selmslie Loc: Fernandina Beach, FL, USA

Why does DOF change when you change the aperture?

BebuLamar

That is why the term "crop factor" is a bad idea. We have a lot of different formats way before digital photography was invented but we didn't have all that confusion because nobody used the term "crop factor".

profbowman Loc: Harrisonburg, VA, USA

Yes, well said. And DOF has nothing to do with post-processing. Once a photo is taken, the DOF is set regardless of magnification. Put 10 markers in the ground 1.0 m apart and focus on the middle one in a condition so that the closer ones and the further away ones are out of focus and thus define the DOF. Now take that photo and crop it in post-processing. The closer markers and further away ones will still be out of focus even though some of them might no longer be on the final image.

(Using AI to do something to the background and foreground can change the DOF. But I still say AI has nothing to do with photographing what is, but it does have to do with art, which is fine.)

As said many times here (and many times not agreed with here <sigh>), DOF is set by the optics and has nothing to do with the sensor put in place. Put a simple piece of white paper in place of the sensor and the image on the paper will still have the given DOF.

My introductory university/college physics students have done this many times in their laboratory exercises using a single double convex lens and a light source. They can use a full page of white paper, a half sheet, a quarter sheet, or even a piece of tan cardboard. The focusing dynamics (position and DOF) will always be the same. --Richard

Silverrails

If My Understanding of ISO is wrong, what does control or affect the "Brightness" of the "Light that reaches the Sensor after passing thru the #1-the Aperture and then #2-the Shutter?
Would it not be reasonable to say that the "Brightness" of the Light reaching the Sensor is controlled by the final aspect of the "Exposure Triangle", which is #3-the ISO.
That logically, ISO is the ONLY thing left in the 3 ingredients or parts of the Exposure Triangle.
And Yes, I still would apply the word "Sensitivity" with "ISO", the Sensitivity of the ISO would be the final determination of how BRIGHT or DARK your Image will appear.

selmslie Loc: Fernandina Beach, FL, USA

After you trip the shutter you can no longer change the aperture setting (or the shutter speed). However, the DoF is not cast in stone.

Take a look at the DEPTH OF FIELD CALCULATOR from Cambridge in Colour. Click on advanced and you will see what can affect DoF after the fact.



All of these factors affect the DoF:

» Max print dimension (or the size of the displayed image) which is a function of the magnification or zoom factor, keeping in mind that if you magnify enough you will crop the image when you run out of paper or screen.

» Viewing distance - As you move away from or toward the print or on the screen it will look sharper or less sharp.

» Eyesight - If your eyesight is better than normal (or if you put your reading glasses) it will be easier to see where the image is no longer sharp.

» Camera Type - Determines the crop factor for the entire image. However, if you crop the image you are, in effect, changing the camera type and the field of view.

» Selected Aperture - Set when you tripped the shutter.

» Lens Focal Length - You can't change this either. But cropping will change the field of view and the "effective" focal length.

» Focus Distance - The perspective of the image is fixed. You can't change it by zooming or cropping. You can change it only by moving closer or further from the image and taking another picture.

If this is not clear, take a look at their TUTORIALS: DEPTH OF FIELD.

I hope that you and SteveR can learn something from this.

selmslie Loc: Fernandina Beach, FL, USA

The "exposure" triangle affects the data recorded in the raw file.

You can change the brightness of the image without changing the exposure or ISO with some of the JPEG processing features provided with your camera.

More commonly, when processing the raw file on your computer, you can change the brightness of the entire image with the Exposure slider which changes the gain and simulates what might have happened if you had used a different ISO.

There is also a Brightness slider that lets you change the brightness of the mid-tones. It has less effect on the highlights and shadows.

