Hyperfocal Distance and DOF some expert advice needed

[HDNitehawk]

I wanted to find some information about DOF for my telephoto lens and that search took me to http://www.dofmaster.com/dofjs.html. I spent several several hours making myself excel charts that showed various DOF at given distances with % of distance in front of and after the focus point. This was good stuff.


Then I started looking in to my landscape lens. Landscape as a prioritesfor me comes after Wildlife and Macro. But...I did get some good landscape pictures this year and thought Ishould learn. This raised quit a few questions:


1, I have the 24mm 1.4L II and the 35mm 1.4L. I took the tripod out did some measuring and test shots what I found is that first the ft/m marks (I did properly line the F stop) did not really match up with what the real focus point would be. Also at the hyperfocal length given with the above chart (for instance at F8 for the 35mm is 16.9') much of the background landscape would be out of focus. When I moved say 4' farther back (about 21') it would come in to focus andat this distance itappeared to be the proper distance. I found the same to be true for the 24mm as well. So myquestion would be, are the charicteristics of these L lens such that the DOF charts and hyperfocal lengths do not apply or have to be adjusted?


2, The DOF chart gives near limit and far limit. Is the actual shape of the DOF of field more of a biconvex shape rather than a flat shape from one side of the frame to the other? If it is this shape is there any way to determine how thin it would be on the side of a ff camera compared to the center?

[HDNitehawk]

I wanted to find some information about DOF for my telephoto lens and that search took me to http://www.dofmaster.com/dofjs.html. I spent several several hours making myself excel charts that showed various DOF at given distances with % of distance in front of and after the focus point. This was good stuff.


Then I started looking in to my landscape lens. Landscape as a prioritesfor me comes after Wildlife and Macro. But...I did get some good landscape pictures this year and thought Ishould learn. This raised quit a few questions:


1, I have the 24mm 1.4L II and the 35mm 1.4L. I took the tripod out did some measuring and test shots what I found is that first the ft/m marks (I did properly line the F stop) did not really match up with what the real focus point would be. Also at the hyperfocal length given with the above chart (for instance at F8 for the 35mm is 16.9') much of the background landscape would be out of focus. When I moved say 4' farther back (about 21') it would come in to focus andat this distance itappeared to be the proper distance. I found the same to be true for the 24mm as well. So myquestion would be, are the charicteristics of these L lens such that the DOF charts and hyperfocal lengths do not apply or have to be adjusted?


2, The DOF chart gives near limit and far limit. Is the actual shape of the DOF of field more of a biconvex shape rather than a flat shape from one side of the frame to the other? If it is this shape is there any way to determine how thin it would be on the side of a ff camera compared to the center?

[Daniel Browning]

the ft/m marks (I did properly line the F stop) did not really match up with what the real focus point would be.

How did you measure the distance? If you measured it from the end of the lens, that may be the cause. The actual distance is from the "witness mark", which is the little circle with a line through it on the camera body that indicates where the sensor is. Another factor to consider is that temperature can change the focus distance (which is one reason why they allow the lenses to focus past infinity).


That said, I wouldn't be surprised if the focus distance marks were inaccurate; I doubt that very many people use them.

Also at the hyperfocal length given with the above chart (for instance at F8 for the 35mm is 16.9') much of the background landscape would be out of focus.

That's because the above web site assumes an 8x10 print size. If you blow the image up larger than that (such as by looking at 100% crops), the DOF will be thinner than what the web site says. If you print smaller (say, wallet size), the DOF will be deeper than it says.

So myquestion would be, are the charicteristics of these L lens such that the DOF charts and hyperfocal lengths do not apply or have to be adjusted?

Although individual lenses can have different DOF characteristics (e.g. overcorrected spherical aberration gives more rear DOF), I think what's going on in this case is that you are assuming a different reproduction magnification than the DOF calculator.

2, The DOF chart gives near limit and far limit. Is the actual shape of the DOF of field more of a biconvex shape rather than a flat shape from one side of the frame to the other?

It can be -- if the lens has an aberration called curvature of field. In Canon lenses that tends to be pretty minor, though.



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If it is this shape is there any way to determine how thin it would be on the side of a ff camera compared to the center?

I don't think the thinness really changes, just the part of the scene in object spaces that it covers.

[Daniel Browning]

That said, I wouldn't be surprised if the focus distance marks were inaccurate; I doubt that very many people use them.

I just test the focus distance scale on my 24mm f/1.4 II and it was accurate. Keep in mind that the little line is thick on purpose -- it doesn't mean that the focus is exactly within the center of the line -- it means it can appear anywhere within the width of that line.

[HDNitehawk]

That's because the above web site assumes an 8x10 print size. If you blow the image up larger than that (such as by looking at 100% crops), the DOF will be thinner than what the web site says. If you print smaller (say, wallet size), the DOF will be deeper than it says.

Thanks for responding


I measured to the camera body, then progressivly moved backfrom the given distance taking sample shots.It wasseveral feetfurther away than thegiven hyperfocaldistance when the picture did come in to focus.


At the given distance it was out of focus on a 8 x 10 size, I did take this in to account when looking at the samples on my 24" monitor.


To curvature of field, I have noticed this with Macro shots having oof areas to the edges on full frame. Some attributed to thesubject not being flat but I wondered how much could be attributed to a curvature of the focal plane.

[HDNitehawk]

I just test the focus distance scale on my 24mm f/1.4 II and it was accurate. Keep in mind that the little line is thick on purpose -- it doesn't mean that the focus is exactly within the center of the line -- it means it can appear anywhere within the width of that line.

I will try the 24mm again some time this week. Most of my time with the 24mm I washard measuring not trying to use the scale. The 35mm I did both the scale and hard measure. But what I did notice on the 35mm lens is that once focused the scale showed I was closer than we actualy were, and for what it is worth in Adobe Bridge in the data it showed that the focus distance was closer than I measured.


The use I see for knowing what the hyperfocal distance is for a given lens, is if you want to take a picture of individuals or subjects close to you in focus and capture the background in a reasonable focus as well. For general landscapes you would try and just catch as much of the area in focus as you can. Am I missing somthing? Is there much more that can be done by using the hyperfocal distance?

[Daniel Browning]

Am I missing somthing? Is there much more that can be done by using the hyperfocal distance?

For me, liveview completely obsoletes all the hyperfocal distance stuff. All I need to do is zoom in 5X (for normal print size) or 10X (for large print size) and rack focus forward until the background is almost out of focus. Then I know for *certain* that I'm at the exact perfect hyperfocal distance. Of course, it's best if you have a tripod. []