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01-07-2016, 12:43 PM   #1
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Shooting APS-C vs FF - perspective, DOF, OOF rendering, FL equivalence...

All, I've asked here before for help in understanding perspective distortion and equivalent focal lengths for lenses used on APS-C and full frame, and I got some very helpful responses.

I thought I understood it, and I think I still do, but last night I got myself a bit muddled up (probably one glass of wine too many! ), so I'd like to confirm my understanding. Can you tell me if the following is correct, or if not, where I've gone wrong?

So... Let's say I have two cameras - one full frame DSLR, and one APS-C DSLR with a 1.5x crop factor. They are fitted with the same 85mm lens set at f/5.6, and the cameras are set to ISO 200 and 1/100 second shutter speed. They are set up on tripods the same distance away from the subject, in order to take a head and shoulders portrait shot, with both lenses focused on the eyes. A little further behind the subject is a brick wall acting as a background for the shot.

Looking through the full frame camera's viewfinder, the head and shoulders are perfectly framed, the subject is in focus and the brick wall is just out of focus enough to give a smooth, pleasing background. I take my shot and I'm happy with it.

I move over to the APS-C camera. I look through the viewfinder and the view is cropped all around compared to the full frame - no good at all. I have two choices: (a) move back until the field of view is the same as with the full frame camera (ie. zoom out with my feet ), or (b) leave the camera where it is, and swap out the lens for a 55mm model which will give close to the same field of view as the 85mm on full frame.

Looking at both approaches individually:

(a) Moving back

I move the APS-C camera back until I get the same framing of the subject as I see through the full frame camera's viewfinder, I re-focus on the eyes, and take my shot. It's OK, but what I've captured doesn't look the same. Since my subject is now further away from the lens, perspective distortion has caused the subject's features to look a little flatter (more "compressed") and the background appears to be closer. The background is also less out-of-focus (thus, more distracting) because the depth of field has increased with the subject distance. There's nothing I can do about the perspective distortion, but I can improve the background by opening up the aperture one stop to f/4, hence reducing the depth of field. To compensate for the extra light coming in, I use a faster shutter speed of 1/200 second or drop my ISO to 100.

(b) Fit a 55mm lens in place of the 85mm

I fit the 55mm lens to the APS-C camera, set it to f/5.6, focus on the subject's eyes, and take the shot. It looks good, but - again - not exactly the same as I got from the full frame setup. The framing, field of view and perspective look about right, and there is no difference in compression of the subject and background. But, the background is less out-of-focus, since the shorter focal length of the lens has resulted in a greater depth of field at the same aperture. I can address this by opening up the aperture to f/4, and using a shutter speed of 1/200 second or setting ISO to 100 to compensate.

Drawing on option (b), if I wanted to emulate a full frame street photographer who is using a 35mm lens at f/8.0 with a shutter speed of 1/60 second and ISO of 400, I could use my K3 with a 24mm lens at f/5.6 and a shutter speed of 1/125 second at the same ISO, or keep the shutter speed the same, and drop the ISO sensitivity to 200.

Have I got this right, or am I badly mixed up?!?

Thanks in advance!


Last edited by BigMackCam; 01-07-2016 at 12:48 PM.
01-07-2016, 12:45 PM - 7 Likes   #2
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Wait. I'm going for popcorn, don't start yet. Back in a minute.
01-07-2016, 12:46 PM   #3
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QuoteOriginally posted by BigMackCam Quote
Have I got this right?
I think so.
But now you're confusing me
01-07-2016, 12:52 PM   #4
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QuoteOriginally posted by BigMackCam Quote
Let's say I have two cameras - one full frame DSLR, and one APS-C DSLR with a 1.5x crop factor. They are fitted with the same 85mm lens set at f/5.6, and the cameras are set to ISO 200 and 1/100 second shutter speed.
Wait...
for them to have the same exposure (F5.6, ISO 100, 1/100 shutter) the pixel size would have to be the same.
So a 36MP FF and 16MP APS-C or 24MP FF and 10MP APS-C, etc...

