selmslie Loc: Fernandina Beach, FL, USA

In Specific question about 35mm vs 50mm (or similar situation) [link in next post] the question was asked was whether cropping an image taken with a 35mm lens would produce the same result if both images were taken from the same camera position.

Some misinformation was posted that was totally incorrect.

Differences between the lenses to start with:
Field of view is different.
DOF is different. (longer on the 35mm)
Field compression is different. (Due to DOF)
Relative perspective is different. (35 will make objects look wider at close range. Details will quickly be lost)

When you crop, you will never be able to have a perfect match.

This response was wrong on every count.

1. Since the camera does not move, the perspective does not change. Cropping makes the field of view the same. Perspective has nothing to do with focal length.
2. DoF is identical. It doesn't matter whether the image is cropped in the camera or later during post processing. Because the cropped image needs to be enlarged to make the same final result, the resulting DoF will end up being the same.
3. Field compression (whatever that means) will be the same because the perspective is the same.
4. You can crop an image taken with a wider lens and end up with a geometrically identical image because of the perspective not changing.

Things that will be different will be:

1. The resolution (the cropped image will have fewer megapixels) and the two lenses will not produce the same sharpness.
2. Vignetting, if any, may be slightly different.
3. Noise (if any can be seen) will be magnified in the cropped image.

selmslie Loc: Fernandina Beach, FL, USA

To see the original post go to https://www.uglyhedgehog.com/t-702400-1.html

My original response there was deleted by the moderator who apparently does not understand how perspective or DoF work.

BebuLamar

DOF are different because the image taken with the 35mm then crop becomes a smaller format. Different format has different DOF.

selmslie Loc: Fernandina Beach, FL, USA

That's only true if you also make a correspondingly smaller image from the result.

But if the final images are the same size, it doesn't matter whether you cropped in the camera (used a Dx crop on an Fx sensor) or if you did a 2/3 stop in post processing.

For more on this see https://www.uglyhedgehog.com/t-698801-1.html#12280939

BebuLamar

But the shot made with the 50mm lens you don't crop at all so it's full frame. The one taken with the 35mm must be crop so it's not full frame any more although still a bit bigger than APS-C.

selmslie Loc: Fernandina Beach, FL, USA

But you crop it to get the same final image. That means that you magnify everything more in the cropped image, including the circle of confusion.

Take a look at the example from Ysarex which includes an explanation from Cambridge in Colour about what is happening.

BebuLamar

Use the formula for DOF and you shall see. Of course the circle of confusion is smaller for the cropped image but the focal length is shorter so for the same aperture the cropped image has more DOF. Similarly a FF 50mm f/1.4 has less DOF than an M43 25mm f/1.4.

CHG_CANON Loc: the Windy City

Since you're on a roll, I know I'm wasting my time .... But, the question was specifically emphasis added:

Consider this: Same camera, same position, same subject (except of obvious field of view), same settings (ss, aperture, iso, white balance, basically everything), same quality and features on lenses - like number and shape of aperture blades …

Our friend Ysarex shows you can achieve the same depth of field with widely different focal lengths if you vary widely the shooting position. His example you've presented as your incontrovertible evidence shows 25mm at 2 feet vs 100mm at 8 feet vs 400mm at 32 feet.

But like I said, I know I'm wasting my time in pointing out you're now raging about a different and un-asked question ...

wdross Loc: Castle Rock, Colorado

As long as the sensor format doesn't change, the other posting information is incorrect and your information is correct. I have proven it at least once on every count. I can only hope that others who read the incorrect information read what you have put forth.

selmslie Loc: Fernandina Beach, FL, USA

The DoF formulas only work if you make an 8x10 image from the original uncropped capture and view it from 25cm.

As Cambridge in Colour explains, as soon as you deviate from those assumptions all bets are off.

If you crop the original image, make a different sized print, view it from a different distance, put your reading glasses on, etc., DoF changes.

wdross Loc: Castle Rock, Colorado

Is only happens with change in sensor format.

selmslie Loc: Fernandina Beach, FL, USA

You are correct, he was addressing a different question.

But he referred us to the Cambridge in Color explanation which is comprehensive. What's more, their DoF calculator is superior to all of the others because it lets us see the effect of changing each of those other variables. The only part they did not cover was the cropping of an image in post processing.

They also do a pretty good job of explaining CoC (circle of confusion).

selmslie Loc: Fernandina Beach, FL, USA

Copping in post processing has the same effect as changing the sensor format. It's just that in post you have an almost infinite choice of cropping factors and aspect ratios.

CHG_CANON Loc: the Windy City

From the calculator of the DOF specifically linked from the Cambridge Post.

(Download)

BebuLamar

He said he crop the 35mm shot to give the FOV of the 50mm in effect changes the the format to approximately APS and thus the DOF is 0.29m which is a less than that of the uncropped image but still more than that of the 50mm shot.