[StarDust]

Just recently I've upgraded from the old first edition kit lens to the Sigma 18-50 2.8 DC Macro. It's not the Pentax equivalent but it's better than the kit lens and that's what I was going for with my tight budget.
I've been shooting alot at 2.8 now and I've noticed that sometimes it has a focus issue (I've forgotten if it's called front or back focus), i.e. it focuses past the object I want to focus on, but not always...
Is this a typical front/back focus issue, that it can nail it 50%-75% of the time, or is it a focus issue with the lens, or is it perhaps me always tilting forward after focus lock?
I've done tripod tests and it almost always nails but on the field I've even had difficulty with the tripod, however the focus was a bit tricky those times.
I haven't had a chance to evaluate photo's with smaller apertures yet, those seem a bit trickier, I tried to take a shot with hyper-focal focus and it's seemingly soft although only at first glance.
I could post images but since this problem is not constant I don't know if there's any point.

[Lowell Goudge]

the problem with macro is that there is virtually no depth of field, especially at F2.8

Additionally, on real world subjects focus sensors can be actually biugger than the subject you are trying to focus on and they may lock on something behind the intended subject.

to compound this, even with a tripod, any movement toward / away from the subject, (perhaps it's windy?) leads to focus error, Hand held is the worst case because shake reduction only works for lateral movement (i.e. at right angles to subject, not longitudinal (towards / awau from the subject)

The best results for macro work are with manual focus, this allows you, not the camera to decide what you want in focus.

[Ron Kruger]

Your problem doesn't sound like a front- or back-focus problem, but the common inconsistency of AF. The nature of AF, regardless the camera make, isn't nearly as reliable as most believe or wish. It is fine if it is a simple snapshot of Aunt Betty standing in an open field, but if the scene is cluttered, or the DOF is narrow, MF is much more reliable. The camera can't read your mind about what you want in focus, and it doesn't know the difference between a leaf or branch and a nose or an eye. Spot focus gives you a little more control, but it still is not totally reliable. I use MF whenever the scene is at all complicated, whenever the DOF is narrow and even when I want to control the hyperfocal distance on scenic shots, which means I use it most of the time. I have, in fact, turned off the AF functon on the half-press and use instead the AF Button whenever I feel safe using AF. The button seems to be faster and more accurate anyway.
For macro, always use MF.
Take control of you shots. If you learn to us MF consitently and accurately, you'll have more control of AF as well, because you will train your eye to recognize when the focus is locked on what you want.

[stevebrot]

What Ron says is so true. I was snap shooting the other day ago and took a photo of a chainsaw sculpture of a bear:



While the nostrils are interesting enough, I would have been much more pleased if the camera had focused on the eyes instead! Even with multiple AF points, it is unlikely that any one of those would have been at the point for best focus. If I had been thinking more clearly, I should have used focus lock to set focus on the eye and them recomposed for the exposure.


Steve

[JeffJS]

If you had a FF/BF problem it would show up a lot more than 25%-50% of the time (based on what you say) even at smaller apertures. Instead, as others have said, check your technique and be mindful of what you are focusing on. To test, set the lens or the camera to Manual focus and see if the camera agrees (the AF sensor will still beep) with what You say is in focus.

[StarDust]

the problem with macro is that there is virtually no depth of field, especially at F2.8

Additionally, on real world subjects focus sensors can be actually biugger than the subject you are trying to focus on and they may lock on something behind the intended subject.

to compound this, even with a tripod, any movement toward / away from the subject, (perhaps it's windy?) leads to focus error, Hand held is the worst case because shake reduction only works for lateral movement (i.e. at right angles to subject, not longitudinal (towards / awau from the subject)

The best results for macro work are with manual focus, this allows you, not the camera to decide what you want in focus.

No argue there, I agree completely.

Your problem doesn't sound like a front- or back-focus problem, but the common inconsistency of AF. The nature of AF, regardless the camera make, isn't nearly as reliable as most believe or wish. It is fine if it is a simple snapshot of Aunt Betty standing in an open field, but if the scene is cluttered, or the DOF is narrow, MF is much more reliable. The camera can't read your mind about what you want in focus, and it doesn't know the difference between a leaf or branch and a nose or an eye. Spot focus gives you a little more control, but it still is not totally reliable. I use MF whenever the scene is at all complicated, whenever the DOF is narrow and even when I want to control the hyperfocal distance on scenic shots, which means I use it most of the time. I have, in fact, turned off the AF functon on the half-press and use instead the AF Button whenever I feel safe using AF. The button seems to be faster and more accurate anyway.
For macro, always use MF.
Take control of you shots. If you learn to us MF consitently and accurately, you'll have more control of AF as well, because you will train your eye to recognize when the focus is locked on what you want.

I pretty much always use spot focus except when I think that I can lose focus due to parallax shifts (is that a good name for it?) and then I choose select focus. MF is so difficult with modern day APS-C cameras. The viewfinder is big enough to compose naturally but not to focus manually. I've been thinking of buying a magnifying cup or something that replaces the eye-cup, if such a thing exists.

