Canon mount Backfocus

[Nils Ruinet]

So, my Epic is ready to ship soon and I'm on the verge of ordering a Canon mount too, but one question remains unanswered : how do people check their backfocus when using the Canon Mount ? I guess you need to check it every time you swap mounts, right ?
AFAIK the RED FOCUS only works with PL mount.

Thanks

 

[Michael Thornton]

If you don't have an instrument.

I would do the poor man's way.

Just measure your distance.

Remember, sometime it's the lens not the mount that is off.

I believe that if your PL mount is in shape then the canon mount would also be in shape too.

 

[Nils Ruinet]

But what instrument is there that can check backfocus on a Canon mount ? Or maybe there's no need to, given that the markings on most Canon lenses are very imprecise anyway ?
I'm just wondering, because this doesn't seem to be a big concern for most people here ?
If your backfocus is spot on with your PL mount, will it stay that way when swaping mounts, or do you need to check it everytime you swap ?

 

[Chosei Funahara]

If your still lens' back focus is off, I guess that you have to send your Canon lens to Canon factory, that I do my still Leica lens.

 

[Stephen Williams]

If you can focus at infinity you will be fine, the marks on still lenses are rather vague & without witness marks.

 

[Mark Phelan]

If you can focus at infinity you will be fine, the marks on still lenses are rather vague & without witness marks.

"How I learned to stop worrying and love the bomb." Obviously, Dr. Strangelove was talking about his EVF, but I think he was in agreement with Stephen. I stopped worrying about the backfocus on my Nikkors because it didn't seem to matter as long as I could focus by sight. The lenses have such a short throw, the marks are worthless and I never used them anyway. For my PL lenses I'll be getting the RED focus. Perhaps there will be an adapter of some sort that a creative machinist will deliver. Any creative machinists out there?

 

[Jacek Zakowicz]

There is a Canon adapter available for the Optimator. As well as Nikon, PL, etc...

 

[Lauri Kettunen]

Nils, as soon as get the Canon mount, can make formal instructions of some advanced algortihms. But, in the mean time, here's a simple iterative process for the collimation;

Pick a lens like the EF 17-35mm L f2.8, set it to 17mm and measure a 1m distance between the sensor and the target. Now, to explain the iterative process, let me formally say that parameter Focus17 gets initially value 1m. Then

1) Set the lens focus to Focus17
2) Set the zoom to 17mm and adjust the collimation until the target is in focus.
3) Set the zoom to 35mm and adjust the lens focus, until the target is in focus.
4) Read now the focus from the lens, and call the value Focus35.
5) If Focus 35=Focus17, then the lens is collimated. Otherwise, if Focus35 is not the same as Focus17, then set the new value of Focus17 to (the old value of Focus17 +Focus35)/2.0, that is

Focus17 := (Focus17 + Focus35)/2.0

and go back to step 1. Iterate until convergence.

6) Finally, check that the lens can focus to infinity when the zoom is 17mm and 35mm. If not, send the lens to Canon maintenance for adjustments.

Adjusting the backfocus means, while you zoom the focus will not change. In addition, you should be able to focus to infinity.

Addition: If your lenses are internally adjusted correctly, you should not need to adjust the collimation when you change the lens. Furthermore, it's important on short lenses but not on the long ones. Some people check the collimation daily. With the Red One and Birger mount I checked it about once in three months, or whenever there was significant changes in the temperature. Before getting the Birger mount used the Nikon mount, and that required more frequent checking. However, should add, I do not tend to zoom while capturing.

The Epic Titan Canon mount should be pretty stable, so guess once every 2 months should be ok. Notice also, collimation becomes more important when you zoom while capturing. I think this is a reason why the DSLRs do not need/have collimation: If one shoots only stills, all what matters is that one is able to focus. For, guess nobody zooms while taking a still and still hopes to get the image in focus.

