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A DOF Focus or IS problem f2.8/200mm
Old 08-20-2006, 12:42 PM   #1 (permalink)
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Hi everyone,

I just came back from a glamourshooting only to see about 1/2 of the pics beeing out of focus or blurry. Now I am trying to figure out why this happened.

The scene was a setup in a garage, and the light was set for f2.8.
As I had my 70-200 "IS" with me, I thought it was a good idea to use 200mm.
I take my pictures with the halfpressed-focus-recompose-and press method (which could be the problem here).

Using the online DOF calculator i found the dof to be about 1 Inch - not much.

Now for my questions:

what happens if I move the lens (!) 1 inch back or forth?
Will the focus plane move 1 inch ? Probably not , right ?

Could the "IS" be the source of the problem (blurry), eg. during recompose, the IS needs some time to settle. And moves the picture. What happens if the shutter is pressed during this "settling" ?

I am seeing a focus error of about 7 inches...


CU & TIA
Peter
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OK, I'll try to take this one ....
Old 08-20-2006, 04:47 PM   #2 (permalink)
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Hi Peter,

First I want to make sure I understand your post and problem.

I assume you have Canon equipment? And the lens you are talking about is the Canon EF 70-200mm f/2.8L IS USM Telephoto Zoom Lens?

So let me further understand, you decided to set the focal length at 200mm for this shoot, correct? And you set the aperture at f2.8, I assume based on a light meter reading, correct?

Sounds like you were using the autofocus?

At 200mm, were you using a tripod? What was the shutter speed? Slow shutter speed is the most common cause of blur (the subject is actually in focus but is blurred). The rule of thumb is that for hand holding a lens, the shutter speed must be about equal to or greater than the focal length. So your shutter speeds should have been at least 1/200s.

If you move back one inch, your plane of focus and the corresponding depth of field moves with you, but the autofocus will adjust for that. This is only something to worry about when you are manually focussing.

Does your Camera use special focusing features - like "average focusing" or does it allow you choices where in the viewfinder you can spot focus? If so, that might be contributing to the problem. What was that focussing setting?

Also, I don't about the features of a Canon, but if you can actually take a picture while the camera is still focussing, then that could also be the problem. You might want to set the camera's feature so that it only takes the picture once the subject is in focus.

I hope that helps a bit.

Regards, Art.

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Last edited by Art; 08-20-2006 at 04:53 PM..
 
Re: A DOF Focus or IS problem f2.8/200mm
Old 08-20-2006, 05:15 PM   #3 (permalink)
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If you only have a 1inch depth of field, and you move the camera forward or back 1inch, then yes, you will have lost focus on your target point.

Yes, the focussed portion of the image does move 1inch when you move the camera forward or back 1inch. No, it doesn't move if you move the camera sideways 1inch

Oh, and IS won't help (or hinder) this.

I understand what you say about pressing the shutter button halfway to focus (and lock focus and exposure), then moving the camera to recompose - you focus on the eyes, for example, then recompose so that, for example, the focus point is on the chin (so the eyes are higher in the frame). That's normal, but you must be awfully careful not to move the camera forwards or back while recomposing if you are working with a very shallow depth of field. 1inch gives you very little room to move, and with the weight of the 2.8 IS (I own one, I know how heavy it is) it would be easy to slip out.

I'd suggest you work with a little more depth of field if you can - maybe stop the lens down to f4 (and increase light to suit)?

Another option is to use an off-centre focus point so you don't have to recompose. If you focus on the eyes with an off-centre point, then take the photo with the focus point still on the eyes, then you won't need to worry about losing focus. You can always crop the image in Photoshop to correct the composition.
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-20-2006, 06:14 PM   #4 (permalink)
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First, of all when you do a lock focus and recompose you take the risk of having a less than sharp shot of the original focus point. This because you are changing the distance between the area you focused. It will be a different distance from the sensor plane when you shoot. So at 2.8, you will likely have your locked focus area not in focus. Sense you don't mention the shutter speed, I can't say for sure, but normally you would want to be shooting at faster than 1/250th with that lens at 200mm (allow for crop factor). But if IS is on you can hand hold at less than that. But I've found that even with IS, you often do not get the sharpness desired. Also keep in mind that at 200mm for that lens you are not at is sweet spot. If you are really seeing a focus error of 7", then you may have some other problem. I shoot with the Canon 20D and a Sigma 70-200 F2.8 all the time and I have no trouble getting sharp eyes. But I always shoot with a tripod if the shutter speed is not at least 50% above the optimum for the mm I'm shooting at. This gets me sharp eyes 97.5% of the time.

Cheers,
rfs
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-21-2006, 08:15 AM   #5 (permalink)
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This may be too simple, but I was having major focus issues with my images, and I finally just tied my camera to my computer with a USB cable, took pictures and compared the oncreen image to the one in the viewfinder. I realized that the diopter on my camera had been moved accidentally. I adjusted the diopter and the problem was solved.
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-21-2006, 10:39 AM   #6 (permalink)
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If using auto focus (as the original poster was) the diopter setting would have no effect on the focus.
Cheers,
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-21-2006, 11:40 AM   #7 (permalink)
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Good point, LOL. I haven't used AF in so long, I forget it's there.
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-21-2006, 11:58 AM   #8 (permalink)
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Ok , I see I forgtot to mention a couple of things:My camera is a Canon EOS 350D , the lens was the Canon 70-200L IS, the crop factor of the camera is 1,6x, resulting in +- 300 mm.

