Live view

[duehew]

After being away from photography for several years I purchased a Nikon D-3200. (use to use a Minolta XG-7) I started out using the Live View but have gone back to the view finder as I noticed that when using the Live View the shutter speed was slow and with the viewfinder the shutter speed is almost instantaneous. Why is that?? I have found no reference to this anywhere. Am I missing something? Just a dabbler and enjoying it.

Du

 

[TCampbell]

After being away from photography for several years I purchased a Nikon D-3200. (use to use a Minolta XG-7) I started out using the Live View but have gone back to the view finder as I noticed that when using the Live View the shutter speed was slow and with the viewfinder the shutter speed is almost instantaneous. Why is that?? I have found no reference to this anywhere. Am I missing something? Just a dabbler and enjoying it.

Du

This has to do with how the focus systems work. The camera has two independent focus systems.

A point & shoot camera doesn't have a viewfinder. The camera samples the image to be focused and it looks for edges of contrast and it compares adjacent pixels to see how rapidly the hue or tonality of the pixels change.

An easy example is to imagine a "bar code" or UPC code. If the image is in sharp focus then what you'd probably expect to see is a "white" background with "black" stripes. You wouldn't expect to see any other tonality (you wouldn't expect to see lots of gray pixels). If the image is in sharp focus then you'd expect to see some black pixels and immediately adjacent you'd expect to see white pixels.

If the image is not focused then you'd expect to see a progression from black to white ... black, then dark gray, then medium gray, then lighter gray, ... and eventually white. That very gradual contrast change from pixel to pixel is an indication that the lens is not focused.

The camera has to sample an image, look for contrasty areas, evaluate the rate of change for contrast between adjacent pixels, then change the focus a little and re-sample everything and re-evaluate to see if the contrast changes is faster or slower. It keeps re-focuses and re-evaluating until it finds the focus point where it's optimized the contrast and THEN it can take the photo.

This system is called "contrast-detect" auto-focus.


The "other" focus system is the "phase-detect" auto-focus. This is the focus system that the camera uses when you look through the viewfinder.

When you look through the viewfinder there is a reflex mirror which is reflecting light at a 90º angle up into the focusing screen, which you see through the eyepiece. But part of that reflex mirror is moderately transparent and there's a second mirror hiding behind it. That smaller mirror is bouncing light down (instead of up). It's bouncing light down and into an array of phase-detect focus sensors.

Each sensor has a tiny lens and light passes through multiple points of that lens. If the image was focused then the points of light that pass through the focus sensor will converge at the exact same point. If the image was out of focus then the points of light will not converge on the same point.. they will be "out of phase".

Now here's the neat trick about phase-detect auto-focus... When an image is out of phase, not only does the camera know it's not focused... it actually knows PRECISELY how far out of focus the lens is AND it even knows in which direction the lens needs to go to obtain perfect focus.

It gets better... DSLR cameras have multiple phase-detect focus points. When the camera evaluates focus, it actually evaluates ALL of the focus points (at least all of the points you allow it to use) simultaneously. Usually if you allow the camera to use all points then the camera will select the point that achieves focus at the closest focusing distance. Since the sensors each know how far focus needs to change to obtain focus and in which direction, in just ONE sample it knows which point can achieve focus at the closest focusing distance and moves the lens to that position.

The reason phase-detect AF is so fast is because the camera basically takes ONE sample... and then moves the lens to the perfect position for focus (there is no extra sampling needed.) It's VERY fast.

When you use "live view" mode, the reflex mirror has to swing up to allow the image sensor to get the image. This means there is no longer a mirror that can bounce light down into the phase-detect AF sensors and it forces the camera to switch to the "contrast-detect" AF system.

Although phase-detect auto-focus is faster (significantly faster) there is a slight potential for contrast detect af to provide a sharper focused image.

This is because contrast-detect AF is actually using the image sensor and evaluating contrast to achieve focus. When contrast is optimal the image is focused and since the sampling is being done by the same image sensor that's about to take your photo, it's in perfect focus FOR THE IMAGE SENSOR.

Phase-Detect AF, on the other hand, has separate focus sensors on the floor of the camera. The sensors are all supposed to be positioned so that light has to travel the same distance to bounce off the mirror and reflect down into the image sensors as it would have had to travel if the mirror had been up and the light traveled straight through to the back of the camera and landed on the image sensor. But this means someone at the factory has to perform some adjustments to make sure that's true (there's a possibly of human error during manufacturing.)

Also, since the phase-detect only takes one sample and then orders the focus motors to move the lens, and does NOT re-evaluate focus after the lens moves, there's a possibility that manufacturing tolerances in each lens can be unique and it might nail the focus perfectly for some lenses... but be just a tiny bit soft for other lenses. Higher end cameras usually have an "auto-focus micro-adjust" feature that lets you dial in some correction. You evaluate how perfect the focus is for any given lens and if you think it's slightly missing focus, you can tell the lens to focus just a tiny amount closer or farther.

 

[dennybeall]

For live view to work part of the mechanism reflecting light to the viewfinder must move out of the way. When you press the shutter release it has to move back, then take the shot and then move back out of the way. Hence more time needed to take the shot. Personally I only use Live View for video and only because that's the only way it will work.