 

[Zeb B]

Nils, as soon as get the Canon mount, can make formal instructions of some advanced algortihms. But, in the mean time, here's a simple iterative process for the collimation;

Pick a lens like the EF 17-35mm L f2.8, set it to 17mm and measure a 1m distance between the sensor and the target. Now, to explain the iterative process, let me formally say that parameter Focus17 gets initially value 1m. Then

1) Set the lens focus to Focus17
2) Set the zoom to 17mm and adjust the collimation until the target is in focus.
3) Set the zoom to 35mm and adjust the lens focus, until the target is in focus.
4) Read now the focus from the lens, and call the value Focus35.
5) If Focus 35=Focus17, then the lens is collimated. Otherwise, if Focus35 is not the same as Focus17, then set the new value of Focus17 to (the old value of Focus17 +Focus35)/2.0, that is

Focus17 := (Focus17 + Focus35)/2.0

and go back to step 1. Iterate until convergence.

6) Finally, check that the lens can focus to infinity when the zoom is 17mm and 35mm. If not, send the lens to Canon maintenance for adjustments.

Adjusting the backfocus means, while you zoom the focus will not change. In addition, you should be able to focus to infinity.

Addition: If your lenses are internally adjusted correctly, you should not need to adjust the collimation when you change the lens. Furthermore, it's important on short lenses but not on the long ones. Some people check the collimation daily. With the Red One and Birger mount I checked it about once in three months, or whenever there was significant changes in the temperature. Before getting the Birger mount used the Nikon mount, and that required more frequent checking. However, should add, I do not tend to zoom while capturing.

The Epic Titan Canon mount should be pretty stable, so guess once every 2 months should be ok. Notice also, collimation becomes more important when you zoom while capturing. I think this is a reason why the DSLRs do not need/have collimation: If one shoots only stills, all what matters is that one is able to focus. For, guess nobody zooms while taking a still and still hopes to get the image in focus.

Awesome! Thank you for the time and effort in producing this

 

[Jacek Zakowicz]

Unfortunately all that fails if your lens s out of focus(if I understand this correctly). That is why you need a reference tool that will adjust your flange correctly fist and let you use the (properly adjusted) camera as a test tool for lenses.

 

[Lauri Kettunen]

Jacek has of course a valid point: Forgot to say, that the procedure yields the collimation with respect to the employed lens. This is why the reference is important.

The good news is, it does not matter what is used as a reference as long as it is correct. So, if one is concerned, the easiest solution is to ask Canon maintenance to check/adjust the backfocus of the wide angle zoom lens one uses as a reference. The last time when bought new Canon wide angle lenses for my Red One I asked the dealer whether the lenses are adjusted. He said he is not absolutely sure, but they will check the focus in their maintenance before sending the lenses to me. So it was that simple.

The other good news is, in practice it is not even that critical whether you know a proper reference. For, the collimation is critical only with wide angle lenses. In rather heuristic but simple illustrative words, in front of the lens wide angle lenses have large depth of field and teles a short one. But behind the lens everything goes the other way around. So, if you do not that have that large number of wide angles and you don't know whether they are adjusted properly, as long as the collimation works out with all your wide angle lenses you are ok. Most likely, it will nevertheless work with the teles.

The collimation and backfocus is, however, critical, if one focus the images relying on the focus readings of the lens (or readings on the LCD/EVF). With still camera lenses the lens markings are anyhow too inaccurate, so they are not reliable at the first place. Second, if one focuses by sight, as I have to do as a wildlife shooter, then the readings just do not matter. All what is important is that the focus does not change while zooming and that the lens focuses to infinity.

Finally, to remember to say everything, one should not expect all lenses are good enough to hold their focus while zoooming. The collimation procedure does fail, if the lens simply can't hold the focus with any collimation. Or it may happen that the focus is ok with the minimum and maximuum zoom settings, but not between. In my experience the best Canon L-series lenses do nout suffer for such problems.