinity marker - 21.1m, 30m, 50m, 100m, 1,000m, 1000,000,000,000,000,0​00,000m? None of those is anywhere near infinity, but does it really matter?

If I've got my reasoning wrong then please correct me.

I agree 100%.
Just focus on something very far and you’ll have 'enough of infinity' in your shots..

You could use this, or this, or our very own Curtis N's DoF calculator.

Or just focus on something far away and go from there.

Tim and Hermeto, please allow me to draw your attention again to the last part of my second paragraph:

' . . . . and can't see to check the focus (like at night, for example).'

That's the reason why I want to focus using the distance scale rather than using my own eyes. That's also the thing that's wrong with focusing at around 21-30m - it's dark!

The distance scale on the lens is equally vague when it comes to manually focusing to the hyperfocal distances (my most used being 17mm and f/8, which is 6' 3.5") as there is only 4mm of nothing between the 3' mark and the beginning of the infinity mark. I know I can solve that one by measuring those distances, focusing at them, then noting the position on the scale, but I haven't got around to it yet. So that brings me back to my original point, a usable infinity mark.

To use your own words, somewhere suitable for all practical focusing purposes.

Mike

Karl C wrote in post #5751203
You could use this, or this, or our very own Curtis N's DoF calculator.

Or just focus on something far away and go from there.

Karl, as I said to Tim and Hermeto, I can't always focus on something far away (or anything else come to mention it) because it's dark! I wrote that in my original post.The DOF Master site is great though. I've had it in my recommended links section for months.

Mike

Markers are not very important on modern day, autofocus lenses, especially zooms.
If you look carefully, you’ll see that even the focal length is not 100% accurate; very often you’ll find that lenses, especially long ones, are couple of mm longer or shorter then specified..