tracey medcalfe photography | You're Smarter Than Your Camera - Focus - More Advanced Topics

You're Smarter Than Your Camera - Focus - More Advanced Topics

April 24, 2015  •  Leave a Comment
You're Smarter Than Your Camera - Focus - More Advanced Topics
Good morning and welcome to the 9th blog on 'You're Smarter Than Your Camera'.  This week we finish with the focus topic and take a look at a few of the more complicated aspects of focus including the detail regarding how modern camera's focus, back button focus - a really cool technique that takes a little time to master, but is well worth it!

Thanks for reading and hope your continuing to enjoy my get in touch in you have any comments or suggestions, or would just like to share your experience so far.

First the Fun Stuff - Back Button Focus - What is it and When can We Use It?

Your camera usually focuses when the shutter release button is pressed half way down and then the image is captured when the photographer presses the button fully. Back button focus changes this by removing the focus from the shutter release button and assigns it to another button, guess what, at the back!

Why bother with this new way?  Here are a few examples of where this method may be preferable to shutter release focus.

1. Shooting portraits.  Normally, as we discussed in a previous blog, you would focus on the subject's eye and then either recompose with the shutter release button half way pressed or take the shot and crop for the composition.  Using the back button focusing method.  The photographer is able to focus the camera for the first shot and then can recompose as many times as they like as long as the distance between the camera and the subject remains the same.

2. Shooting moving subjects.  Ok so a lot of nice camera's have a method of focusing that is continuous called AL-Servo (AF-C on Nikon).  This is good, but still requires that the shutter release button is pressed half way and for each shot.  Enter the back button focus.  Assigning the focus to the back button allows you to continually focus on the subject independently of the shutter release button and then fire off as many shots as your camera and the buffer can deal with.  I was shocked!!!!  The result; a very high percentage of images in focus on a fast moving target.  For one who is is dedicated to the single point focus method. You can bet I'm going to try this during the next soccer game shoot.
For an arty perspective, I took these shots at an interesting staircase using the back button focus and with an aperture of f8.0.  There is a little bit of camera movement as my shutterspeed was only 1/80th of a second and I was turning the camera and shooting a few frames per second (but not on a burst option) and with a bit of creative crops and saturation, they have made some quite interesting shots which you probably wouldn't get using a deliberate focus and compose method.  Naturally, this approach leaves a little to chance, but using it with portraits, you can really get some interesting angles that you wouldn't get if you weren't using the back button focus.
The image below of my camera shows my menu (nikon D7000) that assigns the auto-focus to the back button (AE-L, AF-L) when this is selected as shown in the image.  The focus is removed from the shutter release button and is given to this back button allowing the shutter release button to be just that.
So How Do Modern Cameras Focus..and Why Should We Care?

I couldn't resist going into this a little.  If you are like me, you will feel that a deeper understanding of your tool will help to maximise your ability to use it - right or wrong I don't know, but still I can't resist :-)  Also, at this point I will apologise...most of this section is related to DSLRs and I haven't attempted to cover compact or mirrorless methods of autofocus in this much detail, so there it is...feel free to skip it if it doesn't apply to your camera.

So first of all, there are two types of autofocus - active and passive and they are outlined below - 

1. Active works by shooting a red beam onto the subject uses the light that is reflected back to calculate the distance the camera is from the subject. It then, instructs the lens based on this detail.  It is good for low light situations, but can only be used for stationary subjects and also for close subjects - not more than 15-20 feet.

2. Passive works by having additional sensors in the camera that either detect the contrast from the light that enters the lens (phase detection sensor or AF sensor) or can use the camera sensor itself to detect the contract in the image (also known as contrast detection).  This method is very complex and the intricate balance between all the components that work together to make this perform correctly has to be perfectly aligned to get an accurate result.
1. Ray of light
2. Main/reflex mirror
3. Secondary mirror
4. Camera shutter and image sensor
5. Eccentric pin for adjusting main mirror
6. Eccentric pin for adjusting secondary mirror
7. Phase detection sensor (AF sensor)
8. Pentaprism
9. Viewfinder
So what happens just before an image is taken - 

1. Light rays enter the lens (1) and camera.
2. The main mirror (2) reflects most of the light vertically into the pentaprism (8).
3. The pentaprism 'magically' converts the vertical light back into horizontal and reverts it, so that you see exactly what you get when you look through the viewfinder (yay, one of the huge selling points of the DSLR over compact)
4. A small portion of the light goes through the main mirror and gets reflected by the secondary mirror (3).
5. This light from the secondary mirror reachers the AF sensor (7) which redirects it to a group of sensors (2 per AF point in the Nikon D800).
6. The camera processor then analyses and compares the images from the sensors (similar to how focus is achieved on a rangefinder) and if they don't look identical, it instructs the lens to make adjustments.

As the processor method of evaluating focus is through contract, this explains why when we are photographing something white or in low light, the camera has difficulty and can often be continuously refocusing, often not allowing the image to be taken.

I love this diagram and the short explanation which summarises what happens, just before a picture is taken.  The article is written by  NASIM MANSUROV and if you want can read the full version here -
A Little More Fun - Racking the Lens

So, a point to note is that changing the zoom on any lens has no effect on the focus.  Sometimes it is helpful to zoom in, focus and then zoom out and take the shot, there is no need to refocus.  Racking the lens relies on this fact.  Try setting a shutter speed slow enough to allow you to change the zoom o your lens, use a tripod if necessary.  This technique creates quite an interesting effect and can be used both in natural light as well as at nighttime.
Your Challenge

I'm sure you've guessed it :-)  Find the back button focus on your camera and have a go with a completely different technique of may be surprised just how fun it is, or maybe that's just me!

I would love to hear stories of you finding some creative way to use this that I haven't mentioned here.
Next Time

Wow...we are onto another month already - May will bring ISO - the final variable on most cameras for controlling light.  We usually switch to this when you get to feeling...'Help!  Aperture and shutter speed are just not enough, how do I get that shot?

I will also look at other tools available for controlling the light, such as filters.

Until then, thanks for reading and enjoy your photography :-)
As always, I would love to hear any comments or questions you have.
Thanks again,
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