[troyz]

Well, I had a look to this problem with a DOF calculator, and it seems that on APS-C, you will lose 2 stops of DOF...

I have read many posts on this subject, and the consensus seems to be that in practice, you should set the aperture to one stop slower than the scale f stop you are registering.

So you lose approximately "one stop" of DOF; i.e. use the film-lens marks for f/5.6 for APS-C pictures taken at f/8

The DOF of any lens at a specific focal length and f stop will always be the same. The DOF scales will always be as accurate as they always were.

Well, it's either zero, 1, or 2 stops of adjustment. . . perhaps we should have a poll!

[pschlute]

Everybody really makes this much too complicated.

The DOF of any lens at a specific focal length and f stop will always be the same. The DOF scales will always be as accurate as they always were.

The lens does not know or for that matter care what sort of camera it is attached to. It will project the same image onto a digital sensor, film, ground glass, or a blank sheet of paper.

The only exception I know of is the focus screens on DSLR's. They use a few optical tricks that have the effect of making fast lenses look like they have the DOF of f2.8 or so.

Yes, but the APS-C sized sensor needs more enlargement than a 35mm film frame to produce the same sized print. It is a certain print size viewed from a certain distance that is the benchmark (albeit subjective) of acceptable sharpness in relation to DOF. The circle of confusion (aptly named) will thus be bigger on the print produced from the digital camera, and thus will have less DOF.

So it is safe to say that the focus scale on lenses designed for 35mm film do not work exactly the same on an APS-C sized sensor.

[Steve Beswick]

Yes, but the APS-C sized sensor needs more enlargement than a 35mm film frame to produce the same sized print. It is a certain print size viewed from a certain distance that is the benchmark (albeit subjective) of acceptable sharpness in relation to DOF. The circle of confusion (aptly named) will thus be bigger on the print produced from the digital camera, and thus will have less DOF.

So it is safe to say that the focus scale on lenses designed for 35mm film do not work exactly the same on an APS-C sized sensor.

None of this is true.

[Canada_Rockies]

Everybody really makes this much too complicated.

The DOF of any lens at a specific focal length and f stop will always be the same. The DOF scales will always be as accurate as they always were.

The lens does not know or for that matter care what sort of camera it is attached to. It will project the same image onto a digital sensor, film, ground glass, or a blank sheet of paper.

The only exception I know of is the focus screens on DSLR's. They use a few optical tricks that have the effect of making fast lenses look like they have the DOF of f2.8 or so.

The problem is that you must enlarge the image more, and depth of field calculations are based on the circle of confusion as printed. For Pentax 35mm lenses, the target was described to me as an 8x10 inch print viewed from 10 inches, or normal reading distance. So, given that a particular image were taken by my k10 and the same image taken with my MZ-S, the k10 would require a smaller aperture to be enlarge to the 8x10 print viewable at 10 inches as sharp.

[Szten]

None of this is true.

All of that is true. DOF will be different because the maximum permissible circle of confusion will be different. So even though the lens itself remain the same and none of its optical attributes will change, the DOF with different senzor sizes won't be the same. You should look after the meaning of circle of confusion, and its effects on the DOF. Once things become clear one can find it pretty logical, although I know it sounds like quite a stupidity at fist.

[pschlute]

Steve

I agree with you that a lens cannot change its resolving properties (and thus the image it forms on the film or sensor plane) by moving it from one format to another.

But if you re-read my comments you cannot possibly say none of it is true. Please answer the following questions:

1. Is the APS-c sensor smaller than a 35mm film frame ?

2. To produce an 10x8 print, will the two formats have to be enlarged by the same amount, or by different amounts ?

3. DOF is measured on a given print size, NOT on the sensor or film frame. Yes or no ?

4. The more you enlarge any image, the larger the circle of confusion you create. Yes or no?

Once the above is understood, it is then easy to understand that DOF scales cannot relate to any lens in isolation, but were created by a technician using the lens, and the intended format magnification to a given print size to create those scales. When the 35mm film lens is then used on a different format, the scales are no longer accurate because the assumed of format magnification is not the same.

[troyz]

This thread is turning into Monty Python's "Argument Clinic" sketch!

[pschlute]

The problem is that you must enlarge the image more, and depth of field calculations are based on the circle of confusion as printed. For Pentax 35mm lenses, the target was described to me as an 8x10 inch print viewed from 10 inches, or normal reading distance. So, given that a particular image were taken by my k10 and the same image taken with my MZ-S, the k10 would require a smaller aperture to be enlarge to the 8x10 print viewable at 10 inches as sharp.

