DOF OOF Optics Q uestion

[Likaleica]

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Is there a difference in depth-of-field, at any given aperture, in a 27mm lens cropped to 50mm equivalent, versus a 50mm lens, at the same aperture?

My understanding (for years) is that there is not.

The practical part of this is:  Will an image at e.g. f/2.0 cropped to 50mm on the Q3, have the same OOF look as a 50mm lens at f/2.0 on a M11?  

Lens characteristics aside (eg Summilux vs Summicron), will the two pictures look similar, with respect to OOF areas?  Or does the 27mm lens just have a greater depth of field even when cropped?

[Likaleica]

Ooops.  Meant 28mm lens on the Q3

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[SrMi]

3 hours ago, Likaleica said:

Is there a difference in depth-of-field, at any given aperture, in a 27mm lens cropped to 50mm equivalent, versus a 50mm lens, at the same aperture?

My understanding (for years) is that there is not.

The practical part of this is:  Will an image at e.g. f/2.0 cropped to 50mm on the Q3, have the same OOF look as a 50mm lens at f/2.0 on a M11?  

Lens characteristics aside (eg Summilux vs Summicron), will the two pictures look similar, with respect to OOF areas?  Or does the 27mm lens just have a greater depth of field even when cropped?

A 50mm at f/2.0 has a shallower DOF than a 28mm at f/2.0 even when cropped to 50mm.

[clasami]

Good read: https://bobatkins.com/photography/technical/digitaldof.html

[TDE-Photo]

You have to use the crop factor on the fstop.

[adrianh]

No, it will not. Think it that way: take the whole pic from the Q3 26mm lens at 2.0. It will show you a specific look and a specific DOF. This look will show both in the middle as well as in the corners. If you crop, it means that you look only at the middle of the pic without changing anything in optics, just discarding the rest around it. Therefore, the look and the DOF will be the same as before. You ALWAYS have the same Dof as you had before. Marketing will try to let you believe that you get 4-5 lenses in the Q3. You DON'T. You can just crop even more than before due to more Mpixels, but for example a M10 with a 50 lens will always have less DoF.

I hope that makes sense.  

[01af]

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vor 7 Stunden schrieb Likaleica:

Is there a difference in depth-of-field, at any given aperture, in a 28 mm lens cropped to 50 mm equivalent, versus a 50 mm lens, at the same aperture?

My understanding (for years) is that there is not.

And yet, there is.

.

vor 7 Stunden schrieb Likaleica:

The practical part of this is:  Will an image at e. g. f/2 cropped to 50 mm on the Q3, have the same OOF look as a 50 mm lens at f/2 on a M11?

This is not 'the practical part of it' but, in general, an entirely different question.

And the answer is: no.

.

vor 7 Stunden schrieb Likaleica:

Will the two pictures look similar, with respect to out-of-focus areas?  Or does the 28 mm lens just have a greater depth of field even when cropped?

Again: That's two different questions. Equal depth-of-field does not necessarily mean same rendition of out-of-focus areas. And similar out-of-focus rendition does not generally mean equal depth-of-field. Don't look at depth-of-field when out-of-focus rendition is what you're actually interested in.

However, in your particular case—i. e. shooting from the same point-of-view using equal angles-of-view (equivalent focal lengths)—you can get the same depth-of-field and similar out-of-focus rendition at the same time: stop down the 50 mm shot by 1 2/3 stops (or 2 stops, that's close enough for most practical intents and purposes). So when shooting the Leica Q3 at f/2 and cropping to 50 mm equivalent then use f/3.6 (or f/4) on the 35-mm-format camera with a 50 mm lens.

[pcgarner]

4 hours ago, adrianh said:

No, it will not. Think it that way: take the whole pic from the Q3 26mm lens at 2.0. It will show you a specific look and a specific DOF. This look will show both in the middle as well as in the corners. If you crop, it means that you look only at the middle of the pic without changing anything in optics, just discarding the rest around it. Therefore, the look and the DOF will be the same as before. You ALWAYS have the same Dof as you had before. Marketing will try to let you believe that you get 4-5 lenses in the Q3. You DON'T. You can just crop even more than before due to more Mpixels, but for example a M10 with a 50 lens will always have less DoF.

