Question about using high f-stop

[480sparky]

..........DoF seems to be one of the main technical subjects many amateurs have difficulty understanding.

I think mainly it's due to people wanting to make it easy.

Thinking along the lines of "If I focus my lens at 10 feet, everything from 8 to 15 feet should be in focus."

 

[greybeard]

Dof is controlled by 4 things. 1) f/stop, the smaller opening, the larger the f/ number and deeper the Dof 2) Focal length, the longer the focal length, the shallower the DoF and vissa versa. Focusing distance, the closer you focus, the shallower the DoF for any given F/stop and the last thing that effects DoF is the amount of crop in post processing. The more you crop, the shallower the resulting DoF. Cropping is basically the same as using a longer focal length, image degradation not withstanding.

As for f/stop suitable for portraiture? You have to decide how deep of a DoF you need. I like to keep mine at around f/5.6-f/8 which is the sharpest setting for my 50mm 1.8 that is my go to portrait lens on my crop frame d5100.

When I shoot landscape I set my 18-55 on 18mm set the f/stop to 22 and focus on around 3 feet. Everything in the picture will be in acceptable focus.

 

[480sparky]

.......and the last thing that effects DoF is the amount of crop in post processing. The more you crop, the shallower the resulting DoF. Cropping is basically the same as using a longer focal length, image degradation not withstanding. .........

Explain how that works, please.

 

[analog.universe]

.......and the last thing that effects DoF is the amount of crop in post processing. The more you crop, the shallower the resulting DoF. Cropping is basically the same as using a longer focal length, image degradation not withstanding. .........

Explain how that works, please.

If I understand it correctly, when you crop, you magnify everything, including the CoC. Therefore, fewer CoC's are sufficiently small to appear to be in sharp in focus, shortening the DOF.

 

[480sparky]

If I understand it correctly, when you crop, you magnify everything, including the CoC. Therefore, fewer CoC's are sufficiently small to appear to be in sharp in focus, shortening the DOF.

How does cropping in post magnify anything?

methinks you're confusing DoF in crop sensors v. full-frame sensors and cropping an image in post.

 

[analog.universe]

If I understand it correctly, when you crop, you magnify everything, including the CoC. Therefore, fewer CoC's are sufficiently small to appear to be in sharp in focus, shortening the DOF.

How does cropping in post magnify anything?

methinks you're confusing DoF in crop sensors v. full-frame sensors and cropping an image in post.

It's always relative to the viewing size... When you crop, either by using a smaller sensor, or by discarding pixels in post, each projected CoC takes up a larger percentage of the final view area.

I think the discussion in this thread: http://www.thephotoforum.com/forum/photography-equipment-products/251166-crop-vs-full-frame-dof.html will explain what I'm talking about better than I can quickly reproduce. Helen's comments on CoC specifically are relevant.

 

[480sparky]

It's always relative to the viewing size... When you crop, either by using a smaller sensor, or by discarding pixels in post, each projected CoC takes up a larger percentage of the final view area.

I think the discussion in this thread: http://www.thephotoforum.com/forum/photography-equipment-products/251166-crop-vs-full-frame-dof.html will explain what I'm talking about better than I can quickly reproduce. Helen's comments on CoC specifically are relevant.

OK, so where in my DoF calculations when I'm out shooting in the field do I input my post-processing crop? None of my calculators have that option, so they must be missing this step.

 

[analog.universe]

It's always relative to the viewing size... When you crop, either by using a smaller sensor, or by discarding pixels in post, each projected CoC takes up a larger percentage of the final view area.

I think the discussion in this thread: http://www.thephotoforum.com/forum/photography-equipment-products/251166-crop-vs-full-frame-dof.html will explain what I'm talking about better than I can quickly reproduce. Helen's comments on CoC specifically are relevant.

OK, so where in my DoF calculations do I input my post-processing crop? None of my calculators have that option, so they must be missing this step.

Those calculators do have an input for maximum acceptable CoC, which will vary respective to your crop. You will need a smaller number for the maximum acceptable CoC to retain the same sharpness once the crop is stretched to fill the same size viewing medium. The smaller the maximum acceptable CoC, the shallower the DoF for a given focal length / aperture / subject distance combination.

 

[480sparky]

So this image.......

....has less DoF than this one?

 

[analog.universe]

That particular crop doesn't require the image to be magnified, since the longest side retains the same dimension. For arguments sake, if you were to maintain the aspect ratio, and halve both the length and the width, then yes, you would in fact have less DoF if the crop was viewed at the same physical size as the original.

 

[480sparky]

That particular crop doesn't require the image to be magnified, since the longest side retains the same dimension. For arguments sake, if you were to maintain the aspect ratio, and halve both the length and the width, then yes, you would in fact have less DoF if the crop was viewed at the same physical size as the original.

Why do I need to magnify anything? Perhaps that's the crop I need for the purpose the shot was taken for.

What does aspect ratio got to do with it? Does an 8x10 have a different DoF than a 16x20?

 

[analog.universe]

That particular crop doesn't require the image to be magnified, since the longest side retains the same dimension. For arguments sake, if you were to maintain the aspect ratio, and halve both the length and the width, then yes, you would in fact have less DoF if the crop was viewed at the same physical size as the original.

Why do I need to magnify anything? Perhaps that's the crop I need for the purpose the shot was taken for.

What does aspect ratio got to do with it? Does an 8x10 have a different DoF than a 16x20?

This originated with you asking for an explanation of graybeard's statement: ".......and the last thing that effects DoF is the amount of crop in post processing. The more you crop, the shallower the resulting DoF. Cropping is basically the same as using a longer focal length, image degradation not withstanding. ........."

This whole discussion is made under the assumption that the crop we're talking about gets you the same result as using a longer focal length. The aspect ratio matters because we want consistency in the viewing medium. If you crop an image like the one in your example, drastically changing the aspect ratio but maintaining the dimension of the long side, that doesn't simulate using a longer focal length lens. If you maintain the aspect ratio, then magnification is necessary if the crop is to be viewed the same size as the original.

And yes, if an 8x10 and a 16x20 are made from the same capture, the 16x20 if viewed at the same distance will require a smaller maximum acceptable CoC to retain the same sharpness, and therefore shorten the apparent DoF. This requires that the maximum acceptable CoC doesn't drop below the pixel pitch, and also that the sensor resolution is sufficient to produce an 8x10 and a 16x20 at the same ppi.

 

[480sparky]

But he didn't qualify the statement with, "... and enlarge to the same size as the original."

If I print a 16x20, take a pair of scissors to it and cut out an 8x10 from it, the DOF is the same.

 

[analog.universe]

But he didn't qualify the statement with, "... and enlarge to the same size as the original."

If I print a 16x20, take a pair of scissors to it and cut out an 8x10 from it, the DOF is the same.

Yes, you're right about that.

Quite an unlikely scenario however... "Sorry, I couldn't get you an 8x10 because the edge of the frame was distracting and I didn't want to sacrifice DoF, here's a 6.2x8.5."

 

[Dao]

When we play with the DoF calc, the final result are also based on a predefined viewing distance and final image size. So when we talks about DoF and alter one factor, we need to assume all other factors stay the same, otherwise it will be meaningless. So when a person crop a photo, I will assume the final output is also printed on same size with the same viewing distance. In that case, I believe DoF should be different.