Sigma not focusing dead-on...

[splproductions]

So I finally sat down to figure out if my Sigma 50mm on my 500D is focusing properly. I printed out a black and white ruler off of my printer and sat about 1.5 to 2 feet away from it. It was on a table and I was shooting at an angle so I could see the depth of field.

When focusing right on the 6" mark, my 24-70L was dead-on every time at f/2.8. The Sigma at f/2.8 focused about 75% of the time at the 5.5" mark (0.5" too close). It never focused too far away, only too close.

Is the lens being that far off a pretty big deal? I'm focusing mostly on newborn and baby photography right now, so I'm down in the 1.8-3.5 range a lot, which exacerbates the problem.

Thanks!

 

[Robin Usagani]

were you shooting in the dark? Did you use center focus point?

 

[splproductions]

were you shooting in the dark? Did you use center focus point?

No - I had a 200W equivalent daylight fluorescent bulb about 1 foot away from the paper. Plenty bright. I was at ISO 100, 1/250, and f/2.8. I was using the center dot only (not the "auto" or whatever that mode is).

Same settings on the Canon lens as well. And I had the Canon set to 50mm also.

I was hand-holding, so I could be moving slightly off the mark, but I'm ruling that out since the Canon was always on and the Sigma seemed to love that 5.5" mark most of the time.

 

[Robin Usagani]

Did you go closer than minimum focus distance? Just want to make sure since the 24-70L has an awesome minimum focusing distance.

 

[splproductions]

Did you go closer than minimum focus distance? Just want to make sure since the 24-70L has an awesome minimum focusing distance.

Dang it. I didn't even think about that. Minimum is 17.7 in, and I just tried to recreate where I think I was, and I was probably around 17-23". I guess I probably need to try it again at a further distance to be sure.

Assuming I try it again tomorrow, and I get the same thing... I'm doing a newborn shoot on Wed - a paid gig for a neighbor - should I avoid this lens? It would be nice to get some über-shallow DOF shots, but not if all my photos will be slightly OOF. Hmmmm....

 

[Big Mike]

I don't think you mentioned what particular Sigma lens you're testing. Is it the 30mm F1.4?

Either way, Sigma does have a bit of a reputation for focus accuracy issues.
Keep in mind that focus accuracy settings/adjustments on both cameras and lenses have a tolerance. I don't know what it is, but lets give it a number...say plus or minus 5. If the camera is at -3 and the lens is at +4, you're net is +1, which is well within the tolerance. But if your camera is at -3 and your lens is at -4, then your net is -7 and you're outside the tolerance and might have an unacceptable amount of back focus.

You can have your cameras and lenses adjusted, but they usually means having to send in both the camera and the lens.

I'm not sure if the 500D has micro focus adjustment as an option. Some of Canon's higher end cameras will allow you to dial in a specific amount of focus adjustment. It can even remember a different adjustment for different lenses.

I used to have a Sigma 30mm F1.4. It might have been the sharpest lens I've ever had...but the focus issue made it so that I wasn't confident shooting it at wide apertures, and I eventually sold it.

 

[KmH]

I had a 200W equivalent daylight fluorescent bulb about 1 foot away from the paper. Plenty bright. I was at ISO 100, 1/250, and f/2.8. .

Your 200W equivelent light delivers 200W over a time period of 1 second.

You used a shutter speed of 1/250 so the exposure you made only used 0.8W of light.

 

[splproductions]

I had a 200W equivalent daylight fluorescent bulb about 1 foot away from the paper. Plenty bright. I was at ISO 100, 1/250, and f/2.8. .

Your 200W equivelent light delivers 200W over a time period of 1 second.

You used a shutter speed of 1/250 so the exposure you made only used 0.8W of light.

Sorry... I'm totally derailing my own thread here, but I just have to respond to this. Not saying you're wrong, but I think you are...

If I were given this problem in my Physics classes, this is how I would have reasoned through it...

A watt is a "joule per second". So 200W means 200 joules per sec.

If my sensor was only exposed to the light for 1/250 of a second (0.004 sec), then it received 0.8 Joules. However, it did not receive 0.8 Watts.

200 J / 1 sec = "x" J / 0.004 sec
x = 0.8

But 0.8 Joules / 0.004 sec isn't 0.8W, it's 200W.

So, my exposure was still using 200W.

 

[Robin Usagani]

Not sure why KmH posted that but who cares? LOL... Is this rocket science? 0.8 watt, 200 watt.. its all numbers. Is it bright or not? Is your lens hunting for focus?

 

[splproductions]

Not sure why KmH posted that but who cares? LOL... Is this rocket science? 0.8 watt, 200 watt.. its all numbers. Is it bright or not? Is your lens hunting for focus?

Lol! Sorry - I've been going to school far too long, and I can't let a story problem like that slip by me.When I get home tonight I'll try it from further away and post the results.