Distance-dependent AF behaviour of Canon 35/2 IS

[Albi86]

I started noticing some consistent front-focus problems while I was on a trip, and over some time I settled on a +7 value.

Checking the AF performance of this lens on my 5D3 using FoCal was completely useless due to focus imprecision.

Using DotTune, I confirmed that it needs a +7.5 at distances < 2.5m while it's spot-on at medium distances. I kept the +7 because the DoF is shallower at close distances, but my pictures at medium distances were somewhat affected and with this sort of gap it's very difficult to compromise.

Anyone else who owns this lens encountered similar problems?

 

[Mt Spokane Photography]

Dot tune is not very reliable, its fine to get you in the general area, but the variability is high.

If you find a issue when properly using Focal on a 5D MK III, its very likely a lens issue, and it should be sent in for repair while its under warranty.

Focal will detect focusing issues, and that can result in a failure to give a AFMA value. Also, AF needs lots of light. Daylight bright, as on a sunny day. As light dims, AF gets more and more erratic, and that makes it impossible to get the true AFMA value.

 

[jrista]

I have to agree that Dot Tune is pretty unreliable. There is no direct correlation between when AF is confirmed by the firmware, and the AFMA setting. AF confirmation is a bit more arbitrary, which is why there is variability.

When you use FoCal, do you use the maximum samples option? I forget what it is exactly, I think 10 samples are taken per AFMA setting. If you only do the "quick" AFMA tuning with FoCal, which only uses 2 or 3 samples per AFMA setting, it isn't really all that much better than Dot Tune. You really need to use a high number of samples to get accurate results. Small things during the AFMA tuning process, such as a missfocus (which do happen), wins or something else that might temporarily change the focus distance, etc. can all mess with the focus hits at each AFMA setting. By doing at least 10 samples per, FoCal is then able to use some basic statistics to discard outliers and produce a more accurate curve, and thus find the most accurate AFMA setting.

That said, AF IS often distance dependent. This could be due to spherical aberration in a lens, or possibly other aspects of lens construction. Regardless of the why, tuning AFMA for near focus will often result in improper AFMA for far focus. You might want to run FoCal with a high sample count for both near and far focus, and just try to memorize the settings, or write them down and keep the settings in your camera bag, or something like that, so you can reset AFMA if you need to switch periodically between far and near focus.

 

[Albi86]

Dot Tune produced exactly the same results as what I had previously empirically established over a couple of months. You average the focus confirmation range on both ends of the spectrum, so the inaccuracy is sort of evened out. Also, DotTune excludes the lens inconsistency from the process, something that instead affects FoCal heavily.

With FoCal I use the manual mode and I take 5 shots per AFMA value. The problem is that the variability is so high that the curve is unreliable. I have AFMA'd other lenses with good results. Interestingly, on those lenses FoCal and DotTune measurements don't differ of more than +/- 1. In my experience DotTune is actually quite reliable.

My Sigma 35A had a similar but opposite issue: it was spot-on at short distances but back-focused on medium distances. With the dock I was able to selectively correct that defect, something unfortunately Canon doesn't offer.

I was wondering if it is a design flaw or a specific problem of my copy, in which case it can be sent in for adjustment.

 

[jrista]

With FoCal I use the manual mode and I take 5 shots per AFMA value. The problem is that the variability is so high that the curve is unreliable. I have AFMA'd other lenses with good results. Interestingly, on those lenses FoCal and DotTune measurements don't differ of more than +/- 1. In my experience DotTune is actually quite reliable.

You really need to use at least 10 samples. There is some variability, but with enough samples, it averages out, the outliers are clipped, and the results are much more reliable.

 

[Mt Spokane Photography]

I typically use two or three samples per AFMA setting on focal, and its usually fine, but with a erratic lens, I find that no number of samples is going to fix the issue. I might get a average value of many samples, but if the lens varies all over the place, it needs service. Using a average value when a lens does not work correctly is no help.

 

[jrista]

I typically use two or three samples per AFMA setting on focal, and its usually fine, but with a erratic lens, I find that no number of samples is going to fix the issue. I might get a average value of many samples, but if the lens varies all over the place, it needs service. Using a average value when a lens does not work correctly is no help.

This is good advice, too. If the lens is that erratic, then it probably needs servicing.

 

[Andrew Davies Photography]

I started noticing some consistent front-focus problems while I was on a trip, and over some time I settled on a +7 value.

Checking the AF performance of this lens on my 5D3 using FoCal was completely useless due to focus imprecision.

Using DotTune, I confirmed that it needs a +7.5 at distances < 2.5m while it's spot-on at medium distances. I kept the +7 because the DoF is shallower at close distances, but my pictures at medium distances were somewhat affected and with this sort of gap it's very difficult to compromise.

Anyone else who owns this lens encountered similar problems?

I own this lens and have no issues with it - in fact it is the best focussing lens i have owned , you have a faulty lens and should just send it back and get it checked you will be happy you have done so as the lens is amazing.

www.andrew-davies.com

 

[Albi86]

I typically use two or three samples per AFMA setting on focal, and its usually fine, but with a erratic lens, I find that no number of samples is going to fix the issue. I might get a average value of many samples, but if the lens varies all over the place, it needs service. Using a average value when a lens does not work correctly is no help.

+1

My point exactly.

Another trick that I use is to half-press the shutter button 3-5 times so the lens has more chances to settle on the correct focus.

 

[Sporgon]

Another trick that I use is to half-press the shutter button 3-5 times so the lens has more chances to settle on the correct focus.

Beware !

I think you'll find that is a flawed technique.

 

[Albi86]

Another trick that I use is to half-press the shutter button 3-5 times so the lens has more chances to settle on the correct focus.

Beware !

I think you'll find that is a flawed technique.

Maybe yes, but in my experience it helps reducing random focusing errors

 

[mwh1964]

You may also need to be aware that AF one shot mode is more precise according to Canon. There was a link to that in a post not long ago.

 

[Mt Spokane Photography]

Another trick that I use is to half-press the shutter button 3-5 times so the lens has more chances to settle on the correct focus.

The accepted method is to set the lens to infinity (or MFD if subject is far away) and then do a AF. It should be done before every test point. Focal does this for you, but you can do it manually. By starting from infinity or mfd each time, the results are more consistent.

If the lens and camera is fine, af will not change when you hit the AF button multiple times. If you have to do that to get a goof AF, something is wrong.