Aperture at length question............

admirable

admirable

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Should I increase the f# to obtain the same sharpness at f7 compared with say f2.8 at a focal length of 200mm?

Or should the image be as sharp at 200mm f2.8 as 70mm f2.8?

Might do a test today.
 
I'm assuming that you're asking a theoretical question here and that the lenses in question have the same MTF characterisitcs. That being the case, the f/stop is the defining parameter and the extra distance from the lens' front nodal point effectively cancels out the physically larger aperture circle by converging the light from a point further from the sensor.

Bob

PS....that was the third attempt at putting it into words and it still doesn't look too clear.
 
Canon Bob said:
PS....that was the third attempt at putting it into words and it still doesn't look too clear.
Click to expand...

:lol: had to read that twice!

Well, I've been looking through some old shots from last year and culling the carp. I have noticed that most of my unsharp shots are at longish length eg 200-300mm and are at f2.8-f4.

I've also noticed that my sharpest shots are at max 70mm regardles of F#.

It's not the camera as there are more than one.

It's not the lens as there are more than one.

It could be the aperture setting as I have a habit of shooting everything around f2.8 instead of upping the ISO.

Or, it could be me, maybe time to look at my technique?

I just wondered if upping the f# when using a long lens would improve sharpness?

I'm thinking f2.8 at 300mm gives you a small target to get in focus at that range, yes/no?
 
Certainly the focus would be far more critical shooting 300mm at f/2.8....and more so as you approach the MFD

Bob
 
I think this has to do with the following things

1, Make of lens and know issues.


2, Manufacturing issues and variance and quality thereof

The first copy of the 24-70 L lens I had was razor sharp at 24mm but soft at 70mm using the same aperture.

The copy I am using now is fine at both ends of the scale, but reading the forums it feels like every body has a different experience of the lens.

I have a 70 to 200mm L lens which is razor sharp at 2.8 but a little soft at f8 plus, I think that your original assumption if testing out your lenses is the best way forward.
 
admirable said:
I've also noticed that my sharpest shots are at max 70mm regardles of F#.
Click to expand...

Not anything to do with camera shake is it? Sharper shots at 70mm do hint that maybe focal length:shutter speed might be a cause.
 
pxl8 said:
Not anything to do with camera shake is it? Sharper shots at 70mm do hint that maybe focal length:shutter speed might be a cause.
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As I said above it could be me and I have noticed myself increasing shutter speed of late to try and compensate.
 
It's not clear to me what the real question is here. On first reading it appears to be something related to the fact that lenses deliver greater sharpness at mid-range apertures than when wide open. But...

The OP may be talking about greater depth of field at shorter focal lengths. Or is not focusing accurately at long focal lenghts. Or is suffering camera shake.

I'm just not sure what is meant by the "same sharpness" :thinking:

Richard.
 
Most zoom lenses go soft(er) at the long end of their range. Stopping down a bit helps but you should never have to go beyond f/8.
 
HoppyUK said:
Or is not focusing accurately at long focal lenghts. Or is suffering camera shake.

Richard.
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It's one of those two methinks!
 
Canon Bob said:
I'm assuming that you're asking a theoretical question here and that the lenses in question have the same MTF characterisitcs. That being the case, the f/stop is the defining parameter and the extra distance from the lens' front nodal point effectively cancels out the physically larger aperture circle by converging the light from a point further from the sensor.

Bob

PS....that was the third attempt at putting it into words and it still doesn't look too clear.
Click to expand...

:gag: I felt the need to lie down in darkened room after reading that :lol:
 
admirable said:
Should I increase the f# to obtain the same sharpness at f7 compared with say f2.8 at a focal length of 200mm?
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I've read this more than 3 times and I still haven't got the foggiest idea what the question really means.
 
StewartR said:
I've read this more than 3 times and I still haven't got the foggiest idea what the question really means.
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Does increasing the F# increase sharpness, when using a longer lens? The same way I should increase the shutter speed when using a longer lens.

Nothing to do with DoF.
 
admirable said:
Does increasing the F# increase sharpness, when using a longer lens? The same way I should increase the shutter speed when using a longer lens.

Nothing to do with DoF.
Click to expand...

Yes. Because all lenses are sharper when used at a middle-range f/number. It helps to clear up some optical aberrations, which produce a slight lack of sharpness, that are inevitable at the lowest f/number. (The lowest f/number is also known as the largest, widest, brightest, or full lens aperture.)

By "increase shutter speed" I take it you mean to reduce the shutter duration, also known as a fast shutter speed. This increases sharpness by reducing blur caused by camera movement (camera shake). Be aware that in practise, eliminating the effect of camera movement, either by reducing the shutter duration or by using IS or VR etc, commonly produces more sharpness benefits than using a higher f/number.

Note that while IS and VR reduce the effect of camera movement, they cannot reduce blurring produced by subject movement. And depth of field is another issue in respect to this answer.

Sorry, but that's as clear as I can make it ;)

Richard.
 
A shot at F2.8 can never be as sharp as one taken at a couple of stops down due to the unavoidable distortion at the periphery of the lens being used. Have you never noticed it on your shots.
 
oldfella said:
A shot at F2.8 can never be as sharp as one taken at a couple of stops down due to the unavoidable distortion at the periphery of the lens being used. Have you never noticed it on your shots.
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I'm not quite understanding that OF. If you mean a bit of blurring around the edges at f/2.8 then you're right. But distortion (bowing of straight lines at the edges) is not aperture dependent. It changes with focal length, but the f/number makes no difference.

Regards,

Richard.
 
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