Tuesday, May 17, 2011

Blending Focus

Pete asks why I felt the need to use blended focus, and Helicon Focus specifically, to process the recently posted image (More From Stanley Park).

Several factors come into play.

1) I'm using a full size sensor camera (5Dii) and the larger the sensor the less the depth of field. It's possible that a small point and shoot with it's tiny half inch sensor might have had enough depth of field. Certainly an APS-C size sensor would not in this situation.

2) the reason this image doesn't have enough depth of field is that it is a close up. The area photographed is about 12 inches across, 18 high, the camera two feet away from the furthest object, six inches from the nearest. There is no f stop which will encompass enough depth of field to handle this kind of range, even allowing for blurring the stump in the bottom left. Check out depth of field tables on the net - you will be horrified at how little depth of field there is with subjects a few feet from the camera.

3) my own testing shows me that f16 is the smallest practical f stop for my camera. Beyond that, the sharp bit get fuzzier and there isn't sufficient gain in depth of field to justify this loss (diffraction).

4) even f 11 is sharper than f 16, so when blending, that's the f stop I use.

Why Helicon Focus instead of Photoshop - simple, it does a better job. Helicon has been doing this kind of blending for about 7 years, Photoshop only the last two versions (ie. about two years). Helicon's whole raison d'etre is blending, Photoshop made this one of many add-ons.

Are there limitations? Yes. Two problems occur and both happened here. First, as you change focus, the image size changes. In normal simple design lenses, as you focus further away, you include more (as if you were zooming out or moving away). Oddly, with my 24-105, the exact opposite happens, as I focus further, the image gets larger, ie it crops. So, if I frame perfectly on the near focus, it crops too tight at the far focus. The second and more serious problem is that with wide angle lenses especially, the software has trouble blending the edges of the image and you start to get double or even tripple exposures along the outside of the image. As this only affects about 10% of the image, cropping takes care of it - if you have room to crop 10%. I don't see problems with my 70-200, so in general I'm talking < 70 mm. focal length.

Other than that it works well. My preference is not to do a lot of sharpening on the image before blending - amount 25 in camera raw, considerably less than I'd use on a single image. I do not use any clarity enhancement (increased local contrast) as this tends to be exaggerated in the blending process.


Anonymous said...

Thanks for the detailed reply George.
Pete Arndod

Anonymous said...

George, I am still uncertain about depth of field with APS-C sensor cameras. I have a Nikon D60. I hope you can clarify the issue. If I use a 35mm film lens on my APS-C camera, depth of field at any f-stop will be the same as if I used the same lens on my 35mm film camera. Correct? However, if I use an APS-C lens (DX for Nikon), then I will have a somewhat greater depth of field at any f-stop with this lens. The reason is that the DX lens actually has a shorter focal length, so therefore greater depth of field. A focal length of f11 with my DX lens should give me the equivalent depth of field as f16 with a 35mm lens. Theoretically anyway. The point is, just using an APS-C camera does not automatically provide greater depth of field. You also have to use a lens specifically designed for an APS-C sensor.
In actual fact, it does not work quite so simply. Depth of field is a slippery phenomenon. A sharp lens will appear to have less depth of field than a less sharp lens because everything outside of the exact focus point will appear to be less sharp. Whereas with the less sharp lens there will be less differentiation, so therefore it will appear to have greater depth of field. Regardless, my D60 (with DX lens) at f11 does not have anywhere near the same depth of field as my (less sharp) 35mm film lens on my film camera at f16. All this to say, that depth of field is a very fuzzy subject. What I've learned is that if you want good pictures, focus on your subject and let depth of field fall where it will. This is not to say that depth of field is not good, but often trying to achieve it has spoiled my photos. Hyperfocal distance, bah, humbug! If you want the whole image in focus, than I guess you have to use something like Helicon Focus (or use a small sensor camera).