Author Topic: Apertures, shutter speeds and depth of field  (Read 9954 times)

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Offline Clive

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Apertures, shutter speeds and depth of field
« on: April 03, 2008, 02:53:28 PM »
This is an excerpt from one of my web pages >> http://members.shaw.ca/ocl3/technical1.html

The relationship between apertures, shutter speeds and depth of field is obvious in the images below. Wide apertures (small f number like f 2.8 ) result in faster shutter speeds which is good for stopping action. (For the record, those are potatoes on a conveyor belt moving uphill from left to right.)  If your scene or subject requires a deep depth of field (lots in focus from front to back), then you will need a small aperture (large number). However, the shutter speed will be reduced and that may be a problem. However, if you want to show blurred action, then this is good. The selection is sometimes a compromise.

Note that as the shutter speeds get longer, the potatoes blur more and more because they travel uphill while the shutter is open. The images in the middle column show how depth of field increases as the aperture gets  smaller.



Click here for more DOF images...scroll down Sorry of the bad formatting.
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Offline Simon [aka springtide]

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Re: Apertures, shutter speeds and depth of field
« Reply #1 on: April 03, 2008, 07:18:27 PM »
Cool  :) 


Something that I didn't know until a couple of weeks ago...

Anyone like to guess [ maybe this is common knowledge ??? ] ....

Do you get more or less DOF using the same lens with a FF or APS-C sensor with the same "framing" ?
(i.e. you need to walk closer with the FF camera)

Or, if you had an FF camera with a 300mm lens at f4, and a APS-C camera (1.5 crop) with a 200mm lens at f4 (so same focal length), do they give the same DOF, or is one more than the other?

« Last Edit: April 03, 2008, 07:28:06 PM by springtide »
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Offline Akshay Jamwal

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Re: Apertures, shutter speeds and depth of field
« Reply #2 on: April 03, 2008, 08:02:34 PM »
If you're saying that the magnification is the same (i.e. apparent spatial relationship of objects in respect to one another), then there would be no change in DOF at all, since you'd be moving closer (or farther) to your subject.

Note that focal length isn't a relative term, it is a constant, regardless of the crop factor :P.
If you aren't changing distance, then all (other) things being equal, a smaller format would inherently give you more DOF.
« Last Edit: April 03, 2008, 08:05:35 PM by Akshay Jamwal »
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Offline Asher

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Re: Apertures, shutter speeds and depth of field
« Reply #3 on: April 03, 2008, 09:25:03 PM »
Do you get more or less DOF using the same lens with a FF or APS-C sensor with the same "framing" ?
(i.e. you need to walk closer with the FF camera)

FF would have less DOF in this situation.  With the same focal length (the same lens = same focal length) and the same aperture the shorter the distance the less depth of field you have.
« Last Edit: April 03, 2008, 09:28:48 PM by Asher »

Offline Stef.

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Re: Apertures, shutter speeds and depth of field
« Reply #4 on: April 04, 2008, 12:44:28 AM »
Well- I am not a technician but my guess would be that with a ff camera- same lens and same framing the dof will be greater than with the APS sensor. My guess is purely based on the fact that with the ff frame camera the same lens becomes a wider angle lens and they tend to have more dof.
Come on! Give the answer pleeeeeese!
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Offline Clive

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Re: Apertures, shutter speeds and depth of field
« Reply #5 on: April 04, 2008, 02:15:44 AM »
My brain hurts when I think about super detailed stuff like that. I've no idea what happens to DOF with APS v FF sensors. I've a simple solution to things  don't understand. I just say, "It is what it is -- deal with it." And then I can deal with it. And that's why we use the DOF preview button ...  right boys and girls. :P

This MIGHT help.....sorry I am unable to provide a reference. I shamefully stole it years ago and did not keep the reference.

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Offline Clive

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Re: Apertures, shutter speeds and depth of field
« Reply #6 on: April 04, 2008, 02:48:11 AM »
Found this ... it is all as clear as mud now... :P From here..
http://pentax-toby.blogspot.com/2007/06/depth-of-field-myth-on-aps-dslr.html
==============================================================
Wednesday, June 13, 2007
The depth-of-field myth on APS-DSLR
A common question: "how does the depth-of-field on 1.5X crop factor (APS size) compare to full-frame film?"

