Using depth of field preview to focus the lens in landscapes

[charles_sieracki1]

Returning from a recent shoot in North Carolina I noticed that many

of my photos were out of focus somewhat more often than I liked. I

suppose this happens to everyone to a degree, but I decided to try

and find the reason for this. I had several possible explanations:

 

1) my eyesight is failing or needs testing

2) carelessness with tripod or tripod head being loose or wobbly

3) poor quality lenses

4) improper focus distance set on the lens

5) too slow a shutter speed in the wind

6) any combination of factors above

 

I decided to test using my best lens, the Canon L 135mm f2. I took

several shots of a nearby brick column. The column was some 50 feet

away or so, since I did not want to test close up photography.

 

Using my normal approach, that is, using a good tripod, cable

release and mirror lock up, I tried getting some very sharp photos

of this object. Locking down the ball head securely and making sure

the tripod was steady on a calm day, I focused on the column and

shot at f22. After an entire day of non-sharpness, I remembered the

DOF field preview button. Normally I use this to see if the subject

is in enough focus for a particular aperture. What I did that was

new was set the focus when I held the button down. I didn't do this

before since the viewfinder gets dark.

 

When I did this I was surprised to see incredible detail. I then got

a ground glass focusing loupe and looked inside the viewfinder with

it with the button held down. I then could see the detail that I

needed to see to focus properly. I could then make extremely fine

adjustments to the focus and see the portions in focus and out of

focus.

 

I was wondering if anyone else has tried this or used this method

before. I never see any other small format photographers doing this.

I know our large format friends do this on the ground glass of their

view cameras.

[craig_bridge]

You are experiencing a phenomenon called adaptation where the eye sees an in focus image in front of or behind the focus screen. Near sighted people with bright screens designed for autofocus tend to experience this more often.

 

Use whatever technique works for you to over come this: look away and back, DOF preview to darken image, squint, use the other eye, adjust the eyepeice diopter, etc.

[guytal]

Actually for accurate focusing you want the aperture to be wide open. Stopping down will make things appear sharp (on the small viewfinder screen - tiny circles of confusion) which may not actually be in the plane of focus.

<br>

If the loupe helps you see detail - by all means use it, but keep the lens open for accurate focusing. The DOF preview button is helpful in checking what <i>other</i> elements will appear sharp that are NOT in the plane of focus.<br>

<br>

Guy<br>

<a href="http://www.scenicwild.com">Scenic Wild Photography</a>

[andrew robertson]

Also check your diopter setting.

[david_henderson]

You should be able to focus better by looking through the viewfinder without pressing the dof preview. This is because the image is brightest and as it shows the scene effectively at the largest aperture of your lens, depth of field is most limited and you can see more difference between whats going on at your plane of focus and whats happening nearer and further away.

 

So its easier to establish the plane of focus accurately without using dof preview, but then to press dof preview (which simply stops down the lens to the selected aperture) to check whether objects not at the plane of focus are acceptably sharp to you -or indeed acceptably unsharp as they sometimes need to be. You should not need to focus in the dark nor use any external device.

 

If things aren't working out like that for you then as others indicate, its likely that your viewfinder doesn't suit your eyes, and lets hope your camera permits changing dioptres and/or viewfinder screens. You might find it easier to get accurate focussing using a strong split image device. Also , make it a habit to check the distance at which you're setting the focus for reasonableness. This isn't going to help with fine tuning, but if you find yourself setting focus at 20' for a subject that's clearly about 50' away, you'd notice.

[alex_lofquist]

While I would expect no focus shift of your 135mm f/2, the same cannot be said of some older lenses with strong amounts of spherical abberation. There, the lens must be focussed at its working aperture for sharpest imaging.

[charles_sieracki1]

Thanks for all the answers. I've been reading a lot of articles about this issue. There's really no complete consensus. I'm surprised no one mentioned hyperfocal distance focusing. I knew about it and tried to practice it and sometimes it fails miserably. I thought I was just not doing it right. I found an article called 'The Ins and Outs of Focus' by Harold M. Merklinger. He discusses the problems related to focus in this article:

 

http://www.trenholm.org/hmmerk/TIAOOFe.pdf

 

In a nutshell, he argues that you should focus more toward infinity (or at infinity) than hyperfocal distance suggests. This has to do with rather old standards for the circle of confusion standard built into most lenses. It seems to me he is advocating sacrificing some foreground detail for perfect detail in the background. It certainly is easier to remember to focus at infinity and shoot there. I'm going to try his technique.

 

Anyone else read this article and has comments?