what should I expect at f2.8?

sworks

sworks

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Mark
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I took some photos at the weekend and took a particular shot at f6.3, after taking it I thought it would be better at f2.8, the camera was on a tripod and all I changed was the f number but the photo looks poor in comparison, What went wrong?

IMG_3229 by sworks, on Flickr
f6.3


example shot by sworks, on Flickr
f2.8 - I can tell the depth of field is diffferent but the stop button doesn't seem as sharp as the 6.3 shot. I can't get my head round why it seems softer, the lens has been away for repair so do I have a problem or is it me? The lens is a 24-70L
Thanks
 
It's basic optics. Lenses produce softer results at wider apertures, and diffraction at narrower apertures

You will end up with a "sweet spot" in the middle of the aperture range somewhere

In addition to that, at the wide open end, you need to be ultra careful with your technique, if you move forwards slightly as you shoot, you shift the point in focus forwards etc.
 
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if you tried another shot at your smallest aperture(f22ish?) you would get increased depth of field you would also notice poorer image quality than f6.3. You get one sort of distortion at your widest aperture and a different distortion at the smallest. It's worth taking a set of photos from one to the other to find the best range of apertures for a lens. The more expensive the lens, the less the difference.
FWIW, I don't think your f2.8 is too bad at all.
 
Did you use mirror lockup and a remote or timer release? If not, with the exposure at 0.6 seconds (as opposed to 3.2 seconds for the f/6.3 image) you are in the sweet spot for mirror slap and handling vibration to affect the image.
 
Thanks for the replys, I used a remote but no mirror lockup. If thoughts that it generally is ok looking at the photos and my lack of understanding is the case then I'm happy :) I was worried the 2.8 shot wasn't what was expected after the recent lens repair. I find the science fascinating but don't always understand it
 
Maybe it's worth mentioning that a lens repair does not necessarily imply the the lens is now calibrated with that particular body for the most perfect AF accuracy. There's always the possibility that at f/2.8 a slight focus error will be visible when at f/6.3 it's obscured by the extra DOF.
 
Is the 'stop' button on the plain of focussing? It doesn't look like to me. Look like it's slightly left a bit if you get what I mean?
 
I'd agree, I think the main issue is that the focus point is perhaps not exactly where you hoped for. It definitely looks sharper to the left of the button.
 
It's possible that the focussing was slightly off? That would make me happy, I love user error because I can learn from it :)
 
sworks said:
It's possible that the focussing was slightly off? That would make me happy, I love user error because I can learn from it :)
Click to expand...

Yes, that's exactly it. For whatever reason, the camera has focused to the left of the stop button, as Steve says above. These things are, nine times out of ten, user error ;)

In the first shot, the depth of field has concealed this, but it shows up in the second shot.
 
The focus points on cameras are usually bigger than how they are pictured in the viewfinder - often they are pretty much touching each other.. and the camera focuses on the closest point. So if you focus on the stop-button, the camera may be looking at a larger area and focuses on the high contrast closer to you which still falls into the focus point area. Problem is you can't see that happening with the default focusing screen.

Especially with my 35/1.4 and 135/2 wide open I find that a nudge "backwards" can help on a 5D to get the shallow DoF where the critical focus is. However my default action is to AF, shoot, nudge, shoot again as it's far from easy to change the focus just so slightly. But usually one of the pics will be good :)

On a 7D, I love the Spot AF mode where the AF point is very very small.. I can snipe birds between branches or reeds :love:
 
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