Lens Advice

Feb 12, 2021
6
0
Hello all,

I'm after some advice regarding the below Lenses please.

Camera EOS 6D. All shots taken with manual focus 10X live view, @ f/6.3 Aperture Priority

Lens 1 - Canon EF 100mm 2.8 Macro
Lens 2 - Canon EF 50mm 2.8 II
Lens 3 - Canon EF 50mm 2.8 STM
Lens 4 - Canon EF 50mm 1.4
Lens 5 - Canon EF 28mm 2.8

I've been dissapointed at the sharpness of my 28mm 2.8 (Lens 5), so decided to print out a star chart and take some test photos with a few of my lenses to compare. Now first off, my printer isn't that good, so the star chart is less than optimal. Its a sheet of A4 at around 1m distance from camera. Two flashes at 45 degrees.

Lens 1 - The 100mm Macro. This seems to do a pretty good job. You can see individual dots. I'm using this as my standard to compare the other lenses to.
100mm 2.8 Macro.JPG
Lens 2 - The 50mm 2.8 II - Am I expecting too much of the 50 here?
50mm 1.8 II.JPG
Lens 3 - The 50mm 2.8 STM - Pretty much the same as the 50mm 2.8 II.
50mm 1.8 STM.JPG
Lens 4 - The 50mm 1.4 - Well, I dont see much difference to the other 50's
50mm 1.4.JPG
Lens 5 - The 28mm 2.8. Urghhhhh
28mm 2.8.JPG

I've got a few questions.

As the three 50mm's are about the same, would that indicate that that's about as much as I would be able to get from them, or does it indicate that all three have issues.

The 28mm, well, this has had the front element off and back on again. I tried the trick in the service manual to use paper guides to center the element, which it says can get you close if you don't happen to have a collimator/lens projector handy, but not spot on, but it doesn't seem to matter what I do with the front element, i get no difference in the image quality. Is this pointing at something more substantial. There isn't much that can go wrong with these is there? There are only two groups, fixed together

I tried shooting just out of focus to see if I had any decentering indications on the 28mm, but it looked rock solid.

Am i not comparing the same thing with the same pictures. Should I be moving the camera closer as the focal length gets shorter in order to compare the same thing?

Please don't shoot me down. I don't have much money and tend to collect "cheap" lenses from eBay, so I'm happy to accept the fact that I have a shelf full of duffers. I'm not a professional, just like collecting tech, with a view to sparking off a new hobby.

Thanks for any advice you can give me. Including chuck that 28mm in the bin!
 

AlanF

Stay at home
CR Pro
Aug 16, 2012
8,259
10,327
You would expect he resolution of points on the chart to depend on the focal length of the lens Repeat your procedure by having the chart at different distances so that the image is the same size on the sensor. Eg, the 28 mm 1 m from the chart, the 50mm at (50/28)m and the 100mm at (100/28)m from the chart.
 
Feb 12, 2021
6
0
Yes, that makes perfect sense, if the image is presented the same size on the sensor, then the only variable is the lens.

https://www.gwharton.me.uk/DCIM/28mm@0.5m.JPG
https://www.gwharton.me.uk/DCIM/50mm@0.89m.JPG
https://www.gwharton.me.uk/DCIM/100mm@1.78m.JPG

They all look pretty consisten when taken like that.

I'm a complete novice, so don't have years of experience to call back on as to whether they are acceptable or not. Wouldn't you expect better from these lenses at these distances, or is this a rubbish test.

I'm not even what I would class as an amateur photographer, so I guess the better test for me would be to just get out there and take some pictures with them, and if they look ok, then thats good enough, but as with everything I do, I get obsessed and need to quantify performance to suit my OCD traits!!!! :)

Colors don't look right on the 28mm lens though, Seeing blue shadows/chromatic abberation. I guess that would be symptomatic of having the front element off alignment.
 

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  • 100mm@1.78m.JPG
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Last edited:

AlanF

Stay at home
CR Pro
Aug 16, 2012
8,259
10,327
The 28mm doesn't look as good. Here is a good link for a good intro into testing. http://www.bobatkins.com/photography/technical/lens_sharpness.html
I don't know if you are familiar with AFMA (autofocus microadjustment). Many lenses need various degrees of adjusting the focus on a DSLR like the 6D. You coulkd do some preliminary tests by going into the AFMA menu of the camera and adjusting to say +5 and then -5 and see if it makes a difference.
 
Feb 12, 2021
6
0
Yes, I'm actually reading Bob's site now. As far as I am aware the AFMA compensates for back/forward focusing. I have looked into this before, and saw the Dottune method for getting it spot on. Looked quite useful, but thats not whats going on with my 28. I'm sure its a little more fundamental than that.

The Service manual has a couple of things to try, so I'll take another look at them. It effectively says 1, Loosen screws in front element, 2. wiggle it around until it improves, 3. tighten screws.