DOF scale question

ryan7783 · Apr 14, 2008

I'm reading "Understanding Exposure" by Bryan Peterson and I've gotten to the part of the book that explains DOF scale. The part I'm a little confused about is the part that reads

In regards to Story Telling Composition

"But with the proliferation of high quality zoom lenses, most photographers have abandoned single focus length lenses in favor of zoom lenses. The trade off, of course, is that we are then running around with lenses that don't have depth of field scale. But we do have distance settings. The distance settings are similar to the depth of field scale in that they allow you to preset the DOF before you take your shot. And, since every story telling composition relies on the max depth of field, you would first choose to set your aperture to f/22 and then align the distance-setting mark on the lens. Your focal length will determine which distance you use."

I guess my question is - Am I supposed to know how far away something is from me before I set my DOF? or can I manipulate the way a picture looks by giving it a shallower DOF than I actually need? .... I'm so confused

Oh yeah also he says that when he sets his aperture at f/22 and "presets his focus via the DOF scale", he views the image as fuzzy until he depresses the shutter release at which point, the lens would stop down to f/22, transforming the fuzziness to sharpness - I'm confused about this as well. Why is the image fuzzy? If he presets his focus, shouldn't the image be sharp already? I hate it when I don't understand things.

Helen B · Apr 14, 2008

It sounds like a hugely oversimplified explanation, but I think that he is referring to the use of the hyperfocal distance. Doesn't he explain it in more depth? Anyway, Google 'hyperfocal distance', or look it up in the book's index. It's the distance setting that gives the greatest depth of field for a given focal length, aperture and format (in simple terms).

When you use the hyperfocal distance, much of the viewfinder image will be blurred, because the lens is likely to be wide open - ie it isn't at the working aperture.

This is a subject that deserves a lot more explanation than I've given here. Hopefully there is more in the book, but if not there is plenty on the web. Have an initial browse around, then ask about anything you don't understand.

Best,
Helen

christopher walrath · Apr 14, 2008

ryan7783 typed . . .

Am I supposed to know how far away something is from me before I set my DOF? or can I manipulate the way a picture looks by giving it a shallower DOF than I actually need?

Why is the image fuzzy? If he presets his focus, shouldn't the image be sharp already?

Hi Ryan. You are asking some good questions so that shows that you are absorbing the material and know what questions to ask. Also you are not afraid to ask them. So, don't worry. I was there once as well, you know. We learn and move on. We'll go through this in the order you asked.

It is not necessary to know how far away an object is from the film plane/sensor plane unless you are closer to the object as this can help you to focus better. And can give you more information as to how you would like to manipulate DOF as well.

You can give a shallower depth of field, very useful in macro and close up photography where isolation from the background is the desired result. The closer you are focusing, the shallower the depth of field. The further you focus, the great the depth of field.

Most cameras have a depth of field preview function/button. The reason this is necessary is that TTL viewing allows you to view the subject as the light comes through the lens. Stop the aperture down and you lose a lot of light due to the smallness of the aperture. So most cameras keep the lens opened up, even though set otherwise. This allows for brightest viewing. Activating the DOF preview, releases a latch in the front of the camera body where the lens mounts, allowing the aperture to stop down to the preset aperture setting, allowing the photographer to view what will be in focus, though with reduced incoming light. When the shutter is fired, the lens aperture catch is again released to allow proper exposure to occur.

Hope this helps.

ryan7783 · Apr 15, 2008

christopher walrath said:
ryan7783 typed . . .

Am I supposed to know how far away something is from me before I set my DOF? or can I manipulate the way a picture looks by giving it a shallower DOF than I actually need?

Why is the image fuzzy? If he presets his focus, shouldn't the image be sharp already?

Hi Ryan. You are asking some good questions so that shows that you are absorbing the material and know what questions to ask. Also you are not afraid to ask them. So, don't worry. I was there once as well, you know. We learn and move on. We'll go through this in the order you asked.

It is not necessary to know how far away an object is from the film plane/sensor plane unless you are closer to the object as this can help you to focus better. And can give you more information as to how you would like to manipulate DOF as well.

You can give a shallower depth of field, very useful in macro and close up photography where isolation from the background is the desired result. The closer you are focusing, the shallower the depth of field. The further you focus, the great the depth of field.

