inaccurate focus of some large aperture lenses on some cameras

[lly3988]

Testing an 50 1.8 II, 50 1.8 STM and 50 1.4 USM on 40D and T4i. Set the camera to ISO 400, largest aperture 1.8 and 1.4 respectively. With abundant light and aiming the target with good contrast - center focus point. Shutter speed is about 1/250 to 1/500. Delay shooting is enabled.

Cameras are on a sturdy tripod. Focus confirmed everything.

It turns out that the focus of the three lens on the 2 cameras are not accurate on both bodies. Magnified to 100% and the above results are confirmed. Then have to adjust a bit with manual focus rings to achieve the best results.

Does it mean that camera bodies with afma feature is the only way to go ? (Of course, lenses and bodies can be sent to Canon for calibration but the cost is inappropriate to the value of the equipment.)

Adjusting the aperture to F4 in all three lenses will solve the problem to an certain extend. (depth of field I know) But still not perfect. F5.6 and F8 keep the target in good focus but then why F1.8 and F1.4 lenses.

I will try some F2.8 lens 40 2.8 STM and 24 2.8 STM etc to see if the results are the same.

Please share your experience and thoughts.

[TTMartin]

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@lly3988 wrote:

Testing an 50 1.8 II, 50 1.8 STM and 50 1.4 USM on 40D and T4i. Set the camera to ISO 400, largest aperture 1.8 and 1.4 respectively. With abundant light and aiming the target with good contrast - center focus point. Shutter speed is about 1/250 to 1/500. Delay shooting is enabled.

Cameras are on a sturdy tripod. Focus confirmed everything.

It turns out that the focus of the three lens on the 2 cameras are not accurate on both bodies. Magnified to 100% and the above results are confirmed. Then have to adjust a bit with manual focus rings to achieve the best results.

Does it mean that camera bodies with afma feature is the only way to go ? (Of course, lenses and bodies can be sent to Canon for calibration but the cost is inappropriate to the value of the equipment.)

Adjusting the aperture to F4 in all three lenses will solve the problem to an certain extend. (depth of field I know) But still not perfect. F5.6 and F8 keep the target in good focus but then why F1.8 and F1.4 lenses.

I will try some F2.8 lens 40 2.8 STM and 24 2.8 STM etc to see if the results are the same.

Please share your experience and thoughts.

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Yes, it is normal for there to be variation in the manufacturing tolerances of both the camera and the lens. For cameras without AF MFA there have been two traditional ways of dealing with this, one was as you mentioned sending the gear into Canon for calibration. The second was trying many different lenses until you found the one (ones) that worked best with your camera.

Don't forget for precision focus, liveview (from a tripod) and manual focus are always available options.

[lly3988]

Exactly, that what I have done to compensate for the inaccuracy of AF.

Quoted from my original post : "Then have to adjust a bit with manual focus rings to achieve the best results."

I was doing this in liveview and that may be the way to go when AF cannot be achieved accurately and sending for calibration is not worthwhile. Another way is to of couse step down (may be up to 2 stops) to increase the depth of field to allow more in the range of focus.

Thank you. Martin.

[Waddizzle]

It is not unusual for an inexpensive large aperture lens to have better performance at higher apertures, than when they are wide open.  The reasons are complex, but  I will sum it up in one word...physics. 

Getting high performance out of wide open apertures is why some lenses can cost several hundred dollars, or more.

[ebiggs1]

"Please share your experience and thoughts."

OK.  First question, is anything in the sample photo in focus?  This is DOF and on a Rebel a 50mm acts like an 80mm.  80mm at f1.4 will have a thin DOF.  It is important to make sure the camera focused on what you 'think' it focused on.  This is a good use of DPP as it can tell you the exact focus point.

Second, how far were you.  What is the distance from focal plane to subkect.  This effects DOF, too.

The two plastic 50's are not the same as the ef 50mm f1.4.  I would go with it.  Is its focus any better? Remember most, if not all, lenses get better as they stop down. Until they reach a point that is.  One reason for this is the DOF expands.  But either end, wide open or completely stopped down, is not its best performance area.

The inability to bring all the light to a single focus point in either extreme is very difficult. 

[lly3988]

Thank you, ebiggs1

You have also bring up a very good point. I have some basic knowledge of DOF and I have an app installed on my smartphone for checking this out. Sometimes, the in focus range (DOF) is really very thin, e.g. 50mm @ f1.4 with focus distance of 20 inches is less than 0.2 inches.

I also enabled the "focus point display" in the menu and check in the Canon DPP to ensure that I am aiming right.

Testing with the focus distance increased will definitely change the DOF and included more in focus. But testing with the MFD has not been quite successful.

Testing all the 3 lenses after afma on my 5DIII yield far better results. Compare the two plastic 50 1.8 and the 1.4 @ f1.8. The f1.4 is only slightly better.