70-200 range switch question

I shot a few "glamor" photos yesterday of my daughter using the T4i and 70-200 f4 (focusing was in single shot mode). I was using an off-camera flash so shutter speed was at 1/200, and the lens focal length was near the low end of the scale, say from maybe 70 to 100 mm.

I noticed that some of the photos weren't as sharp as I expect from this lens, since I've never had a bit of trouble from it in the past I assume it was something I was doing. The only thing I could find that was different than usual was that the range switch was on 1.2 m instead of 3 m where I normally leave it (I was probably between 5 and 10 meters away). Could that range switch possibly have anything to do with it? (I understand that having it at 3 helps it to focus faster when you're far enough away from your subject but beyond that I have to admit that I'm somewhat ignorant on exactly how it works).

Not if it was 1.2m to infinity, no. I don't know what settings that lens has but unless it was something like 1.2m-5m and you were just over 5m away it would have no effect at all.

If your settings are both to infinity, the only difference will be how close the lens will focus, on the 3m setting it will just stop it focusing any closer. 1.2m would give you the entire range.

I have the IS version of this lens, and find the focus limiter HAS to be set properly. I can take a shot from 2 meters @ far setting, or 10 meters @ close setting, and AF confirmation will lock and beep, but the shot will be OOF.

sandpiper wrote in post #16160887
Not if it was 1.2m to infinity, no. I don't know what settings that lens has but unless it was something like 1.2m-5m and you were just over 5m away it would have no effect at all.

If your settings are both to infinity, the only difference will be how close the lens will focus, on the 3m setting it will just stop it focusing any closer. 1.2m would give you the entire range.

That lens' settings are 1.2m to infinity and 3m to infinity. So either setting would be fine for a subject 5-10m away.

My 70 - 200 f4L focuses accurately above 5m no matter what position the switch is in.

Had to look again, yes, the two settings are 1.2-infinity and 3-infinity. Guess I'll go back and see if I can find anything else that was different than normal. (Or possibly this is one of those times when it's the lens' fault but it cures itself when it's on a tripod ).

1Tanker wrote in post #16160895
I have the IS version of this lens, and find the focus limiter HAS to be set properly. I can take a shot from 2 meters @ far setting, or 10 meters @ close setting, and AF confirmation will lock and beep, but the shot will be OOF.

I haven't noticed this. I just shot a cereal box at about 5m with both settings and they're the same.

1Tanker wrote in post #16160895
I have the IS version of this lens, and find the focus limiter HAS to be set properly. I can take a shot from 2 meters @ far setting, or 10 meters @ close setting, and AF confirmation will lock and beep, but the shot will be OOF.

That sounds like something is faulty. If you are on 3m to infinity, the lens should not focus at all at 2m, no lock, no beep. On 1.2m to infinity it should work perfectly at 10m, the setting should have no effect, the only thing it would do, is slow down how long it takes the lens to focus if you start off OOF and it starts hunting down towards the near end, as it will have further to go before it starts coming back, but again it wouldn't lock or beep until it had focus, and at that point it should be correct.

The switch should have no bearing on OOF shots at 10m whatsoever, either it is a complete coincidence and you are just relating missed focus with the switch position in your mind or you have a problem somewhere. I suspect you are just getting OOF shots and (incorrectly) blaming the switch setting. Unless on the 3m + setting and shooting closer than that of course.

OK, thanks for the replies. I guess I was just wondering if the autofocus was maybe a little "less perfect" with the switch on 1.2. Doesn't sound like that's the case, so I'll have to search for something else to blame my bad photos on (Lord knows if I ever take a bad photo it's not my fault !!! )

In fact I just looked over a few I took today and they're great, so I don't know what was different other than the lighting.

The AF may be a tad slower, but it shouldn't affect accuracy. I leave mine on 1.2 all the time unless I'm tracking fast moving subjects at a distance. In both situations I get plenty of bad shots because of me.

