# AF error tolerance



## WarStreet (Jun 16, 2011)

After posting here, I got the 400mm 5.6 lens, but it is front focusing. The front focus is about 2cm. The total DOF is about 6cm, which makes it only 3cm from the center to the front/behind, when lens positioned about 7m @ 5.6. Measurements are rough and not very accurate.

I noticed that the longer the subject distance, the bigger the error.

Shooting the moon, the phase detection focus showed softer results consistently compared to contrast detect focus when viewed 100% crop. I used tripod, cable release and shot all photos in live view (including the phase detection focus).

I don't know if this is within acceptable tolerance, but even if it is, I don't like the results. Since I don't have micro adjustment, I don't know if the error can be fixed with it for different subject distance. Also, the focus ring wobbles a bit too much, enough to make 'loud' wobbling noises, and there was some dirt on the lens body, which I removed without any problem. 

Got this from Amazon uk, do you think I should replace it ?


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## motorhead (Jun 16, 2011)

If you had got it from a local shop I'd suggest putting some of their stock through the same test and having the best, sadly you don't have that option with amazon.

As you probably know, it's a combination of errors that create the problem. The lens contributes, but so does your camera. What you need is a lens with errors that help cancel out the built-in errors your camera already exhibits.

In your situation I'd certainly talk to the supplier. If they are genuine they will want you to be happy with your purchase and hopefully will help you resolve the problem. But don't expect them to pick up all the extra transport expenses.

If the supplier refuses to cooperate then if you send the lens and camera to Canon they can tweak the lens to match. Yes, it will take time and no, it will not be cheap.


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## Flake (Jun 16, 2011)

You don't have to send the lens & body to Canon, any authorised repairer can do it much quicker & cheaper. But you shouldn't do this without including all of your lenses, because if they adjust the body it'll just throw every other lens out. If you know anyone else with a Canon camera you could always do a quick test before returning it.


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## WarStreet (Jun 16, 2011)

This lens will mainly be used on the future 5DIII. Here in Malta you won't find any shop to try out the lenses, and I think that the repairs are actually done by sending them abroad. There are only official resellers in Malta since the market is very small in this tiny island.

I think that 2cm front focusing from a 3cm center to front DOF is unacceptable. I would like to know if this is outside Canon tolerance. I remember I have read about Canon tolerance somewhere but I can't find them again. I did read that an error of 1/3 of DOF is acceptable on a 1D. I don't know if they are measuring DOF from center or total DOF, and I don't know about rebel tolerance. Any one know about Canon camera - lens tolerance regarding focusing ?

I never returned / replaced anything from Amazon yet, I think that I should not have any problem replacing the lens after telling them that it is not working fine as expected.


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## epsiloneri (Jun 16, 2011)

It sounds like micro-adjust should be able to help you with the problem, and 5D3 will certainly have it (whenever it arrives). I don't either know what the tolerances are, but if you're not happy, it is probably better to return the lens if you can. It could be a while before the 5D3 shows up.


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## Admin US West (Jun 17, 2011)

WarStreet said:


> After posting here, I got the 400mm 5.6 lens, but it is front focusing. The front focus is about 2cm. The total DOF is about 6cm, which makes it only 3cm from the center to the front/behind, when lens positioned about 7m @ 5.6. Measurements are rough and not very accurate.
> 
> I noticed that the longer the subject distance, the bigger the error.
> 
> ...



See http://en.wikipedia.org/wiki/Auto-focus

Autofocus accuracy is said to be about 1/3 the depth of field at maximum aperture, so, if I understand properly, 2cm of 7cm is within the expected value.

If the focus is 2cm in front of the 7cm zone, then it may be that the combination of lens and camera may be causing it, since each has a tolerance.


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## neuroanatomist (Jun 17, 2011)

scalesusa said:


> Autofocus accuracy is said to be about 1/3 the depth of field at maximum aperture



I'm not sure that this is the case, even though that's exactly what the wiki page states. Canon notes that the f/2.8-sensitive center AF point on some bodies is 'high-precision' but precision ≠ accuracy. The terms accuracy and precision are sometimes (improperly) used interchangeably - accuracy is 'closeness to true' whereas precision is repeatability. Here's a diagram that illustrates the concepts:







In an email exchange, Chuck Westfall has told me that, "_The AF precision for the standard precision sensors is within the depth of focus for the maximum aperture of the lens, while the AF precision for the high precision sensors is within 1/2 or 1/3 the depth of focus for the maximum aperture of the lens, depending on the camera model under discussion._" Note that depth of focus is measured at the image plane of the sensor, and is different from (but proportional to) the depth of field. I interpret that to mean that although the precision is specified, for example with the f/2.8 sensor all the shots will be focused within a region that is 1/3 of the depth of focus deep for that lens' maximum aperture, focusing could still be inaccurate (i.e. it would look like case C above). That's the situation that AF microadjustment is designed to correct - a systematic focusing error that results in inaccurate focus. 

