Hm. Seems like my 50mm 1.4 is a weird one..

So my Sigma 40mm 1.4 is lovely. I really do love it. It's awesome in the daylight, even at 1.4 all the way up to wherever I wish to use it..
But it seems that at night, it doesn't like focusing on what I want it to. It'll wander around, grabbing what seems to be random things and choosing to focus on them by it's own volition. Has anyone experienced this? How can I accurately test what this thing is doing?

I've calibrated it using that 45 degree test chart at 1.4, but I think that might be throwing things off. I didn't know that was possible.

Hi there,

Your problem usually has more to do with the body and AF system more than the lens. Some lenses do hunt more than others but in many cases it's that the AF system is easily fooled and focus is pulled away based on lack of contrast or greater contrast in the foreground or background than on the actual desired focal plane.

I think Robert is right. Focusing at night is a bear. When I'm doing night photography I'll bring with me a fairly strong flashlight and will use it to light up my subject and then turn off the AF after I have obtained a good focus before I take any images. Another thing that's helpful is to pay attention to exactly where your lens will go when focusing on things far away during the daylight hours. For instance, when I focus on a pier approximately 1/3 of a mile away I'll make a mental note of where it lands relative to the infinity sign. And I'll also do the same when focusing on the moon or a very distant mountain range that would equal an infinity shot. So then when I'm taking night photographs and I can't easily illuminate the subject because it's too far away, I'll use the mental markers for the 1/3 mile and infinity and guesstimate within that range. It also helps to take down the lens a stop or a stop and a half to cover up what little I'm off in my guesstimation.

yup,, also,, in LV,, set it to manual focus, and with my camera you can way up the ISO until it becomes visible in the viewer, manually focused ,, then turn down the ISO for less noise.

I am now under the impression that 1.4 is pretty unpredictable in almost all situations. If I shoot this lens at say f2.8 or higher, I don't have many issues. Its a damn shame because I might as well have bought a 1.8 version!
I suppose I could always manual focus for better accuracy, but still. Part of what I pay for is the AF.

Are you using the center focal point? I'm wondering with such shallow DoF if an off center focal point is thrown off by the curvature of the lens.

I usually do, yeah.

I really want to love this lens because the IQ is so far up there. But, I'm finding it harder and harder to use because of this. If I sell it, I don't think I'll get anything for it however because when trying to sell my 16-85, I was only offered $310 for it at best..:/

Robert Mitchell said:

Hi there,

Your problem usually has more to do with the body and AF system more than the lens. Some lenses do hunt more than others but in many cases it's that the AF system is easily fooled and focus is pulled away based on lack of contrast or greater contrast in the foreground or background than on the actual desired focal plane.

Click to expand...

I stil wonder if it's a user error type of thing (though when shooting at at least 60th of a second, it should freeze my own motion on the camera just fine..) a lens type issue or a body not being in perfect harmony with the lens.

I'm doing another test with a Fuji X5Pro body today at a super bowl party as well as the D7K with no AF fine Tune on either and see what happens. I'll also bring a tripod and test everything we can and see what the results are. Will post back later after reviewing the evidence.

Nowadays I am firmly convinced that the only reason I need a fast fast lens is just to gain a stop or two of shutter speed. I'm still going to stop just about anything down to about 5.6 if I can.

BTW give the 50 some time. Perhaps try and discover how the 50 "sees" and don't try to force it to see something if that makes any sense. Just trying to be helpful sir.

Well, in low light a stop or two can mean a non blurry photo versus a throw away.
I like having the option

Thanks, Jeff. I know you're trying to help and I appreciate it.

I had an opportunity to test the lens on two different cameras. I used my mate's Fuji S3 Pro and my D7k. Both set to single point auto af, at f1.4 at ISO1600. The lens seemingly had no issues with grabbing focus. Understandably, the lens was a bit soft at 1.4, but it didn't have the issues that I was experiencing previously. I am not quite sure what to make of that result. I honestly thought that it would have some sort of issue somewhere. Hurumph. I suppose that it could be that my ISO wasn't cranked up enough and I really overestimated the performance of f1.4. I really was under the impression that f1.4 was a lot wider than f1.8, but I guess it's not as large of a difference as I thought? I will continue to shoot as normal with it being mindful that and will report further issues as/if they arrive. I'll chaulk it up to user error for the moment, though I do believe that slower apertures in general allow for a LOT more leeway and user error!

Depending on the camera to subject distance, f/1.4 can be a paper thin depth of field. Combine that with the fact that the 50mm f/1.4 lens is not especially sharp at f/1.4 and you've got a recipe for disaster brewing.

The difference between f/1.4 and f/1.8 is 2/3 stop and while that's not a huge difference, it makes large enough when it comes to both light gathering ability and depth of field, especially if you love paper thin depth of field.

Having said that, if you don't need that shallow depth of field, f/1.8 is a beautiful thing and if you don't need to shoot wide open then f/2 and smaller tends to give much better results with a crisper, sharper image with greater contrast.