optimal distance for accurate focusing

[vargyropoulos]

I've been doing some testing lately with my 7D and different lenses including my 50mm f/1.4, 100mmf2.8L, 400mmf/5.6L and the 600mmf/4 +1.4 extender. I seem to notice a steep drop in image quality if the focus point is farther away.

the 50 for example is tack sharp when I have a standing model filling the frame in portrait orientation but I found the images softer when I shot further back to have the standing model with room in frame while on landscape orientation.

today I borrowed a 600 + extender and I went out to one of my regular birding areas where I take my 400 normally. I framed a white egret in a similar manner but I found the images from the 600 softer, especially with the extender on. even though the camera beeped to signal focus lock, the whole image was blurry. close-ups were no issue, I got some very sharp images when the bird was 20 yards or closer.

is this normal behavior? is there a guide anywhere that would describe what distances are more optimal for different focal lengths? do I need to simply stop down my aperture for subjects that are further out?

 

[Logan]

im not 100% sure but i will take a guess, i feel your pain, inconsistency like that bothers me too.

the focus sensor is looking for a line of contrast. the closer your subject is, the easier time you are going to have keeping the focus point on your chosen contrast point. the further away the subject is, the more chance that you will shake off of the target just enough at the key instant it is taking a reading, leading to missed focus of varying amounts depending on how exactly the algorithms work. say it take 100 readings, and needs 50 good ones for bang on focus. in your "good" examples, you are on target for say 70-90 out of 100 readings, but the longer distance lowers your time on target just enough to start lowering accuracy.

blow my theory out of the water by testing it on a tripod with unmoving subjects.

 

[vargyropoulos]

hi Logan,

thanks for your initial response. I did get a chance to play with the 600 setup over the past weekend, using a 6D & 7D.

I think the focusing issues may be more camera related. I did notice quite an improvement in focus accuracy when I had good light on my subject. the egret in question on my original post was sort of back lit... I had a 3pm sun, shooting towards that direction... white egrets in general gave me a lot of problems.

it could be a FF vs crop sensor too, but the images off the 6D with the same lens, on the same subject/distance/light were much sharper.

I will agree with you that it requires good lines of contrast to lock on focus. I think this is where I was headed in my initial question... if the subject is too far to accurately resolve the details, focus is off... not to mention that the actual focus point in the image had a lot of variation with the focus point that was selected... in some images, the focus was in an area 2-3 points away from the point that supposedly had the "AF lock"