01-07-2016, 01:02 PM   #5
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QuoteOriginally posted by bertwert Quote
Wait...
for them to have the same exposure (F5.6, ISO 100, 1/100 shutter) the pixel size would have to be the same.
So a 36MP FF and 16MP APS-C or 24MP FF and 10MP APS-C, etc...
That's not my understanding - I thought exposure is exposure is exposure, whatever the sensor - so 1/100 sec at f/5.6, ISO 100 is the same (or very close) on any sensor (assuming calibration is consistent)...
01-07-2016, 01:04 PM   #6
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QuoteOriginally posted by BigMackCam Quote
That's not my understanding - I thought exposure is exposure is exposure, whatever the sensor - so 1/100 sec at f/5.6, ISO 100 is the same (or very close) on any sensor (assuming calibration is consistent)...
Okay.
Let's say both cameras are 24MP.
The full frame pixels are 2.25x the size of the APS-C pixels gathering 2.25x more light, do you follow?
Therefore the FF would be overexposed or the APS-C underexposed.
01-07-2016, 01:06 PM   #7
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QuoteOriginally posted by bertwert Quote
Okay.
Let's say both cameras are 24MP.
The full frame pixels are 2.25x the size of the APS-C pixels gathering 2.25x more light, do you follow?
Therefore the FF would be overexposed or the APS-C underexposed.
Yes, I follow, but I thought the whole point of the ISO standard was to normalise the expression of light gathering capability - so ISO 100 on one medium (or sensor) is the same as ISO 100 on another. If that's the case, and I believe that's correct, then shutter speeds and apertures fall into line.
01-07-2016, 01:09 PM   #8
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QuoteOriginally posted by BigMackCam Quote
Yes, I follow, but I thought the whole point of the ISO standard was to normalise the light gathering capability - so ISO 100 on one medium (or sensor) is the same as ISO 100 on another. If that's the case, and I believe that's correct, then shutter speeds and apertures fall into line.
Ok...
That makes sense.
If you provide me with two cameras (one FF and one APS-C) of 24MP I'll test and prove you right

I don't know really I'm not an expert in this field and I've never shot a FF camera.

01-07-2016, 01:13 PM   #9
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QuoteOriginally posted by BigMackCam Quote
That's not my understanding - I thought exposure is exposure is exposure, whatever the sensor - so 1/100 sec at f/5.6, ISO 100 is the same (or very close) on any sensor (assuming calibration is consistent)...
You are right. Your description ob (b) mostly is right as well.

The one thing to avoid though is that DoF and background blur are not the same, not in sync.
Shoot a 1m high subject in front of a distant mountain on APSC with two lenses:
a) 50/1.8 at 1.8
b) 200/4 at 4
and to the surprise of many you will find that b) here has twice MORE DoF than a) (just like APSC "has more DoF than FF") but also twice MORE background blur (just like "FF has more background blur than FF").
Just one example why looking at DoF is such a waste.

Enjoy the ride.
01-07-2016, 01:13 PM   #10
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QuoteOriginally posted by bertwert Quote
Ok...
That makes sense.
If you provide me with two cameras (one FF and one APS-C) of 24MP I'll test and prove you right

I don't know really I'm not an expert in this field and I've never shot a FF camera.
That is not how exposure works.

Anyway, perspective is entirely based on the distance from the camera to the subject, regardless of lens or format. Taking a ultrawide 12mm shot and cropping it or using a nice 85mm from the exact same spot will give you the exact same perspective.
01-07-2016, 01:19 PM   #11
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QuoteOriginally posted by beholder3 Quote
You are right. Your description ob (b) mostly is right as well.

The one thing to avoid though is that DoF and background blur are not the same, not in sync.
Shoot a 1m high subject in front of a distant mountain on APSC with two lenses:
a) 50/1.8 at 1.8
b) 200/4 at 4
and to the surprise of many you will find that b) here has twice MORE DoF than a) (just like APSC "has more DoF than FF") but also twice MORE background blur (just like "FF has more background blur than FF").
Just one example why looking at DoF is such a waste.

Enjoy the ride.
Ah, thanks for this. Interesting - I hadn't understood the DoF / blur aspect. Very useful!

---------- Post added 01-07-2016 at 08:20 PM ----------

QuoteOriginally posted by enoeske Quote
That is not how exposure works.

Anyway, perspective is entirely based on the distance from the camera to the subject, regardless of lens or format. Taking a ultrawide 12mm shot and cropping it or using a nice 85mm from the exact same spot will give you the exact same perspective.
Right, I get that. Hence, a 55m lens and 85mm lens shot at the same distance give the same perspective, as in my example (b), yes?
01-07-2016, 01:23 PM - 2 Likes   #12
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More or less. I don't really buy the whole equivalence stuff, mainly because I find it dwells on the need to find ultimate shallow depth of field. You can shoot plenty shallow portraits on APS-C at f2.8. No one takes a photo with a full frame camera and then asks the question "How can I re-create this photo?" They pick up a camera, choose a lens and then compose in such a way as to flatter their subject.