WarStreet, if you haven't already, check out the article posted here on CR (reposted, actually, from Roger at lensrentals.com). The bottom line is that your camera and lens are 'mismatched' (on opposite ends of the manufacturing tolerance spectrum, such that the errors add to something unacceptable). Your options are to 1) live with it, 2) send body + lens in for calibration, or 3) return the lens and hope you get a better match. I'd opt for #3. Amazon (at least in the USA) is very good about handling returns.


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## WarStreet (Jun 17, 2011)

neuroanatomist said:


> WarStreet, if you haven't already, check out the article posted here on CR (reposted, actually, from Roger at lensrentals.com). The bottom line is that your camera and lens are 'mismatched' (on opposite ends of the manufacturing tolerance spectrum, such that the errors add to something unacceptable). Your options are to 1) live with it, 2) send body + lens in for calibration, or 3) return the lens and hope you get a better match. I'd opt for #3. Amazon (at least in the USA) is very good about handling returns.



I did read already both articles regarding camera - lens matching and the one how to test a lens. They are very helpful ! I have went with option 3, they let me print return label so to return the package without any hassle. But now I have received an email from them saying this : 

_*Weâ€™ll be glad to help as much as we can but first weâ€™d suggest that you consult the manufacturer to check whether this is a known issue. Theyâ€™ll have specialist knowledge of their own products and in many cases they may be able to diagnose and resolve the problem immediately over the telephone.

If not, they may be able to get your item replaced quickly because of their technical expertise. Theyâ€™ll be able to advise you whether a repair, a replacement or a return to us is necessary. 

Please tell them that you bought your item from us*_

Would they do the body + lens calibration for free or I will end up paying for it ?

Will they fix something in the camera or just fix the lens ? I am asking since I don't won't that my camera performs differently with my other lenses. 

Note, that currently this lens is out of stock, and it is marked as available 1 - 3 months 

Thanks again for the help


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## neuroanatomist (Jun 17, 2011)

WarStreet said:


> Would they do the body + lens calibration for free or I will end up paying for it ?
> 
> Will they fix something in the camera or just fix the lens ? I am asking since I don't won't that my camera performs differently with my other lenses.
> 
> Note, that currently this lens is out of stock, and it is marked as available 1 - 3 months



I believe that they'll do it for free, since the lens is new and therefore covered by warranty (even though the camera is not). However, they might adjust the body to fix the issue (as Flake mentioned earlier in this thread), which might affect your other lenses. It's a bummer that Amazon.uk is out of stock.  One option, if you have the time/patience, would be to send just the lens to Canon for adjustment. They may find a problem and correct it (but they may not, meaning they'd then ask for the body as well). I suppose another option, if you can find stock at another retailer, would be to just return the lens to Amazon for a refund then buy elsewhere (but check their return policy first!).

FWIW, an issue like this one was what induced me to buy a 7D, which AF microadjustment (I started with a T1i/500D, and later bought an EF 100mm f/2.8L Macro IS which backfocused slightly - I could have sent it back or to Canon, but by that time I was shooting birds and a running toddler, and the Rebel's AI Servo was having trouble keeping up, so the 7D was the best option).

Good luck!


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## ystasino (Jun 17, 2011)

Those of us who have been trying to focus the 85 mm f/1.2 with the cross-type original 5D AF sensor at f/1.2 or other wide apertures know the limitations of Canon's AF precision all too well... It was hit or miss with 1 in every 3 photos in acceptable focus. IR patterns didn't help much.

Since shooting digital is cheap I learned to live with it. However, the lens's blades recently failed and I took them to Canon NJ. They charged $168 flat repair fee which they said was for most repairs. I assume that's unless something really expensive needed to be done. 

Since they repaired the blades they automatically adjusted the focus of the lens with my 5D and in the few times I have tested the lens the situation seemed improved. Maybe they also cleaned the the AF sensors, which would have fallen in the $30.00 charge I paid for sensor and eyepiece cleaning of the camera.

Overall it was a good experience and the repair only took 3 business days. I would opt to have your equipment sent to Canon like Amazon suggested.


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## WarStreet (Jun 18, 2011)

Thanks all. I have opened a ticket with Canon online, telling them all the story and asking them what's the best route. Amazon uk now removed the product !! there are only Amazon re-sellers which sell this lens, so I think it is better to deal with Canon. 

The lens has a very good image quality, it is small and lightweight and easy to handhold. The focusing seems to be very fast too, it's just the front focusing problem which I don't like when matched with my camera. Maybe if I had micro adjustment the lens would have worked fine. And although neuroanatomist tried to tempt me to get a 7D  , I will resist since otherwise it will change my plans 

I will keep you updated.


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