Need help with focusing issue.

Started 2 months ago | Questions thread
MOD Jerry-astro Forum Pro • Posts: 14,478
Re: Adequate depth of field for Focus/Recompose

Truman Prevatt wrote:

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.

Umm, whatever, Truman.  Plain and simple if you shoot two people standing side by side at a large aperture, and focus on one person, you can run the risk of having the other one slightly OOF.  And the reason for that is exactly the point you made, which is that you have a curved rather than linear field.  Thus, the distance from the camera of one person may be a touch further than the other in spite of the fact that they're standing side by side.  So, please explain how your explanation differs from that (other than being a lot longer) or what I said earlier?

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