Advice on focusing a third of the way into a scene

[Guest]

On Saturday I'll be revisiting one of our medieval churches to see what the 7D can do.

I recall from a previous post,I was advised to focus a third of the way into a scene to get acceptably sharp DoF at the furthest point in the scene,ie a chapel or chancel etc.

So,do I just lock focus on a point on the floor at 1/3rd and then recompose the shot?

Here's an example of what I mean,taken with the 550D on f6.3,I'll retake this shot,but approx where should I focus on to get everything in the scene acceptably sharp front to back?And does picking a point on the floor generally apply to similar scenes?

Advice gratefully received thanks.
IMG_0024 by Roy Widdowson, on Flickr

[@imagesBV]

I would focus on the closest edge of the yellow mat....of course the wider angle and smaller aperture the better chance you have of front to back sharpness (but I wouldn't go smaller than f16).
I assume you can use the depth of field preview...although it will be fairly dark and hard to see.
Perhaps take five shots with slightly different focus points might be a good tactic - at points between the mat and candle holder/font.
Good luck!

[colin C]

Yes Roy, do exactly that.

You would naturally assume that if you focus half way through a scene, you would get maximum depth of field. However that is not so. Owing to the laws of Physics, optics, inverse square law and no doubt many other things I only half remember, the limits to acceptable sharpness from the focussed point extend one third towards you and two thirds away from you.

If you have lenses with a focus window and focus one third into the scene, you can see each side of the focus line what is likely to be in focus. I have just done this with my 24-70mm F2.8L, focussed on 3m, everything from1m to nearly infinity would have been acceptably sharp @ F16.

In the days of film and prime lenses, there was a scale of Fstops each side of the focussed line and if you focussed the lens with your chosen Fstop against infinity on the right of the focus line, you could see the minimum focus distance on the same Fstop on the left of the focus line.

[Guest]

I would focus on the closest edge of the yellow mat....of course the wider angle and smaller aperture the better chance you have of front to back sharpness (but I wouldn't go smaller than f16).
I assume you can use the depth of field preview...although it will be fairly dark and hard to see.
Perhaps take five shots with slightly different focus points might be a good tactic - at points between the mat and candle holder/font.
Good luck!

The"yellow mat" is actually daylight on the floor,but I will focus in that area,then try 5 shots between that and the font etc.

Thanks for the advice.

[Guest]

Yes Roy, do exactly that.

You would naturally assume that if you focus half way through a scene, you would get maximum depth of field. However that is not so. Owing to the laws of Physics, optics, inverse square law and no doubt many other things I only half remember, the limits to acceptable sharpness from the focussed point extend one third towards you and two thirds away from you.

If you have lenses with a focus window and focus one third into the scene, you can see each side of the focus line what is likely to be in focus. I have just done this with my 24-70mm F2.8L, focussed on 3m, everything from1m to nearly infinity would have been acceptably sharp @ F16.

In the days of film and prime lenses, there was a scale of Fstops each side of the focussed line and if you focussed the lens with your chosen Fstop against infinity on the right of the focus line, you could see the minimum focus distance on the same Fstop on the left of the focus line.

The lenses I'll be using don't have a focus window unfortunately,but it's all good advice thanks.

[Guest]

Just one more question,do I use a single AF point and use multi-zone metering? I'll be taking my flash for some of the darker corners.

[Guest]

I've overlaid five red rectangles on this to show the suggested points I need to lock focus on before recomposing the shot.Comments please?

IMG_0024B by Roy Widdowson, on Flickr

[colin C]

At a rough guess I would go for the second from last, which is the dark band across the floor.

[Guest]

Thanks Colin,using five different points to see which gives the best result was suggested by Brian,so I'll use your idea for the first shot,and then take the next four points.

I have an LR add-on which displays where the AF points landed,so should be easy to see which particular one proves to be the best for the DoF I'm after.

The light tomorrow might not be as good as it was in September when I last visited,so I'll see what I can do if that's the case.

[colin C]

The very best of luck Roy. It is generally my experience that the more thought you put into a shot in advance, the more you consider the variables, the better the end result. I look forward to seeing a "Cracker" of a shot.

[Guest]

Thanks Colin,I fully agree that pre-planning generally gives better results.

Unfortunately several things have combined today and I've had to write off the church visit.However,I will go in the very near future and of course use all the good advice given.

It's within 10 minutes walk from home and open to visitors every Saturday.

In the meantime I've got a daytrip to Fakenham and Cromer on Thursday and plan to visit the church near Fakenham market.