Food for Thought: Depth of Field

10/28/2003 03:30:27 PM · #1

Food for Thought Question #1:

If I have a 100mm f/2.8 lens and I add a 2x teleconverter between the lens and the camera body to achieve the effect of 200mm with the lens, will the depth of field be affected by the 2x converter?

Tip #1: Depth of field is determined by aperture size, focal length, and distance to subject.

Tip #2: A 2x teleconverter reduces the amount of light allowed to the film plane / sensor by two stops.

10/28/2003 03:39:11 PM · #2

if you keep the aperture the same (i.e., at least f5.6 in each case), and move so that the subject appears the same size in both cases, the the depth of field will be essentially the same.

glimpses

Sony DSC-F707

10/28/2003 03:44:39 PM · #3

Originally posted by Gordon:

if you keep the aperture the same (i.e., at least f5.6 in each case), and move so that the subject appears the same size in both cases, the the depth of field will be essentially the same.

Then, why should I have mounted a 2x teleconverter?? =))
To lose 2 stops on the shutter speed?

Sorry John, but I think that question sucks a little bit or there is some hidden (to me) knowledge.

Excuse my ignorance.

10/28/2003 03:48:22 PM · #4

Originally posted by glimpses:

Originally posted by Gordon:
if you keep the aperture the same (i.e., at least f5.6 in each case), and move so that the subject appears the same size in both cases, the the depth of field will be essentially the same.

Then, why should I have mounted a 2x teleconverter?? =))
To lose 2 stops on the shutter speed?

Sorry John, but I think that question sucks a little bit or there is some hidden (to me) knowledge.

Excuse my ignorance.

Because you've doubled the magnifcation, so to keep the same DoF you have to move twice as far away ?

glimpses

Sony DSC-F707

10/28/2003 03:53:51 PM · #5

Originally posted by Gordon:

Originally posted by glimpses:
Originally posted by Gordon:
if you keep the aperture the same (i.e., at least f5.6 in each case), and move so that the subject appears the same size in both cases, the the depth of field will be essentially the same.

Then, why should I have mounted a 2x teleconverter?? =))
To lose 2 stops on the shutter speed?

Sorry John, but I think that question sucks a little bit or there is some hidden (to me) knowledge.

Excuse my ignorance.

Because you've doubled the magnifcation, so to keep the same DoF you have to move twice as far away ?

Yeah, but you specified that, in order DOF to be the same, the subject must appear the same size in both cases.

Evidently, if you want to enjoy the magnification, the depth of field will be certainly narrowed a lot, assuming the same light conditions.

That's why, IMHO, the question is "questionable". =)
To get the magnification you lose 2 stops which means that you risk to have an under-exposed shot or a sensibly reduced DOF (which is a well-know property of tele lenses anyway).

Message edited by author 2003-10-28 15:54:52.

10/28/2003 03:54:10 PM · #6

Does the magnification offered by the 2x converter change any of the physical properties that define depth of field?

glimpses

Sony DSC-F707

10/28/2003 03:58:33 PM · #7

Originally posted by jmsetzler:

Does the magnification offered by the 2x converter change any of the physical properties that define depth of field?

Not directly.

You should say if aperture and shutter speed of the shot are the same or not.

10/28/2003 04:13:36 PM · #8

Originally posted by jmsetzler:

Does the magnification offered by the 2x converter change any of the physical properties that define depth of field?

yes it changes the magnification, hence the focal length, which changes the depth of field, unless you move.

But for all intents and purposes, changing the focal length and keeping the subject size constant, means that the DoF will remain the same, all other things being equal.

Your question isn't really phrased with enough detail to ask a specific question though.

If you use an aperture of f5.6 or above, the teleconverter will have no effect on the aperture, though it will double the effective focal length. If you want the main subject to take up the same amount of the frame, you have to move, which will keep the depth of field constant, but significantly change the field of view. If you don't move, you will change the subject size by 2x and the depth of field will change appropriately, for the same aperture/ shutter speed.

10/28/2003 04:15:25 PM · #9

Or is your question, is there a difference between using a

100mm macro lens, + 2x teleconverter and a
200mm lens at the same location, with the same aperture and shutter speed, focused on the same point ?

Sigma 12-24mm f/4.5-5.6 EX Aspherical DG for Canon

10/28/2003 04:35:27 PM · #10

There is a really good article on depth of field (and how it works with the digital crop factor) at photo.net. This might be of some help. Consider a 100mm f/2.8 lens with a 2x teleconverter attached a 200mm f/5.6 lens for equation purposes ('cause that's what it is ;)).

