Depth of Field explained

I wrote a little article about explaining depth of field and going into some of the finer points on what you could do to increase or decrease depth of field in your images. Though you might benefit from it.

http://www.aperturetou​rs.com …/depth-of-field-explained

First up your definition of Hyperfocal distance is completely incorrect. To quote your article: Hyperfocal: The closest distance where everything from this point until infinity will be in focus. The hyperfocal distance is actually the distance that for that focal length and aperture combination will give the maximum depth of field. This is also the one situation where you get the classic 1/3 - 2/3 DoF split around the focal distance. So for the Hyperfocal distance your 2/3 DoF behind the focal distance will take you to "infinity" and your 1/3 in front is also pretty simple to figure out. So you actually have a DoF from 1/3×HFD to Infinity. In the section that you talk about HFD you mention the 1/3 2/3 split at HFD, then proceed to quote figures in your example at half HFD. If your HFD is 30m then your DoF will be 20m to infinity, not 15m. For a 6m HFD you will have 4m to infinity, not 3m.

Your definition of DoF is also poor, since only at the focal distance is the image actually in sharp focus. DoF relies on the perception of the viewer that for only slightly out of focus parts of the final image the viewer perceives the blurry spot as a point, so that it seems sharp.

Alan

I like it. Nice explanations with good examples.

One thing to note is your inconsistency with lens speed. In some sections you refer to a fast lens having a minimum aperture of f/2.8. Fast zooms have a maximum aperture of f/2.8. The discrepancy between fast and smaller numbers will confuse newbies.

You also reference fast lenses (f/1.4, f/2) as giving 'remarkable DOF'. I think you should specify 'remarkably thin' DOF. I can get remarkable DOF out of my 12mm f/2.8 FE lens at f/5.6. Remarkably DEEP DOF, that is. You do a really good job with the definitions section. Remarkable, by itself, does not convey your intended meaning.

First a point of correction on terms:

• That which is 'in focus' falls within the DOF zone
  
• That which is 'out of focus' is 'out of focus blur' (or 'background blur') it is absolutely NOT 'bokeh' which common misuse spreads bad information.
  

I agree with Wilt, totally, on the FL/DOF equation. I can get razor thin DOF out of my 12mm f/2.8 FE @ f/2.8 and 8" focus. I can get DOF a 1/2-mile deep with my 100-400L focused at 1000 yards. Distance matters. Aperture matters.

Of course, focusing my FE at 1000 yards or my 100-400L at MFD (it won't do 8") would yield very different results.

Wilt wrote in post #18009340
If you use various FL at proportionally changing focus distances, so as to keep the subject the same size in the frame, the DOF zone for a WA lens is the same depth as the DOF zone for a telephoto lens!

Qualifier, Wilt: at the same aperture.

It really surprises me that most folks who take photos don't realize this. It is way to easy to prove to yourself. This will get as many arguments as the old filter/hood debate.

Bassat wrote in post #18009349
Qualifier, Wilt: at the same aperture.

It really surprises me that most folks who take photos don't realize this. It is way to easy to prove to yourself. This will get as many arguments as the old filter/hood debate.

Yes, that's why my examples did state the aperture for which I had compiled the DOF values.

While admittedly FL and aperture enter into the DOF considerations when we want to increase/decrease DOF zone depth, the classic equations fail to indicate explicitly how the subject magnification -- the percentage of occupation of the frame height -- has a very specific influence on DOF zone depth when comparing different FL to one another, as my table proves.

Thanks guys for your input of the article. Listening to some of the things said here I have changed some small points to clarify and make sure that we're all on the same page. Espically the part about distance compression. Hopefully it should be better now.

Alexander J.E. Bradley wrote in post #18009817
Thanks guys for your input of the article. Listening to some of the things said here I have changed some small points to clarify and make sure that we're all on the same page. Espically the part about distance compression. Hopefully it should be better now.

Alexander, you got yourself into trouble when you stated in the revised text,

"Focal length and Distance Compression
Even though your depth of field in actual terms will be relatively the same at 24mm as 80mm and again as 200mm, the image will appear to be more out of focus when zoomed in. The reason for this is caused by ‘distance compression’; as we walk back and zoom in, keeping our subject the same size within the frame, the background has been brought closer.
Look at the example below; the chair has relatively the same amount of blur in each photo, but as we compress the background and make it closer to the image, it gives the impression of being more out of focus, by amplifying the out of focus area in comparison to our subject.
If, by instance, your subject and background were both in focus in the 24mm image, they would still be in focus in the 200mm image as well. Only when it is out of focus can you amplify the effect."

Some critiques

1. Unfortunately, 'compression' is a term usually associated with 'telephoto compression' in which distant objects seem to be brought closer to the subject, because their apparent size is magnified (by camera position, relative to the distant object vs. your subject). You risk confusing the beginner with using that same term relative to DOF discussion, when it is not applicable and normally not part of DOF discussions.
   
2. Your statement, "as we walk back and zoom in, keeping our subject the same size within the frame, the background has been brought closer" is indeed true, but not in the context of what is 'in focus' vs. 'out of focus'. Relative size ('brought closer' vs 'moved back farther' is a characteristic of 'perspective', not 'DOF'.
   

3. You state, "the image will appear to be more out of focus when zoomed in". Again, to avoid confusion in use of terms, most folks think of 'zoomed' as pertaining to FL. The image is NOT 'more blurred' due to FL, per se, as the blur chart proves. The DOF is shallower 'when the subject occupies a larger percentage of the frame' at the same f/stop, but the farfield blur magnitude is related to the aperture diameter.

• At the same f/stop, the DOF zone is related to the size of the subject in the frame
  
• At the same aperture diameter (millimeters), the farfield blur magnitude is the same
  

BigAl007 wrote in post #18009268
The hyperfocal distance is actually the distance that for that focal length and aperture combination will give the maximum depth of field. This is also the one situation where you get the classic 1/3 - 2/3 DoF split around the focal distance. So for the Hyperfocal distance your 2/3 DoF behind the focal distance will take you to "infinity" and your 1/3 in front is also pretty simple to figure out. So you actually have a DoF from 1/3×HFD to Infinity. In the section that you talk about HFD you mention the 1/3 2/3 split at HFD, then proceed to quote figures in your example at half HFD. If your HFD is 30m then your DoF will be 20m to infinity, not 15m. For a 6m HFD you will have 4m to infinity, not 3m.

I have a problem with the 1/3:2/3 split of DOF zone, as it is an urban legend, and is only true at ONE specific distance (not at the Hyperfocal distance):

In the case of the 50mm lens at f/4 (and assuming 20/20 visual acuity of the viewer), Hyperfocal distance is 58m, and the closest 'in focus' distance is 29m (1/2 the Hyperfocal), and the far distance 'in focus' is 65616m away, IOW...

• the DOF zone is 65587m deep,
  
• the near distance in focus is 29m in front of the Hyperfocal point or 0.00044% IN FRONT
  
• the far distance is 65529m behind the Hyperfocal point, or 99.9116% BEHIND