[Class A]

Now put on your really expensive 35mm f2.8 lens. Does it magically become a 70mm lens ? Does it magically change from f2.8 to f2 ?

Now tell me who has claimed what you are saying?

If you really think anyone has said this then you are not reading carefully enough.

[RioRico]

Now tell me who has claimed what you are saying?

If you really think anyone has said this then you are not reading carefully enough.

On this particular thread, no.

On many similar threads, yes, people say/ask, "HEY ON A DSLR, A 50MM LENS BECOMES A 75MM LENS, RIGHT?"

And too often, even the knowledgeable say that a 55mm on APS-C is 'like' an 85mm portrait lens on 135/FF.

That is wrong, and it just feeds the confusion.

A concept this prone to confusion is a bad concept.

[Class A]

And too often, even the knowledgeable say that a 55mm on APS-C is 'like' an 85mm portrait lens on 135/FF.

That is wrong, and it just feeds the confusion.

What is wrong about it? Surely there is room for a more precise phrasing but given how sloppily people talk about other issues and everyone is OK with it, I don't see the big deal.

A concept this prone to confusion is a bad concept.

Most people find algebra confusing. Does that make algebra bad? Not in my book.

If someone gets confused by crop factors then, AFAIC, that's because the underlying principles are not necessarily intuitive.

[Lowell Goudge]

If someone gets confused by crop factors then, AFAIC, that's because the underlying principles are not necessarily intuitive.

I think the problem here, with the crop factor is that it is a gross over simplification that attempst to explain the change in field of view caused by the different formats.

Like most over simplifications, there are limits to the applicability, but unfortunately people grasp onto it as though it is the golden rule for photography. Otherwise we wouldnt have people asking if focal length changes, then surely doesnt F stop?

The error is, we have in the "over simplification" chosen to create a variable, which we apply to an object that has fixed physical properties (i.e. the lens), in order to explain a difference in the camera itself.

I don't know exactly who created the expression, but I think they unknowingly generated a real mess.

[mattdm]

I am amazed why it is so hard to comprehend that a smaller sensor only captures a smaller part of the projected image.

Take your really expensive DSLR and with a black felt-tip draw on the sensor a rectangle half its size and blank out the outside. Now put on your really expensive 35mm f2.8 lens. Does it magically become a 70mm lens ? Does it magically change from f2.8 to f2 ?

If you print the image at the same size you were printing before (and have the sensor resolution to spare):

1. The field of view is that of a 70mm lens before your conversion
2. Depth of field is approximately equal to that given by f/5.6 (not f/2) before your conversion. (Math: 35mm ÷ 2.8 = 12.5mm; 70mm ÷ 12.5mm = 5.6. Or of course just double f/2.8.)
3. Exposure is unaffected by the change, as the ration between the actual focal length and actual aperture size is unchanged.

[Colbyt]

Follow up question if I may, I really want to understand this.

Depth of field is approximately equal to that given by f/5.6 (not f/2) before your conversion. (Math: 35mm ÷ 2.8 = 12.5mm; 70mm ÷ 12.5mm = 5.6. Or of course just double f/2.8.)

Are you saying an old A series lens has a greater DOF at any given aperture when used on a digital camera?

If yes or even if you are saying the opposite, what DOF table would one use for a given lens to get some approximate ranges?

[Class A]

The error is, we have in the "over simplification" chosen to create a variable, which we apply to an object that has fixed physical properties (i.e. the lens), in order to explain a difference in the camera itself.

The error is to confuse an "equivalent focal length" or an "equivalent f-ratio" with an actual focal length or an actual f-ratio.

No one with a correct understanding of crop factors applies factors to an object with fixed properties (in the sense that the fixed properties change). Instead a hypothetical object with different properties is assumed.

This is not a "mess" but can be useful. Just take the classic range of focal lengths for portraits of ~85mm-135mm. Of course, a better way to express the range would be an AOV range. But it is a convention to use focal length instead (here, based on the 35mm film format).

What lens(es) on APS-C do I need to cover this classic range? A crop factor conversion answers this question in seconds.

[mattdm]

Follow up question if I may, I really want to understand this.



Are you saying an old A series lens has a greater DOF at any given aperture when used on a digital camera?

If yes or even if you are saying the opposite, what DOF table would one use for a given lens to get some approximate ranges?

I'm saying that small sensors give an effectively-greater depth of field for the same relative aperture. This is familiar from looking at the depth of field from point & shoot cameras.

