General focusing questions (and 24-105)

skywalkerbeth · Mar 16, 2008

Hello everyone

I bought my second lens Friday night - 24-105 F4L IS. (what a mouthful).

Last night's photos, indoors with flash, mostly seemed to turn out well. I posted some examples here:

http://www.thephotoforum.com/forum/showthread.php?t=115541

At any rate, I'm still getting the feel for this lens and some practice photos this morning taken from the door to my deck (it's raining out) weren't quite as good (in fact to be honest, crap) so I've got some general focusing questions and hopefully can learn something. I always consider that a focusing issue has to do with me, and not the lens, unless something convinces me otherwise (not that I know enough to figure it out just yet).

1. Do you always set ONE focusing point on your camera (and if so, where - usually center, or always shot specific) or do you typically let the camera pick 3-4 spots - or self select 3-4 spots? I'm mostly talking about handheld, walking around photos. I know if you are setting up for a portrait of something you want to have the focus point on the eyes for instance.

2. What does this term "front focus" versus "back focus" mean - I have read about that in relation to folks sending it back to be calibrated. But what does it mean? And how can you tell?

3. At what point do you decide that a lens needs to be recalibrated? How would you know - just that every photo is soft? Or how would you figure out it was YOU and not the lens?

4. I've noticed this before, mostly with my little P&S Canons.. for some reason, the photos seem more in focus when the flash fires. Just from last night and today's practice, it seemed that could be one reason. Is that true, if so, why? I didn't select one point to focus last night, I merely set it on auto and let er rip. This morning I used Tv at about 1/80, at 105mm. I just wanted to take shots of the tree limbs behind my house.

5. A friend of mine told me that the reason he would not get the 24-105 is b/c the F4 means that in lower light sitches, the AF is useless - too dark, and the camera doesn't know where to focus. Is this true, if so, why? (I know, basic question). I thought when you are looking through the viewfinder, no matter what speed is selected before you depress the shutter, you still see the same thing. Maybe I just don't understand the metering/focusing tie in. I know that if it is REALLY too dark, AF won't work - but "too dark" for F4 and "too dark" for F3.5 are not insanely different - will the AF really not work for a scene that is just a teeny bit too dark?

6. For a 24-105, what are the safe slow shutter speeds to select with the IS on? I suppose at 24, using 1/40 on a normal camera is ok, so what does the IS translate that to? and for 105, I suppose 1/200 is a good speed for a normal camera, so what would IS do for that?

Thank you everyone, I am sure I will think of more later.

Socrates · Mar 16, 2008

1. For personal convenience, I always use center focus. When it's appropriate for me to focus on something other than the center, I focus first, hold the setting and move the camera.

2. As far as I'm aware, "front focus" and "back focus" refer to the direction of incorrect focusing.

3. In over forty years of SLR usage, I've never had a need for lens calibration.

4. I'd need much more information before drawing any conclusion. Flash, of itself, makes no difference.

5. I don't know the Canon system but my Nikon has an illuminator to solve the problem. The illuminator functions with no flash and with the pop-up (which I never use). With my external head, an IR lamp on the head provides the focus assist.

6. Again, I don't know the Canon system but, with my Nikon 18-200 VR, I can shoot 200mm at 1/15 no sweat. With some extra caution, I can go as slow as 1/8 and, with some luck thrown in, 1/4.

By the way, the DoF is greater behind the focus point than it is in front.

Warhawk · Mar 16, 2008

I currently have the EOS 40D
here are some answers:

1. I mainly use the centre spot, because mostly that is what i want in focus, but i do sometimes use the other spots if i want a certain angle. I dont use the automatic setiing, because the camera focuses on the thing closest to you and that might not be the thing i wanted in focus.
Here is an example of where i move the focus spot:

I dont recompose with my EOS like Socrates, because if i do, i sometimes get less in focus images. It has to do with the focus distance that moves.

2. Front focus (according to the HEAD TECHNICIAN on Canon SA - I say this, because alot of people dont agree with this) is when the focus lies between you and the thing you want to take a photo of. Back focus is when the focus lies behind the subject you want to have in focus. You can test this by using a focus chart, set your camera at 45 degrees and taking the photo. Ive had 3 EOS 30D bodies that backfocused and its irretating, because (especially at lower f-stops) you get fewer in focus photos.

3. If you use the focus chart, you take your camera and lens in to be tested, they will callebrate the one that is out, so dont worry. After you get use to your camera, you will know if its you or the lens, but like i said, test the camera and lens on the focus chart, if something is wrong, take both of them in.

4. 1/80th of a second is to slow, try shooting at higher shutter speeds. I try to keep my shutter speed at a minimum of 1/125th of a sec.

5. The f-stop has nothing to do with focus, it has to do with the depth of field and how much light gets into the lens. You will have to shoot at higher ISO's to get the same shutter speed of a person shooting with a f2.8 lens. The focus is sometimes a bit difficult in darker situations and also depends on the type of surface you try to focus on.

6. My lens doesnt have IS, so i cannot say, but i know my one friend shoots at 1/100th of a sec with his 24-105mm f4 lens without any problems.

Socrates · Mar 16, 2008

Warhawk said:
I dont recompose with my EOS like Socrates, because if i do, i sometimes get less in focus images. It has to do with the focus distance that moves.

What?

