APS-C Lens on Full Frame Camera - DoF Question

[jaapv]

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Why don't you use a Summilux-M on your SL then?  A TL lens will lose you half the pixels and will never be the same in DOF as a FF 1.4.

[Guest]

I do, but I'd prefer an autofocus option, especially a (relatively) small and light lens which also pairs nicely with the fp.

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[jaapv]

Yes, but I notice that you mention the Sigma 16/1.4. At such angles AF holds very little advantage.  TL lenses are not THAT small, compared to M lenses,

[ramarren]

8 hours ago, AndyGarton said:

Hi guys, a hopefully simple question which I've seen conflicting answers on. When using, for example, a 16mm f1.4 lens on a full frame body, I understand the change to field of view (24mm) and also the reduction in overall image size (e.g. 24mp down to 16), but I'm not sure about depth of field. When used wide open, do you get the depth of field of a 24mm f1.4, or a 24mm f2? (Or something else?)

I've seen so many unclear and incorrect discussions of this topic it is mind boggling. Haven't read through this particular thread, but just thought I'd post my standard response. 

• Lens focal length and aperture are optical properties of a lens, constant regardless of what format you use the lens to illuminate. 
• Field of View and Depth of Field are properties of the combined imaging system of lens plus recording medium. The same lens on a FF format nets more field of view and less depth of field in total then it does when used at the same focus setting and aperture number on an APS-C format, and vice versa. 

For example, a 50mm lens fitted to a FF camera and focused to 10 feet at f/8 nets Field of View numbers thus:

  f	  Hor       Vert      Diag    
 50mm	 39.5978   26.9915   46.7930 in degrees

And nets DoF numbers thus: 

Subject distance

10 ft

 

Depth of field 

Near limit 

7.77 ft

Far limit 

14 ft

Total 

6.28 ft

 

In front of subject 

2.23 ft (36%)

Behind subject 

4.04 ft (64%)

 

Hyperfocal distance 

34.3 ft

Circle of confusion 

0.03 mm

THE SAME 50mm lens fitted to a 1.5x crop APS-C camera and focused to 10 feet at f/8 nets Field of View numbers thus: 

  f	  Hor       Vert      Diag 
 50.0	 26.6662   17.8452   31.7393 in degrees

and DoF numbers thus: 

Subject distance

10 ft

 

Depth of field 

Near limit 

8.39 ft

Far limit 

12.4 ft

Total 

3.98 ft

 

In front of subject 

1.61 ft (40%)

Behind subject 

2.37 ft (60%)

 

Hyperfocal distance 

51.4 ft

Circle of confusion 

0.02 mm

I hope that makes the situation clear. 

G

[Guest]

18 minutes ago, jaapv said:

Yes, but I notice that you mention the Sigma 16/1.4. At such angles AF holds very little advantage.  TL lenses are not THAT small, compared to M lenses,

At 1.4, even at 24mm (!), I find I need AF. The Sigma lenses (24, 45 and 85 equivalents, all F1.4) aren’t super small, but they are light, and cheap, so for me represent a good option I think.

[jaapv]

Hmm. I use my Summilux 24 without AF and without any problems, both on my Ms and on my CL and TL.

[Guest]

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No need to Hmm, everybody is different.

[bags27]

5 minutes ago, AndyGarton said:

No need to Hmm, everybody is different.

Hmm, very true. 😀

[jaapv]

3 hours ago, AndyGarton said:

No need to Hmm, everybody is different.

But the focusing is the same...

[Guest]

Ok then, you’re right and I’m wrong.

[Marac]

I have all 3 Sigma lenses you are talking about, I also have a CL and an S1R. I will put the 16mm on both, on a tripod and shoot the same scene at f1.4 then post them both, untouched in post.

I'll post the result here very soon for you.

[Guest]

Thanks, I really appreciate the offer, and would certainly be interested, but really the comparison I'm talking about is between a 16mm APS-C lens and a 24mm full frame lens, both f1.4, both on a full frame body. But I think it's pretty clear now what I'd see, so I don't even need that.

(I've ordered all three lenses now by the way, I'm confident they'll be very useful to me, although perhaps the 30mm won't offer much over the Sigma 45mm f2.8 FF lens I already have. Still a whole extra stop though, if I assume f1.4 becomes f2. 30 megapixels is still more than enough for me too.)

Edited August 6, 2020 by Guest

[jaapv]

2 hours ago, AndyGarton said:

Ok then, you’re right and I’m wrong.

That was not the point - the point is that very few of us, not just me,  will see a difficulty in focusing a 24 mm lens on a rangefinder - there must be another reason. For instance the availability of a histogram in an EVF, especially interesting for exposing wideangle shots. Or an artificial horizon.

[Guest]

21 minutes ago, jaapv said:

That was not the point - the point is that very few of us, not just me,  will see a difficulty in focusing a 24 mm lens on a rangefinder - there must be another reason. For instance the availability of a histogram in an EVF, especially interesting for exposing wideangle shots. Or an artificial horizon.

I have difficulties manually focusing a 1.4 lens quickly enough, or indeed accurately enough, even on an EVF camera. Hence I'd prefer an AF option, especially for indoor low light people scenarios, where reacting quickly is important. Not sure why that is so hard to understand! If others can do it quickly enough with manual focus lenses, great, I'm happy for you, and a bit jealous (i could mostly do it when I was younger with better eyesight!).

[jaapv]

3 hours ago, AndyGarton said:

Ok then, you’re right and I’m wrong.

Well, failing eyesight is a very good reason to move from RF to AF. However. I find the magnification feature on Leica's EVFs quite good and fast.

[Guest]

I find it quite good also, but I'm still not fast enough. On the fp also I only have the rear screen most of the time (I do have the separate viewfinder, but it's too big to use often), and I definitely can't see accurately enough to focus there (to be honest I struggle even with composition!). Don't get me wrong, I enjoy using my M lenses when the time is right, e.g. outdoors in good light with static subjects.

[tom0511]

Am 5.8.2020 um 10:24 schrieb AndyGarton:

Hi guys, a hopefully simple question which I've seen conflicting answers on. When using, for example, a 16mm f1.4 lens on a full frame body, I understand the change to field of view (24mm) and also the reduction in overall image size (e.g. 24mp down to 16), but I'm not sure about depth of field. When used wide open, do you get the depth of field of a 24mm f1.4, or a 24mm f2? (Or something else?)

Using a 16/1.4 mm lens on dx sensor will show approx. the DOF like shooting a 24mm lens at f2.0 on a FF camera.