I don't want to go into the theoretical and mathematical details of depth-of-field. So I will use a Palm program to solve this. The circle-of-confusion used by the program is 30um for full-frame, and 25um for APS-DSLR.

In short, the DOF is affected by: focal length, shooting distance, aperture, and film (sensor) format. So the above question is a tricky question because it has 2 different answer. The DOF on 1.5X DSLR is bigger then 135 full-frame, and it's also smaller. It all depends on how you look at the question.

First, if you are using 'equivalent' lens on your DSLR, shooting at the same position, using the same aperture.... the DOF on DSLR is greater than full-frame film. eg:
full-frame: 75mm lens, f/2, distance=2m
APS-DSLR: 50mm lens, f/2, distance=2m
You will get the same framing and perspective with the above 2 setting. But the DOF with APS-DSLR will be bigger than full-frame 135:
DOF of full-frame: 1.96m to 2.04m, ie. 8cm
DOF of APS-DSLR: 1.92m to 2.08m, ie. 16cm

So, many people say full-frame has more 3D feeling than APS-DSLR. The main reason it the difference in DOF: a shallower DOF gives more 3D feeling.

On the other hand, if you are using SAME LENS, shooting at SAME DISTANCE, than the DOF of APS-DSLR will be shallower than full-frame 135, eg:
full-frame: 50mm, f/2, distance=2m, DOF=1.91m to 2.10m, 19cm
APS-DSLR: 50mm, f/2, distance=2m, DOF=1.92m to 2.08m, 16cm
So if you are going to use the DOF scale on your lens, the DOF on APS-DSLR will be approx. 1/4 stop shallower.

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Offline Fud

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Re: Apertures, shutter speeds and depth of field
« Reply #7 on: April 04, 2008, 07:53:07 AM »
OK had a play around with my DOF Calculator spreadsheet.  I assume that if you are shooting from 10m away on a 1.5x crop sensor with a 100mm lens you would need to shoot from 6.67m away on a full sensor to fill the same picture - is this right?

If so then the FF sensor would have a much smaller DOF.  I have assumed that the Circle of Confusion for a FF sensor would be 0.03mm which is generally regarded as about right compared to the cropped sensor at 0.02mm.  If that is the case here are the depths of field in metres for a cropped sensor at 10m

       f5.6       f8
200   0.55   0.79
100   2.25   3.25
50   11.12   21.42

and for a FF at 6.67m

       f5.6       f8
200   0.36   0.52
100   1.49   2.15
50   7.39   14.49


As you can see the DOF is about a third less on a FF sensor.
A900+VG, A77+VG, Minolta 17-35G,  Sony CZ24-70 F2.8,Sony 70-200 F2.8 SSM, Sony 70-400 SSM, Sony 135STF,85 F1.4, 200 F4 Macro, 600 F4 HS APO

Offline Asher

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Re: Apertures, shutter speeds and depth of field
« Reply #8 on: April 04, 2008, 08:00:17 AM »
My guess is purely based on the fact that with the ff frame camera the same lens becomes a wider angle lens and they tend to have more dof.
It gives you a wider angle of view but the focal length is the same.

Offline Simon [aka springtide]

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Re: Apertures, shutter speeds and depth of field
« Reply #9 on: April 04, 2008, 10:00:05 AM »
My guess is purely based on the fact that with the ff frame camera the same lens becomes a wider angle lens and they tend to have more dof.
It gives you a wider angle of view but the focal length is the same.

:)   This took me a little time to figure out in my head..

And that statement was the one that make it all click!

So a 200mm lens on FF, still gives you a "200mm image" on APS-C, it's just that you've "cropped" the picture.
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Offline fother

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Re: Apertures, shutter speeds and depth of field
« Reply #10 on: April 04, 2008, 10:33:48 AM »
Bingo! :mrgreen:

Offline Asher

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Re: Apertures, shutter speeds and depth of field
« Reply #11 on: April 04, 2008, 11:01:35 AM »
I think the most important thing to recognise is DOF as related to the scene you capture.

The larger the sensor, the smaller the DOF when capturing the same scene (assuming that aperture is the same).

What do I mean by the same scene?  The same angle of view and the same perspective so that the foreground, background and subject are all in the same position in the frame.

By the way, this is not a new discussion brought about by DSLR.  The same concepts and understanding are also related when comparing 35mm and medium/large format.