focus distance

[sedwards]

is there a rule of thumb as to the maximum focus distance vs focal length for accurate focusing ? with my 100-400 set at 400 , it seems to do pretty good out to about 150 feet. after that its very hard to get sharp focus. is it my cameras focusing system that isnt good enough or am i going past the normal limits of the lens. i have tried with a tripod and with high shutter speed (1/2500+) and it doesnt seem to help.

[HDNitehawk]

I think you need to clarify.
The Auto Focus of the camera doesn't perform well after 150'?
Or the lens is not sharp after 150' even in manual focus?
Of course Manually you should be able to get a sharp picture at any distance, of course taking other external factors in to account.

If you are talking about the AF system it sounds like an adjustment might be in order if your camera has that ability.

[neuroanatomist]

With long lenses and long distances, atmospheric conditions play a role - thermal haze and/or humidity reduce the sharpness of your image, simply due the amount of air between you and the subject.

[sedwards]

it seems like its the autofocus that isnt getting things as sharp as i think they should be. could it be the size of the area its trying to focus on ? i dont know how big of an area the sensor is looking at. i mostly use the center focus point only and i figured that with good light it would be better but it doeasnt change much. i mostly take pics of deer and wild turkeys . at 400-500 feet the red dot for the focus point fits on the animal pretty easy but if the sensor is looking at an area that covers 20 feet all around it , that would explain my problem.

[HDNitehawk]

it seems like its the autofocus that isnt getting things as sharp as i think they should be. could it be the size of the area its trying to focus on ? i dont know how big of an area the sensor is looking at. i mostly use the center focus point only and i figured that with good light it would be better but it doeasnt change much. i mostly take pics of deer and wild turkeys . at 400-500 feet the red dot for the focus point fits on the animal pretty easy but if the sensor is looking at an area that covers 20 feet all around it , that would explain my problem.

Yes the point will cover more area and may be more prone to picking up a spot you do not want. One of the big advantages I found when I bought my 1D IV is that it had an option to make the area smaller and you can tune in on a small object.

However at farther distances the AF is going to be more forgiving. For instance if you are shooting a deer at 500 feet rather than 50 feet your depth of field is greater.

The first thing I would do is make sure it was not the lens. Set up on a tripod, take image stabilization off and shoot in live view at a subject and manually focus in 10x magnification. If you can get sharp pictures then it is just probably the AF system. (Any time you can set your camera up and manually focus like this on a subject it will be far superior to your AF system)

I would check with the IS on and off as well, at high shutter speeds and on the tripod you shouldn't need IS. If you are using it in those situations I would test that out to.

You can send the camera and lens to Canon and they can do an AF adjustment on the lens and camera. The higher end bodies have an AF adjustment you can do, the T4i does not.

[sedwards]

yeah , if i had done some homework before getting my camera i would have gone with the 7d. right now my plan is to get a 5d3 but that probably wont be till next year so i have to put up with the t4i for now. if i can scrape up some cash i might trade it for a used 7d but cash seems to keep vanishing from my wallet lol.
ill try the liveview manual focus on the weekend to see if its better. got to wait at least a couple days as we are getting a nasty storm starting today

[sedwards]

here is an example of what i mean. this is uncroped and unedited. they were roughly 500 feet away. i took the same shot about 12 times using back button focus and trying to refocus for each shot . this was the best of the 12

IMG_4674 by sedwards679, on Flickr

[Busted Knuckles]

here is an example of what i mean. this is uncroped and unedited. they were roughly 500 feet away. i took the same shot about 12 times using back button focus and trying to refocus for each shot . this was the best of the 12

looking at the grass it looks like it might be front focusing, focusing just in front of the subject - easy enough to test, focus w/ through the optical view finder, switch to live view and 10x box and see if the image gets sharper because you have to focus further or closer.

[HDNitehawk]

If you are at 500', and I looked at your EXIF and you shot at F/8 your Depth Of Field would have 147' by the calculator.
At 400' your DOF would be 93.6'.
Knowing the average size of deer and the framing it looks to me like you are about 450' away.

You should have enough area in focus at that distance to cover the deer. Your lens would have to be way off.

Also use RAW format, the files from the t4i should sharpen nicely. If you are using JPEG you are losing out on that advantage.