[NOOB] Depth of Field


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refraXion

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Mar 24, 2008
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Hougang
#1
I'm slightly confused over this concept and hope to gain some understanding from all my seniors over here. I'm thinking this confusion is a result of me mixing two concepts of "different frequencies" together and as a result failing to piece two sets of understandings together, so I hope I could be brought back to the right tracks again. :)

Quote from DPReview:

Lenses with shorter focal lengths produce images with larger DOF. For instance, a 28mm lens at f/5.6 produces images with a greater depth of field than a 70mm lens at the same aperture.
I've also noticed that in macro photography — which is an application that often requires shallow DOF if I'm not wrong — it is preferred to shoot at the telephoto end. Putting aside the more practical/execution issues such as wanting to shoot from a distance to prevent scaring the insects and animals (or if it helps, imagine the case that I want to shoot an inanimate object for practice/abstract purposes), does telephoto also help to give that desired DOF?

My train of thought is as such: consider the case that I wanted to take a picture of a subject at a given size with a shallow DOF. By basic principle of DOF, if I want to have a greater blurring of the background, I should decrease the distance between the subject and my camera, as well as increase the distance between the subject and the background, keeping all other things constant.

However, after I "zoom in" on my subject, I have to move back further than if I were to shoot the same subject at the wider end, in order to obtain that same given size. Doing so effectively violates my basic understanding of obtaining a shallow DOF as stated in the previous photograph. So why is shooting telephoto preferred?

Having said that, to complicate matters further: some lens have higher F-numbers when at their telephoto ends. So using telephoto would mean using a larger F-number while at the same time increasing subject-camera distance.

To give a concrete example with figures: would shooting at 35mm f/2.8 or 140mm f/4 be better to give the desired shallow DOF?

Granted, in this case the F-numbers are different so it's no longer about the focal length only, but since shorter focal lengths (35mm in this case) provide greater DOF, does the f/2.8 of 35mm more than compensate to give a shallow DOF compared to f/4 at 140mm? Don't really know how to phrase this, hope it isn't too confusing!

Sorry for the long read, I thought it was better to describe my question in greater detail. :)
 

yannh

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Dec 10, 2007
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Woodlands
#3
Not sure if I understand your question correctly.
You've mentioned 3 variables, f-number, focal length & object distance.
All statements that you've mentioned about DOF are true, when 2 variables are fixed and changing the other one.

To simplify a bit, when you combine the focal length & object distance, DOF change according to "magnification". Means if you're using same f-number, no matter what focal length & distance you adjust, if the size is the same on your viewfinder or image sensor, the DOF is about the same. This is not 100% true but rather close.

Hope this help.
 

refraXion

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Mar 24, 2008
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Hougang
#4
Thanks for your replies and the link to the very useful tool.

Okay, could you please tell me if this is correct? Just to verify if I have understood you correctly. :) Let's assume I have a set up such that I place an object on a table and I shoot it "horizontally" along the plane of the table such that the background is a wall maybe 4m away. Can I take it to mean that regardless whether I shoot the object with my camera 5cm away or I walk 1m away and zoom in to the same object size, the background will have similar blur, assuming constant f-number?
 

yannh

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Dec 10, 2007
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Woodlands
#5
In general yes.
One of the factor that makes it not 100% true is, hyperfocal distance of the lens. You may check it out on the definition.
 

refraXion

New Member
Mar 24, 2008
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Hougang
#6
Alright, I'll be sure to read up. Thank you very much for the info!
 

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