Thursday, May 26, 2011

Further On The Value Of Projects

Another advantage of doing projects is that we tend to get better with time. Not only was Pepper # 30 Edward Weston's thirtieth attempt at a pepper, he'd also been working with a variety of other vegetables and shells. Although his other vegetable images do get included in shows and books, it is the single wonderful image that many consider one of the greatest photographs ever. So it would appear that even though this was part of a project, it's ok to come out the other end with a single iconic image.

Given a choice between being known for 20 mediocre images vs. one great one, I think most of us would vote for the one great one, in the hope that having made one, it wasn't a fluke and we can do it again, eventually.

Sunday, May 22, 2011

Depth Of Field

Wayne asks for further clarification about depth of field and comments on lenses for APS-C cameras and so on.

Right. Some basic facts.

1) depth of field is proportional to the f stop - an opening half as large doubles the f stop (2 stops) and doubles the depth of field.
2) image magnification is invesely proportional to depth of field - make a print twice as big (in linear dimension) and you have half the depth of field. Start with a smaller sensor and you have to magnify the image more, so less depth of field there too.
3) depth of field is inversely proportional to the SQUARE of the focal length. Double the focal length and you get one quarter the depth of field - ouch.

And believe it or not - that's it, no other factors play a big role in measuring depth of field.

So, in Wayne's example of a 35 mm. lens on a DX sensor Nikon and comparing it to 35 mm. film camera (or a full size sensor camera), you have a sensor 1.5X smaller. The actual focal length is 35 mm., whether designed for a dx camera (APS-C size sensor) or full frame.

Wayne notices that a 35 mm. lens on his film camera seems to have more depth of field than when he has a 35 mm. lens on his Nikon APS-C sensor camera. He's right and here's why.

We have three factors, two of which didn't change - focal length stays 35 mm., and f stop remains, say f 11. All that changes is which camera the lens is on (or which 35 mm. lens you are using, but that's the same thing).

Put the 35 mm. lens on a larger format camera (bigger sensor or bigger film) and you don't have to magnify the image as much to make the same size print - so more depth of field. It has nothing to do with angle of view. It has nothing to do with some magic lens design for DX size cameras. It applies regardless of what 35 mm. lens you use.

Now, if you did want the same angle of view, you'd need to use a wider lens on the small sensor camera, so you would be changing the focal length. Here you would still have the magification issue, but you'd have the square of the change in focal length - essentially 1.5 squared and divided by 1.5 - that is, the lens you'd need for the DX sensor camera, would be (35 divided by 1.5) mm. long - approximately 21 mm. The depth of field would be approximately 1.5 times better with this lens on this camera when compared to the same view, same print size on the full frame camera.

This explains why it is so difficult to get shallow depth of field with a point and shoot camera with a sensor one quarter the diameter of 33 mm. film.

What does twice as much depth of field mean - well a simple way to look at it would be sharpness from infinity to 4 feet instead of infinity to 8 feet, or a range of sharpness of approximately a foot behind and in front of the subject instead of 6 inches.

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.

Friday, May 13, 2011

Stanley Park

just back from a visit to Vancouver, to see our daughter and take a break. Found this partly burned stump on the beach at Stanley Park. Blended focus with Helicon Focus, a little bit of Akvis Enhancer to the centre only and otherwise pretty straight. Canon 5DII.

Monday, May 02, 2011

Travel Tripod

When I was in Victoria last fall, I couldn't find the ideal tripod for travel purposes, and ended up picking up an inexpensive Slik travel tripod, complete with built in ball head and mini-quick release plates. It was adequate and I got some lovely shots thanks to that tripod. The head though was the problem, especially for vertical shots. I normally use an L bracket from Really Right Stuff for my 5D2, so what I've done is order the smallest lever release head from the same company and will place that on this super light tripod. It's aluminum but reasonably sturdy and tall enough for travel purposes.

The big thing with a travel pod is it's nice if it can go on your backpack, which theoretically huge tripods can do, but in practice don't very well. Also, it needs to fit inside luggage when flying, and there are times a tripod that doesn't tie up both hands to carry is very nice.

Some might wonder 'why even bother with a tripod', especially with modern cameras great high ISO Imaging. For me the reasons are:

1) to be able to stop down for max. depth of field
2) shoot under marginal light - often more interesting light
3) to make blended images for incredible depth of field using Helicon Focus
4) blended exposures for HDR without the worry of misalignment of images.

These justify lugging a tripod, and if it can be a small light one without too much compromise, well all the better.

Let you know how the new combo works when I return.