Quick question regarding wide focusing.

[satay16]

Hi to all.

Just want to know across all camera systems about this case.

When you have a wide to standard zoom lens (eg. 17-35, 18-50, 17-40), and you lock focus at wide, do you find it blur when you zoom in? (assume parfocal lens)

thanks!

 

[ExplorerZ]

Hi to all.

Just want to know across all camera systems about this case.

When you have a wide to standard zoom lens (eg. 17-35, 18-50, 17-40), and you lock focus at wide, do you find it blur when you zoom in? (assume parfocal lens)

thanks!

if parfocal, why would it be blur? if its blur it is not parfocal (assuming not lens softness problem) :dunno:

 

[satay16]

if parfocal, why would it be blur? if its blur it is not parfocal (assuming not lens softness problem) :dunno:

i was thinking along the line of whether focus is less accurate at the wide part, causing it to be blur sometimes when i zoom in. cos right now, i have some issues with the tamron 17-50 that when i focus at wide, then zoom in, it is blur most of the time.

 

[lsisaxon]

i was thinking along the line of whether focus is less accurate at the wide part, causing it to be blur sometimes when i zoom in. cos right now, i have some issues with the tamron 17-50 that when i focus at wide, then zoom in, it is blur most of the time.

If you do it the other way round, focus at the higher magnification, then zoom out, and still sharp, then your lens is parfocal. If not, then the lens is not parfocal.

The reason why it will be blur when you zoom in is because at the widest end, the magnification is the lowest. When the AF sensor thinks it's already sharp, it is already sharp within the tolerance of the sensor. But when you zoom in, the magnification is higher, so you will see that what you and the AF sensor thinks is sharp is still not very sharp.

So for a parfocal lens, the logical flow is to focus using the highest magnification then zoom out. This way you will be certain that the focus is the sharpest.

 

[Wenjin]

does this mean that the focusing depends on the lens and not on the type body
eg. 350d VS 30d or DIGIC II vs DIGIC III

i always thought that the body tell the lens to focus by a certain amount

 

[lsisaxon]

does this mean that the focusing depends on the lens and not on the type body
eg. 350d VS 30d or DIGIC II vs DIGIC III

i always thought that the body tell the lens to focus by a certain amount

I thought the same until I bought a Sigma 24/1.8 which has front focus. With other Nikon lenses the focus are all ok. Then I sent the lens back to Sigma for calibration and they were able to reprogram the lens to et the focus right again. So the information in the lens is also important for the body to know how much to focus... :sweat:

 

[Wenjin]

I thought the same until I bought a Sigma 24/1.8 which has front focus. With other Nikon lenses the focus are all ok. Then I sent the lens back to Sigma for calibration and they were able to reprogram the lens to et the focus right again. So the information in the lens is also important for the body to know how much to focus... :sweat:

recalibration? so a lens should be calibrated for a certain model of camera?

 

[lsisaxon]

recalibration? so a lens should be calibrated for a certain model of camera?

Luckily not. I brought my D70s body for them to use as reference and after the calibration (they don't touch the body, only use it to shoot) the focus was ok even for D200 and D2X. The strange thing is that the amount of front focus was different for all the bodies. I believe it's a backlash setting or something like that.

It doesn't happen with my Nikkors, all of them are spot on.