DOF on 8x10 vs. 4x5?

[LFLarry]

I am trying to better understand the reasons why there is less depth of field (DOF) with 8x10 vs. 4x5 at the same aperture.

I think I understand the basic principles of light and that larger film at the same aperture as a smaller format like 4x5 has less DOF.

Can someone help me better understand the "why" and the science behind this a little better?

Thanks

[Dan Fromm]

Short answer, if the image size on 8x10 is the same size relative to the format's size as on 4x5 it will be twice as big. Magnification will 8x10 will be twice magnification on 4x5. Since DoF is controlled by, and only by, magnification and relative aperture (f/stop) at the same magnification and f/stop 8x10 will have less DoF than 4x5. 4x5 will have less DoF than 24x36. And so on.

[LFLarry]

Makes total sense Dan. Thank you.

Short answer, if the image size on 8x10 is the same size relative to the format's size as on 4x5 it will be twice as big. Magnification will 8x10 will be twice magnification on 4x5. Since DoF is controlled by, and only by, magnification and relative aperture (f/stop) at the same magnification and f/stop 8x10 will have less DoF than 4x5. 4x5 will have less DoF than 24x36. And so on.

[Eric Woodbury]

It's all very confusing. Thing to remember is longer lenses reduce apparent DOF and there is no winning.

Same f/# and same lens gives same DOF. Lens doesn't know how big your image is. However, if you compare "normal" lens for different formats, this will change the circumstances.

DOF approximately equals

2 * (D ^ 2) * N * c /(f^2),

where D is subject distance, N is f/#, c is chosen circle of confusion diameter, and f is focal length. [I know, I'm not fond of the math either.] This shows what changes DOF. If you hold the focal length constant, the f/# constant, the confusion constant, and the distance to subject constant, then DOF doesn't change. If you switch from 4x5 to 8x10 AND twice the lens, then you can see that there is a change. Change just the lens by twice and DOF changes by 4X since focal length (on the bottom) is squared. Chances are you'd change your distance twice, too, and it is squared on top, cancelling the focal change. However, with a longer lens, there is "compression", that means a given distance looks shorter, but measured it's the same.

(Usually with 8x10, it is assumed that the enlargement is less, too, and that the circle of confusion need not be so small.)

[Eric Woodbury]

Yeah, what he said.

[William Whitaker]

Format does not affect depth of field.

[Jim Jones]

When the same images captured on various formats are presented at the same size, the only factor that affects DOF is the diameter of the lens aperture. That is the most practical explanation I know. It also leads to a simple way of determining the hyperfocal distance, which is typically 2000 times the aperture diameter as seen through the front of the lens, regardless of the focal length or the camera format. However, that figure of 2000 can vary widely, depending on the degree to which the image is enlarged and the preferences of the photographer.

[LFLarry]

Eric, this is very helpful and interesting. Thank you for sharing this.

This all opens up another question that I have recently started thinking about.

Long story short, because of health reasons, I am not able to get out and do landscapes with my 4x5 right now so I have been looking at options to keep me photographing. So I decided to explore doing still life in a little make-shift studio in a spare bedroom. This is a new area for me and I quickly have realized that I need to get my mind around reproduction ratios, exposure compensation based on bellows extension, and depth of field.

I am just starting to think about this, but maybe you already know this off the top of your head.

I was thinking for any given exposure, that I need to follow a process or a checklist. This is just how my mind works.

Do you have any recommendations on an approach? For example, let's say that I am photographing some donuts using my 240mm lens and it turns out that I am using 260mm of bellows extension. And just to be clear, do I measure from front standard to rear standard to determine this or do I have this wrong?

I would like an easy way to document the magnification/reproduction ratio for the exposure and determine how much exposure compensation I need based on my bellows extension.

Based on what I have read in a variety of large format textbooks, most tend to indicate that I probably don't want to use an aperture greater than f/16 or so as a general rule.

Possibly I am forgetting something else?

Hopefully, I am explaining this correctly and it makes sense.

Any insights on doing the "math" correctly would be appreciated.

-Larry

It's all very confusing. Thing to remember is longer lenses reduce apparent DOF and there is no winning.

Same f/# and same lens gives same DOF. Lens doesn't know how big your image is. However, if you compare "normal" lens for different formats, this will change the circumstances.

DOF approximately equals

2 * (D ^ 2) * N * c /(f^2),

where D is subject distance, N is f/#, c is chosen circle of confusion diameter, and f is focal length. [I know, I'm not fond of the math either.] This shows what changes DOF. If you hold the focal length constant, the f/# constant, the confusion constant, and the distance to subject constant, then DOF doesn't change. If you switch from 4x5 to 8x10 AND twice the lens, then you can see that there is a change. Change just the lens by twice and DOF changes by 4X since focal length (on the bottom) is squared. Chances are you'd change your distance twice, too, and it is squared on top, cancelling the focal change. However, with a longer lens, there is "compression", that means a given distance looks shorter, but measured it's the same.

(Usually with 8x10, it is assumed that the enlargement is less, too, and that the circle of confusion need not be so small.)

[Jody_S]

It's pretty easy to understand that if you take an image with a 240mm lens at f22 on your 4x5, then mount the lens on your 8x10 and re-take the image still at f22, the resulting images will have the same depth of field.


If, on the other hand, you use a 'normal' 135mm lens at f22 on your 4x5, and compare the image to one taken with a 'normal' 300mm lens at f22 on your 8x10, the depth of field will differ.

[LFLarry]

Very interesting Jim. I was reading in the Simmons "Using the View Camera" book today, and I think he mentions a value of 1500 vs. 2000, but I could be remembering that wrong? Do you think hyperfocal is useful or relevant when doing product/studio work? I am just starting to explore photographing small objects in my little makeshift studio which range from less than 1:1 up to 1:1 and I have been thinking more about depth of field and how to best manage it.

When the same images captured on various formats are presented at the same size, the only factor that affects DOF is the diameter of the lens aperture. That is the most practical explanation I know. It also leads to a simple way of determining the hyperfocal distance, which is typically 2000 times the aperture diameter as seen through the front of the lens, regardless of the focal length or the camera format. However, that figure of 2000 can vary widely, depending on the degree to which the image is enlarged and the preferences of the photographer.