I hope that makes sense.  

The clearest answer and the correct. The crop lines are simply to help visualize what a 35, 50, etc lens would capture.

[01af]

vor 1 Stunde schrieb pcgarner:

The clearest answer and the correct.

No, it's incorrect.

Cropping (at constant focal length) will reduce depth-of-field. However, a longer focal length (at constant sensor size) will reduce depth-of-field even more.

[pcgarner]

39 minutes ago, 01af said:

No, it's incorrect.

Cropping (at constant focal length) will reduce depth-of-field. However, a longer focal length (at constant sensor size) will reduce depth-of-field even more.

Technical justification for your opinion, please. 

[01af]

vor 19 Minuten schrieb pcgarner:

Technical justification for your opinion, please. 

It's not an opinion but a simple fact.

If you settle upon a certain degree of (minimal) blur to accept as 'sufficiently sharp' in the final picture then depth-of-field will be proportional to the linear sensor size because smaller sensors need more magnification for the same picture size, hence a smaller diameter of the maximum permissible circle-of-confusion. On the other hand, depth-of-field is inversely proportional to the square of the focal length.

So, half the linear sensor size means half the depth-of-field (and cropping is just that: reduction of the effective sensor size). Half the focal length means four times the depth-of-field. Half the sensor size and half the focal length (= equivalent focal length) means 0.5 × 4 = twice the depth-of-field. Have a look at the depth-of-field formulae in a pertinent textbook of your choice.

And please: Think about this for a minute or two before asking your next question.

[dpitt]

14 hours ago, Likaleica said:

The practical part of this is:  Will an image at e.g. f/2.0 cropped to 50mm on the Q3, have the same OOF look as a 50mm lens at f/2.0 on a M11?  

 

No it will not be the same. Cropping to 50mm from a lens that is close to 26mm is almost 2x crop.
Your Q3 lens cropped to 50 and shot at F2.0 will have a DOF similar to a 50mm FF lens at F4.0 .

See this thread:

 

[Corius]

35 minutes ago, pcgarner said:

Technical justification for your opinion, please. 

I’m going to take a stab at this because I always find this confusing, so please correct me

The DoF depends on, amongst other things, the viewing distance. When you crop you are effectively moving closer to the subject and the circles of confusion increase and hence the DoF decreases.

(Just noticed 01af post a much better technical description)

[Likaleica]

57 minutes ago, dpitt said:

No it will not be the same. Cropping to 50mm from a lens that is close to 26mm is almost 2x crop.
Your Q3 lens cropped to 50 and shot at F2.0 will have a DOF similar to a 50mm FF lens at F4.0 .

See this thread:

 

This link was a very clear explanation. Thanks. And no snarkiness!

“If you start cropping an image taken by a 28mm lens at F1.7, you will keep the same DOF distance no matter how much you crop at the same distance. So if you shoot a subject and you have 20cm DOF, you will keep that no matter how the crop is.

But because you are only using part of the available light (you crop the rest away) it is as if you are closing the aperture as you crop further and further. So your 'virtual' lens gets slower and slower:

28 at F1.7
cropped to 35mm gives F2.0
cropped to 50mm gives F2.8
cropped to 70mm gives F4.0

So with the Q you can simulate an M with a 50mm Elmarit, but 50 Summicron and 50 Summilux or Noctilux will have much shallower DOF at the same distance when you use them wide open.”

Edited August 29, 2023 by Likaleica

[pcgarner]

3 hours ago, Likaleica said:

This link was a very clear explanation. Thanks. And no snarkiness!

“If you start cropping an image taken by a 28mm lens at F1.7, you will keep the same DOF distance no matter how much you crop at the same distance. So if you shoot a subject and you have 20cm DOF, you will keep that no matter how the crop is.

But because you are only using part of the available light (you crop the rest away) it is as if you are closing the aperture as you crop further and further. So your 'virtual' lens gets slower and slower:

28 at F1.7
cropped to 35mm gives F2.0
cropped to 50mm gives F2.8
cropped to 70mm gives F4.0

So with the Q you can simulate an M with a 50mm Elmarit, but 50 Summicron and 50 Summilux or Noctilux will have much shallower DOF at the same distance when you use them wide open.”