Warhawk said:
4. 1/80th of a second is to slow, try shooting at higher shutter speeds. I try to keep my shutter speed at a minimum of 1/125th of a sec.

Why? This is an IS/VR lens that only goes up to 105mm focal length.

Warhawk said:
5. The f-stop has nothing to do with focus...

Yes, it does. Smaller apertures combined with low light conditions prevent the autofocus mechanism from receiving sufficient light.

Warhawk said:
6. My lens doesnt have IS, so i cannot say, but i know my one friend shoots at 1/100th of a sec with his 24-105mm f4 lens without any problems.

He's got to be able to do better than that. A 1/100 shutter speed is reasonable even without IS/VR.

Warhawk · Mar 16, 2008

To Socrates:

1. The focus problems is visible when you take photos thats closer to you, on far away shots this doesnt matter so much, because the depth of field is bigger. If you want, i can draw a picture where i can demonstrate to you that if you recompose, the distance on what the camera focuses and the subject you want to take a photo of does indeed differ.

2. There is still a possiblity that the wind is blowing outside and that could result in blury images, thus a higher shutter speed would solve this problem.

3. I know that the f-stop allows more light to enter the lens and thus help the camera to focus easier, but what i understood from the OP is that according to his friend he is going to almost have no focus in low light situations, where he still will have focus. I know this because i have the 24-70mm f2.8 and my friend has the 24-105mm f4 lens and we both took photos at events, in low light and there was no issues, only difference was that he had to use a higher ISO.

4. Like i said, i dont have IS, so i only posted what i know from my friend that has the same lens as he does. Its just important to remember that IS does nothing if the subject is moving, it only helps for the camera shake.

table1349 · Mar 16, 2008

Good read on Re-composing and focus.
http://visual-vacations.com/Photography/focus-recompose_sucks.htm

Warhawk · Mar 16, 2008

At least now i dont have to draw a picture

Socrates · Mar 16, 2008

Warhawk said:
To Socrates:

1. The focus problems is visible when you take photos thats closer to you, on far away shots this doesnt matter so much, because the depth of field is bigger. If you want, i can draw a picture where i can demonstrate to you that if you recompose, the distance on what the camera focuses and the subject you want to take a photo of does indeed differ.

2. There is still a possiblity that the wind is blowing outside and that could result in blury images, thus a higher shutter speed would solve this problem.

3. I know that the f-stop allows more light to enter the lens and thus help the camera to focus easier, but what i understood from the OP is that according to his friend he is going to almost have no focus in low light situations, where he still will have focus. I know this because i have the 24-70mm f2.8 and my friend has the 24-105mm f4 lens and we both took photos at events, in low light and there was no issues, only difference was that he had to use a higher ISO.

4. Like i said, i dont have IS, so i only posted what i know from my friend that has the same lens as he does. Its just important to remember that IS does nothing if the subject is moving, it only helps for the camera shake.

What does any of that have to do with the questions that I posed to you?

By the way, don't bother attempting to demonstrate how recomposing changes DoF because it just doesn't happen.

Socrates · Mar 16, 2008

gryphonslair99 said:
Good read on Re-composing and focus.
http://visual-vacations.com/Photography/focus-recompose_sucks.htm

The author of that web page clearly does not understand the correct method for recomposing.

Warhawk · Mar 16, 2008

Socrates, that website shows exactly how it works.

If you focus on something thats not 90 degrees infront of you and you move your camera to the thing thats infront of you, the 2 distances will never even in a million years be the same distance! To get a perfectly infocus image, you will have to refocus on the subject you want to take a picture of.

my previous reply i stated the reasons for my answers in my original post for the parts you had a problem with.

Socrates · Mar 16, 2008

Warhawk said:
Socrates, that website shows exactly how it works.

If you focus on something thats not 90 degrees infront of you and you move your camera to the thing thats infront of you, the 2 distances will never even in a million years be the same distance! To get a perfectly infocus image, you will have to refocus on the subject you want to take a picture of.

my previous reply i stated the reasons for my answers in my original post for the parts you had a problem with.

Like I said, the author clearly does not understand the correct method for recomposing and, obviously, neither do you.

Reasons? Heck, there was no relationship to the subject matter!

Warhawk · Mar 16, 2008

Ok, socrates, you can even test it YOURSELF since you dont want me to draw a picture...

Get a focus chart, focus above the "focus line" on the chart, move the focus point to the "focus line" and that the photo, do the same below the "focus line".

Compare the 3 images with each other and you will be able to see that the focus is infact different.

Just for interest sake, i see recomposure as:

When you focus on something and keep the trigger in and you move the camera a bit to the left or right and then press the trigger in deeper to take the photo.

in your first post, you say this:

When it's appropriate for me to focus on something other than the center, I focus first, hold the setting and move the camera.

so this shows that you also recompose the same way as i do.

Also, do you maby have your camera on Servo or AI Focus? Because if you do, your camera will automatically refocus anywhere you point it, so the distances wont remain the same.

If not, can you please tell us how you recompose...

skywalkerbeth · Mar 16, 2008

Thanks everyone! This is apparently a passionate subject.

I went out today and took some practice shots. Some turned out very well, others were a bit soft but overall I'm pleased.
What do you think of the below? There is one in particular that is really soft, even though I was standing right in front of it AND the focus light showed in the middle.
http://www.flickr.com/photos/skywalkerbeth/sets/72157604135911113/