Correct..... for comparing two images taken with the same lens at the same distance to subject

[Canada_Rockies]

Correct..... for comparing two images taken with the same lens at the same distance to subject

No. It is correct for the same field of view. The distance to the subject is not the criteria, it is the degree of enlargement made from a 24x36 mm frame versus the enlargement made from a 16x24 mm frame. That's all I can say. Take your D-SLR and take an image of, say, a building. From the same location, take your film camera with Velvia 50, and set the field of view to the same at the same distance. Your 8x10 inch enlargement will be "sharper" from the film camera. No contest.

[KungPOW]

However, because the DOF calculations are different with APS sized sensors, we can no longer rely on these lens DOF scales. We will have less DOF than they show us. But while we get less DOF from each lens, the smaller sensor is actually giving us greater DOF at equivalent fields of view (FOV).

This was written by Clayton Jones. I think it is one of the better explinations for DOF effects on the web. And he uses a Pentax lens in his example.

Here is the link:

Hyperfocal Focusing With APS Sized Sensors

It was a good read.

[Steve Beswick]

Steve

I agree with you that a lens cannot change its resolving properties (and thus the image it forms on the film or sensor plane) by moving it from one format to another.

But if you re-read my comments you cannot possibly say none of it is true. Please answer the following questions:

1. Is the APS-c sensor smaller than a 35mm film frame ?

2. To produce an 10x8 print, will the two formats have to be enlarged by the same amount, or by different amounts ?

3. DOF is measured on a given print size, NOT on the sensor or film frame. Yes or no ?

4. The more you enlarge any image, the larger the circle of confusion you create. Yes or no?

Once the above is understood, it is then easy to understand that DOF scales cannot relate to any lens in isolation, but were created by a technician using the lens, and the intended format magnification to a given print size to create those scales. When the 35mm film lens is then used on a different format, the scales are no longer accurate because the assumed of format magnification is not the same.

First let me answer your questions.
1. Yes, but it's not relevant.
2. I'll explain below why this is not a valid question.
3. No, no, and no. I'm not being redundant - none of these possibilities are correct.
4. See #2.

Your theory of "format magnification" is inherently flawed in the following ways:
1. Traditional enlarging is an optical process. Does the DOF vary based on the enlarger? How about the brand of photo paper, or for that matter the brand of film?
2. Digtal printing is just that, completely digital. Further more, digital files have no physical size.
3. According to your logic, switching to a different type/speed/quality of film would change the apparent DOF of the same shot with the same lens. The same goes for switching from a 6 MP to a 14 MP sensor.

To thoroughly answer your third question, DOF is relative to the sharpest point in the image, within reason. Obviously if the whole image is out of focus, then there is no DOF.

Edit: Notice I never said anything about field of view, which is a completely separate subject.

[dlacouture]

Steve : well, how do you explain that some of my pics seems in focus on my camera's LCD, but when I zoom in, I see that they are out of focus? Same thing applies to thumbnails on the computer, or between a 4x6 and a 20x30 paper output.

It's simply because I enlarged the pic... And the more I enlarge it, the less the DOF will be!
So, as we HAVE to enlarge an APS-C more than we would a FF photo, DOF is smaller...

[Steve Beswick]

Steve : well, how do you explain that some of my pics seems in focus on my camera's LCD, but when I zoom in, I see that they are out of focus? Same thing applies to thumbnails on the computer, or between a 4x6 and a 20x30 paper output.

It's simply because I enlarged the pic... And the more I enlarge it, the less the DOF will be!
So, as we HAVE to enlarge an APS-C more than we would a FF photo, DOF is smaller...

Tell me this: when you make a print from an image taken with a digital camera, what exactly are you "enlarging"?
I'll tell you one thing - it's not the sensor.

[Szten]

Tell me this: when you make a print from an image taken with a digital camera, what exactly are you "enlarging"?
I'll tell you one thing - it's not the sensor.

Yes, it's the sensor. More specific, it's the CoCs in the sensor. The problem is that it seems like you don't know what a circle of confusion is, and thus understanding the DOF is not possible.
Take a look at a DOF calculator. The calculated DOF will be different with cameras with different sensor sizes. Why? Because the maximum permissible circle of confusion will be different.
You should look after and understand the meaning of CoC first, and everything will be clear.
A few links:

Circle of confusion - Wikipedia, the free encyclopedia
Understanding Depth of Field in Photography
Circle of Confusion: Optical: Glossary: Learn: Digital Photography Review

Read them and you will understand what Coc is, and how it affects the DOF.

[newarts]

Here's a useful form of standard theory for non-macro small DOF for a display viewed at a constant distance:

DOF=2*Display_Pixel_width*(Field_width/Display_width)*F-number*(Field_width/Sensor_width)

The field width is the actual size of what's projected on the sensor (nominally, what you see through the viewfinder.) If you decrease the sensor size for a particular field width the DOF increases; similarly if you decrease the display width the apparent DOF increases (this is like standing back from the display - things look sharper.)

Dave in Iowa

PS the last factor is (1+Field_width/Sensor_width) for the general case including macros.