Keep in mind that with your camera at 5 meters away, 200mm and f/4.0, the depth of field will only be 0.09 meters evenly divided fore and aft. Slight movement after focus lock could be a problem.


http://www.dofmaster.c​om/dofjs.html

Phil

Zivnuska wrote in post #16162905
Keep in mind that with your camera at 5 meters away, 200mm and f/4.0, the depth of field will only be 0.09 meters evenly divided fore and aft. Slight movement after focus lock could be a problem.

Yes, but I don't think that is likely the case here, I wouldn't rule it out but DOF is unlikely to have been anywhere near as shallow as you suggest.

The OP stated that he was shooting at the short end of the lens (between 70 and 100mm) and at 100mm the DOF becomes 37cm so allows for a fair bit of movement. And that is a worst case scenario, as he stated he was shooting somewhere between 5 and 10m away, so DOF would increase dramatically as he gets further back than that 5m. Taking the midpoint at 7.5m as more likely, he would have 85cm of DOF and if he was at 10m the DOF is around 1.5m.

We also don't know what aperture he was using. You chose f/4 because it was an f/4 lens, that doesn't mean he was shooting wide open. If he stopped down at all, then the DOF would be even greater.

Yes, 9cm of DOF does allow for the shot to be OOF with little movement, but that is using settings and distances that just aren't the case. Assuming the maximum focal length in the range he used, the minimum distance, and assuming the lens was wide open, the DOF would be 37cm at a minimum. Using the mid points of those two ranges (85mm, 7.5m) gives 1.18m at f/4, and allowing for stopping down just one stop increases that to 1.67m.

I don't know why the images weren't sharp, but there should have been enough DOF unless movement was quite significant.

sandpiper wrote in post #16163210
Yes, but I don't think that is likely the case here, I wouldn't rule it out but DOF is unlikely to have been anywhere near as shallow as you suggest.

Standard DOF calculators assume poorer vision of the viewer than 20/20 standard vision. If we assume 20/20 vision...

5 meters subject distance, 200mm f/4 has DOF zone of 3.2 cm...a mere 1.59 cm (0.626") in front of plane of focus
5 meters subject distance, 100mm f/4 has DOF zone of 13.1 cm...about 6.5 cm (2.5") in front of plane of focus

Probably the number one cause of OOF images with any of the zooms is not being aware that they are varifocal designs. That means they go out of focus whenever you change the focal length. So when using One Shot, you must refocus if you change the focal length of the zoom. (If you are hearing a beep, you're using One Shot.)

Wilt wrote in post #16163517
Standard DOF calculators assume poorer vision of the viewer than 20/20 standard vision. If we assume 20/20 vision...

5 meters subject distance, 200mm f/4 has DOF zone of 3.2 cm...a mere 1.59 cm (0.626") in front of plane of focus
5 meters subject distance, 100mm f/4 has DOF zone of 13.1 cm...about 6.5 cm (2.5") in front of plane of focus

DOF calculators assume all sorts of things, size of print, viewing distance, what is "acceptably sharp" etc. You can change the DOF number by altering any variable, why not assume a 30x20" print as well and reduce it further. That is why I don't like DOF calculators, they just give an arbitrary number that only applies under a strict set of conditions, but look like they are "accurate" by the way they are presented.

But that is beside the point, I was just pointing out that it wasn't likely to be just 9cm, using Dofmaster, because the figures that were used were flawed. Even your reduced DOF figures for assuming 20/20 vision give about 50% more DOF than Divnuska suggested, and even that is still making big assumptions that the OP was at the minimum distance they may have been at, the lens is at the longest focal length they might have been using and that the lens was wide open, when they never mentioned an aperture that was used.

I'm not arguing with the DOF figures at those settings, just that these shallow DOF figures being bandied about are probably much shallower than was the case, as the settings that have been used are all "worst case scenario", rather than taking a midpoint in the OPs distance and FL estimates, or that they may just have stopped down a bit.