# D.O.F.; does equal framing cancel out focal length differences?



## ScottyP (Jul 17, 2016)

Thinking a lot about DOF as I am getting into macro. I know DOF gets shallower as you get closer to the subject, and also as you increase the focal length of the lens. Conversely DOF gets deeper as you go to a shorter/wider focal length and also deepens as you increase distance to subject. 

Does the difference in DOF between a wider angle lens and a more telephoto lens equalize if you frame exactly the same? In other words if you stand closer to the subject with the wide lens and farther away with the longer lens, so the framing is the same, will the DOF be the same? I realize the longer lens will compress the scene more and produce more bokeh, but what about the DOF? There might be some kind of way to use geometry along with a DOF calculator and an AOV chart for lenses but that just hurts my head.


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## Zeidora (Jul 17, 2016)

Correct. Notice that DOF formula does not contain focal length (or any equivalent) but only magnification, which is a different way of saying "framing". Then there is the circle of confusion (usually one takes 0.03 mm = 30 µm), which is a constant, and f-stop; for macro that is the f-stop set on lens barrel, not the effective f-stop at sensor plane.


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## privatebydesign (Jul 17, 2016)

The dof stays the same for the same subject magnification irregardless of focal length. But it often doesn't look like it due to perspective.

The framing can never be 'the same' if different focal lengths are used from different places, your point of view creates the perspective and subsequently the framing. If the subject is two dimensional and fills the frame it is impossible to tell the difference in focal length used, but our subjects are not normally two dimensional (copy work aside).


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## neuroanatomist (Jul 17, 2016)

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Zeidora said:


> Then there is the circle of confusion (usually one takes 0.03 mm = 30 µm), which is a constant...



The CoC is constant for a given sensor size. The value varies directly with sensor size, for example the CoC for Canon's APS-C is 0.018.


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## StudentOfLight (Jul 17, 2016)

When shooting macro, I think of focal length more with regards to how much you want to show of the background. Longer focal lengths will show less background while wider focal length lenses will show you more background for a given sensor size.


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## neuroanatomist (Jul 17, 2016)

StudentOfLight said:


> When shooting macro, I think of focal length more with regards to how much you want to show of the background. Longer focal lengths will show less background while wider focal length lenses will show you more background for a given sensor size.



Conversely, the longer FLs will blur the background more, while the additional background seen with the wider FLs will be less strongly blurred.


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## ScottyP (Jul 17, 2016)

neuroanatomist said:


> StudentOfLight said:
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> > When shooting macro, I think of focal length more with regards to how much you want to show of the background. Longer focal lengths will show less background while wider focal length lenses will show you more background for a given sensor size.
> ...



These are interesting to think about. Thanks to all for the input.

I had been thinking about how different it might be shooting either a 50mm macro or a 180mm macro instead of my 100mm, primarily because I find it so challenging to get good bug shots. Especially flying ones. The bug hardly stays on one flower long enough to get the camera in there and focused (by leaning in and out). The wind is moving the flower too. Using the f/2.8 isn't even a possibility due to the tiny DOF. The small apertures (f/16 and higher) I end up using need light, but the movement requires a fast shutter, so I find I am cranking up the ISO. 

I am thinking I need to draft my 9 and 10 year olds as photo assistants to hold a reflector for some light, and maybe some kind of wind-blocker too, but they are just about as wiggly and hard to keep on-task as the bugs are. 

Still fun though.


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## danski0224 (Jul 18, 2016)

The 50mm will bring the camera much closer and the 180mm will allow more distance. I don't know if it is a linear change in comparison to your 100mm.

The 180mm will blur out the background much more than the 50mm. 

An extension tube on your 70-200 will give a shorter MFD at the expense of infinity focus- your working range may be only a few feet. The extender should also work. Don't know if that would get you close enough.


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## kphoto99 (Jul 18, 2016)

ScottyP said:


> neuroanatomist said:
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> > StudentOfLight said:
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You may want to consider using flash, on camera or off with either a cable or wireless transmitter. 
If the flash supports HSS then you can shoot at 1/1000 and shorter to minimize blur from camera movements. 
If you shoot above 1/2000 the background gets very dark. 
If you like to do many continues frames then get yourself Godox PB960 external battery for the flash, with it I can do 6 continues frames at full power with Yongnuo 600EX-RT flash when connected to both battery outputs.


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## Zeidora (Jul 18, 2016)

neuroanatomist said:


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If you get really serious about it, then output size/viewing distance also has to be considered. The 30 µm is for FF with enlargement to 8x10" viewed at about 2 feet (reading distance). For smaller final output, the CoC can be quite a bit larger. That's why contact prints (web thumbnails) look very sharp, but may be unusable at 8x10 or larger.

If you know that your image will be printed at a small size, then you can stop down more, get more depth of field, and the image still look sharp at that final production size. Reverse is true for large size viewed at close distance (pixel peeping, billboard up close).

Additionally, there is a psychological component to sharpness/blur as well. Casual viewers will tolerate more blur than experienced ones.


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## mukul (Jul 18, 2016)

ScottyP said:


> Does the difference in DOF between a wider angle lens and a more telephoto lens equalize if you frame exactly the same?



Brian on TDP has illustrated the case here







Ref: http://www.the-digital-picture.com/Reviews/Canon-EF-180mm-f-3.5-L-USM-Macro-Lens-Review.aspx


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## kaihp (Jul 18, 2016)

neuroanatomist said:


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Interestingly, Canon seems to disagree with this. If you look at the EXIF data, you'll find that the 5D3, 5DS and 1DX2 reports 0.025mm, but the 1DX reports 0.026mm. Baffled me a bit when I saw that.

Strictly speaking, the CoC is also a function of magnification and expected viewing distance, but I'm sure Neuro is much better versed in this than me.


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## Mikehit (Jul 18, 2016)

ScottyP said:


> Using the f/2.8 isn't even a possibility due to the tiny DOF.



Not sure what you mean by that - you can use the 100mm f2.8 at f11....
The disadvantage of the 50mm/60mm is the very short working distance - for macro you are often a matter of centimetres from thee subject if you want true 1:1. This can be a distinct problem for bugs. The other advantage of something like a 180mm macro is that you are less likely to get so close you are reducing the amount of light on the subject.


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## Valvebounce (Jul 22, 2016)

Hi Mikehit. 
I think that what ScottyP was trying to convey is this. 
Using the 100mm f/2.8 at f2.8 isn't even a possibility due to the tiny DOF. At lest that is how I read it. 

Cheers, Graham. 



Mikehit said:


> ScottyP said:
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> > Using the f/2.8 isn't even a possibility due to the tiny DOF.
> ...


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## Mikehit (Jul 22, 2016)

Valvebounce said:


> Hi Mikehit.
> I think that what ScottyP was trying to convey is this.
> Using the 100mm f/2.8 at f2.8 isn't even a possibility due to the tiny DOF. At lest that is how I read it.
> 
> ...



That makes sense. Thanks, Graham.


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