Front Focusing

[ppp]

OK I think my Sigma 100-300 f4 has a front focus issue, have checked with MTF? charts and if I AF on either body it front focuses, MF is fine, if I AF focus on the 0 line it appears the number 1 and sometimes nearly 2 are more in focus, is there anything I can do or is it over to Sigma to calibrate? Only I want to use the lens at easter weekend BSB at Brands!

 

[Uneducated_Rick]

Do the memory card line up.

Place 3 or 5 memory cards in a line next to each other.

Now stagger the line so that the back edge of one is in line with the front edge of another.

Autofocus on the middle card and see which is actually in focus.

Far superior to the 45 degree charts which are fundamentally flawed in their design

 

[ppp]

ok will give it a try, gimme a minute

 

[ppp]

ok its not looking too bad but think maybe the front card should be more OOF??

Centre Focus point is bang over the square symbol above the word Lexar on the middle card, camera solid mounted so no movement/hand shake
1.5 inch gaps between cards

F4 300mm, 10sec Self timer before shot

F8 300mm, 10sec Self timer before shot

 

[tdodd]

I know you're in the Nikon/Sigma camp here, but for what it's worth, Canon recommends performing AF accuracy testing/calibration at no less than 50X focal length. For a 300mm lens that should be 15m or more. I think Sony recommends either 40X or 50X as well. I'm not sure about Nikon.

Furthermore, with a lens slower than f/2.8 the AF accuracy is only supposed to be sufficient to have the focus plane fall within the depth of field (speaking for the Canon AF system). It can vary from one AF attempt to the next, perhaps sometimes bang on target, sometimes a little front focused and sometimes a little back focused. You should start with the lens defocused manually and then repeat your AF test at least three times, defocusing again between each test. Then you might get a statistically more reliable view of what is actually going on. A single AF attempt is not really enough to provide any degree of certainty about how the body/lens are performing overall.

In your f/8 test, while I appreciate the DOF is increased over f/4, it looks to me like the focus is sharpest on your target CF card. Certainly it appears sharper than either of the others. Also, while testing AF at distances of 10' or so might be interesting to kill some time, if you are going to be shooting BSB I personally think you should be concentrating your testing on AF performance at more representative distances. 15m or maybe 50m would make a lot more sense to me, for this specific excursion you have in mind. Of course, if you have doubts about the lens then you should get it seen to, but testing at 10' (ish) may not provide a reliable indicator of performance at much greater distance.

Just my 2c

 

[ppp]

Yeah thinking about it that makes complete sense and tbh looking at the results above I am happy with the target card, plus when shooting in the real world at bikes or BTCC cars I don't think there will be a problem, as you say 15m plus is a more realistic details, I think its best to go to the BSB and see what the results are like and go from there. Thanks to both of you.

 

[Tobers]

Are you using mirror lockup for your focus tests? As well as the advice above, I highly recommend it as a long lens even on a tripod will be susceptible to mirror slap.

Also, you should always do the test at f/4 to minimise the depth of field to determine exactly where the focal plane is.

 

[Canon Bob]

Are you using mirror lockup for your focus tests? As well as the advice above, I highly recommend it as a long lens even on a tripod will be susceptible to mirror slap.

If you're using objects at slightly different distances then any movement caused by not using MLU will show across the subject area of the frame and not appreciably have any effect on the results....the observation is comparative and not absolute. If doing several shots for an average ....al la micro adjusting, then MLU becomes more of a neccessity.

Bob

 

[ppp]

I wasn't using MLU just the self timer, but am happy enough with the results for now, will know for sure when i use it in anger, if i'm not happy i'll ping it back to Sigma