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08-14-2017, 01:52 AM   #1
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Question about Depth-of-field & focal length relation

(Sorry if my questions are obvious for you, I''m still learning things )

In searching for a good portrait lens, I've seen many discussions and debates, about the DA 70 vs DA* 55, or FA 50, for example.

For depth-of-field specifically, people said 2.4 is still big enough with the DA 70, because with the 70mm focal length, you would achieve more shallow depth-of-field at the same aperture, than with the 50s. This is also what I read in theory.

My concern is: if I'm using a 70mm, standing 2m away from my model, to have a head and shoulder portrait, shooting at f2.4. If I switch to a 50mm, still at 2.4, the DOF would not be as shallow. However, to achieve the same composition, I have to move closer, say 1.5m. Wouldn't that make the DOF shallower, effectively canceling the advantage of the longer focal length? I have a Sigma 30 1.4, and at f1.8 it is so blurry, I'm not sure if 2.4 of the 70mm can give that

Also, another question: why a bigger sensor will give you shallower DOF providing the same aperture? a 70mm 2.4 on crop sensor would be equivalent to a 45mm f? on FF?

Thank you so much

08-14-2017, 02:07 AM - 1 Like   #2
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Hi Bui, I hope this DOF calculator helps to answer some of your questions. Good luck.


Online Depth of Field Calculator
08-14-2017, 03:49 AM - 1 Like   #3
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QuoteOriginally posted by Bui Quote
For depth-of-field specifically, people said 2.4 is still big enough with the DA 70, because with the 70mm focal length, you would achieve more shallow depth-of-field at the same aperture, than with the 50s
That rule would only apply if you did not change your camera to subject distance. If you did move closer to the the subject with a 50mm lens, the same aperture gives the same DOF.

The "apparent" narrower DOF you get with a telephoto is due mainly to the squeezing of perspective of the subject to background.
08-14-2017, 04:00 AM - 2 Likes   #4
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QuoteOriginally posted by Bui Quote

[snip]

Also, another question: why a bigger sensor will give you shallower DOF providing the same aperture? a 70mm 2.4 on crop sensor would be equivalent to a 45mm f? on FF?

Thank you so much
I think there's a lot of confusion regarding DOF on FF versus crop. The DOF of a specific lens at a certain aperature at a certain distance from the subject is going to be the same regardless of the image sensor size. Where I think people get hung up on this is if you're moving the lens forward or backwards to frame the same area of the subject to account for the difference in what you see framed with FF versus crop, you will change the DOF in the process. If you stay in one position and change lenses to have the same framing or field of view going between FF and crop, you'll change the perception of DOF because you've changed the lens, and the DOF of the different focal lengths will be different. So while the actual DOF of an object 10 feet away for a 70mm at 2.4 will always be the same regardless of the sensor size, if you start changing the distance of the lens to the subject - get closer with the 70 on FF to get the same field of view as the 70 on crop let's say, then you'll "change" the perception of DOF, and this case, it will get "narrower".

Hope I said that in a way that makes sense


Last edited by clickclick; 08-14-2017 at 04:28 AM. Reason: clarity.....
08-14-2017, 04:21 AM - 1 Like   #5
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QuoteOriginally posted by Bui Quote
(Sorry if my questions are obvious for you, I''m still learning things )

In searching for a good portrait lens, I've seen many discussions and debates, about the DA 70 vs DA* 55, or FA 50, for example.

For depth-of-field specifically, people said 2.4 is still big enough with the DA 70, because with the 70mm focal length, you would achieve more shallow depth-of-field at the same aperture, than with the 50s. This is also what I read in theory.

My concern is: if I'm using a 70mm, standing 2m away from my model, to have a head and shoulder portrait, shooting at f2.4. If I switch to a 50mm, still at 2.4, the DOF would not be as shallow. However, to achieve the same composition, I have to move closer, say 1.5m. Wouldn't that make the DOF shallower, effectively canceling the advantage of the longer focal length? I have a Sigma 30 1.4, and at f1.8 it is so blurry, I'm not sure if 2.4 of the 70mm can give that

Also, another question: why a bigger sensor will give you shallower DOF providing the same aperture? a 70mm 2.4 on crop sensor would be equivalent to a 45mm f? on FF?

Thank you so much
You need to go back to basics.

Depth of field is based upon the definition of what is acceptably sharp. This all goes back to the film era where the common print format was an 8" X 10" print.

Acceptably sharp was considered when a point source still appeared to be less than 1/100 of an inch. When looking at a 35mm negative blown up to fill the 8" X 10" format.


It all relates to this definition, and to the amount of magnification in your processing. If you enlarge beyond the 8x10 print size depth of field goes down.

Overall you need to consider the full magnification subject through lens, to sensor then enlarged to print.

Play with any of the DOF calculators on line and you will see the relationships form.

Subject distance , lens and sensor size are all in the equation, (but they seem to always omit the print size) but if you brake it down, you will soon see that in the end focal length, distance and sensor size (assuming you fill the sensor with the image) really only come back to the magnification of the system.

