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DPChallenge Forums >> Tips, Tricks, and Q&A >> Focus Question
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11/19/2005 03:35:54 PM · #1
My question is I have glasses but donít wear them much even though I should. So I have to adjust the eyepiece to make it clear when looking through my 350XT. Am I taking a sharp picture or what I perceive to be sharp?

I kind of confused on this because when I take the picture most of the time it seems it could have been sharper. BTW Iím shooting in AF mode.

11/19/2005 03:41:12 PM · #2
Your camera when set to AF auto-focus should give you a sharp image dependent on aperture and exposure time which is not related to the image you see in the viewfinder.
11/19/2005 03:56:09 PM · #3
If your eyes are bad and your viewfinder is not set right, when you use AF...and the cam locks in, you may see the object as blurry.

If your eyes are bad and you use manual focus, then you may see the object as "sharp" when in reality it is out of focus to your cam and all of us!

KenSkid
11/19/2005 04:01:50 PM · #4
Scott,

if you adjust your diopter in the eypiece so the focusing marks on the ground glass are sharp, then whatever appears sharp in the image will be "in focus". The marks are on the plane of focus, the ground glass.

However, even absent eye problems, focus on ground glass is problematic, because we can "adjust the focus" in our eyes. So we can get, relative to the ground glass, either front focus or back focus. The correct way to focus with an SLR is to "look at" the marks, not the image, and then focus so the image swims up into sharpness. In other words, you want your eyes focused at the ground glass so you can bring the image up into focus. It's tricky at first, but it becomes second nature.

Robt.
11/19/2005 04:01:52 PM · #5
Originally posted by kenskid:

If your eyes are bad and your viewfinder is not set right, when you use AF...and the cam locks in, you may see the object as blurry.

If your eyes are bad and you use manual focus, then you may see the object as "sharp" when in reality it is out of focus to your cam and all of us!

KenSkid


So if I'm using the AF mode and the camera locks in (properly) focused. Can I then set my eyepiece to look sharp while holding the shutter button half way allowing me to see what the camera sees? And if so would this help me in Manual focus mode because I have set the eyepiece to my site?
11/19/2005 04:03:47 PM · #6
Originally posted by SDW65:

Originally posted by kenskid:

If your eyes are bad and your viewfinder is not set right, when you use AF...and the cam locks in, you may see the object as blurry.

If your eyes are bad and you use manual focus, then you may see the object as "sharp" when in reality it is out of focus to your cam and all of us!

KenSkid


So if I'm using the AF mode and the camera locks in (properly) focused. Can I then set my eyepiece to look sharp while holding the shutter button half way allowing me to see what the camera sees? And if so would this help me in Manual focus mode because I have set the eyepiece to my site?


This is not a good idea. The focused image still has DOF, it's hard to tell which portion of it is "really" in focus, you can move the focal plane, in effect, by front or back focusing your eyes. As stated above, focus on the marks on the ground glass and then bring the image up into that focus plane.

R.

Message edited by author 2005-11-19 16:04:18.
11/19/2005 04:06:21 PM · #7
Originally posted by bear_music:

Scott,

if you adjust your diopter in the eypiece so the focusing marks on the ground glass are sharp, then whatever appears sharp in the image will be "in focus". The marks are on the plane of focus, the ground glass.

However, even absent eye problems, focus on ground glass is problematic, because we can "adjust the focus" in our eyes. So we can get, relative to the ground glass, either front focus or back focus. The correct way to focus with an SLR is to "look at" the marks, not the image, and then focus so the image swims up into sharpness. In other words, you want your eyes focused at the ground glass so you can bring the image up into focus. It's tricky at first, but it becomes second nature.

Robt.


Sounds like the best argument for a hi-res EVF i've ever heard! hehehe. ;-)
11/19/2005 04:10:16 PM · #8
Originally posted by bear_music:

Originally posted by SDW65:

Originally posted by kenskid:

If your eyes are bad and your viewfinder is not set right, when you use AF...and the cam locks in, you may see the object as blurry.

If your eyes are bad and you use manual focus, then you may see the object as "sharp" when in reality it is out of focus to your cam and all of us!

KenSkid


So if I'm using the AF mode and the camera locks in (properly) focused. Can I then set my eyepiece to look sharp while holding the shutter button half way allowing me to see what the camera sees? And if so would this help me in Manual focus mode because I have set the eyepiece to my site?


