technical issue 48mm vs 70 mm

[carrbowl]

Well I've been noticing some of my photos are not as sharp as they should be and I haven't been able to tell why. Tonight I took some photos to send to "grandma" and after some investigation noticed that two shots in comparison, one is very nicely focused and one isn't. I looked to see where the camers focused and both are on the same spot? So why the difference? Both settings were 1/20 2.8 100 ISO, one was auto white balance and the other was flourenscent (which doesn't matter for our focus issue) but the biggest difference was one was 48 mm and the other was 70 mm. So can one of you "guys" who loves the technical side of this photography business explain in non laymans terms why this affects the focus differently. Does this all go back to my favorite subject--DOF?????

I think I might be halucianting (sp??) are they both focused well and one is just closer than the other??? (I think I might just need to get to bed!!)

[engstrom]

Renae, you're shutter speed is too slow. I think it's not so much out of focus as it is motion blur from having the shutter open too long. The rule of thumb is a minum shutter speed of 1/focal length. THat mean 1/48 second (1/60) or 1/70 second (1/100). Also not that your camera's crop factor should be put into the equation so if you have a 1.5 crop then it's 1/(48 * 1.5) = 1/60 and 1/(70 * 1.5) = 1/105.

[AndrewCCM]

John,

Are you positive that the crop factor has to be taken into account? The focal length is the same, just a portion of it is cropped out. The steadiness factor should be the same as full frame in theory (or at least I thought so).

Regardless, I concur with your suggestion that the problem lies in shutter speed.

BTW: What in the world is that pic for? Grandma?

[engstrom]

Originally Posted by AndrewCCM

John,

Are you positive that the crop factor has to be taken into account? The focal length is the same, just a portion of it is cropped out. The steadiness factor should be the same as full frame in theory (or at least I thought so).

Yeah, I'm positive. It has to do with the size of the circle of confusion being smaller on cropped sensors. Think about this - a point and shoot at 20mm needs a very high shutter speed. That 20mm is really a 20mm focal length but the cropped sensor is like 6x so it's equivalent to 120mm on a full frame 35mm sensor. Try taking a picture at 1/30 of a second with that point and shoot and it will have a lot of motion blur.

[AndrewCCM]

Good to know. I was thinking the opposite...but learn something new each day...

[Bartman01]

EXIF shows 1/20th of a second for both images. If that is correct, then I would agree with motion blur from either subject/camera/photog movement. Even if you are using a tripod getting a live subject to stay perfectly still for 1/20th of a second is hit and miss.

[carrbowl]

Ok...but it's a 5d, so I don't have to worry about the crop factor correct? But if I was using my 10D I would..correct? But the 48 vs 70 isn't the problem, just the shutter speed.

And about the photo, my son was in the hospital last weekend and had a "loop recorder" put in his chest, thus the thing sticking out. We were trying to get a photo to show Grandma what it looks like. Beautiful don't you think...hope we don't have to do the defibulator next..it's much larger!!

[DavidsStudio]

Originally Posted by carrbowl

Well I've been noticing some of my photos are not as sharp as they should be and I haven't been able to tell why. Tonight I took some photos to send to "grandma" and after some investigation noticed that two shots in comparison, one is very nicely focused and one isn't. I looked to see where the camers focused and both are on the same spot? So why the difference? Both settings were 1/20 2.8 100 ISO, one was auto white balance and the other was flourenscent (which doesn't matter for our focus issue) but the biggest difference was one was 48 mm and the other was 70 mm. So can one of you "guys" who loves the technical side of this photography business explain in non laymans terms why this affects the focus differently. Does this all go back to my favorite subject--DOF?????

I think I might be halucianting (sp??) are they both focused well and one is just closer than the other??? (I think I might just need to get to bed!!)

Looking at the pictures, camera shake is not an issue. In the top picture, the tape is in sharp focus. In the bottom picture, the nipple is sufficiently sharp.

It looks like the focus point has changed, shich shifted the DOF forward in the bottom picture from where it was in the top picture. My best quess would be the focus point shifted forward between the two shots.It doesn't look like there was a significant change in the depth of the DOF.

David

[engstrom]

Originally Posted by carrbowl

Ok...but it's a 5d, so I don't have to worry about the crop factor correct? But if I was using my 10D I would..correct?

Correct and correct.

[carrbowl]

"It looks like the focus point has changed, shich shifted the DOF forward in the bottom picture from where it was in the top picture. My best quess would be the focus point shifted forward between the two shots.It doesn't look like there was a significant change in the depth of the DOF."



It has changed somewhat. I''m not sure if I can explain it well enough, but one photo the the bottom square (when I show the auto focus points) is right beside the nipple and on the other photo the bottom square is above the nipple a little bit. Would that be enough to change things??

[amadjuster]

You are shooting at F/2.8 so the DOF is not very deep. I am not sure how the Canon differs from a Nikon on auto focus but I have found that I have to override the AF on closeups or I get the wrong part of the picture in focus. I also find this can happen on other shots with similar details. The lens seems to have a mind of its own. I now take a little extra time to see which little "box" is highlighted, showing where the darn thing is focused. I also try and have as few focus "sensors" in the viewfinder. This just lets the little gremlins (yes, there are little bitty creatures there - and you thought it was electronics) have fewer choices on how to mess you up.