What depth of field calculators do not do, is give you any idea of the rendering of the out of focus area or bokeh. This is far more important that depth of field on its own, and generally is "nicer" with longer focal lengths, because the out of focus points appear as big blobs, compared to small circles (or what ever shape your aperture is)

Also consider when shooting portraits, it is as important to control the subject to background distance and background pattern as it is to control depth of field
08-14-2017, 04:24 AM - 1 Like   #6
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QuoteOriginally posted by Bui Quote
Also, another question: why a bigger sensor will give you shallower DOF providing the same aperture? a 70mm 2.4 on crop sensor would be equivalent to a 45mm f? on FF?
DoF depends on the lens, not the sensor. The focal length, aperture, and focus distance. But bigger sensor has wider FoV, so the DoF seems more shallow.
The confusion comes from the idea of "equivalence". This is when people want to take the exact same photo with two cameras with different sensor size. This causes all kinds of headaches, because you will have to adjust one of the things I mentioned above. You will have to step closer, adjust focus, or switch lens focal length, or change ISO and shutter speed. This is why the equivalists claim ASPC has wider DoF and more noise than FF. Basically, you can just ignore that whole issue unless you have two cameras with different sensor sizes and for some reason you want to take same photo
Get the lens you want, for example one that gives you perspective and bokeh that you want. Unfortunately, the only way you can figure this out is by looking at sample photos and doing your own tests. You need experience. Start with a decent lens that you can afford without trouble and then learn from it. You can look at those DoF calculators, but I recommend you just buy a used DA 50mm f1.8 (or FA 50mm f1.4, whichever you can get in good condition), see how it works, and then decide if you want to upgrade to a DA* 55mm or DA 70mm or whatever.

Last edited by Na Horuk; 08-14-2017 at 04:33 AM.
08-14-2017, 04:48 AM - 2 Likes   #7
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QuoteOriginally posted by Bui Quote
My concern is: if I'm using a 70mm, standing 2m away from my model, to have a head and shoulder portrait, shooting at f2.4. If I switch to a 50mm, still at 2.4, the DOF would not be as shallow. However, to achieve the same composition, I have to move closer, say 1.5m. Wouldn't that make the DOF shallower, effectively canceling the advantage of the longer focal length?
For DoF, yes it will be about the same. But this doesn't mean the background blur will be. There's a calculator for that, you can adjust the parameters but it's set right now for your example:

How Much Blur? - A Bokeh Calculator - Asklens

With the parameters you chose the 70mm will provide noticeably more blur of the background (assuming it's a few metres behind the model). It will also include less of it. This can be an advantage if there are icky areas behind you'd like to avoid, and it can also make finding an area with a more consistent colour much easier.
08-14-2017, 07:17 AM - 1 Like   #8
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You've had great answers so far, but I'd like to add one.

Unless you are shooting both FF and Apsc, you should only consider the one you are shooting. 70mm on FF is 70mm, on an Apsc (crop) it equates to approximately 105, just as your 50mm equates to approximately 75mm on Apsc. All lens focal lengths are measured for the 35mm full format.

08-14-2017, 07:38 AM   #9
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QuoteOriginally posted by twilhelm Quote
You've had great answers so far, but I'd like to add one.

Unless you are shooting both FF and Apsc, you should only consider the one you are shooting. 70mm on FF is 70mm, on an Apsc (crop) it equates to approximately 105, just as your 50mm equates to approximately 75mm on Apsc. All lens focal lengths are measured for the 35mm full format.
i have to say I think this statement is very misleading. Focal length is a property of the lens only and has nothing to do with format. Whether one is using FF or aps-c a 70mm lens is a 70mm lens. Assigning a "35mm equivalent" to aps-c usage serves what purpose ? As far as I can tell it just confuses people into thinking they are getting greater magnification on the crop format as a product of the lens/camera combination.
08-14-2017, 07:38 AM   #10
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It took me a while to get my head around the crop versus full frame thing as far as DOF goes. Yes, the focal length of a lens goes up by 1.5 when you put it on an APS-C camera body. But you also "gain" depth of field because of the smaller image - roughly one f/stop. This means that a 50mm f/1.4 lens will take about the same pictures as a 75mm f/2 on a full frame.

This explains why cell phone cameras have such enormous depth of field - it is down to the teeny tiny sensor. It also explains why a medium format camera is not easy to focus manually - bigger image, less DOF.

Another APS-C vs. 35mm example - a 200mm f/2.8 becomes a 300mm f/4 and not a 300mm f/2.8. As far as DOF goes anyway. The extra light from a faster aperture is still yours to take advantage of. Hope I have it right and that this helps.
08-14-2017, 07:55 AM   #11
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QuoteOriginally posted by Wasp Quote
Yes, the focal length of a lens goes up by 1.5 when you put it on an APS-C camera body
Nope, it just doesn't. What does change is the angle of view. You get a narrower AOV on a smaller sensor. When you view that image on a computer screen or print it "appears" you have gained a longer focal length, but that is because your image has been "enlarged" more than a FF sensor (in fact in pixel terms it has been reduced less, assuming identical pixel pitch).

