Depth of Field (Bokeh) lesson

[minicoop1985]

I took this shot today as a bit of a lesson for anyone interested. This is called depth of field, the field being the in focus area. The shallower the depth of field, the smaller the area in focus is. What controls the size of that field is your aperture. The lower the number for your aperture, the larger the diameter, so basically the opening of a 1.4 lens at 1.4 is much wider than one shot at 5.6. Now here's the fun part. MATH! The longer MM a lens is, for a given distance at same aperture, the DOF for the longer lens will be thinner. Also, sensor size plays into this as well-the larger the sensor, the longer the lenses needed to effectively cover the sensor area, so the larger the sensor, effectively, the thinner the DOF. This was shot on a Hasselblad H3D 39 medium format (aka a sensor 2X+ the size of the one in a full frame Canon or Nikon) with a 150mm f3.2 lens, shot wide open. No, I didn't blur it in post.

Fun with DOF! by Michael Long, on Flickr

 

[Ysarex85 79]

5.2mm lens (only 3.5% of 150mm) and shot on a sensor less than half the size of a postage stamp. I did leave the lens wide open at f/1.4 (cause I could).

Joe

 

[KmH]

While depth-of-field (DoF) is adjustable, bokeh is fixed and not adjustable.

The aesthetic quality of the blurred foreground/background, bokeh, varies with each lens make/model.
Lens construction, the quality of the glass elements used in the lens, the lens element coatings, the number of aperture blades and the shape of the aperture blades all influence the visual aesthetic quality of the blurred regions created by controlling the DoF.

 

[minicoop1985]

Yeah, I meant bokeh separately from DOF. But that is a good point to bring up.

 

[Bebulamar]

Didn't see any bokeh in either posted images. Lack of Depth of Field isn't a good thing.

 

[Ysarex85 79]

Didn't see any bokeh in either posted images. Lack of Depth of Field isn't a good thing.

Bokeh is apparent in both images. And both photos exhibit depth of field. How can a photograph lack depth of field?

Joe

 

[Tim Tucker]

I would have to disagree with a number of points here.

The first is that aperture controls dof. It doesn't, it controls exposure. A lot of the nonsense I read is based on the assumption that aperture controls dof, but really all aperture does, and is calibrated for, is the accurate control of light intensity hitting the sensitive recording media.

There are four variables that affect dof; distance to subject, aperture, focal length, and image magnification. Focal length is irrespective of sensor size as the only variable here is image magnification for final viewing. A 150mm lens has the same dof on any sensor before you magnify the image, one of the main problems with larger formats is that you have to work with the inherently shorter dof of the longer "standard" lenses.

Shoot the stars with a 35mm at f1.4 and how much is in focus? Thats right, from about a few hundred yards to billions of miles away. Quite a dof. So is it really aperture or subject distance?

I do not dispute that aperture affects it, but it does not control it, it controls exposure.

Bokeh, or the character if the OOF is not set by the lens design alone but by lens design and aperture. The way a lens corrects, or under-corrects, aberrations and the way it blends the various corrections of many aberrations is what creates the "character" of the bokeh. And the aberrations are directly affected by aperture.

 

[Ysarex85 79]

There are four variables that affect dof; distance to subject, aperture, focal length, and image magnification.

Don't forget the size of the recording media. All else held equal to take the same photo with cameras of different sensor size, there will be a DOF difference due to sensor size.

Joe

 

[Tim Tucker]

There are four variables that affect dof; distance to subject, aperture, focal length, and image magnification.

Don't forget the size of the recording media. All else held equal to take the same photo with cameras of different sensor size, there will be a DOF difference due to sensor size.

Joe

Not really. The difference in dof is due to the magnification of the image to the final viewed size. With all being equal then a 150mm lens will produce the same circles of confusion regardless of the sensor size, it's just how much you magnify it in the finished image, (and how that relates to viewing distance).

 

[Ysarex85 79]

There are four variables that affect dof; distance to subject, aperture, focal length, and image magnification.