The setup was a studio setup with 4 flash heads, so I put my shutter to 1/250s to be sure. The wireless trigger could just handle this speed.

As I wanted to shoot verticals I already put the AF (yes, shooting auto focus) field to the right, so after turning the camera to a vertical position, the AF field is at the top. This way I tried to reduce the movement needed to recompose after half pressing (and so fixing the focus on the eyes) the shutter.I know I could have turned the IS off, as it probably is redundant shooting flash.

Now I still ask myself if moving the camera by , lets say 1 inch to the front or the back, would only move the plane of things being in focus also by one inch?
(of course fixing the focus, not letting the AF correct the shift of the camera)

My guess is that this depends on the lens magnification.
Right ? I mean , wouldn't the shifting (after shifting the camera) of the plane of things beeing in focus be bigger if I use a 500mm lens than using a 50 mm lens ?

CU
Peter
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Re: A DOF Focus or IS problem f2.8/200mm
Old 08-21-2006, 02:14 PM   #9 (permalink)
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Quote:
Originally Posted by DHDSP
Ok , I see I forgtot to mention a couple of things:My camera is a Canon EOS 350D , the lens was the Canon 70-200L IS, the crop factor of the camera is 1,6x, resulting in +- 300 mm.
The setup was a studio setup with 4 flash heads, so I put my shutter to 1/250s to be sure. The wireless trigger could just handle this speed.
As I wanted to shoot verticals I already put the AF (yes, shooting auto focus) field to the right, so after turning the camera to a vertical position, the AF field is at the top. This way I tried to reduce the movement needed to recompose after half pressing (and so fixing the focus on the eyes) the shutter.I know I could have turned the IS off, as it probably is redundant shooting flash.
Now I still ask myself if moving the camera by , lets say 1 inch to the front or the back, would only move the plane of things being in focus also by one inch?
(of course fixing the focus, not letting the AF correct the shift of the camera)
My guess is that this depends on the lens magnification.
Right ? I mean , wouldn't the shifting (after shifting the camera) of the plane of things beeing in focus be bigger if I use a 500mm lens than using a 50 mm lens ?
CU
Peter
I think you're overthinking the whole thing. But a few points.
1. Turn the IS off. It will actually give you poorer results (if on) under the circumstances you mention. Keep in mind that the flash will freeze the action (the shutter speed in this case is not relevant for freezing action).
2. The most sensitive AF sensor is the one in the middle. If at all possible you should use it.
3. IF you are shooting at f2.8 and you move the sensor plane 1 inch after focus lock, you have just set yourself up to have the point you locked on out of focus. What you want, is the area that you locked focus on to be the same distance from the sensor/film plane at the time you shoot. If it is less or greater than this distance, then it will no longer be in sharp focus. Period.
4. When using flash heads you are always better off to be 1 stop below the maximum sync speed. So for Canon at 1/250th, you will usually be much safer to use 1/125th. Keep in mind that in a normal studio with strobe lighting the ambient light is almost never of any concern and would not alter the photograph unless you got the shutter speed down below 1/30th in most cases. The exposure is solely determined by the fstop and the power of the flashes. Here is an example (from an outtake) of where I wanted the background lights exposed properly. I dragged the shutter (1/15th), but the fstop was based on the strobes that lit the model (Candace):

5. When shooting with studio strobe, you will usually get better results if you are in the f8 to f11 range. This will also give you better DOF and less problems with focus issues. Also, the sweet spot of most lens is more likely in that area than at 2.8.
Have you done the tests I mention earlier to determine if you have a lens or camera problem rather than just a problem in technique? Once you have verified that the lens and camera are working properly, then you can address the other issues.
Cheers,
rfs
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Let's reshuffle the deck and deal again.
Old 08-21-2006, 02:57 PM   #10 (permalink)
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"My camera is a Canon EOS 350D", "The setup was a studio setup with 4 flash heads, so I put my shutter to 1/250s", "I thought it was a good idea to use 200mm."

None of this makes sense. You are shooting with strobes in a small room and you are shooting at f2.8? I can't imagine four strobes putting out so little light. I assume you are shooting head shots and you are 20 feet away using your lens at 200 mm (you would have to be that far away with a 350D and a 200 mm lens in order to get the head and shoulders in)? To shoot full length shots you'd have to be in the garage across the street with a 1.6 crop camera and the lens set at 200 mm. I would need a radio to talk to the model under those circumstances.

And a 1" depth of field? At 20 feet away, according to the online depth of field calculator the depth of field would be 4'' at f 2.8. Again, if you got any closer with a 200 mm lens you be lucky to get the eyes and nose in the picture, forget the mouth and chin (about 4 inches of subject top to bottom from 6' away with a 200 mm lens on a 350D).

And last but not least, your camera (an EOS 350D) has a maximum sync speed of 1/200 of a second, not 1/250. If you are shooting with strobes at 1/250th, you are not in sync. This whole discussion seems weird.

Post a picture from the series that gave you problems, and it will be a lot easier to figure out what happened. But from the information you have given us so far, I doubt it is anything that has been discussed.


Lisa Menuto, 85 mm lens, Canon 10D (1.6 crop), at f4 from 10 feet away at 1/500 second (no flash).

Fish
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