Makes complete sense. Thanks for dogging this.

[01af]

vor 3 Stunden schrieb Likaleica:

... because you are only using part of the available light (you crop the rest away) it is as if you are closing the aperture as you crop further and further. So your 'virtual' lens gets slower and slower: [...]

This is utter nonsense.

.

vor 3 Stunden schrieb Likaleica:

28 mm at f/1.7
cropped to 35 mm gives f/2
cropped to 50 mm gives f/2.8
cropped to 70 mm gives f/4

That's wrong. Correct is this:

28 mm at f/1.7,
cropped to 35 mm equivalent, gives the same DOF as a 35 mm lens at f/2 on 35-mm format
cropped to 50 mm equivalent, gives the same DOF as a 50 mm lens at f/2.8 on 35-mm format
cropped to 75 mm equivalent, gives the same DOF as a 75 mm lens at f/4 on 35-mm format
(f-numbers rounded to the next full f-stop).

Of course, to realize this depth-of-field, the longer focal lengths require slower shutter speeds (or higher ISO settings) in order to get proper exposure. After all, the 28 mm 1:1.7 lens does NOT become slower by cropping.

 

[dpitt]

On 8/29/2023 at 10:02 PM, 01af said:

That's wrong. Correct is this:

28 mm at f/1.7,
cropped to 35 mm equivalent, gives the same DOF as a 35 mm lens at f/2 on 35-mm format
cropped to 50 mm equivalent, gives the same DOF as a 50 mm lens at f/2.8 on 35-mm format
cropped to 75 mm equivalent, gives the same DOF as a 75 mm lens at f/4 on 35-mm format
(f-numbers rounded to the next full f-stop).

Of course, to realize this depth-of-field, the longer focal lengths require slower shutter speeds (or higher ISO settings) in order to get proper exposure. After all, the 28 mm 1:1.7 lens does NOT become slower by cropping.

 

Thanks for clarifying. This is what I meant to say in my previous post. You get an equivalent DOF when you crop, but fortunately you can still use the same F1.7 opening and the same exposure times as a F1.7 lens.

Cropping has the 'advantage' of providing larger DOF for a given opening. But this is not always what you want. That is why with my Digilux2, I struggle getting any useful narrow DOF. It has a wonderful Summicron zoom with 90mm equivalent at the long end, but because of the 4x crop factor sensor everything is sharp, even at F2.0. wide open the DOF is equivalent to using a 90 mm at F8.0 on a FF sensor.

[jmschuh]

 

Q3 Crop Conversion

The math for this is pretty simple.  

Just divide the crop focal length 35, 50, 75 or 90 by 28 to get the crop factor.

Multiply the crop factor by 1.7 to get the equivalent aperture in terms of depth of field.

No matter what crop factor is used, the exposure will always be that of an f1.7 lens.

For the pixel count, divide 60.3 by the crop factor squared. 

You get:

28mm f1.7 = 1x 35mm format - 60.3 MP (9520 x 6336)
35mm f2.1 = 1.25x (larger than the M8 APS-H sensor) - 38.6 MP (7616 x 5072)
50mm f3.0 = 1.79x (larger than MFT) - 18.9 MP (5328 x 3552)
75mm f4.5 = 2.68x (same size as a 1" sensor) - 8 MP (3552 x 2268)
90mm f5.5 = 3.21x 5.8 MP (2960 x 1968)

 

[Likaleica]

Thanks to everyone's efforts.

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Hallo,

Look here: Leica Q

[thighslapper]

Went round in circles of confusion with this for a while .......

The salient point is in the definition:

DOF relates to a print or other reproduction of an image. It's NOT an intrinsic property of a lens. If you put a lens on an optical bench you can measure focal length, you can measure aperture, but you can't measure depth of field

So from what I can see it's an entirely fabricated concept dependent on the final medium that the image is displayed on and at what size it is viewed. 

 

 

 

Edited September 3, 2023 by thighslapper