Depth of Field and Digital

Format related Depth of Field

Since I don't want to scare readers way with a page of algebra showing the derivation of the following equation, I'll just state it. You can derive it (as I did) from the simple lens equation.

Here F is the focal length. D is the subject distance, c is the circle of confusion and fn is the f# (f-stop) of the lens. Now this equation doesn't reduce to some simple rule of thumb. However we can say that over the range of focus distances which aren't in the macro range (where D is close to F) and which aren't close to the hyperfocal distance (where D = F*F/fn*c) you can "guesstimate" that the depth of field ratio between two lenses used at the same aperture and focused at the same distance by assuming it's proportional to the size of the circle of confusion and inversely proportional to the square of the focal length.

Again, this simple analysis only applies at "intermediate" distances, but we have to have that limitation if we want a "simple" formula. It only really breaks down when the lens is focused further than about halfway to the hyperfocal distance or when we get to magnifications near 1:1.

Message edited by author 2003-10-28 16:37:10.

robsmith

Canon EOS-300D Rebel

10/28/2003 04:42:54 PM · #11

Jimmy

Thanks for the link, however when you quote from anothers work it is best to acknowledge this.

10/28/2003 04:46:06 PM · #12

Originally posted by jimmythefish:

Consider a 100mm f/2.8 lens with a 2x teleconverter attached a 200mm f/5.6 lens for equation purposes ('cause that's what it is ;)).

I think that might have been the question...

10/28/2003 04:46:42 PM · #13

The link and the italics weren't enough?

soup

Canon EOS-30D

10/28/2003 04:56:16 PM · #14

you could acheive the same DoF buy using a slower shutter -
2 stops slower to make up for the 2xconverter.

the same amount of light would hit the sensor.

EDIT - err focal length - so no - thats why a 200mm f2.8 lense is so expensive ;}

soup

Message edited by author 2003-10-28 16:57:55.

10/28/2003 05:00:05 PM · #15

nope. shutter speed affects light only, not depth of field. the circles of confusion will remain the same no matter which shutter speed is used. depth of field is a product of the lens' focal length, camera's sensor size, aperture and distance to the subject. shutter speed is one of the variables used to expose correctly, given a constant aperture...the other being ISO speed.

Originally posted by soup:

you could acheive the same DoF buy using a slower shutter -
2 stops slower to make up for the 2xconverter.

the same amount of light would hit the sensor.

soup

Message edited by author 2003-10-28 17:01:31.

Sigma 18-125mm f/3.5-5.6 DC for Canon

10/28/2003 05:41:07 PM · #16

Let me rephrase the question:

We have a 100mm f/2.8 lens with a 2x converter. We also have a 200mm f/2.8 lens. We make the same photo with both lenses. The camera settings are f/2.8 and 1/125". The subject is the same distance from the camera for both photos.

Will the depth of field be different?

EDIT: Assume that f/2.8 and 1/125" are correct exposures for both lenses for both photos.

Message edited by author 2003-10-28 17:51:01.

Kavey

Canon EOS-400D Rebel XTi

10/28/2003 05:41:53 PM · #17

Is this meant to be a class or are you actually asking?

Canon PowerShot Pro 90 IS

10/28/2003 05:51:18 PM · #18

Originally posted by Kavey:

Is this meant to be a class or are you actually asking?

It's a lesson.

coolhar

10/28/2003 05:51:43 PM · #19

I think the answer is no.

Canon RF 24-70mm f/2.8 L IS

10/28/2003 05:54:49 PM · #20

Originally posted by coolhar:

I think the answer is no.

Why?

kirbic

Canon EOS R5

10/28/2003 06:01:26 PM · #21

The answer is actually "yes".

The aperture is F2.8 for the 100mm lens, but becomes F5.6 with the converter since the aperture diameter does not change, but the focal length doubles.

10/28/2003 06:02:33 PM · #22

The DOF is greater with the 100mm + teleconverter because the aperture is smaller with that lens vs. the 200 f/2.8.

Canon RF 24-70mm f/2.8 L IS

10/28/2003 06:05:53 PM · #23

Originally posted by jimmythefish:

The DOF is greater with the 100mm + teleconverter because the aperture is smaller with that lens vs. the 200 f/2.8.

But the 2x converter does not change the size of the aperture.

kirbic

Canon EOS R5

10/28/2003 06:06:49 PM · #24

Originally posted by jmsetzler:

Originally posted by jimmythefish:
The DOF is greater with the 100mm + teleconverter because the aperture is smaller with that lens vs. the 200 f/2.8.

But the 2x converter does not change the size of the aperture.

But an F2.8 aperture on a 100mm lens is smaller than an F2.8 aperture on a 200mm lens.