Canon 70-200mm f/2.8L IS USM Focusing Switch

[Canon88]

Hey guys,

This will sound totally ignorant but I'm prepared to admit it. I've owned by Canon 70-200mm f/2.8L IS USM for several years, and I'm completely aware of the flush focusing switch built onto the lens ranging from 1.4 meters to 2.5 meters.

My question to those who use this lens: Why would a photographer ever position the switch to 2.5 meters?

If I'm understanding correctly, the photographer has a range of 4.5 feet to infinity when the switch is set to 1.4 meters. And when the switch is set to 2.5 meters (or 8.2 feet), the photographer must be a minium of 8.2 feet away to focus on the subject - so why not just leave the switch set to 1.4 meters?

I'm seeking an answer to this question in the hopes that I can solve a bigger problem, that is, frequent focusing issues. I use the center AF always, IS, and back-focus, at 200mm f/2.8 my images are more often blurry than they are sharp, even at fast shutter speeds. Two examples of the focusing issue are below with respective cropped areas of the models face magnified in Lightroom 3.0 on my Macbook Pro. I'm wondering if I should send both my lens and body into Canon as this happens often.

[ptrd]

Not an empirical answer but my educated guess is:

1. that extra 1.x meters of focus represents a good deal of focus movement, so if you are shooting something far, it will always be in that further range and not hunting back
2. shooting behind a fence or something and you want it to always focus past that particular point (maybe stretching a bit with this one)


Also if you find the focus is out, does your body have microadjustments for lenses? (sorry not familiar with a 5dmkII, I have a 50d which has adjustments for this sort of thing). Also if you can't adjust it, maybe it's time for a freshing up from Canon?

[shnitz]

Exactly as mentioned above, it's for if you are using the lens where there is something like a baseball net in front of you, or you are shooting some event where there is a large group of people.

It has nothing to do with your focusing issues, I'm guessing. Image stabilization causes softness in images; if you're using it in regions where you would otherwise not need it, you are just hurting your image quality.

Also, what do you mean by backfocusing? You are focusing behind the subject? That is not helping either. At 200mm and f/2.8, your depth of field is less than 1.5 inches, if you're focused 10 feet away. You need to stop down more, or use a tripod and Live View to really catch focus precisely, because even breathing will throw your subject out of focus.

You say you're using center AF. Are you moving the camera? For example, if you halfpress to focus with the camera on his face, and then recompose the shot before taking the picture, you will move the depth of field too far away.

[Canon88]

Originally Posted by shnitz

At 200mm and f/2.8, your depth of field is less than 1.5 inches, if you're focused 10 feet away. You need to stop down more, or use a tripod...

This is new information to me. Can you source this, by chance? Are there any charts you know of with focal length & f-stop relationships (with respect to DOF)?

[shnitz]

Google is your friend, this took me about 4 seconds to find:
Online Depth of Field Calculator
Depth of Field Calcularor

Also, to convert from feet to inches, you can also use google. For example, in your example, a fullframe/35mm camera, with a 200mm lens, focused 10 feet away at f/2.8, gives you 0.12 ft of DOF. If you open up a new tab, go to google.com, and type "convert .12 feet to inches" into the search box and press enter, it will say at the top of the search results page:
.12 feet = 1.44 inches

Conversely, you can use the distance scale on your lens. Again, if you need any assistance, google is just a mouseclick away.

[kenw]

Dofmaster is great and has an ap for iPhone and perhaps other mobiles

[Dobick]

In the first image, your focus point is actually in front of the subject. it would appear that you probably focused and recomposed and the camera regrabbed focus on the center point -- or it could be the simple act of recomposition that caused a focus shift.

[PhotoDen]

Originally Posted by Dobick

...it would appear that you probably focused and recomposed and the camera regrabbed focus on the center point -- or it could be the simple act of recomposition that caused a focus shift.

Okay, you guys have got my curiosity up. Ariel (shnitz) said "if you halfpress to focus with the camera on his face, and then recompose the shot before taking the picture, you will move the depth of field too far away."

And Kent (Dobick) said

Originally Posted by shnitz

Are you moving the camera? For example, if you halfpress to focus with the camera on his face, and then recompose the shot before taking the picture, you will move the depth of field too far away.