This is not a good idea. The focused image still has DOF, it's hard to tell which portion of it is "really" in focus, you can move the focal plane, in effect, by front or back focusing your eyes. As stated above, focus on the marks on the ground glass and then bring the image up into that focus plane.

R.

I sorry to sound stupid but could you explain "ground glass". I have never heard of that term.
Thank bear.
11/19/2005 04:10:58 PM · #9
Originally posted by wavelength:

Originally posted by bear_music:

Scott,

if you adjust your diopter in the eypiece so the focusing marks on the ground glass are sharp, then whatever appears sharp in the image will be "in focus". The marks are on the plane of focus, the ground glass.

However, even absent eye problems, focus on ground glass is problematic, because we can "adjust the focus" in our eyes. So we can get, relative to the ground glass, either front focus or back focus. The correct way to focus with an SLR is to "look at" the marks, not the image, and then focus so the image swims up into sharpness. In other words, you want your eyes focused at the ground glass so you can bring the image up into focus. It's tricky at first, but it becomes second nature.

Robt.


Sounds like the best argument for a hi-res EVF i've ever heard! hehehe. ;-)


The trouble with EVF, aside from resolution, is that it's not real-time. The great advantage of an optical viewfinder is that you see things precisely as they happen. Plus EVF draws a LOT of power. One reason dSLRs can run so long on a single battery charge is that they use much less current since they don't have to continually draw an EVF picture.

R.
11/19/2005 04:14:30 PM · #10
Originally posted by bear_music:

Originally posted by wavelength:

Originally posted by bear_music:

Scott,

if you adjust your diopter in the eypiece so the focusing marks on the ground glass are sharp, then whatever appears sharp in the image will be "in focus". The marks are on the plane of focus, the ground glass.

However, even absent eye problems, focus on ground glass is problematic, because we can "adjust the focus" in our eyes. So we can get, relative to the ground glass, either front focus or back focus. The correct way to focus with an SLR is to "look at" the marks, not the image, and then focus so the image swims up into sharpness. In other words, you want your eyes focused at the ground glass so you can bring the image up into focus. It's tricky at first, but it becomes second nature.

Robt.


Sounds like the best argument for a hi-res EVF i've ever heard! hehehe. ;-)


The trouble with EVF, aside from resolution, is that it's not real-time. The great advantage of an optical viewfinder is that you see things precisely as they happen. Plus EVF draws a LOT of power. One reason dSLRs can run so long on a single battery charge is that they use much less current since they don't have to continually draw an EVF picture.

R.


Yes, but then there's the mirror slap to power!

Never heard the part about focusing my eyes on the marks on the glass though Robert, and I do use manual focus a fair amount. I'll have to try it!
11/19/2005 04:16:45 PM · #11
Originally posted by SDW65:

[quote=bear_music] I sorry to sound stupid but could you explain "ground glass". I have never heard of that term.
Thank bear.


An SLR optical viewfinder uses mirrors to project the image from the lens onto a textured, "ground glass" screen, and then a pentaprism of mirrors above that to reflect the image on the ground glass into the eyepiece horizontally and in the correct orientation. The glass is "textured" to a matte ("ground") finish because it's very difficult to focus an image accurately when it's floating in transparent space, as the eyes can front and back focus at will, and they eed a point of reference to define the focal plane to use.

The ground glass creen is located precisely the same optical distance from the lens as is the sensor plane; whatever is in focus on the ground glass will be in focus on the sensor. When you depress the shutter button to capture the image, a mirror flips up, blacking out the viewinder, and the transmitted light from the lens goes directly back to the sensor plane. Youc an see this mirror withint he body when you remove a lens from the camera. The loud noise an SLR makes when shooting is this mirror slapping up and down; it moves very fast.

Robt.
11/19/2005 04:20:27 PM · #12
Originally posted by nshapiro:

Yes, but then there's the mirror slap to power!


Yah. The slap itself is mechanical (spring-loaded) but it takes power to hold the mirror up. This is why they require that you have a full battery charge when you're cleaning your sensor; terrible things would happen if power cut off while you were cleaning and the mirror tried to drop back while you had an object in there :-) For sure, if you take very long exposures consistently, your battery will power fewer shots before needing a recharge, as it is drawing substantial power to keep the mirror up.

R.

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