What Ron says is so true. I was snap shooting the other day ago and took a photo of a chainsaw sculpture of a bear:

While the nostrils are interesting enough, I would have been much more pleased if the camera had focused on the eyes instead! Even with multiple AF points, it is unlikely that any one of those would have been at the point for best focus. If I had been thinking more clearly, I should have used focus lock to set focus on the eye and them recomposed for the exposure.


Steve

That would be how one of my pictures look like although focus would be on the ears. Did you use spot focus? It's a snap shot, but do you remember what you focused on?

If you had a FF/BF problem it would show up a lot more than 25%-50% of the time (based on what you say) even at smaller apertures. Instead, as others have said, check your technique and be mindful of what you are focusing on. To test, set the lens or the camera to Manual focus and see if the camera agrees (the AF sensor will still beep) with what You say is in focus.

Again, I find MF difficult. But yes, I've never played with such small DOF except on my Tamron 70-300 Macro and those are usually spot on. I've never had front/back focus problems with that lens, just unfocused due to the quality of the lens.

[Ron Kruger]

I pretty much always use spot focus except when I think that I can lose focus due to parallax shifts (is that a good name for it?) and then I choose select focus. MF is so difficult with modern day APS-C cameras. The viewfinder is big enough to compose naturally but not to focus manually.

I hear this kind of thing all the time, but I never noticed any increased difficulty using MF when I switched to digital. I think the problem is modern photographers rely too heavily on the P&S aspects of DSLRs and simply never developed a good MF technique.
MF wasn't any easier with older cameras, but it was all we had, so we had to LEARN how to use it.
First of all, don't just look for focus. Go from one side, all the way through focus to where it just begins to blurr. Then go back through to the other side to where it begins to blur. Then slowly ease back into focus to where the image sort of pops.
Don't look at the entire image or subject to check focus. Pick one particular part, such as an eye. Don't look at or concentrate on anything else.
Some of the problem with using MF, however, has to do with the lens. You can't MF a kit lens. You need older lenses make for MF, Limited or * lenses that have smooth, accurate focus rings.

[klh]

I pretty much always use spot focus except when I think that I can lose focus due to parallax shifts (is that a good name for it?) and then I choose select focus. MF is so difficult with modern day APS-C cameras. The viewfinder is big enough to compose naturally but not to focus manually. I've been thinking of buying a magnifying cup or something that replaces the eye-cup, if such a thing exists.

I have had similar problems to what you describe, but recently installed a new focusing screen with a split screen and microprism ring. I bought it with the intent of allowing me to use MF with my older lenses as I did in my film days, but I found that it has dramatically improved my AF as well. Now I can clearly see what the AF is choosing, and make a change if it is not what I want. What I thought was a backfocus issue was really just the AF choosing the wrong subject to focus on.

[JeffJS]

Let's define then what you mean by 50-75 percent of the time your focus is correct. I was under the impression that was with a single lens. If the Sigma is off ALL the time then yes, you likely have a focus problem. From your description, it sounds like Back focusing.

[StarDust]

First of all, don't just look for focus. Go from one side, all the way through focus to where it just begins to blurr. Then go back through to the other side to where it begins to blur. Then slowly ease back into focus to where the image sort of pops.
Don't look at the entire image or subject to check focus. Pick one particular part, such as an eye. Don't look at or concentrate on anything else.
Some of the problem with using MF, however, has to do with the lens. You can't MF a kit lens. You need older lenses make for MF, Limited or * lenses that have smooth, accurate focus rings.

This is exactly what I do. I think my problem might be that I have contacts. When I stare really hard for a longer period of time these tend to blur our faster than when using glasses. Switching to glasses is not an option now since I'm comfortable with contacts that I only change once a month.

Also, I know exactly what you mean when you say you can't use new lenses for MF. The Sigma has an even shorter focusing ring and it really bothers me, it's really annoying when you want to focus manually.

I have had similar problems to what you describe, but recently installed a new focusing screen with a split screen and microprism ring. I bought it with the intent of allowing me to use MF with my older lenses as I did in my film days, but I found that it has dramatically improved my AF as well. Now I can clearly see what the AF is choosing, and make a change if it is not what I want. What I thought was a backfocus issue was really just the AF choosing the wrong subject to focus on.

I've also considered this but I would prefer a "quick shift" solution since I use auto focus alot and rely on my viewfinder to show me exactly what I want.

Let's define then what you mean by 50-75 percent of the time your focus is correct. I was under the impression that was with a single lens. If the Sigma is off ALL the time then yes, you likely have a focus problem. From your description, it sounds like Back focusing.

Back focusing Ok, but I think it might not be that since I don't always have the problem and when I do tripod tests with good lighting I get a very accurate focus.


Personally I think that this lens has such a short focus ring that even the motor drive has problems to accurately focus enough for me. But ( !! ) when on a tripod it seems to be perfect, mostly...
This is a good example. I do a couple of tripod tests, pictures turn out perfectly. I take the camera off the tripod and aim at a subject, 5-10 cm off... Lighting is indoors during daytime.