Assuming you focus and recompose, but don't release the shutter button (i.e. keep it pressed down to maintain focus) could the recomposition still throw your focus off? Or are you asssuming he released the shutter button? If holding focus and recomposing can throw focus off please explain how. Is it just the physical act of moving the lens with a 1.5 inch DOF or is there something else?

I know that as your subject moves away from the center of the lens focus tends to become a little softer with a lot of lenses, but, is there something else that would be throwing it off (assuming he maintains his original focus point by holding down the shutter button)?

Thanks for any clarification.

[shnitz]

That focus shift occurs especially when you're continuing to half-press the shutter. I was not at all referring to accidentally focusing on a different spot. Although, to go off on a tangent, your 5D and 7D both have an AF-on button, so consider trying out having a decoupled autofocus from your shutter! It's very liberating to have AF controlled by AF-on, exposure controlled by AE lock, and the shutter only for opening the shutter. You're taking 3 aspects of creating an image and taking full control of them.
focus - What is the advantage to back-button autofocus? - Photography - Stack Exchange
Canon EOS 40D Digital Field Guide - Charlotte K. Lowrie - Google Books


Think about how the camera focuses: there is a flat sheet of focus parallel to the sensor/film (in most cases, unless you're using a technical camera, or a tilt/shift lens). If you point the camera at a subject to focus, and then move the camera, then you've moved the plane of focus. Here is an illustrative website:
Don't recompose!

[PhotoDen]

Originally Posted by shnitz

That focus shift occurs especially when you're continuing to half-press the shutter...[/url]

Ariel, thanks a lot. VERY enlightening and makes perfect sense. The links were very helpful.
Thanks,
Dennis

[Dobick]

take a look at this:

http://press.hasselblad.com/media/19...e-focus_en.pdf

it is a pdf from hasselblad on a tech that they call APL (absolute position lock) which overcomes the problem that you might be having.

With such a long lens, it is possible that this is exactly the problem that is occurring

[Canon88]

Originally Posted by shnitz

Exactly as mentioned above, it's for if you are using the lens where there is something like a baseball net in front of you, or you are shooting some event where there is a large group of people.

It has nothing to do with your focusing issues, I'm guessing. Image stabilization causes softness in images; if you're using it in regions where you would otherwise not need it, you are just hurting your image quality.

Also, what do you mean by backfocusing? You are focusing behind the subject? That is not helping either. At 200mm and f/2.8, your depth of field is less than 1.5 inches, if you're focused 10 feet away. You need to stop down more, or use a tripod and Live View to really catch focus precisely, because even breathing will throw your subject out of focus.

You say you're using center AF. Are you moving the camera? For example, if you halfpress to focus with the camera on his face, and then recompose the shot before taking the picture, you will move the depth of field too far away.

I'm a little confused about the suggestion to stop down more. Although at 200mm f/2.8 focused from 10 feet away the DOF is less than 1.5 inches, it should never the less be ACCURATE when focused on the subject. After looking at the images I posed, it appears to me that the focus has missed entirely and is, as Dobick suggested, in front of the subject.

[shnitz]

My suggestion to stop down more is because, while theoretically you are focused on the subject, in any shoot many things can happen, like:
-You breathe, sway, etc. This can move your body, and therefore the depth of field enough to make you miss the shot.
-Your subject shifted, swayed, etc.
-Phase detect AF can have errors. Your camera has a micro-adjustment feature, for problems like this. It involves doing some careful tests, but if you have a tripod, and you wish to do such precise work like this, you should really consider getting your focus tuned in with each lens that you have, or consider using manual focus and live view for this critical work.

Because of all of these things, if you stop down, you ensure a greater depth of field, which will ensure that the subject is in focus.

Or, it could just be a case of user error. If you focuses on the guy, then recomposed, it could be that you accidentally re-focused on the brick wall. I work in technology, and it is very common for this to occur. The problem is that computers, cameras, etc. are very literal machines; they do what you tell them to do. However, what you are telling them to do vs. what you are wanting them to do are sometimes two very different things.

When a good friend of mine shoots landscapes, he has his SLR on a tripod. He uses Live View to carefully focus on his subject manually. He even uses a special loupe to really get critical focus. When working with such a narrow depth of field, you have to be very precise in your work.

[Canon88]

Hey shnitz, thanks for the helpful information. I meant to say thank you sooner but I've been really sick. This issue has been troubling me for a long time, I really appreciate your informative responses.

[larrycumba]

What picture style are you using?