APS-C Lens on Full Frame Camera - DoF Question

[Guest]

Advertisement (gone after registration)

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?)

[jaapv]

Search the forum, there are extensive threads aplenty. Admittedly,they are highly confusing for the amount of twisted thinking and misconception displayed. 
If the subject distance and aperture remain the same DOF will remain the same, if the print size is cropped by the same amount. (it is equivalent to cutting the edges op the FF print). If you match the print size, DOF will decrease because you blow the final image up more. If you match the framing by increasing subject distance and keep the same print size, DOF will increase because of the longer distance, and perspective will change. Rule of the thumb: assume that DOF will decrease by one stop when switching to APS-C.

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[Guest]

I've read lots of threads which confused me, hence phrasing the question the way I did! Forgetting printing, cropping etc, will a 1.4 APS-C lens act like a 1.4 or 2.0 DoF wise, on a full frame camera?

[jaapv]

If you mean on an SL, the camera will "crop" the sensor down to APS-C automatically, so it will behave exactly the same on the SL as on a CL or TL.

I don''t know about the S1 but if they don''t crop down, the lens will not cover the frame.

[Guest]

SL2 and Sigma fp, and both crop down automatically (optionally in the fp's case, which I wish Leica would do too). So the answer is 1.4? Or does a 1.4 lens on a CL/TL (which I don't have) behave like an f2?

Edited August 5, 2020 by Guest

[LocalHero1953]

This does seem to be a subject on which everyone has their own (occasionally twisted) way of explaining in words the actual visible facts and underlying science. My best suggestion is that you try it for yourself and reach your own subjective understanding.

FWIW, when using a lens on my CL, I mentally multiply the focal length by 1.5 to get the equivalent field of view that I would see on my SL: a 35mm lens on my CL, whether TL or L mount, shows me the same field of view I would get if using a 50mm on my SL.

And FWIW in the same scenario, the depth of field I get with a 35 mm lens on my CL is the same depth of field I would get if I were to use a 50mm lens on my SL.

That's how I look at it!

[jaapv]

Advertisement (gone after registration)

15 minutes ago, AndyGarton said:

SL2 and Sigma fp, and both crop down automatically (optionally in the fp's case, which I wish Leica would do too). So the answer is 1.4? Or does a 1.4 lens on a CL/TL (which I don't have) behave like an f2?

No - the sensor is the same  size (artificially) so the behaviour is the same

[Guest]

No to which question please?

[Guest]

The answer is 2.0 if the camera automatically crops to an APS-C size sensor.

[jaapv]

If the APS-C lens was 2.0.  There is absolutely no DOF difference between a naturally APS-C sensor and a cropped-to-APS-C Full-frame (or medium format etc) sensor. Joachim is right if you were using a full-frame Summilux on an APS=C sensor. Provided subject distance and aperture are unchanged. 

In other words: wearing a TL lens, the SL is just a big CL to all intents and purposes. (nothing wrong with that  )

[Guest]

So the answer is 1.4?

[lct]

So the answer is.... use a good DoF calculator

http://www.dofmaster.com/dofjs.html

 

[Guest]

Fun that nobody is willing to commit to a straight answer! I'm going to assume it's 2.0 amounts of DoF, 1.4 amounts of light gathering (i.e. for the exposure, so I do "gain" the extra stop, allowing faster shutter speeds or lower ISO).

[Guest]

7 minutes ago, AndyGarton said:

Fun that nobody is willing to commit to a straight answer! I'm going to assume it's 2.0 amounts of DoF, 1.4 amounts of light gathering (i.e. for the exposure, so I do "gain" the extra stop, allowing faster shutter speeds or lower ISO).

Your question was:

When used wide open, do you get the depth of field of a 24mm f1.4, or a 24mm f2?

My answer was:

The answer is 2.0

Not a straight answer? What else do you want?

[lct]

3 minutes ago, AndyGarton said:

Fun that nobody is willing to commit to a straight answer! I'm going to assume it's 2.0 amounts of DoF, 1.4 amounts of light gathering (i.e. for the exposure, so I do "gain" the extra stop, allowing faster shutter speeds or lower ISO).

You'll get a straight answer if i understand your question. Must be my poor english, never mind... Just to be clear: 
1. What do you mean by a 16/1.4 lens? A lens made for FF or APS-C camera?
2. What do you mean by 16mm? The actuel focal length of the lens or anything else (so-called "equivalent" things)?
3. What do you mean by f/1.4? The actual aperture of the lens or anything else (so-called "equivalent" things)?
4. Do you intend to use the same lens on both FF and APS bodies or two different lenses?

[earleygallery]

4 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?)

OK let me try, going back to your original question and making a couple of assumptions ..........

If you have a 'full frame' lens that is a 16mm f1.4 and you use that on an APS-C camera then you will get an equivalent field of view as a 24mm lens (lets say 1.5x as not all APS-C are the same). 

It is still a 16mm f1.4 lens but taking the reduced field of view into account it will give the equivalent DoF as an f2 lens - the 16mm f1.4 gives an image result that will be approximately the same as a 24mm f2 lens made for an APS-C camera (on a like for like comparison).  

You would still make the correct exposure for a 1.4 lens however as the physical characteristics of the lens don't change just because you're using it on a camera with a smaller sensor than it was primarily designed for. 

Edited August 5, 2020 by earleygallery

[jaapv]

Light gathering is misleading nonsense; forget about it and certainly in this context. A lens is a certain speed, say 1.4, and will remain that speed regarding exposure in all circumstances, the DOF can only change if you change the sensor format (which not when using a cropped SL(2) ), change the magnification throughout the system or change subject distance and/or aperture. In other words: Use a TL lens on the SL as you would use it on a CL and you will get exactly the same results in all respects (except pixel count). Basically you are using an APSC camera and throwing away the rest of the sensor surface.

[jankap]

And now the question when one uses a lens booster (= focal reducer)! So a full frame lens on a APS-C camera concentrating all the light it collects on the smaller sensor.

A FF 16mm  f/1.4 draws as the equivalent of an 24mm f/1.4 with a normal adapter (on APS-C), but with a lens booster it draws as the equivalent of an 16mm f/1.0 lens. 

What DOF in the last case?

[jaapv]

2 hours ago, jaapv said:

In other words: wearing a TL lens, the SL is just a big CL to all intents and purposes. (nothing wrong with that  )

Edited 2 hours ago by jaapv

 

1 hour ago, AndyGarton said:

Fun that nobody is willing to commit to a straight answer!

What isn't straight here?

[Advertisement]

Hallo,

Look here: Leica L-Mount

[Guest]

1 hour ago, Joachim_I said:

Your question was:

When used wide open, do you get the depth of field of a 24mm f1.4, or a 24mm f2?

My answer was:

The answer is 2.0

Not a straight answer? What else do you want?

You didn't quote your full answer! "The answer is 2.0 if the camera automatically crops to an APS-C size sensor." I didn't understand the qualifying part.