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PaulCowan · Joined: 24 Feb 2005 Posts: 4181 Location: Evolving

Pixel density and size isn't relevant at all for DoF, it matters for diffraction.

http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

Note that the only factors he takes into account for the calculator are the sensor/film size, the focal length and the aperture - and Cambridge in Colour is very authoritative on the technical stuff.

The assumptions for DoF are that it is for a 10x8 print viewed from 1ft away.

mikenorton · Posted: Thu May 10, 2012 1:27 am

cpaulfell,
Before you read this look at my portfolio. http://www.shutterstock.com/g/mikenorton

I wrote this some time ago and I pull it out and rerun it every now and then. So it must be now or then.

The physics behind depth of field is very rigid but once you understand how and why it works the way it does then it becomes a very fluid technique to use. Depth of field is controlled by the f-stop, the distance from the camera to the subject and the part of the universe being photographed. All lenses, wide angle, telephoto and normal have the same depth of field at the same f-stop if they are photographing the same part of the universe. When a photographer changes a lens but does not change the camera position the photographer has changed the part of the universe being photographed. This change in the part of the universe being photographed is responsible for the change in depth of field, not the lens. If a photographer was to change lenses and then recompose the same picture by moving back or forward, he or she would get the same depth of field.

The smaller the part of the universe the smaller the depth of field. Remember when you were in school and looked at a drop of water under a microscope? You could focus on the top of the drop, in the middle of the drop or on the bottom of the drop. The microscope's lens saw such a small part of the universe that its depth of field would not even cover a drop of water. On the other hand if you photograph more of the universe you will have more depth of field. A lens can have sharp focus on a tall building, a 1/4 of a mile away, and also have sharp focus on the moon, a 1/4 of a million miles away, at the same time.

Try this if you think a wide angle lens has more depth of field than a telephoto lens. Put a wide lens on a camera and pick out 3 objects. Compose your picture so that the first object is on the extreme left of the picture and the second object is on the extreme right of the picture, focus on the third object placed half way between the other two, and shoot a picture at f11. Now put a long lens on the camera, don't move the objects and compose the same picture with the same object on the extreme left, the same object on the extreme right, focus on the same middle object and again shoot a picture at f11. Your depth of field will be the same in both pictures because in both pictures you photographed the same part of the universe at the same f-stop. The difference between the two pictures is the distance from the camera to the middle object, you will have to be much farther away from your middle object when using the long lens.

Sometimes I focus on a background object and then I focus on a foreground object then shift the focus somewhere in between the two and check the depth of field by stopping down the lens, that is using the depth of field preview button. Sometimes this leaves my lens focused on the air between the 2 objects so that when I look through the view finder nothing is in focus but everything comes into sharp focus when I look through the view finder and stop the lens down, that is use the depth of field preview.

So to get the entire scene in focus this is what I would do: Put the camera on a tripod. Then I would manually focus on the tree marked by the red X and I would note where the lens was focused. Next I would refocus on the tree marked by the yellow X, and again note where the lens was focused. Then I would move the focus to about 1/3 of the distance between the yellow X and the red X. Next I would choose the f-stop I wanted to use and press the depth of field preview. While this button is being pressed in and held I would look through the viewfinder to see if the flowers in the foreground at the bottom of the frame and the trees all the way in the back were in sharp focus. If they were then I would set the shutter speed for that f-stop and shoot the picture. But if the flowers were not in sharp focus I would move the focus a little closer to the point where the yellow X was sharp and press the depth of field preview again and check the flowers again. I would do this until the flowers in the foreground and the trees in the background were both sharp.

If I could not find a place where everything was sharp at the f-stop I chose (and this might very well happen) I would move the tripod back about 5 feet and start all over again.