For a table, you can use Depth of Field Table

The conversion breaks down in the macro range, and is only approximate because of several real-world factors, not the least being that one might expect to print larger from a larger-sensor camera. Also, while it's easy to use in the real world with a 2x crop factor, 1.5x isn't so useful unless you have aperture options like f/2.1, f/3, f/4.2, etc. So I do have to agree with everyone saying that real-world application is dubious.

The important thing is: small sensor, same lens, same print size, more DoF.

[Class A]

Are you saying an old A series lens has a greater DOF at any given aperture when used on a digital camera?

This question is ill-formed because you are not specifying what you want to keep constant. The camera position? The AOV? The f-ratio?

If you leave the lens in the same position and just exchange the camera then a crop camera will produce an image with less AOV (you'll get a crop of the original image) and -- assuming you are going for the same output size -- an image with less DOF. Lowell opposes to the idea that the output size is kept constant, but it is a quite commonplace assumption.

Just skip to the bottom of Bob Atkin's piece about DOF and look at the image demonstrating that one must re-interpret the DOF markings on a FF-lens when used on an APS-C body (again, using the "same output size" assumption).

BTW, Matt's answer implied that you are attempting to capture the same scene and hence move back to undo the cropping. The increase in subject to camera distance will increase the DOF. The latter DOF increase is crop-factor^2 and the DOF decrease caused by the smaller sensor size is 1/crop-factor, resulting in a net effect of (crop-factor times) increased DOF.

If yes or even if you are saying the opposite, what DOF table would one use for a given lens to get some approximate ranges?

Use an online DOF calculator. A good one will ask you for the sensor size (often indirectly by asking for the camera model).

[kh1234567890]

Use an online DOF calculator. A good one will ask you for the sensor size (often indirectly by asking for the camera model).

Here we go again. It asks you the camera model, from which it guesses the sensor size, from which it guesses the circle of confusion size. It is this that changes when you change sensor size (or amount of enlargement). The lens properties do not change.

[Parallax]

Here we go again. It asks you the camera model, from which it guesses the sensor size, from which it guesses the circle of confusion size. It is this that changes when you change sensor size (or amount of enlargement). The lens properties do not change.

Nope, lens properties don't change; but DOF does. I have no idea why, but it does.


APS-C Sensor, 50mm lens, f1.4,
Subject distance 10 ft:

Depth of field
Near limit 9.67 ft
Far limit 10.4 ft
Total 0.68 ft

In front of subject 0.33 ft (48%)
Behind subject 0.35 ft (52%)

Hyperfocal distance 290.2 ft
Circle of confusion 0.02 mm


24x36 Sensor, 50mm lens, f1.4,
Subject distance 10 ft

Depth of field
Near limit 9.52 ft
Far limit 10.5 ft
Total 1.02 ft

In front of subject 0.48 ft (47%)
Behind subject 0.54 ft (53%)

Hyperfocal distance 193.5 ft
Circle of confusion 0.03 mm

[kh1234567890]

Nope, lens properties don't change; but DOF does. I have no idea why, but it does.

Circle of confusion 0.02 mm
Circle of confusion 0.03 mm

Because the circle of confusion size changes -argggh !

Smaller sensor -> have to blow up the image more to make it same size as one from a bigger sensor -> bigger image makes it easier to see more detail -> smaller circle of confusion.
Ignoring pixel or grain size and assuming that your eyesight does not go wonky in-between.

[Parallax]

Because the circle of confusion size changes -argggh !

If you don't want to bother answering a question, then don't answer it. The "arggggh" and cute little, "I'm angry because I know more than everybody else" smileys aren't going to get you very far.

[Colbyt]

This question is ill-formed because you are not specifying what you want to keep constant. The camera position? The AOV? The f-ratio?

I will agree to the ill formed question. That was a nice article but did not answer my question. The tables at dofmaster report certain DOF numbers. I have reviewed those. Are they accurate for older lenses on modern digital cameras at the f-stops as listed in the tables? Yes is all I need to hear. A no requires so additional information.

[Class A]

The lens properties do not change.

Who said when that lens properties change?

The tables at dofmaster report certain DOF numbers. I have reviewed those. Are they accurate for older lenses on modern digital cameras at the f-stops as listed in the tables?

A DOF table makes certain assumptions (e.g., regarding the relationship between output size and viewing distance). One of the assumptions is that the same output size is targeted, independently of the sensor size. Because of the latter assumption, the entries in a DOF table will change if you change the sensor size (because the enlargement factor changes). This is why the DOF Table Matt gave you a pointer to, asks you to put in the sensor format first.

So, "No", you cannot use a DOF table calculated for 35mm film for your crop camera.