Sigma 85 1.4 - Flash required

My first one was back focusing inside of 4 feet and outside of about 12 feet. Shipped it back and got the new. Initial testing showed it was back focusing at distances greater than 10 feet.

I decided to put it through some real world testing this weekend and used it at my nephew's birthday party with bounced flash. It performed perfectly and the AF was spot on. I tested it again Saturday night the BF problem is still there.

Can the use of flash make that big of a difference? Your opinion - likely focus issue or not?

How are you testing for back focus? Could it be motion blur as opposed to back/front focusing issues? Are you using focus recompose when testing? Is your shutter speed fast enough for the focal length when testing?

The fact it was fine with flash (and I assume a faster shutter speed than when testing) suggests motion blur to me for any of the above reasons

Tripod mounted, release cable using Dot Tune. Once I zoom in on LiveView and establish focus, lens goes to manual. Basically, the way I notice the BF is looking at a post-MA'ed picture. My target isn't in focus period but the object 3 inches behind it is.

huntersdad wrote in post #16764555
Tripod mounted, release cable using Dot Tune. Once I zoom in on LiveView and establish focus, lens goes to manual. Basically, the way I notice the BF is looking at a post-MA'ed picture. My target isn't in focus period but the object 3 inches behind it is.

In that case then it sounds like a lens focus issue. This is the exact scenario I had before I had to sell it as I couldnt cure it. I cant explain why flash cured the issue though (sorry not much help to you)

Minimum focus distance is about three (3) feet for this lens.

frugivore wrote in post #16764599
Minimum focus distance is about three (3) feet for this lens.

I'm aware, slight typo. I'll correct.

Anecdotal only.. but I find my Sigma 30mm 1.4 seems to focus better with the flash. I'm usually quite aware of motion blur due to slow shutter speeds, but I can't think of any other explanation.

The flash itself shouldn't have anything to do with the focus at all. It fires after the shutter is open. The AF doesn't move while the shutter is open. If it's something that is really happening, then it's more likely an AF assist beam causing the better AF. But that would indicate that your AF is actually missing the target and the AF assist beam is helping to get it right.

Hows the lighting set up on your test chart? Maybe its too dark?

Maybe because the flashes infrared focus assist beam is doing its job

I redid my testing last night using a good bit more light (good thing studio lights come with model lamps). Still tripod mounted and cable released, my range of focus confirmation was +2 to -20. I settled in at the midpoint of -10. Did some handheld testing after that with a minimum SS of 1/200. Something is still off, but I can't put my finger on it. After reviewing my pics from the weekend, I noticed the sharpest were relatively close, 3-6 feet. Out side of that, they seemed to get a little hit or miss.

Looks like more information is in order. How about some samples? And what body do you have it on? Do you have access to a second body you can borrow for additional tests?

huntersdad wrote in post #16764406
My first one was back focusing inside of 4 feet and outside of about 12 feet. Shipped it back and got the new. Initial testing showed it was back focusing at distances greater than 10 feet.

I decided to put it through some real world testing this weekend and used it at my nephew's birthday party with bounced flash. It performed perfectly and the AF was spot on. I tested it again Saturday night the BF problem is still there.

Can the use of flash make that big of a difference? Your opinion - likely focus issue or not?

Heya,

It's not the flash. Your focus tests were done at F1.4. I'm going to bet that your birthday shots were not at F1.4, so naturally, were probably more likely to be closer to in-focus.

Very best,

They were at 1.6 just because I could. I tried several other tests including going outside in daylight and got the same results - under 10 feet good, over 10 ft. either AF was off or softness got really bad. It went back to Adorama yesterday for a refund.