So if I, for instance, have my cat sitting 2 feet from me, and I set my focal distance for 5 feet then not only will my cat be in focus, but everything behind him for 5 feet should be in focus too?

Most cameras have a depth of field preview function/button. The reason this is necessary is that TTL viewing allows you to view the subject as the light comes through the lens. Stop the aperture down and you lose a lot of light due to the smallness of the aperture. So most cameras keep the lens opened up, even though set otherwise. This allows for brightest viewing. Activating the DOF preview, releases a latch in the front of the camera body where the lens mounts, allowing the aperture to stop down to the preset aperture setting, allowing the photographer to view what will be in focus, though with reduced incoming light. When the shutter is fired, the lens aperture catch is again released to allow proper exposure to occur.

I see now, so the camera recognizes that the picture is blurry so when you hit the preview button, it shows you the picture as it will appear on digital media (clear)... even though the picture it's showing you isn't as bright as the picture that will appear after you release the shutter. Am I right?

ksm · Apr 15, 2008

Ryan ,
You are almost there. That is a great book you are reading but I think you need to re-read it, photo captions/notes and all. Try undersatnding the beginning of each chapter especially on aperture and shutter speed.

Anyway I'll try to explain as well as I can and try not to create more questions.

Aperture: Is a fraction (i.e 1/3, 1/11, 1/22) refering to the actual openning size of the shutter. Because they are fractions of the number 1, for simplicity manufactureres omit the "1" and end up using the denominator only (i.e. f3, f11, f22). That is why f22 is actually smaller openning than f3 ( because 1/22 is smaller than 1/3).

Ok now depth of field. Dof is depended on various things some of them being the distance of the lens to the film plane or digital sensor which is your focal length (i.e on a zoom lens for the same aperture setting the dof will be different throughout the focal length of the lens 14mm to 400mm).

Another factor is aperture, the opening of the shutter. The smaller the aperture (bigger number i.e f22) the most dof (sharpest image throughout the photograph) you will have. The bigger the aperture (smallest number i.e f1.8) the shallower the dof (sharpest image in the center while losing focus towards the corners of the photo.

Another factor is the distance of the subject you ar photographing and the difference in distance of the actual subject and its background. For a given focal length and aperture how close you are to the subject will affect the dof. Also if you, for example, are taking a close up picture of a person's face against a wall where the subject and background are at the same distance you might as well just use f/8 because pretty much everything is going to come out sharp no matter what aperture you use because everything is the same distance from the camera.

Now focal length, aperture, and distance of subject all interact for what we call d.o.f. Any change of one of those will effect the final result.

Now where to focus. That all depends on the hyperfocal distance of the lens for a given aperture. I'm not going to get in to that but I'll give you a rule of thumb and as you take more photographs you will get more comfortable.

Large DOF. Small Aperture and focus 2/3 of the distance to the main subject and everything should turn out in focus.
Shallow DOF. Large Aperture. Get closer to the subject and focus ON THE subject. As long as there is a background around the subject at a different distance it should coume out blurry. (Do you understand why focusing behind your cat in your post wouldn't work?)

Lastly I think your question was about the viewfinder. When you step down the aperture i.e f22 the opening of the shutter becomes really small. As a result less light gets through and if you would try to look through the viewfinder everything would be a lot darker. For that reason manufacturers have the viewfinder looking through a wide oprn lens all the time. So eventhough you have f22 setup you won't see the effect until you take the picture. If you have a DOF preview button you can press that while looking through the viewfinder and itwill actually stop down the lens and you get to see how dark it gets and of course the effect of the apertureon sharpness.

Man, hope I explained this ok for you. I would hate to have made everything a bigger headache for you

One Sister · Apr 15, 2008

Good discussion. Thanks for your comments, ksm. There is also a very good explanation, complete with charts on http://www.dofmaster.com/index.html. Check out the hyperfocal distance chart and DOF calculator, very instructional.

ksm · Apr 15, 2008

Just hope I didn't get to confusing. Sometimes I get ahead of myself when trying to explain something.

ryan7783 · Apr 15, 2008

ksm said:
"...."

Thanks for your comments. I think, as I read more into this book, that I am beginning to understand this stuff. I'm going to go talk with one of my mothers friends (a professional photographer) on Saturday about this stuff in more depth and ask her to physically show me what all this means. I should have it by this weekend. Thanks for all the comments and advice, everyone