Need help with focusing issue.

Started 2 months ago | Questions thread
Truman Prevatt
Truman Prevatt Veteran Member • Posts: 9,691
Re: Adequate depth of field for Focus/Recompose

Jerry-astro wrote:

ttusteveo wrote:

Jerry-astro wrote:

Nas from the Valley wrote:

charley5 wrote:

Hi Everyone:

I am having a frustrating time with a focusing issue and I need help to solve it. I focus on the face of the person initially by pressing down the shutter half way. Then, because I want the face to be at the top third of the frame, I move the camera (with the shutter still half-way depressed) usually to chest level, and press all the way down. Their chest then is well focused in the final image but their face is not. This is really frustrating. I have lost several good shots in this way. What may be the problem? I use single point focusing and 9 squares. Thanks for any suggestions. Here are two examples:

I couldn't read all of the comments above but of course you will have a problem with focusing. It is obvious!

When you focus (in single focus mode) and hold the shutter button and move the camera you are changing focus plane! Don't expect camera to remember where exactly you focused and lock that focus spot forever!

Let me give you an example: you focus on the face which is exactly let's say 2.37 meters away from the camera and focus plane will stay on the same distance (2.37 meters) as long as you are holding the shutter button. Then you move down the camera a little bit and the face will be on let's say 2.32 meters away from the camera but the chest now will be on 2.37 meters. So the focus plane will be changed and the face will no longer be the sharpest spot since it's not on the focus plane anymore. Simple as that.

The only way to do it the way you want it is with wide tracking, face/eye detection or the best is just use the focus points situated at the top of the sensor where the face will be.

Focal plane changes when you re-focus.

I know drawing is funny but that's what I wanted to show you. The red line is the focal plane.

The point you're making is spot on, however, it's worth noting again that the focal plane is actually curved, not straight as you've illustrated, and equidistant to the camera.

Question...

An online depth-of-field calculator shows that for the first image:
Focal length 39mm, f/8, that the depth of field

I think he had to be at least 6 feet from his subject when he took the photo, but the OP can confirm.

At a distance of 8 feet, the depth of field would have been:
In front of subject: 1.58 ft
Behind subject: 2.61 ft

Wouldn't that have been sufficient for Focus+Recompose to have captured sharp detail, in spite of the slight change in the focal plane?

If you were responding to my comments, they were intended to be more generic as opposed to relating to this specific example. The larger the aperture and thinner the DOF, the more likely you are to run into issues with focus and recompose. Shooting portraits, for example, with the 56/1.2 wide open, can most definitely get one in trouble with focus and recompose as opposed to simply moving the focus point directly on your intended point of focus.

Not really. The image below was taken with a 56 f1.2 no cropping except to knock off the long dimension to make it a 4x5 aspect ratio. The distance from the camera to Dad was about 6 to 7 feet. This was taken at f1.2 at ss 1/60 using a Pro2. The center focus point was used and the focal point was right between his eyes. I then recomposed. The DOF is in fact narrow as the two design patches on the shirt front and back of the focal plane are soft, the Marine tattoo on his right bicep is soft and the "Semper" tattoo on his fore arm is quite soft.

The reason focus recompose worked on this images and really on most most lenses have a curved field of focus instead of a planer field of focus.  I know that there is all sorts of material on the internet about how one should not use focus recompose because of flat focal field.  However, it is strictly a academic exercise since very few lenses have a flat field.  If you look at the MTF of a lens - wide open you can easily tell the field of focus is not flat.  Even the  the MTF spec (on the Fuji website) is not flat.  The 56 f1.2 MTF spec is flatter than the 50 f2 although neither are bad.  Production lenses most likely will have an MTF a little worse in field curvature at a random focal distance than the spec since the spec is defined at one specific focus distance usually where the lens performs best. Any lens that is soft in the corners wide open and most lenses are - the field of focus is not flat.

If I have time I will use the joystick, but when I photography people I prefer to work fast camera to my for less than a second.  If I had held the camera up and messed with the joy stick, by the time I got ready to fire the shutter I would have been looking at Dan looking at me with his tongue sticking or or his middle finger up - or both!    What are friends for?

With people unless it is a formal portrait with the camera on a tripod, I normally focus recompose - even with the 56 wide open. If it is a formal portrait I would probably use the joy stick.

Clearly using the joy stick is one option if the shooting situations allows in some cases it should be the default.  However, one should not be afraid to use focus recompose based on the academic arguments on the Internet which are based on lenses that don't really exist in the wild.  The only true flat plane lens I nave ever owned is my Nikkor enlarging lens from my 4x5 enlarger.  But for a simple enlarging lens it was very expensive - even used.

The key is to know your camera, know your lens so you won't be surprised and you will have skill craft to make good realtime decisions.  That comes with a lot of experimentation which is difficult to come by sitting online.  

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Truman
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