Why recommend a superzoom for deep dof?

Started May 16, 2012 | Discussions thread
Veteran MemberPosts: 3,088
In reply to Great Bustard, May 18, 2012

Great Bustard wrote:

jrtrent wrote:

Great Bustard wrote:

Neither lenses, focal lengths, nor apertures have a DOF

Who's saying they do?

People who say the DOF depends only on the focal length and use the DOF scales on some lenses as "evidence".

Anyone looking at such scales on a lens will see that subject distance and aperture also have a direct affect on the depth of field.

The youtube video simply said that depth of field is determined by aperture, focal length, and distance from subject.

It also depends on display size, viewing distance, and visual acuity.

If you don't want to hold viewing conditions as a constant, then you're saying there are six variables instead of three, but focal length still affects depth of field because it is still one of the six variables.

What's at issue here is Barrie's statement, "Focal length doesn't affect Depth of Field." Well, of course it does, otherwise it wouldn't be part of a formula for calculating depth of field.

I don't know the context of his statement,

See James Cafferty's post above, in which he provides a link to an Adorama video on depth of field.

but let's give it a new one. Consider a person with a 4/3 sensor using a lens at 50mm f/2. The photo will have the same DOF as one taken with a FF sensor at 100mm f/4, if the focal distance is the same, the display diagonal is the same, and the photos are viewed from the same distance and visual acuity.

Which is why the online depth of field calculators also have a place to specify the format you are using. Note that focal length is still one of the variables; it still has an effect on depth of field.

But let's say we stay within a system. If we keep the framing the same, then the same f-ratio will result in the same DOF for the same framing. For example, 50mm f/2 10 ft from the subject and 100mm f/2 20 ft from the subject on the same format will result in the same DOF.

In this case you are holding format and viewing conditions a constant, and the three variables you list are focal length, aperture (also held constant in this example), and subject distance, the same ones listed by James Cafferty above. Focal length still affects depth of field because it is one of the variables, along with subject distance, that changed to maintain the same depth of field.

So, as always, the context for the statement is really essential.

Your second example could just as easily be used to "prove" that subject distance has no effect on depth of field. See, all I have to do is change focal length to keep the framing the same, and you can see that distance to the subject has no effect on depth of field. Both arguments are specious, because both focal length and subject distance continue to be factors used to calculate the depth of field you get when shooting "50mm f/2 10 ft from the subject and 100mm f/2 20 ft from the subject."

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