Don't forget the size of the recording media. All else held equal to take the same photo with cameras of different sensor size, there will be a DOF difference due to sensor size.

Joe

Not really. The difference in dof is due to the magnification of the image to the final viewed size. With all being equal then a 150mm lens will produce the same circles of confusion regardless of the sensor size, it's just how much you magnify it in the finished image, (and how that relates to viewing distance).

Not really, to take the same photo with two different cameras using different size recording media the camera with the smaller sensor/film will produce deeper DOF all other factors adjusted so as to produce the same photo. No comparison is otherwise appropriate except one in which you use the two cameras to take the same photo.

To take the same photo both cameras must be in the same place. The field of view at least against the long side of the media (assuming possible different aspect ratios) must be the same and the exposure must be the same.

Joe

 

[freddy21]

I use an Android app on the go simply named DoF. Its an excellent calculator to have with you and at hand. Especially if you let the DOF get too shallow like I do with my 50mm f1.8 all the time.

 

[Tim Tucker]

Not really, to take the same photo with two different cameras using different size recording media the camera with the smaller sensor/film will produce deeper DOF all other factors adjusted so as to produce the same photo. No comparison is otherwise appropriate except one in which you use the two cameras to take the same photo.

To take the same photo both cameras must be in the same place. The field of view at least against the long side of the media (assuming possible different aspect ratios) must be the same and the exposure must be the same.

Joe

View attachment 130472

I really do not hold with this "equivalence", or same photo nonsense, (in fact it annoys me ), because it's so meaningless and misleading.

If in your instance you hold exposure (or same f-stop) as constant and to satisfy the condition of the same photo you also have changed the focal length and the amount you magnify the finished image to the final print. The sensor size does not change the way the light passes through the lens, it's only affect on dof is the amount you have to magnify it to produce the final image.

Take for example the following case which is encountered in normal photography:

You take a photo with a camera and a certain lens, you print it 10"x8". You then crop it, (after the brilliant advice you receive off this site), and print again at 10"x8". How does that affect the dof? Are you going to tell me that the case is irrelevant because it does not satisfy the condition of the same photo? Of course not.

The whole system of exposure/fov/dof is very symmetrical and if you hold some things constant then others cancel out. In equivalence where you take the same photo (and same dof) with a different sensor is a very special case where a number of variables must be held constant. In this special case a number of other variables cancel out.

To explain, in order to satisfy the condition of the same photo (with the same dof) then subject distance, fov and aperture diameter in mm must be held constant. In this instance exposure must vary but focal length cancels out of the equation. But we do not take photos like this.

In your case you substitute exposure for aperture diameter in mm but still insist that other things must be held constant such as fov and subject distance and the conclusion is that sensor size affects dof, which it does not directly do. A fuller understanding is that with smaller sensors you use shorter focal length lenses and to maintain exposure you are using a smaller aperture diameter in mm which results in the increased dof, it happens before it reaches the sensor and happens when you use shorter focal length lenses on the same sensor.

 

[Braineack]

 

[Ysarex85 79]

Not really, to take the same photo with two different cameras using different size recording media the camera with the smaller sensor/film will produce deeper DOF all other factors adjusted so as to produce the same photo. No comparison is otherwise appropriate except one in which you use the two cameras to take the same photo.

To take the same photo both cameras must be in the same place. The field of view at least against the long side of the media (assuming possible different aspect ratios) must be the same and the exposure must be the same.

Joe

View attachment 130472

I really do not hold with this "equivalence", or same photo nonsense, (in fact it annoys me ), because it's so meaningless and misleading.

Yeah, it annoys me too. But you can't use that as an excuse.

To explain, in order to satisfy the condition of the same photo (with the same dof) then subject distance, fov and aperture diameter in mm must be held constant. In this instance exposure must vary but focal length cancels out of the equation. But we do not take photos like this.