 

[duehew]

After being away from photography for several years I purchased a Nikon D-3200. (use to use a Minolta XG-7) I started out using the Live View but have gone back to the view finder as I noticed that when using the Live View the shutter speed was slow and with the viewfinder the shutter speed is almost instantaneous. Why is that?? I have found no reference to this anywhere. Am I missing something? Just a dabbler and enjoying it.

Du

This has to do with how the focus systems work. The camera has two independent focus systems.

A point & shoot camera doesn't have a viewfinder. The camera samples the image to be focused and it looks for edges of contrast and it compares adjacent pixels to see how rapidly the hue or tonality of the pixels change.

An easy example is to imagine a "bar code" or UPC code. If the image is in sharp focus then what you'd probably expect to see is a "white" background with "black" stripes. You wouldn't expect to see any other tonality (you wouldn't expect to see lots of gray pixels). If the image is in sharp focus then you'd expect to see some black pixels and immediately adjacent you'd expect to see white pixels.

If the image is not focused then you'd expect to see a progression from black to white ... black, then dark gray, then medium gray, then lighter gray, ... and eventually white. That very gradual contrast change from pixel to pixel is an indication that the lens is not focused.

The camera has to sample an image, look for contrasty areas, evaluate the rate of change for contrast between adjacent pixels, then change the focus a little and re-sample everything and re-evaluate to see if the contrast changes is faster or slower. It keeps re-focuses and re-evaluating until it finds the focus point where it's optimized the contrast and THEN it can take the photo.

This system is called "contrast-detect" auto-focus.


The "other" focus system is the "phase-detect" auto-focus. This is the focus system that the camera uses when you look through the viewfinder.

When you look through the viewfinder there is a reflex mirror which is reflecting light at a 90º angle up into the focusing screen, which you see through the eyepiece. But part of that reflex mirror is moderately transparent and there's a second mirror hiding behind it. That smaller mirror is bouncing light down (instead of up). It's bouncing light down and into an array of phase-detect focus sensors.

Each sensor has a tiny lens and light passes through multiple points of that lens. If the image was focused then the points of light that pass through the focus sensor will converge at the exact same point. If the image was out of focus then the points of light will not converge on the same point.. they will be "out of phase".

Now here's the neat trick about phase-detect auto-focus... When an image is out of phase, not only does the camera know it's not focused... it actually knows PRECISELY how far out of focus the lens is AND it even knows in which direction the lens needs to go to obtain perfect focus.

It gets better... DSLR cameras have multiple phase-detect focus points. When the camera evaluates focus, it actually evaluates ALL of the focus points (at least all of the points you allow it to use) simultaneously. Usually if you allow the camera to use all points then the camera will select the point that achieves focus at the closest focusing distance. Since the sensors each know how far focus needs to change to obtain focus and in which direction, in just ONE sample it knows which point can achieve focus at the closest focusing distance and moves the lens to that position.

The reason phase-detect AF is so fast is because the camera basically takes ONE sample... and then moves the lens to the perfect position for focus (there is no extra sampling needed.) It's VERY fast.

When you use "live view" mode, the reflex mirror has to swing up to allow the image sensor to get the image. This means there is no longer a mirror that can bounce light down into the phase-detect AF sensors and it forces the camera to switch to the "contrast-detect" AF system.

Although phase-detect auto-focus is faster (significantly faster) there is a slight potential for contrast detect af to provide a sharper focused image.

This is because contrast-detect AF is actually using the image sensor and evaluating contrast to achieve focus. When contrast is optimal the image is focused and since the sampling is being done by the same image sensor that's about to take your photo, it's in perfect focus FOR THE IMAGE SENSOR.

Phase-Detect AF, on the other hand, has separate focus sensors on the floor of the camera. The sensors are all supposed to be positioned so that light has to travel the same distance to bounce off the mirror and reflect down into the image sensors as it would have had to travel if the mirror had been up and the light traveled straight through to the back of the camera and landed on the image sensor. But this means someone at the factory has to perform some adjustments to make sure that's true (there's a possibly of human error during manufacturing.)

Also, since the phase-detect only takes one sample and then orders the focus motors to move the lens, and does NOT re-evaluate focus after the lens moves, there's a possibility that manufacturing tolerances in each lens can be unique and it might nail the focus perfectly for some lenses... but be just a tiny bit soft for other lenses. Higher end cameras usually have an "auto-focus micro-adjust" feature that lets you dial in some correction. You evaluate how perfect the focus is for any given lens and if you think it's slightly missing focus, you can tell the lens to focus just a tiny amount closer or farther.

Thank you very much for your thorough explanation. Very helpful.

 

[duehew]

For live view to work part of the mechanism reflecting light to the viewfinder must move out of the way. When you press the shutter release it has to move back, then take the shot and then move back out of the way. Hence more time needed to take the shot. Personally I only use Live View for video and only because that's the only way it will work.

Thank you for your reply. Very helpful.

 

[wfooshee]

Live view is not for general photography. It's for video, or for critical framing and focusing while on a tripod. As TCampbell said, in more words than this, all of the camera's useful systems are disabled while in Live View because the mirror is up and those sensors are blind.