AI Servo AF on a tripod

[adjohnson]

I have just upgraded from a Canon G3 to an EOS Digital Rebel. Now I have an unexpected problem. I create "action panoramas" of kids www.kidsbeingkidsportraits.com/panoramas.html where I used to focus lock in the center of my studio and let the kid go crazy without touching my camera. Now I understand that the EOS Digital Rebel will not focus lock when my subject is moving and will start to try to continually focus on it. Does this mean that I have to try to move my camera around (it's on a tripod) to keep my subject on one of the autofocus points or is the camera smart enough to interperlate focus length when the subject is moving between the autofocus points. I would prefer to avoid manual focus since I don't do as well as the autofocus.

Thanks for any input!

[roanjohn]

Can't you manual focus on the center of your studio and shoot away?? The DOF of the Rebel might be a tad shallow compared to the G3 so I would probably increase my aperture.

..............I hope I answered your question right.

Ro1

[scottbergerphoto]

Can't you manual focus on the center of your studio and shoot away?? The DOF of the Rebel might be a tad shallow compared to the G3 so I would probably increase my aperture.

..............I hope I answered your question right.

Ro1
The DOF of the 300D is controlled by the lens and aperture used and isn't a function of the 300D body. P&S cameras by virtue of their single lense with smaller maximum aperture and smaller sensor have a large depth of field.
Scott

[adjohnson]

The smaller depth of field with the Digital EOS Rebel is definitely a problem because I don't have really bright studio lights so I'm hanging around 4.5 or 5.0, using flash shutter speeds. I don't want to reduce my shutter speeds too much since I'm doing action.

So I just need to figure out how to focus quickly on my moving subjects which brings me back to the original question. Does the EOS Rebel do the AI Servo AutoFocus only at the focus points - i.e. does the subject have to be in one of the focus points at all times?

Thanks!

[roanjohn]

I don't think the Rebels AI servo mode would be sufficenet for tracking your subject indoors. But to answer your question, I think the Rebel will only use the center focus point under sports mode for AI servo. Though I could be wrong on this.............BUT, I strongly suggest you pick only the center focus point to track your subjects accurately.

RO1

[wolf]

If you were locking the focus previously with the G3 you can basically do the same with the Drebel. Auto focus on the subject then click the lens to manual focus and shot away.

[scottbergerphoto]

The smaller depth of field with the Digital EOS Rebel is definitely a problem
:?: Please look at a depth of field scale and show me where it says anything about camera body.
:cry:

[roanjohn]

I think that due to sensor size............similar apertures between the Rebel and G3 will yield different DOF. So in fact, it has a lot to do with the body.

Ro1

[scottbergerphoto]

I think that due to sensor size............similar apertures between the Rebel and G3 will yield different DOF. So in fact, it has a lot to do with the body.

Ro1
Excuse me for noticing, but you didn't say "for similiar apertures" in your two previous posts. You just said that the 300D has a smaller depth of field, and that just isn't correct. Words matter. In a DSLR as in a 35mm SLR, DOF has little to do with the body. Just use a different lens, a different focal length, or a different f stop. If you want a large depth of field in a 300D, use a short lens and a small aperture.
Scott

[roanjohn]

But we're talking digital. And correct me if I'm wrong but digital bodies have different sensor size.........so between the G3, Rebel, 1D and 1Ds, DOF should slightly vary from body to body. And this is especially apparent on the G3 (the posters original camera) against the Rebel.

So technically speaking (and in conclusion) - different digital bodies (even with similar lens under similar apertures) will have an effect on your DOF.

I hope I said this correctly :?

Ro1

[adjohnson]

Very interesting - thanks everyone for the discussion. Now a newbie question:

You mentioned that a short lens will give me a better DOF. Does that mean a 28mm will have greater DOF than a 55mm at the same aperture? Also, does a short 28mm lens have a better DOF than a longer zoom lens set to 28mm at the same aperture?

Thanks again!

[scottbergerphoto]

DOF is mainly affected by the focal length of the lens and the lens aperture in use. Long lenses and large apertures give a narrow DOF and short lenses and small apertures give a wide DOF. To compare the DOF for two lenses of different focal lengths at different F stops, you need to look at a DOF calculator like this one:
http://www.dofmaster.com/dofjs.html
The DOF for a zoom at 28mm and a 28mm lens at the same f stop are the same.
Regards,
Scott

[psychedelic_never]

roanjohn, you are right smaller the sensor greater the DOF (only true for P&S) for a similar aperture on a ful frame body. The reason to this might be that a 28 mm lens on a G3 will actually be a much wider lens on a full frame body. The 28 mm mentioned is the 35 mm equivalent. Since the focal length of the lens on the G3 is much less than 28mm, the DOF will obviously be greater than an actual 28mm lens on a full frame body.
The issue here is that the focal length mentioned on the P&S bodies are all 35mm equivalent and the actual focal length of these lenses are much lesser hence the increased DOF.

[gcogger]

Just to clarify - the DOF is determined by the (actual, not equivalent) focal length, aperture and sensor size. For calculators such as dofmaster, you need to enter a circle of confusion (COC) that depends on sensor size. I find it easier to use one such as:

http://www.outsight.com/hyperfocal.html#nearfar

that lets you simple enter the 'multiplier' factor (1.6 for a 300D) as part of the calculation.