That is how I focused on this image to get the Bluebonnets in the foreground and the aspen trees in the background both sharp. Oh and I shot at f45 but the technique will work at any f stop, you might just have to be a few feet further away from the foreground object.

http://image.shutterstock.com/display_pic_with_logo/51753/51753,1232164355,1/stock-photo-wild-flowers-growing-in-an-forest-of-aspen-trees-23407915.jpg

PaulCowan · Joined: 24 Feb 2005 Posts: 4181 Location: Evolving

That's an excellent point about stopping down, I meant to mention that. I've a nasty feeling that the stop-down button on a lot of DSLRs is at risk of seizing up through lack of use (I actually had to use WD40 on an old Leica to get that lever working again!)

Apart from that, you are really saying exactly the same as I did in a different way.

If the camera is in a fixed position, a smaller sensor or longer lens will capture a smaller portion of the universe. If you enlarge that portion to 10x8 and compare the photograph with a 10x8 shot from the same spot with a shorter lens or bigger sensor, then the second photo will have greated DoF than the first.

If you adjust the enlargement sizes so that the objects in both photos appear exactly the same size, then the DoF will be identical, regardless of the lens or sensor.

The reason is that DoF is an illusion caused by the limits of our eyes ability to resolve detail. In any photo, only an infinitesimally thin plane is truly in focus, on either side of that plane it greadually becomes more and more out of focus.

However, as long as the picture is small our eyes can't make out the fact that some of it isn't in focus. The bigger it is enlarged, the more obvious the lack of focus either side of the perfect plane becomes to us. Just the same as an illustration in a book looks sharp but if you enlarge it with a magnifying glass it becomes obvious that it's just a bunch of dots.

To look at it another way, depth of field is entirely a creation of the resolving power of the human eye. Changing the camera set-up simply changes the strength of the illusion. To maximise the illusion, stop down, stand back and magnify everything as little as possible, to minimise it use large magnifications, wide open apertures and close proximity.

Back to the question of the settings for hyperfocal distance. This was shot at f/14 with the focus set at 30ft with a standard lens, which the old camera makers used to recommend as the best combination for maximum DoF on 6x9 cameras (the flat contrast is due to a 70 year old lens). The focus is roughly where the boat in the foreground is but the scaffolding round the minaret in the town a mile away has also been resolved.

Oh, and don't stop down to f/22 or f/32 on a DSLR, because diffraction effects will blur the image a bit. Mike could use f/45 because he was shooting with a 4x5 large format camera which is not affected by diffraction at high f-numbers.

dlovely · Joined: 26 Oct 2009 Posts: 626

Most helpful thread! Thank you all.

ps Mike, I really like your use of the term "universe" in your description.
From now on, I only photograph small and large parts of the universe!

ajancso · Joined: 18 May 2009 Posts: 1891 Location: Right Behind You

cpaulfell · Joined: 07 Dec 2011 Posts: 2435

cpaulfell · Joined: 07 Dec 2011 Posts: 2435

ruxpriencdiam · Posted: Thu May 10, 2012 12:20 pm

Diffraction chart for reference.

semmickphoto · Posted: Thu May 10, 2012 12:36 pm

What does that mean? That above f8.4 my 450D will start having diffraction?

cpaulfell · Joined: 07 Dec 2011 Posts: 2435

Thanks Barry. Very interesting for my sensor according to the table diffraction sets in above f6.8 but, my eyes just don't see it until it gets up to around f18.

ruxpriencdiam · Posted: Thu May 10, 2012 12:38 pm

PaulCowan · Joined: 24 Feb 2005 Posts: 4181 Location: Evolving

Those figures are the theoretical limit. In practice, you probably wouldn't notice until you are maybe a stop beyond what the theory says. I'm generally happy with the results I get from a 5D Mk2 at f/14 - but it's always a good reminder to see these lists and remember when you are pushing the limits.

semmickphoto · Posted: Thu May 10, 2012 12:42 pm

PaulCowan · Joined: 24 Feb 2005 Posts: 4181 Location: Evolving

ruxpriencdiam · Posted: Thu May 10, 2012 12:48 pm