Now DOF is different between the formats, but this is not an exact science. It is related to camera to subject distance, and what size you are viewing your output. As has been pointed out DOF has no meaning at all unless you reference it to the output format. If I take identical pictures with a FF K1 and an aps-c sized sensor but print the K1 picture at half the size of the aps-c one my K1 picture will have more DOF.

Last edited by pschlute; 08-14-2017 at 08:36 AM.
08-14-2017, 08:26 AM - 1 Like   #12
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QuoteOriginally posted by Bui Quote
In searching for a good portrait lens, I've seen many discussions and debates, about the DA 70 vs DA* 55, or FA 50, for example.

For depth-of-field specifically, people said 2.4 is still big enough with the DA 70, because with the 70mm focal length, you would achieve more shallow depth-of-field at the same aperture, than with the 50s. This is also what I read in theory.
Yes, this is the theory, but not that relevant or important in practical use. Because, as you noted yourself, we don't use these lenses for the same reasons or the same situations. And also because, as other mentioned above, many other factors contribute to the appearance of out of focus areas, not even considering the results you're trying to achieve... The 70 works very very well for tight head and shoulder portrait, and 2.4 is large anough sperture for these. For full body portrait, a 50mm or even a 35mm might be preferable. Not because of DOF considerations, but because you don't want to be, or can't be, 50ft from your subject... Even if in theory you could do it...
08-14-2017, 10:33 AM - 1 Like   #13
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QuoteOriginally posted by Bui Quote
My concern is: if I'm using a 70mm, standing 2m away from my model, to have a head and shoulder portrait, shooting at f2.4. If I switch to a 50mm, still at 2.4, the DOF would not be as shallow. However, to achieve the same composition, I have to move closer, say 1.5m. Wouldn't that make the DOF shallower, effectively canceling the advantage of the longer focal length? I have a Sigma 30 1.4, and at f1.8 it is so blurry, I'm not sure if 2.4 of the 70mm can give that
We can calculate this! I did a test involving a shelving unit. With the 77mm lens, I had to be 12ft away to capture it. At 55mm, I had to be 9ft away. The K-1 was used.

At f/2.4 with the 55mm: 8.64ft to 9.61ft for 1.15ft of DoF
At f/2.4 with the 77mm: 8.71ft to 9.31ft for 0.59ft of DoF

The thing to remember is that the DoF depends nonlinearly on the focal length and other parameters:
Depth of Field Equations

What will also change with focal length is the strength of the blur. It will be stronger at higher focal lengths, even though you have to move back farther.

QuoteOriginally posted by CarlJF Quote
Yes, this is the theory, but not that relevant or important in practical use. Because, as you noted yourself, we don't use these lenses for the same reasons or the same situations. And also because, as other mentioned above, many other factors contribute to the appearance of out of focus areas, not even considering the results you're trying to achieve... The 70 works very very well for tight head and shoulder portrait, and 2.4 is large anough sperture for these. For full body portrait, a 50mm or even a 35mm might be preferable. Not because of DOF considerations, but because you don't want to be, or can't be, 50ft from your subject... Even if in theory you could do it...

Just FYI, with the FA77, I need to be about 12-15 feet away from an average woman (5'2" to 5'5", maybe as tall as 5'9" or 5'10" with heels) to capture her head to toe if she's standing. Mari here is probably about 5'4" or so and I was 4m away (13.2 ft) according to Photoshop when I took this:

08-14-2017, 11:38 AM   #14
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QuoteOriginally posted by Bui Quote
My concern is: if I'm using a 70mm, standing 2m away from my model, to have a head and shoulder portrait, shooting at f2.4. If I switch to a 50mm, still at 2.4, the DOF would not be as shallow. However, to achieve the same composition, I have to move closer, say 1.5m. Wouldn't that make the DOF shallower, effectively canceling the advantage of the longer focal length?
Is your intent to maintain framing or a particular DOF? You can, of course, simply test it the difference in DOF yourself using a meter stick. This is not rocket science and does not require calculation. The time-honored approach to this problem is:
  • Choose the lens and focal length based on optical characteristics and desired working distance.
  • Choose aperture narrow enough to ensure adequate DOF for your subject. DOF preview is handy for this task. The challenge with most focal lengths 50mm and longer is adequate DOF, not the opposite.
  • Position subject relative to background elements appropriately for subject isolation. The deeper the background elements, the greater the isolation. One does not need razor-thin DOF to accomplish this.
FWIW, your larger concern moving from 70mm to 50mm should be the change of perspective to maintain framing. The purpose of a longer focal length for portrait is to move the camera (lens actually) further from the subject to equalize the relative size of features (e.g. nose to rest of face).


Steve

Last edited by stevebrot; 08-14-2017 at 11:45 AM.
08-14-2017, 11:53 AM   #15
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Perhaps more simply--DOF varies with f stop and magnification.
So different FL lenses at different distances have same DOF if:
-- the same object in focus is the same size (same magnification)
-- and the same f stop
-- and you print/view result at the same magnification (same size/same crop)

Last edited by dms; 08-14-2017 at 01:44 PM. Reason: deleted redundant/confusing words at end on 1st line
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