- Messages
- 11,513
- Name
- Stewart
- Edit My Images
- Yes
We encountered a technical conundrum in the office yesterday that's got us stumped. I wonder if anybody out there can suggest a solution?
The short version is that we want to find an easy, reliable, repeatable way of checking that a lens can focus to infinity.
Here's why. Recently a customer hired a lens (Canon EF 24mm f/1.4 L II USM) for a trip to Norway to photograph the aurora borealis. But when he got back, he told us that he hadn't been able to get any good pictures because the lens wouldn't focus to infinity. The two professionals leading the trip he was on agreed with the diagnosis.
We were surprised because we test every lens every time it comes back to us, and we had never noticed anything wrong with this lens. So we started to investigate, but after half an hour of head scratching we realised that we were struggling.
Out of our office window the most distant object we can see is a factory chimney which is about 400m away. We photographed it with the suspect lens and with another identical lens which we knew to be good. (Ironically, because another customer had taken it on the very same Norway trip and had had no problems.) We took photos with the lens focussed manually at infinity, and then focussed automatically on the chimney (starting from the infinity stop), and pixel-peeped the results.
Here's what we got:
- suspect lens, manual focus at infinity - sharp image
- suspect lens, auto focus - sharp image (and no discernable AF movement)
- good lens, manual focus at infinity - soft image
- good lens, auto focus - sharp image (and a slight AF movement)
What it looks like is that when the suspect lens is manually focussed at what we think ought to be infinity, its actually focussed at some medium distance of several hundred metres. That would explain why the AF doesn't move when the lens focusses on the chimney, and it would explain why the manually focussed image is sharp.
But - we haven't actually proven whether or not the lens can actually focus at infinity. And we don't have an infinitely distant subject in or near the office to test it with. We tried using clouds but there's not enough contrast there to do a reliable test. If the moon had been out we could have tried that, but that wouldn't be a procedure which we could employ day-in day-out.
Any suggestions?
The short version is that we want to find an easy, reliable, repeatable way of checking that a lens can focus to infinity.
Here's why. Recently a customer hired a lens (Canon EF 24mm f/1.4 L II USM) for a trip to Norway to photograph the aurora borealis. But when he got back, he told us that he hadn't been able to get any good pictures because the lens wouldn't focus to infinity. The two professionals leading the trip he was on agreed with the diagnosis.
We were surprised because we test every lens every time it comes back to us, and we had never noticed anything wrong with this lens. So we started to investigate, but after half an hour of head scratching we realised that we were struggling.
Out of our office window the most distant object we can see is a factory chimney which is about 400m away. We photographed it with the suspect lens and with another identical lens which we knew to be good. (Ironically, because another customer had taken it on the very same Norway trip and had had no problems.) We took photos with the lens focussed manually at infinity, and then focussed automatically on the chimney (starting from the infinity stop), and pixel-peeped the results.
Here's what we got:
- suspect lens, manual focus at infinity - sharp image
- suspect lens, auto focus - sharp image (and no discernable AF movement)
- good lens, manual focus at infinity - soft image
- good lens, auto focus - sharp image (and a slight AF movement)
What it looks like is that when the suspect lens is manually focussed at what we think ought to be infinity, its actually focussed at some medium distance of several hundred metres. That would explain why the AF doesn't move when the lens focusses on the chimney, and it would explain why the manually focussed image is sharp.
But - we haven't actually proven whether or not the lens can actually focus at infinity. And we don't have an infinitely distant subject in or near the office to test it with. We tried using clouds but there's not enough contrast there to do a reliable test. If the moon had been out we could have tried that, but that wouldn't be a procedure which we could employ day-in day-out.
Any suggestions?
Last edited: