DOF questoin Locked

Started 7 months ago | Questions thread
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Bill Ferris
Bill Ferris Veteran Member • Posts: 6,603
Re: Yeah, no.

photolando wrote:

Camera Nerd wrote:

Okay, you know how increasing focal length increases depth of field.

No. Neither does decreasing focal length. At least if you fill the the subject exactly with each lens. Meaning backing up or getting closer to match.

"Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length! This would of course require you to either get much closer with a wide angle lens or much farther with a telephoto lens". taken from Cambridgeincolour.com

Some of the disagreement in this thread may be a product of how folks use language.

For example, if we imagine a scenario in which a photographer is using a camera with a 70-200mm zoom to make a portrait. Standing in the same spot with their subject a few steps distant, the photographer may choose a combination of focal length and f-stop of 70mm, f/2.8, and make a photo. The photographer may then zoom in to a 200mm focal length keeping the f/2.8 f-stop and make a second photo. Let's compare:

Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 70mm, f/2.8

Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/2.8

As we compare the above two photos, let's keep in mind the difference between bokeh and depth of field. Bokeh is the quality of the out-of-focus specular highlights. Is there a visible "onion ring" effect within the broken balls - the defocused glints of sunlight reflecting off the Christmas ornaments - or are the bokeh balls evenly illuminated across their disks? Depth of field is the distance in front and to the rear of the subject within which focus is sharp enough that a physical element within that range is perceived as being well-focused.

Looking at the second photo, the photographer may say, "The size (diameter) of the bokeh balls relative to the hula dancer toy grew larger at 200mm, f/2.8. I kept aperture constant at f/2.8 in the two photos. All that changed was an increase of focal length from 70mm to 200mm. Therefore, increasing focal length makes depth of field more shallow."

I want to focus on the use of the term, aperture, in the above. It's commonly used as a reference to f-stop. In a casual setting, as long as there is an implicit agreement to use the term to describe the f-stop used to make a photo, that's all well and good. However, the more formal meaning of the term, aperture, is an opening. In photography, aperture describes the virtual entrance pupil of a lens. It's the diameter of the virtual opening through which light passes while en route to the sensor.

One way of evaluating depth of field is to compare the size of the bokeh balls relative to the size of the subject. If the bokeh balls appear larger in size with respect to the subject, we perceive that as a photo having a shallower depth of field. If the size of the bokeh balls is smaller with respect to the subject, we perceive that as a deeper or greater depth of field.

In the above photos, the bokeh balls are smaller with respect to the hula dancer toy in the 70mm photo than they are with respect to the toy in the 200mm photo. Also, the degree to which the folding tables are blurred is greater in the 200mm photo than the amount of blur they display in the 70mm photo. Visually, we interpret these differences in appearance as being the product of a difference in depth of field. Depth of field in the 200mm photo is shallower than depth of field in the 70mm photo.

Our hypothetical photographer attributes this difference in appearance to the change in focal length. After all, the camera hasn't moved. The subject hasn't moved. Aperture is f/2.8 in both photos. Only focal length changed. Therefore, that must be the central factor affecting depth of field. Unfortunately, that understanding is incorrect and it's the casual use of the term, aperture, that produces the misunderstanding.

The f-stop of a lens describes a focal ratio. This is the ratio of lens focal length to the virtual entrance pupil diameter. For example, a 70mm, f/2.8 lens has a virtual entrance pupil diameter of (70/2.8=25) 25mm. A 200mm, f/2.8 lens has a virtual entrance pupil diameter of (200/2.8=71.4) about 71mm.

It is this increase in aperture diameter that produces the perception of a shallower depth of field in the 200mm photo. To test this, let's take a look at this photo:

Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/8.0

This photo was made with the same camera and lens as the first two photos. The camera & lens are mounted to a tripod in a fixed position. The tray tables, hula dancer toy, and ornaments are all in the same physical location with respect to each other in the three photos. While the f-stop of the lens in this third photo is different (f/8.0), the virtual entrance pupil diameter (200/8=25) of 25mm is the same as was used to make the first photo. The same aperture diameter at the same distance from the subject produces a perception that the two photos - despite the very different angles of view - have similar or same depths of field.

Because depth of field is a perceived quality of a photo, we need to allow room for folks to have different perceptions of depth of field in photos presenting subjects at very different image scales. That acknowledged if we asked a random group of folks to compare the depths of field in these three photos. most folks would describe the 2nd photo as having a very different, much shallower depth of field than the other two. Most people would also describe the first and third photos as having the same or similar depths of field.

The primary factor producing these perceptions is not focal length. It's aperture diameter - the size of the lens entrance pupil - that produces the change in perceived depth of field.

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Bill Ferris Photography
Flagstaff, AZ
http://www.billferris.photoshelter.com

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