Each tonality range - shadows, mid-tones, highlights - can be changed independently from the other two although there is some overlap.

baron_silverton Loc: Los Angeles, CA

Yes, I see your point. I will be researching the COC issue to understand this issue. From what you are saying the DOF changes when you crop - even in post - which means this is a percieved issue versus an actual issue - obviously the pixels do not change after the shot is taken and as such cropping in (or blowing it up to use old terminology) should not cahnge the actual DOF of the shot - this is fixed into the image that was taken.

As such the perception must change based on what remains in the COC as you change the image size - this is interesting.

In the end, that all said - what stands is that larger sensors display shallower DOF (maybe this is the wrong terminology) maybe we should say display a perceived shallower acceptably 'in focus' area that smaller sensors.

In the case of APS-C it seems that multiplying the aperture by the crop factor gives a reasonable approximation of what that perceived 'acceptable focus' zone will be.

Maybe this quik calculation is not right on, but from what I have tested and seen from others as well, it is pretty close making it a simple bench mark to understand the result of your shot in your head while in the field rather than having to call upon a DOF calculator when out shooting.

In short, this seems to work as a 'rule of thumb' - a guideline if you will.

In the end, if you want a shallower perceived or acceptable focus area then you will get that better with FF over APS-C and even better still with medium format over FF.

Thanks again :)

Ysarex Loc: St. Louis

Nothing. The luminance of the original scene is an external factor and given when taking a photo. Only the shutter and the lens/aperture then affect the amount of light that reaches the sensor.

With a constant scene luminance:

1/250 sec.
f/8
ISO 200

and

1/250 sec.
f/8
ISO 800

Both above setting combinations deliver the same amount of light to the sensor. ISO does not affect how much light reaches the sensor.

Ysarex Loc: St. Louis

We don't measure DOF on film, we never did, or now on the sensor, DOF hasn't changed. DOF is defined, yes, as the perception of acceptable sharpness in the final print. A lot of folks get caught thinking DOF must be fixed at the film/sensor when the photo is taken. Just above Scotty correctly pointed out to pfbowman the error of that thinking and showed the advanced version of the Cambridge in Colour DOF calculator which allows you to alter the standard assumed factors that are part of the equation. Historically, long before any of us were born, the photo/optical scientists who codified the definition of DOF and formulae we use to make the calculations made the right call. DOF is defined only when the print is nailed to the wall and we are standing back an appropriate distance and viewing it.

selmslie Loc: Fernandina Beach, FL, USA

Yes, perception is everything. If Sarah has better eyesight than Felix it will be impossible for them to agree.

Everyone tries to prove that DoF can be described in mathematical terms. But it can only be be calculated approximately.

That's what makes all of these endless discussions about DoF so pointless.

Ysarex Loc: St. Louis

DOF is not and has never been defined at the film/sensor. It is defined in the final print. We're using the industry standard definition of DOF in this the photography industry on this a photography forum. Here in the discipline of photography sensor size, enlargement and cropping are all factors that we incorporate into determining DOF.

You're either attempting to redefine DOF which is not your call or you don't entirely understand it. Below is an example of DOF changing with cropping and enlargement.

Silverrails

Well, I didn't mean to offend anyone, and I will definitely apologize to you, I do have much to learn, and I thought I understood this subject, but I have to listen to the more Experienced Photographers.

Ysarex Loc: St. Louis

You didn't offend me and you don't need to apologize. And I'm happy to help as much as I can. Photo can be difficult not because it's hard to understand but because there's such a huge volume of misinformation out there. I'm sure you've encountered plenty of sources that told you ISO changes the camera/sensor's light sensitivity. I'm sure I could spend the next 20 minutes searching and quickly find a page of Youtubes and blogs and tutorials that say exactly that. No reason for a beginner then to question that. But it's not true. So it's tough trying to learn when you're being handed bad info.

This is a pretty good tutorial from one of the better sites: https://photographylife.com/what-is-iso-in-photography

Make sure to at least scroll down to the end where he lists Common ISO Myths and Misconceptions. He gets it right.

Silverrails

Thank You