RF 28-70 F2 Back Focus/Slow Focus

ghost123238
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p.1 #1 · p.1 #1 · RF 28-70 F2 Back Focus/Slow Focus

Hey all I recently purchased a 28-70 F2 from Best Buy. The lens is amazing its tack sharp, gives an amazing pop to any images and will honestly be replacing alot of my primes. However I was shooting with it today just simple kids in the backyard playing the splash pad and when I came inside to review a few images and notices that on faster action the lens seemed to be back focusing/slow to grab focus. I understand that f2 is a very thin DOF especially up close. I was just curious if anyone else had exp with this or did I get a lens that had been dropped or something. Also I was shooting in Servo and my shutter speed was 1/3200 or above so I don't think it was motion blur. It seem to happen when I was up close and personal with the lens more than when I was further away.

Jul 01, 2024 at 11:10 PM

rscheffler
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p.1 #2 · p.1 #2 · RF 28-70 F2 Back Focus/Slow Focus

Lots of variables at play. Even 'just' kids moving around normally at close distances can be a challenge for AF systems, particularly at wide apertures. It will depend on the camera and light quality on the subjects - front, side, backlit, etc. Mirrorless can have difficulties with backlit and low contrast situations. Sometimes it might focus on object edges with higher contrast against a background rather than central but lower contrast features away from the edge transition to the background, if that makes sense. For example, strong backlighting or low contrast lighting on the face can be a situation where face/eye detection/tracking instead focuses on the ears, hair, etc, causing focus to be behind where you would want it. That could be just one possible scenario.

I wouldn't classify the 28-70 as a very fast focusing lens. Having just used the new 24-105/2.8 Z for a couple weeks to compare against it, the Z had noticeably faster AF and IMO was able to better follow focus in some types of poor lighting situations.

Something else to keep in mind is that the relative change in subject distance as the subject approaches the camera at a steady speed is from the camera's POV with a wide angle lens much greater at closer distances. To the camera it will look like the subject is accelerating and it's possible that at closer distances the AF may not be able to keep up.

Difficult to say if the lens is bad or if it's a situation that is on the edge of what the AF system can reliably focus on, but I'd guess latter without having a chance to see some images. Likely the only way to rule out a bad lens would be to compare it directly against another copy, if possible.

Jul 01, 2024 at 11:32 PM

ghost123238
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p.1 #3 · p.1 #3 · RF 28-70 F2 Back Focus/Slow Focus

rscheffler wrote:
Lots of variables at play. Even 'just' kids moving around normally at close distances can be a challenge for AF systems, particularly at wide apertures. It will depend on the camera and light quality on the subjects - front, side, backlit, etc. Mirrorless can have difficulties with backlit and low contrast situations. Sometimes it might focus on object edges with higher contrast against a background rather than central but lower contrast features away from the edge transition to the background, if that makes sense. For example, strong backlighting or low contrast lighting on the face can be a situation where...Show more →

So to answer your question I am shooting on an R5 I was in great lighting around 6pm shooting at 1/3200 at F2 with iso of like 200. I don't feel like it's the camera or light. I regularly shoot in those conditions, and don't drop focus on the 2.8, I get that 2.8 is different than F2 but still.

Jul 02, 2024 at 09:50 AM

rscheffler
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p.1 #4 · p.1 #4 · RF 28-70 F2 Back Focus/Slow Focus

Only way to know for sure, IMO, is to compare with another copy...

Jul 02, 2024 at 12:31 PM

ghost123238
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p.1 #5 · p.1 #5 · RF 28-70 F2 Back Focus/Slow Focus

rscheffler wrote:
Only way to know for sure, IMO, is to compare with another copy...

Fair I was hoping someone else could tell me oh yeah mine does that sometimes as well. I will keep researching for a few days I still have a fair amount of time to return.

Jul 02, 2024 at 12:42 PM

fgphoto
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p.1 #6 · p.1 #6 · RF 28-70 F2 Back Focus/Slow Focus

I have noticed from time to time that the lens seems to focus more slowly in comparison to the 24-70 and 24-105 2.8.

I mostly use it for environmental portraits and other things that don't move fast. Normally use the 24-70 or 24-105 for fast moving things.

Jul 08, 2024 at 06:46 PM


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