Question on focusing in backlighting condition

[kelvin8x]

Hi bros,

I have a question on focusing in backlighting condition (e.g., shooting people in the shade or whatever with quite bright backlighting).

Situation 1: When I do the metering to get the background not overexpose (of course the subject, e.g., face of the person, will be a bit dark), the pix looks ok in full screen of my 17'' monitor but not very sharp in 100% crop (face of the subject). In 100% crop it looks a bit blur like miss focus.

Situation 2: When I try to increase exposure compensation to get the subject exposure right (of course in this case the background will overexposure), the pix looks very very sharp even in 100% crop.

Situation 3: Back to sittuation 1, now i am using the fill flash to obtain both background and subject exposure, then the pixs are sharp.

My question is how to obtain the sharp person face in situation 1. I am not sure but it's like when the subject is a bit underexposure with bright background, I can not get a very sharp subject (focusing or whatever).

Please advice me, thanks a lot

 

[catchlights]

use focusing lock, check out your camera manual.

 

[kelvin8x]

Hi catchlights, it's always focusing lock in all situation by half pressing the shutter. I even used the center focus point @ the face without recomposing to avoid the DOF. Still manage to get very sharpix in Situation 2 & 3, only situation 1 pix looks ok but not in 100% crop.

 

[catchlights]

situation 1, take meter reading, AE lock, focus, recompose and fire off.

 

[kelvin8x]

What I meant is only in Situation 2 & 3 when the exposure of subject is correct (either by EC+ or fill flash), the px is sharp. In Situation 1, when I purposely get only the bg exposure correct (the subject will be a bit dark so), then I got problem with focusing to get very sharp pix in 100% with whaterver I tried (AE lock, focus lock...). Is it because in this case, the subject will be very low contrast or dark and very difficult to focus.

 

[cantaresg]

I think in this case, the camera tries to focus through the lens. In this case, if there is not enough light, the camera will not be able to focus. What you can do is expose for the subject, do a focus lock, then expose for the background, recompose, and fire off?

 

[Adelfin]

I think in this case, the camera tries to focus through the lens. In this case, if there is not enough light, the camera will not be able to focus. What you can do is expose for the subject, do a focus lock, then expose for the background, recompose, and fire off?

erm... i think in all cases.. the camera focuses through the lens...
by right, focusing should not change throughout any of the situations, given the same lighting conditions.. at focusing, the camera will focus based on the actual light entering the lens at the widest aperture... so it will not have to do with any form of compensation or flash use..
flash may, however, help to make the pix look sharper because it will "freeze" the subject..

how many times did you try this test out? coz it might just be a case of mis-focusing coz lenses are not always 100% accurate, esp in low light... another thought could be that the shadows are so dark on the subject that it seems blur.. try pumping up the brightness with some software to have a better look...

 

[cantaresg]

Erm. true. The focus should be done in the largest aperture. My mistake.

But not all cameras focus through the lens. What about IR focusing? Compacts normally use that.

 

[Adelfin]

Erm. true. The focus should be done in the largest aperture. My mistake.

But not all cameras focus through the lens. What about IR focusing? Compacts normally use that.

hmm..i'm guessing he uses a dslr from the kind of qns he's asking... i may be wrong though...

but i feel compacts have pretty good focusing systems.. unless the IR sensor is totally off... the only problem is compacts are bad at choosing the appropriate point to focus on... but that's a different matter all together...

 

[cantaresg]

Or... can it be due to diffraction? Perhaps the aperture used is more than f/13, and diffraction cause some softening in the image?

 

[ExplorerZ]

Erm. true. The focus should be done in the largest aperture. My mistake.

But not all cameras focus through the lens. What about IR focusing? Compacts normally use that.

eh I believe all AF camera focus thru the lens... if not how do the camera know if the lens is in correct focus? :bsmilie:

 

[cantaresg]

The IR focus works by measuring distance of the subject from the camera, then adjust the lens to the distance.

 

[kelvin8x]

Sorry you guys, busy to pack some of my stuffs for my trip thus cannot follow the thread.

@Adelfin: I've tried quite a lot, not in low light condition but in sunny day with the subject in the shade. Yes, pumping up the brightness can have a better look but as I said the pix look ok in full screen mode but not in 100% crop. In 100% crop can see blur, in situation 2 & 3, 100$ crop the pix is very sharp.

@ cantaresg: Im using dSLR with 50 1.4. Taking 3/4 body with Av = 3.5 -> 4 for the test. This makes me irritated coz in situation 1, with a PnS the pix seems sharp in 100% crop (maybe because the pix size a bit smaller and PnS makes the pix more sharpen then dSLR does in post processing)

 

[ExplorerZ]

The IR focus works by measuring distance of the subject from the camera, then adjust the lens to the distance.

not sure, but ain't IR focusing a MF method specially for IR film or something like that? :dunno:

Sorry you guys, busy to pack some of my stuffs for my trip thus cannot follow the thread.

@Adelfin: I've tried quite a lot, not in low light condition but in sunny day with the subject in the shade. Yes, pumping up the brightness can have a better look but as I said the pix look ok in full screen mode but not in 100% crop. In 100% crop can see blur, in situation 2 & 3, 100$ crop the pix is very sharp.

@ cantaresg: Im using dSLR with 50 1.4. Taking 3/4 body with Av = 3.5 -> 4 for the test. This makes me irritated coz in situation 1, with a PnS the pix seems sharp in 100% crop (maybe because the pix size a bit smaller and PnS makes the pix more sharpen then dSLR does in post processing)

possible to post a crop of the "blur"?

 

[Gunjack]

I think the blur caused in the first situation could be that the contrast is too high and the camera didnt focus correctly, but got the other 2 situations correct, OR the pic for the subject is underexposed and so appears not sharp due to noise...

 

[kelvin8x]

Above is the 100% crop sample of blur in situation 1 & sharp in situation 2. Both cases, taking full body.

 

[catchlights]

metering and exposure mode got nothing to do with camera focusing,

when doing auto focus, a few factors affect the accurately of auto focusing, refer to your camera manual.

 

[ExplorerZ]

Above is the 100% crop sample of blur in situation 1 & sharp in situation 2. Both cases, taking full body.

eh I think gunjack got the point... the image is soft due to noise in underexposing part, not focusing...