It's exactly how we take photos. We take photos with our attention on the content of the photo and not the peculiarities of the camera. We go on vacation and take landscapes from where we visited. We photograph the kids and the dog. We photograph our friends and our activities. And when we heap all the photos together there is deeper DOF on average in the photos if our camera has a smaller sensor/film and shallower DOF on average if our camera has a larger sensor/film. No more evidence is needed than all the people with phone cameras who keep asking why they can't get blurry backgrounds in their pictures.

In your case you substitute exposure for aperture diameter in mm but still insist that other things must be held constant such as fov and subject distance and the conclusion is that sensor size affects dof, which it does not directly do.

The value for sensor size is no less direct or less integral than lens focal length.

Sensor size is just as integral a value in that equation as focal length. H (hyper focal distance), f (focal length), N (f/stop), c (circle of confusion). You can't remove any value from the equation and the value for c is directly derived from the size of the sensor (see chart below).

reference: Circle of confusion - Wikipedia

We take photos like this. A vacation landscape:

I show up to take that photo with my phone camera and you bring a FF DSLR. Click and click we take the same photo. Infinity is a limiting factor. Exposure is a limiting factor. The dirt road on the left is a limiting factor (looks bad) and the campground on the right is a limiting factor (ruins the composition). So we take the same photo and at any given f/stop value hyperfocal distance is closer to me than to you, yes because I have a shorter lens focal length, BUT ALSO because the circle of confusion is a smaller value for my camera than yours and that's because my camera has a smaller sensor (see chart above).

Joe

 

[Tim Tucker]

I show up to take that photo with my phone camera and you bring a FF DSLR. Click and click we take the same photo. Infinity is a limiting factor. Exposure is a limiting factor. The dirt road on the left is a limiting factor (looks bad) and the campground on the right is a limiting factor (ruins the composition). So we take the same photo and at any given f/stop value hyperfocal distance is closer to me than to you, yes because I have a shorter lens focal length, BUT ALSO because the circle of confusion is a smaller value for my camera than yours and that's because my camera has a smaller sensor (see chart above).

Joe

Joe, the circle of confusion is not a function of the sensor, it is not caused or defined by the sensor but the lens and primarily the diameter in mm of the effective aperture. Any camera with a smaller sensor uses a shorter focal length lens to take "similar" photos, (let's drop the "same" tag as I see what you mean). To maintain the same exposure you use the same f-stop which as a ratio of the focal length to effective aperture diameter means any similar photo shot on a smaller sensor with the same exposure is always using a narrower effective aperture. This is what affects the circles of confusion and gives the greater dof on smaller sensors.

If we make the assumption that two sensors have the same fov when one is shot with a 50mm lens and the other with a 35mm lens and that to maintain the same exposure (shutter speed and ISO the same) we use f4 on both. Then the image shot through the 50mm lens is shot through an effective aperture with a diameter of 12.5mm, and the image shot through the 35mm lens is shot through one of 8.75mm. With the fov being the same the smaller effective aperture limits the angle of light entering the lens making it more parallel and thus giving the greater dof. Exactly the same thing happens when we put the 35mm lens on the larger sensor, we get greater dof because of a shorter focal length and how it affects effective aperture while maintaining exposure. This effect is not in this case anything to do with sensor size but focal length, the optics and the change in the circles of confusion are the same in both cases. It's only when you specify "same photo with same fov" that you are forced to change sensor size in proportion to focal length and it's this condition alone that brings sensor size into the equation. The relationships that govern dof work just the same when we do not take photos with the same fov.

The chart you show defines circles of confusion used to calculate dof for various format sizes based solely on image magnification (the size defined in your chart is magnified x number of times) to a 10"x8" print to be viewed at a certain distance. It is because you magnify the image made by the smaller sensor more than that of a larger sensor that the circle of confusion must be smaller to create the same dof in the 10"x8" print. The sensor does not create them in any way, though the amount you have to enlarge the image is an integral part of calculating dof. (The decrease in dof through larger magnification of the smaller image is smaller than the increase in dof by the smaller effective aperture diameter).