# Dottune distance: from lens or sensor?



## bereninga (May 28, 2015)

This may be a dumb question, but when setting up dottuning, do you measure 50x the focal length from where the sensor approximately is or from the end of the lens?


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## Valvebounce (May 28, 2015)

Hi bereninja. 
Having made the mistake during AFMA of measuring to the front of the lens and being corrected here that I should have been measuring to the sensor you need to measure to the sensor (focal plane) which is marked on the outside of the body with the circle on a line. 

Cheers, Graham.


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## AlanF (May 28, 2015)

It shouldn't make any difference. 50x is only a rough guide. Anyway, you will be lucky to get dottune to work. It never did for me and several others here.


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## neuroanatomist (May 28, 2015)

AlanF said:


> It shouldn't make any difference. 50x is only a rough guide. Anyway, you will be lucky to get dottune to work. It never did for me and several others here.



+1

You're talking about a ~2% difference. It's like worrying about your speedometer being 2% off when your car is missing a piston.


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## Valvebounce (May 29, 2015)

Hi Folks. 
I think there are situations where it will make a significant difference, when I had problems it was during the calibration of a Sigma Contemporary lens, Sigma's dock software gives the opportunity to set AFMA for four different focal distances at four different zoom points, at mfd the difference between the front of the lens zoomed in to zoomed out was a significant proportion of the distance, however the distance to the focal plane did not change. Given that the software is expecting a set distance and I was moving the camera to get the distance to the front of the lens correct it is no surprise that I had flakey results. 

Cheers, Graham.


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## Zen (May 29, 2015)

I know there are those who denigrate the dot tune method, and I know it's not "scientific", but it CAN be effective. Either I'm lucky or easily satisfied, but whatever the reason, I've had few problems with any of my dozen or so lenses. I've dot tuned all of them and have only found two that needed adjustments, neither of which were more than +3. So, I'd suggest to anyone that they try the method before spending the money for Focal. If it doesn't work, you've lost nothing, right?

BTW, the recommended distance is x20 to x50 the focal length, and it matters little whether you measure from the front element or the focal plane. In my experience, x50 the focal length is more effective that the shorter distance.

zen


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## neuroanatomist (May 29, 2015)

Valvebounce said:


> I think there are situations where it will make a significant difference...at mfd



In this case, the OP was asking about testing at 50x the focal length. However, you do have a point - with an ultrawide lens and its retrofocal design, it could matter. 50 x 10mm is ~20", and the front element of the 10-22mm is ~5" from the center, so that's 25%.

But in fact, the 25-50x guideline is based on the fact that AFMA changes with distance, but that change decreases with increasing distance, and the value is pretty stable by 25-50x FL. So if you measure from the sensor (correctly), you're fine, and if you measure from the front of the lens, you're at greater distance than needed, and again you're fine.


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## AlanF (May 29, 2015)

neuroanatomist said:


> Valvebounce said:
> 
> 
> > I think there are situations where it will make a significant difference...at mfd
> ...



That's spot on. Further, if you routinely use your lens at a single fixed distance from the target rather than a variety of distances, do AFMA at your most used distance.


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## bereninga (May 29, 2015)

Valvebounce said:


> Hi bereninja.
> Having made the mistake during AFMA of measuring to the front of the lens and being corrected here that I should have been measuring to the sensor you need to measure to the sensor (focal plane) which is marked on the outside of the body with the circle on a line.
> 
> Cheers, Graham.



Oh, that last part is very interesting. I'll check that out. Thanks for the info!



AlanF said:


> That's spot on. Further, if you routinely use your lens at a single fixed distance from the target rather than a variety of distances, do AFMA at your most used distance.



Does this mean that if you use your lens at varying distances, it's mostly pointless to AFMA? If AFMA changes w/ distance, there sure are a lot of distances to test.


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## AlanF (May 29, 2015)

bereninga said:


> AlanF said:
> 
> 
> > That's spot on. Further, if you routinely use your lens at a single fixed distance from the target rather than a variety of distances, do AFMA at your most used distance.
> ...



It's not mostly pointless, but you have made a good point. Most of my telephoto use is for distances greater than 25xf, so the AFMA is constant enough over that range when I measure it at 25-50xf. I suppose better still would be to calibrate AFMA like you do for a zoom lens - Canon lets you do it for minimum and maximum zoom then extrapolates the intermediate zooms for you. The next step would be for Canon to have settings for "far" and "close" and then extrapolate. I believe the Sigma dock allows you to do a distance calibration but I have no experience of that. Perhaps a Sigma dock user could elaborate.


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## Valvebounce (May 30, 2015)

Hi Alan. 
Quote from my post above 
"Sigma's dock software gives the opportunity to set AFMA for four different focal distances at four different zoom points."
End quote. 
The distances at which the Sigma software lets you calibrate the AFMA are pre determined as are the zoom values, this is for my 17-70 contemporary lens. 
You get four zoom values at mfd, four at the next distance, maybe 6ft, another four at the next distance, perhaps 15ft, and the last four of the values are for infinity I think. I have not used the sigma software for a while and the distances are guessed (dragged out of a cullender style memory ;D) 

Cheers, Graham. 



AlanF said:


> It's not mostly pointless, but you have made a good point. Most of my telephoto use is for distances greater than 25xf, so the AFMA is constant enough over that range when I measure it at 25-50xf. I suppose better still would be to calibrate AFMA like you do for a zoom lens - Canon lets you do it for minimum and maximum zoom then extrapolates the intermediate zooms for you. The next step would be for Canon to have settings for "far" and "close" and then extrapolate. I believe the Sigma dock allows you to do a distance calibration but I have no experience of that. Perhaps a Sigma dock user could elaborate.


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