The whole "full frame flatters my subject more" is fallacious, in my opinion. Subjects are flattered by choice of light, correct focal length and appropriate composition. This is true regardless of format. Shooting with 55mm on APS-C doesn't really give different results from 85mm on full frame, even though many people seem to think it does.


Last edited by Rondec; 01-07-2016 at 01:40 PM.
01-07-2016, 01:27 PM   #13
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QuoteOriginally posted by BigMackCam Quote
Yes, I follow, but I thought the whole point of the ISO standard was to normalise the expression of light gathering capability - so ISO 100 on one medium (or sensor) is the same as ISO 100 on another. If that's the case, and I believe that's correct, then shutter speeds and apertures fall into line.
Correct (until someone starts whining about DOF equivalence). If however you were at ISO 3200, then the FF camera (with pixels double the size) would have lower noise levels for the same ISO/exposure.

Aperture, DOF, and bokeh, this is when the popcorn comes out.

I shoot 67 film, where I use a 105mm lens at f2.4. If I wanted exactly the same shot on FF, I'd need a 50mm f1.4 lens. For apsc, I'd need a 35mm lens with <f1.2.

I really hope that's the end of this thread.
01-07-2016, 01:31 PM   #14
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QuoteOriginally posted by Rondec Quote
More or less. I don't really buy the whole equivalence stuff, mainly because I find it dwells on the need to find ultimate shallow depth of field. You can shoot plenty shallow portraits on APS-C at f2.8. No one takes a photo with a full frame camera and then asks the question "How can I re-create this photo?" They pick up a camera, choose a lens and then compose in such a way as to flatter their subject.

The whole "full frame flatters my subject more" is fallacious, in my opinion. Subjects are flattered by choice of light, correct focal length and appropriate composition. This is true regardless of format. Shooting with 55mm on APS-C doesn't really give different results from 85mm on full frame, even though many people seem to think it does.


https://flic.kr/p/upkuVp
Thanks, Vincent. Maybe equivalence is the wrong way for me to describe it... I guess my motivations in understanding this might help frame the point... I read about various established and published photographers whose work I appreciate, and I look at the equipment and settings they often shoot with. I then like to think, if I was trying to achieve the same outcome with my equipment, how would I do it? It doesn't mean I *want* to re-create it, and certainly not *exactly*, but I like to understand how I would do so. Also, many articles in magazines and on the internet explain how a certain look or effect is achieved in a photograph. But some of these articles relate to 35mm film / full frame DSLR, while others relate to APS-C, others still to medium and other formats. By understanding the fundamental conversions to achieve broad "equivalence" (sorry! ), it helps me understand how such advice relates to my own setup.

EDIT: The example I gave of a street photographer was based on Bill Cunningham, the New York photographer, who I believe shot (and still shoots) a lot of his work with a 35mm film camera using a 35mm lens

---------- Post added 01-07-2016 at 08:34 PM ----------

QuoteOriginally posted by robthebloke Quote
Correct (until someone starts whining about DOF equivalence). If however you were at ISO 3200, then the FF camera (with pixels double the size) would have lower noise levels for the same ISO/exposure.

Aperture, DOF, and bokeh, this is when the popcorn comes out.

I shoot 67 film, where I use a 105mm lens at f2.4. If I wanted exactly the same shot on FF, I'd need a 50mm f1.4 lens. For apsc, I'd need a 35mm lens with <f1.2.
Excellent, I understand that perfectly. Thanks, Rob.

QuoteOriginally posted by robthebloke Quote
I really hope that's the end of this thread.
If you have (as I think you have) confirmed that all of my understanding in my original post is correct (with the exception of the DoF / blur issue already raised), then I'm happy for the thread to end too

Last edited by BigMackCam; 01-07-2016 at 01:39 PM.
01-07-2016, 01:35 PM   #15
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QuoteOriginally posted by bertwert Quote
Wait...
for them to have the same exposure (F5.6, ISO 100, 1/100 shutter) the pixel size would have to be the same.
So a 36MP FF and 16MP APS-C or 24MP FF and 10MP APS-C, etc...
As someone who happily shoots APS-C and FF, "Oh, no'! 😀
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